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WillMill - HSM Benchtop Composite Mill
Goals
- Mill aluminium moulds
- Good surface finish in aluminium
- Have a very damp machine
- Get experience with casting composites
- Have fun!
Requirements
- Max deflection: 70N/?m
- Positional accuracy over 100mm: 0.05 mm
- Part flatness over 200mm: 0.03 mm
- Vibration dampening properties
- Must contain dust and chips
- Benchtop machine
Specifications
- Gantry clearance: 120mm
- Part size: 300x600mm
- 1.5 kW HF spindle
Features
- Square linear rails and ball screws
- Epoxy granite base
- T-slot table
I have not specified a part accuracy, but I'm fairly certain that I can get the positional accuracy to be better than 0.05 mm. The goal with the tolerances is to be able to machine somewhat of a press fit in aluminium, and I'm fairly confident I can achieve that by using precision milled stock (0.01 mm flatness) to mount the linear rails onto. I think the flatness of the t-slot table and aluminium stock to mount the linear rails on, will be a little better than specified by the manufacturer, to give them some room for error. Therefore the machine will probably be a little more precise and increase my chance of milling "somewhat" of a press fit. The base of the machine will either be a thin t-slot table cast into epoxy granite, or just a solid piece of cast iron t-slot plate. The t-slot table will also have a flatness of 0.01 mm. The linear rails for the gantry will simply be mounted directly to the t-slot table.
I've specified a maximum stiffness of 70 N/?m for the whole machine, this means that the machine should not be able to deflect more than 0,001 mm from where it previously was, every time we apply 70 N.
Now there's still a lot for me to figure out, but for now I'll start designing the gantry for the machine. I'm still not sure weather or not I will use epoxy granite for the gantry or if it will be made of a composite consisting of fibers. - It depends on what I can get my hands on for cheap/free.
Any questions and input are always welcome:)
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Re: WillMill - HSM Benchtop Composite Mill
Hi William - Your gantry clearance will have to be much bigger for a mill.. unless you are only doing plate work. but you state moulds so how tall will they be... Plus 70N/um is very stiff, look at Bamfords work that was his design target and look at the size of his machine to do the job...Peter
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You're right, it's very stiff. I'll aim for 70N/um at the moment and then perhaps adjust that spec a bit if it proves to difficult. For what I'm intending to use the machine for, 120 mm clearance is almost the minimum required (tool length considered) for what I intend to do with the machine. I'm a little concerned with how stiff I'll be able to make the z-axis when it's fully extended, so that's mainly the limiting factor of the gantry height. When I get further into the design I'll be able to see just how much deflection it will have and decide weather or not it would make sense to increase the gantry height. I intend to have two vices to hold metal pieces on and if stiffness becomes a problem I could simply put the vices onto some kind of riser block. This would increase rigidity as the gantry would be retracted further, but we will discuss options like that if it becomes relevant.
Yesterday I was doing some simulations on the gantry. I've continued with the design today and have currently ended up with a gantry that has a deflection of 2 to 5 microns (depending on where the forces come from) with a force of 1400N.
1400N should the force required to cut steel on a small scale.
I have simulated the gantry with a composite that has an estimated modulus of elasticity of 50 GPA and a density of 2 g/cm^3. I've force is applied to the front faces of "abstract" 20x20mm square rails 90 below the lowest point on the gantry, and the whole gantry body weighs in at just 13 kg. At the moment I'm not sure how stiff my composite will be, as I haven't made it yet. My plan is to mix carbon fiber cutoffs together with washed aluminium swarf from a machine shop. Aluminium has a young's modulus of 70 GPa and steel has a young's modulus of 200 GPa. Look up "modulus of elasticity" or "Young's modulus" on google, if you'd like to learn about "bending stiffness" as a material property.
For everyone reading along, Peter has helped me a lot already, and I'd like to mention that a lot of design workflow and inspiration has come from reading Peteeng's thread on one of his http://machine builds, there is lot's of great advice and information to read there. You can see some of his router kits and read about some exotic and machinable composite materials that he has made here: https://cncrouterkits.com.au/tetrium/.
Now regarding the gantry... I think it's lacking a bit of torsional stiffness, so I'll get working on that.
- William
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Re: WillMill - HSM Benchtop Composite Mill
Good morning William - The scope is coming along. BUT its time to go back to the beginning. At first I imagined you wanted to build a moving gantry machine. They are also called bridge and double column machines. The machine exists to move the tool around so there are two fundamental things to discuss. A) The tool holder and spindle B) the machine foundation (or structure). Now you are clear its a mill we understand it needs to be very stiff. Your 70N/um is a clear indication of that.
A) The tool holder and spindle - what do you intend here? A R8, Morse taper, ER collet, BT-20 or 30 etc etc..... This is the pointy end of the machine so its really important to get it settled.
Now ideally a machine is a pyramid with broad foundations and the tool at the top. Pyramids are statically and dynamically stable. Skyscrapers however are unstable and a gantry machine is equivalent to a skyscaper. This has to do with its bearing arrangements. Its difficult to place the tool load within the kern (or the centre) of the bearings. If you designed a chair where the seat was outside the 4 legs when you sit in it it would fall over. This is exactly what a gantry machine could do if we did not use bearings that where captive to the machine. So you need to look at other machine configurations before you commit to the gantry style of machine.
B) A moving gantry machine is inherently unstable. Ideally the tool point should be within the footprint of the bearings at all times. This makes it stable. With a 3 axis machine it is possible to have 2 of these within the foot print but not all three (I think). So have a look at mills in the table size you are thinking of and broaden your thoughts on the machine config. There are a couple I've seen and I can't find their image at the moment but you should have a think about this. A gantry machine is used to maximise the working envelope vs the footprint. If you do not have a foot print issue then you would not use a gantry config. Plus its for very wide tables. 400mm is not very wide.
as an aside I just found a machine builder in Denmark - maybe useful for you https://cnc-nordic.dk/
Enough for now. Peter
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Some alternative configs and some structural muses - Peter
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Hi William - I have been mucking about with alternative geometry for gantries (or any element really) Especially for casting as making hollows can be tricky. So I made two models to compare. One is from 2mm thick steel with an internal isogrid. The other is a hollow 5mm thick with the same profile (100x150mm) I placed a 100N load at the same place and there deflections are nearly the same. But the isogrid weighs 15.7kg and the hollow weighs 20.8kg. Triangles work!! Peter
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Re: WillMill - HSM Benchtop Composite Mill
Quote:
Originally Posted by
Williamlii
Yesterday I was doing some simulations on the gantry. I've continued with the design today and have currently ended up with a gantry that has a deflection of 2 to 5 microns (depending on where the forces come from) with a force of 1400N.
1400N should the force required to cut steel on a small scale.
- William
Wow. Where are you getting these numbers from (polite answers only please)? I'm trying to think of cutters of this size that could take cutting forces equivalent to the weight of one seriously fat bloke without breaking - or even deflecting noticeably, let alone anything like 2-5um. Then there's the machine itself....
There is a danger you might end up blowing smoke up each other's ass if you aren't careful.
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Re: WillMill - HSM Benchtop Composite Mill
Hi William - Now you have settled on a machine static stiffness you can apply 70N or 700N to the tool point and expect the machine to deflect 70N = 0.001mm or 700N = 0.01mm. Machines are designed to be stiff and very linear. The machine components will be very low stressed at these load levels so stress is not important at the moment. 140kgf is a very, very large tool load so would be unexpected. So maybe 700N is a good "design" load (vs actual tool load). I think you need more Z which will mean more deflection and is this a moving gantry design or a fixed gantry design? The bearings will have compliance and the base structure will have compliance so you need to start adding these to your model....
1400N / 0.005mm is 280N/um which is a very stiff component. A 20mm square car has a stiffness of ~200N/um Regards Peter
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Hi William - If you are considering a mouldless composite construction then consider using G10 or FR4 laminate. They are made to high tolerances and have good mechanical properties. A good Eglass quadaxial laminate will have same properties as these if infused. I have made laminates from 1mm to 55mm thick with about the same properties. So you could have these machined although shops don't like machining it, mill it in your shop or have it water jet cut to rough or finish. Flat construction has drawbacks so making moulds will allow shapes and features that you can't get otherwise. Peter
Perhaps a combination of G10 and laminated metal wil be a good compromise....
https://www.professionalplastics.com/G10FR4SHEET
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HI Peter - My space for the mill is very limited, so it has to be an moving gantry mill. I'd like it to be similar to the machines that Datron builds. Interesting pictures and constructions Peter, I'd also like to design a small scale 5 axis some day, but that would be far into the future... I have not settled on a spindle yet, but I will likely end up with a high speed spindle that has a common spindle diameter of 80 mm. I was initially thinking about buying a common 1.5 kW HF spindle with ER collets and then upgrade to a nicer spindle in the future. At the moment I'm not sure weather or not I should buy 1.5 kW, 2.2 kW or 3.0 kW, but I'll read up on how much power is required when it becomes relevant. If I end up needing 3 kW i'll just update the clamp from Ø80mm to Ø100mm. Interesting simulations Peter, I had never really though about that kind of triangle construction for a gantry, that's a great and simple idea!
Peter the leverage of a moving gantry and the forces applied to the gantry's bearings on the base is something that I have though about a lot. I didn't know that 20 mm rail cars "only" had a stiffness of 200 N/um, from the beginning I've intended to use roller bearings for increased stiffness, but I might just have to choose bigger rails, that should solve the problem. I'll look into this when I know what the z-axis will look like. Great input. I've worked a bit on the gantry in the mean while...
Yesterday I did something I thought was a little clever... I decided to redesign the gantry from the ground up using shape optimization in Fusion 360 (their generative design features are for premium users only). When you make a shape optimization simulation in fusion 360, it's optimizing the shape for 1 load, so I just combined results from multiple simulations and created 1 solid shape from that. Now the optimization workspace in F360 currently doesn't consider shapes for different kinds manufacturing, it just find's the critical load "inside" the body that's simulated, so it doesn't always create results that makes sense in the real world. See the linked picture. Now what is interesting is that I was able to make a gantry about 1 kg lighter than the first design and have about 20% less deflection. The body of the gantry currently weighs 14 kgs with a density of 2,00 g cm^3. Another thing that's really great about using shape optimization in the way I described, is that you get an end result that deflects equally when the load comes from the directions you simulated. This method is probably being used by machine designers everyday, but I though it was worth sharing anyways:)
Hi Muzzer, welcome. I've done a lot of reading the last two weeks on machine design and all sorts of materials that be used to build machines... I'm by no means an engineer and I'm in deep water with this project, but I read in an article somewhere that chatter can be generated when the end mill deflects more than 0.02 - 0.04 mm (somewhere around there, can't remember exactly but it was almost the width of a human hair). Now I don't know much about how much force cutters can take at their given size, but there's lot of variables to consider. I saw a guy on YouTube compare old end mils with newer end mills that were made with better materials. All of the compared cutters was of course the same size. Some of them were roughing end mills and others had variable flutes. The strongest cutter could take something like 4 times the load compared to the weakest cutter and potentially more, so there's lot's of stuff to consider before before going bezerk with a long finishing end mill for a roughing operation. If the machine deflects while taking a cut, then I could imagine that the machine's body would move back and fourth as well, as the edges off the tool cut into the material, which could produce vibrations and chatter. Therefore I think a stiffer machine would allow to tool to be used more effectively. Any thoughts? Take a look at this video for example, in my head the cutter should break under these cutting loads:
https://www.youtube.com/watch?v=M27CA9KYzbQ
Regards William
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Hi William - Choosing a HS spindle such as you are talking about is a poor decision for a mill. Those are router spindles not mill spindles and is inconsistent with your dialogue. If you are using Datron as a benchmark you will see that they are not using such a spindle. You need to research this area more. If space is limited and parts are 300x600 (so you need a table 400x700 minimum then consider a moving column design or a fixed 700mm high axis like a photo prior vs a gantry or bridge. This will be a stiffer approach. Plus synchronizing two columns seems easy but if you are to cut steel (is steel in there?) this is not a trivial consideration (small red flag being flown here) Driving two screws means that they will always be at least the screw tolerance delta from side to side, this introduces chatter and inaccuracies. Driving each axis with one screw is better for your concept letting the bearings do their work properly. Daltron will have very precise screws with parallel scales for displacement feedback. I have not seen them cut steel (has anyone seen this?), they are light cut flyers....Peter
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Hi Willkiam - Datron publish some interesting white papers. I found info that they do cut steel. Light fast cuts, so maybe 80mm spindle OK. Anyone have experience in this application? Peter
https://www.datron.com/project/cnc-c...n-white-paper/
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Hi Peter - That was a great little read. I've seen plenty of videos of people cutting steel with HF spindles, I found a guy that had build a very stiff fixed gantry frame and had excellent results:
https://www.youtube.com/watch?v=Klk6T1IBowU&t=189s.
Now regarding power it looks like a 2-2.5 kW spindle will be great for my machine, but I'll have to do a deep dive on HF spindles, when I have to decide which one to get.
When I have designed more of the gantry I'll simulate the rails according to their actual stiffness and decide which size of linear rails will be fit for the job.
Didn't know that Datron had displacement feedback, that sounds very clever and advanced. Your comment made me think about the "synchronization" problem with having two ball screws for the X-axis. I initially thought of wiring up two motors in a way that would allow me to use a single driver for the motors, but then I realized that this would't allow me to adjust how much the specific motor rotates according to it's given signals. If a tell two motors of the same model to rotate 360 degrees, does anyone know how much they can differ from the desired amount of rotation?
It could be that motor 1 rotates, 360,01 degrees, and motor 2 rotates 362 degrees. Is this a real world problem, or just something that I should ignore? - I guess it depends on what motors you get your hands on, but does anyone know just how "precise" generic stepper motors are compared to an identical model. If both turns 370 degrees then they would be off by the same amount, and I could just use a single driver to account for that. What if I used servo motors instead? Any input is very welcome.
I might just end up with a single ball screw under the machine base to move the gantry back and fourth... I'll think about this and make then decision and comparison when it's time to design the base of the machine.
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Hi William - The 20mm rail will be fine for your mill. Its the foundation that's the issue. It needs to be very stiff. Using single driver is possible and I have done it to test. Cheap small machines use a single driver BUT it may have mid range resonance which has been my experience. A driver receives feedback from the motor (back EMF or back voltage) this is used to change the waveform to improve smoothness. If two motors are used on one driver this signal can be complex and the driver does not work properly. Especially at high speed (more back voltage). So I recommend one motor one driver.
Standard stepper motors are 5% accurate mechanically with 200 steps per rev. You can get 400step/rev motors as well (I use these on one of my machines) and you can get precision grades. A stepper moves pole to pole and this is an electrical pole so its a bit wobbly on position. So 360/200= 1.8degs. So if you have a 10mm lead and direct drive 10/200= 0.05mm per mechanical step and 0.05*0.05(%)= 0.0025mm so your motion accuracy +/-0.0025mm So the possible delta side to side is 0.005mm from the motors. This is within your stated tolerance so not a big deal at the moment. Its more of a dynamic movement issue (walking/racking one side will always lead the other and as it moves, this changes ( Look up stick slip behavior ) and a bearing arrangement issue.
You do need to move on and flesh out the entire machine. A machine is complex and inter related. The first milestone in a design is to have a general arrangement that at face value meets the scope and has no obvious fatal errors. You are not getting close to that yet. Peter
Please note the the steel video is a fixed gantry and will be significantly stiffer then your current moving gantry design. Its a very good space frame design and its steel. It sounds very solid. Peter
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Hi William -The space frame mill in the video is very interesting. It's a great solution to the machine stiffness problem vs whats called open loop structures ie large blocks of material and cantilevers. Here's a paper on closed loop machines. Also included is a nice small extrusion closed loop machine. Peter
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Hi Peter - Oh you meant that the base had to be stiff, and not the rails... Yes, the base will end up very stiff indeed, no worries. Thank you for your explanation of drivers and motors peter, that settles it. I'll have to use two drivers for two motors, even though they are synchronized.
The I'll design the machine will not affect the shape of the gantry much depending on weather or not I use a single or two ball screws. But point taken, I agree that this should already be sorted out. It initially was, but you made a few good points that made me lean more towards using a single ball screw instead of two.
Ouhh, interesting paper. I really like these types of machines, but there's also a lot I don't like about them too. The paper introduced to quite a lot of new constructions in parallel kinematics that I have not seen before. I do hope these machines will get accepted in the industry in the future, all though it doesn't seem like it.
Regards William
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Hello, I'm back with some design updates. I've been tinkering a bit around with some FEA and shape optimization in an attempt do design a rigid Z-axis/clamp. Again this has been designed for a material with a Yong's modulus of 50 GPa, and the results a rather impressive. For the simulations I have used a hollow tube to represent a 2.2 kW JGD-80 spindle (about 8 kg) , although I'm sure it will be a little stiffer than what I've made it in CAD. At the moment the Z-axis also functions as a Ø80mm clamp for the spindle, and it weighs in at just 7 kg. The idea with the current z-axis is that it's very damp as a composite, and I'm hoping it will do wonders compared to aluminium clamps. I'm not sure how well a composite material will grip the spindle when it functions as a clamp, therefore I could potentially cast a sleeve into the casting. I'll have to figure this out if this becomes the winning design. The screws for the clamp will of course have washers if this end up as the final design. Another very noteworthy thing to mention is that I've increased the initial z-axis travel so that it can travel 150 mm. The gantry will therefore also have to be higher than it currently is. The Z-axis clamp/plate is currently a bit tricky to cast, I'll have to sleep on it and figure out whether or not it would be worth it to go though the trouble. I do have a simple solution for casting it, but it does make the mould a little more complex.
Other alternatives and materials. I have multiple designs that I think could work well. The simplest of them all would be to buy a stress relieved cast iron plate and mount two generic Ø80mm clamps for the spindle. A more exotic method would be to laminate metals together using epoxy and glass veil as Peter suggested as an option. This would also be a more damp compared to solid aluminium. Some articles mention that epoxy granite has up to 10 times better dampening properties than cast iron. My composite material will made be made of aluminium and carbon swarf and will hopefully have a similar dampness to that of epoxy granite.
At the moment I really like the current design. It has superior damping properties compared to conventional materials, and I think it looks really cool and professional. I know pretty doesn't cut, but it makes me happy..
I'll return with some alternative solutions and compare the results.
- William
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Hi William,
Found this today good read. Your 70N/um is way up there but as usual aim high....Peter
https://books.google.com.au/books?id...N%2Fum&f=false
you do need to include bearings : 1) you have to be able to assemble the assemblies 2) bearings take up real estate and their packaging size may drive other dims c) you need to think about tramming the spindle. How are you going to do that? These things get sorted at assemble level. Parts are always very stiff, assemblies are 30% to 50% efficient in stiffness at least.
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Hello William - On the point of modelling. You are using a "design" material with E=50GPa this is possible from CF offcuts using infusion to consolidate the laminate. The process would be to model the part hollow to establish what thickness the skin needs to be to get to what deflection you need. Now F36o uses isotropic theory to do the modelling. Isotropic theory says that the shear modulus (G) will be about E/(2.6)= 19GPa. Unfortunately composites have a G~9GPa so the isotropic FE will under estimate the shear deflection by about half. So if there is a dialogue in F360 in which you can enter in the G separately use 9GPa.
Aluminium swarf, from tests it is unlikely you will get a volume fraction better then 45% so E=70*0.45*efficiency. The efficiency of longish bits is about 0.6, Spherical bits is 0.5 and long fibres is 1.0. So your filler will be 70*0.45*0.6= 18.9GPa say 19GPa isotropic. This will bring down your filled volume stiffness a fair bit over the 50GPa you are using. So make a hollow part and see how its deflection changes vs the solid part. Determine if there is a thickness that does not influence the deflection much say 10% delta the solid deflection. That's the skin thickness you want to aim at.
So casting would be to make the mould, lay in the CF at the design skin thickness, infuse, let cure. Next day fill with Al swarf infuse, next day release part done. Peter
CF by the way E laminate = 200GPa*0.45* 1.0 = 90GPa if all fibres in one direction. If in 2 directions then E=90/2=45GPa if in random directions its a bit better then 45 so 50GPa is good.
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Hi Peter - Great numbers regarding machine stiffness. You're right 70N/um is perhaps a too high then. I'll see how strong I can make the Z-axis without adding absurd amounts of weight. Then when I get to simulate the whole gantry including the z-axis we can determine if we need to work on adding more stiffness (probably not). I've heard that FEA overestimates the stiffness sometimes, and if we then also include the "lost" stiffness by simply having joined parts, then the 70N/um would drop to something quite a bit lower in reality, right? I'll take a closer look at that book soon, seems like it's packed with great information that I can use right now.
Oh yes, the Z-axis cannot be trammed as it is right now, I totally made a brain fart. In my head I could just tram the whole z-axis, but by combining the clamp into the same part I forgot that this removed the possibility tram the spindle separately. I'll have to revise the current design. I know that the bearings and actual thickness take up an important role, but since you point it out, I think I'll try and get a cad model from the manufacture. Then everyone can benefit of having that information available.
Thank you for mentioning the thing about fusions calculations on the shear modulus. It's currently at 26 Gpa, I'll edit it to 9 GPa under "manage materials" - Thank you Peter :)
Volume fraction. That's a great word. I've been a little worried about how much a vacuum back could compress aluminium swarf in a mould, but you introduced me some numbers that sounds realistic. It would indeed be a lot better to use milled aluminium strands. I've thought a bit about crumbling random aluminium swarf with some kind of simple and crude process to make a more compact and dense swarf.... If we assume that I were to make a "pure" aluminium composite, I was hoping that I could get swarf dense enough to have a volume fraction of 70%. Do you think this will be possible by using strands?
Great idea, using woven carbon fibers for the essential skin thickness. Would you use the unidirectional type? It just makes sense, because it will give us the optimal stiffness if pointed in the right direction. Perhaps a mix of unidirectional and twill will work out better.
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Morning William - FEA on a part basis will be spot on with stiffness if it is constrained and loaded realistically. In simsolid I can model the friction in connections and the bolt loads so it will model assembly deflections accurately. In your case it will need to be a bonded connection? How does F360 deal with assemblies? If the connection is bonded I assume it to be 50% efficient a a first guess.
Using close pack fibres that are well stacked (hexagonal close pack or square close pack) theoretically you can get 90% plus but in reality 60% Vf is the top figure. Random bits I expect to be 40% Vf. Hard to get to 50%. Using CF will be uneconomical for you unless you can get scrap. I'd use a random scrap stack. Need specialist software to design laminates and FE laminates I use Strand7.
You need to get some aluminium swarf, place in a known volume container on a scale weigh the swarf then fill the swarf volume with water and weigh. You can then convert weight fractions to volume fractions and you will know the Vf of the system. Same with sand if you go the sand route.
With glass and CF laminates under 1atm pressure usually works out Vf=50%. Autoclaves at 6bar can get to 60% Vf in laminates. Internal hydraulic pressure and friction within the laminate prevent squeezing it much further. It is possible to go further, I did read Boeing went a lot further but you do need resin around fibres and the autoclave technology becomes complex. Peter
https://www.compositesworld.com/news...s-to-tennessee
there are recyclers in europe as well
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Hi William - heres a link to common bulk densities of stuff. take sand its dry bulk density is 1650kg/m3 its crystalline density is 2200kg.m3 (silicon dioxide) weight = density x volum so volume = weight/density so 1650kg/2200= 0.75m3. so the volume fraction of sand is 0.75 which is really good. Using the water fill test you can determine this yourself. So if you used sand E=70GPa then the E for the epoxy mix will be 70 x 0.75 x 0.5 = 26GPa . which is about the published stiffness of many EG mixes. Some get to 35GPa but I think they measure this in compression which is the stiffer direction. Peter
https://www.simetric.co.uk/si_materials.htm
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Hi Peter - Glad to hear that FEA will estimate the stiffness in real life, assuming that one would constrain it correctly. F360 is great with assemblies, but sometimes there can be trouble when simulating something like a hollow body filled with a different material. That's what I've been having trouble with anyways.
Great idea with using water to find the potential volume fraction of the swarf used. I'll definitely experiment a little with that. Oh that's genius, I can estimate the stiffness by knowing Vf (volume fraction) and E (Young's modulus) of the materials! Thank you Peter
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Hi William - Then just model the hollow and the core is a bonus. Have you found a suitable scrap CF yet? Its time to nail down the material you will use otherwise your time in FE is wasted. Peter
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Hi Will,
Here's a box of offcuts from one of my engineering customers. I get a box every 2 weeks or so from them. It adds up over time. I'm sure there must be a composite fabricator nearby that can do same. Any "dry" carbon offcut ask them to put in a box for you. Peter
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Hi William and others - I was watching a video of the Taiwan Machine show from 2015 and saw this, a CF gantry on a Hiwin machine. This particular gantry is a cantilever. I'm sure we shall see more of these around. Peter
https://www.youtube.com/watch?v=03L-ayrKEYw
I do like air bearings and intend to make a machine with these soon.
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Hi William - another article for you, but settle the material!! . One papers machine is about 30N/um the other is about 150N/um measured on VMC sorry that papers too big to upload.... Peter
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Hi Peter - Things have been moving a bit slow here, lot's of things going on right now. Yes regarding materials, the plan is to use woven carbon fiber for the skin of the components, and then use carbon fiber swarf next to the skin to maximize the potential stiffness. Aluminium swarf will be used for the core as a filler. The amount of carbon fiber used will depend on how much I can get my hands on. I'll focus on getting my supply of materials sorted today.
Update: The revised z-axis is finished. It's a composite clamp for the spindle that mount's to a cast iron plate with a bolted connection. In this way the head can be trammed with shim stock or other means of "adjustments". I have though of a second configuration for the z-axis. I will post some pictures and do a little comparison of the two configurations.
I received my 4.8 CFM vacuum pump with the mail today. I miss being in the workshop so I have decided to make my own vacuum chamber as a little excuse to getting my hands dirty again. I'll just mount some vacuum hose fittings on a thick acrylic lid and put on some silicone sealing paste. Then vacuum lid can the go on just about every pot. I'll do the same for a resin trap. This also saves me a bit of money ;-)
The Mill Stiffness PDF was a great read!
Regards William
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Fwiw..
I have actually built a mill, really a VMC, for milling steel, somewhat efficiently.
2.4 x 2 x 1.5 meters in size, 1600 mm table, 1200 mm movement, 32 mm ballscrews.
When I stood on top of the spindle, 85 kgf, originally it deflected 0.28 mm.
After 5 revisions and 1000 kg more in tool steel and 400 bolts, the deflection was around 0.12 mm (v3,v4) before the latest revisions.
At the moment I needed to take off the front beam, since the auto toolchanger was too large, and I need to put a spacer in to accomodate it.
Once the beam is back in place, I can measure current deflection.
Anything around 0.05 mm will be great.
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Re: WillMill - HSM Benchtop Composite Mill
Hi Hanermo - That sounds like an beefy machine. 0.12 mm deflection is more than what I would have expected for a machine that can mill still "somewhat efficiently" as you state. I'd love to see a clip of the machine milling some steel.
On another note, that means that my machine is way more stiff than it has to be. This is of course not bad news, it means I can slack a little with the composite the machine will be made of. It doesn't has to be as stiff, which potentially means I don't have to use any carbon in my composite mix... I'll stick to the plan for now: Aluminium swarf, carbon strands and woven carbon fiber skin.
Thank you for the numbers, good to know.
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Re: WillMill - HSM Benchtop Composite Mill
Hi William
1) Resin trap - You need to select a suitable container to go inside your vacuum pot. eg a icecream container, a std 10L bucket or something that is readily available and a throw away. I use std 10L buckets in mine. I made my trap by laminating fibreglass around a large plastic bucket. If resin flows into your trap (and being a learner it will) its a pain to clean it so by having the resin flow into a container you just throw that away. Plus make it big enough to get your hand in there. You need to wax it very well to make resin removal easy. Do not seal the pot with silicon. You need to be able to get the lid off and on easily. Use some foam or plastic sheet and cut a seal. Test your containers, vacuum will collapse thin or non circular objects.
2) You have not designed the entire machine yet. The entire machine will deflect quite a bit more then you think once it is fully modelled. A machine can never be too stiff it always works out less stiff in reality then on paper for various reasons.
3) You may plan to use CF cloth for the outside but do you have a source yet? and why go to all this trouble yet use a cast iron plate for the Z Axis? What is CF swarf? If you have a suitable supply of CF then you can make a slab of CF for the Z axis. This is then in-line with your objectives....Regards Peter
Re: woven cloth vs stitched cloth. Woven cloth has non-straight fibres (kinked) and therefore is less stiff (at least half the stiffness) then stitched cloth which has straight fibres. So try to get stitched cloth offcuts not woven. If you get woven then place this internally not near the surface. Plain weave is often used for its appearance on the surface as this is the "look" but structurally weaves are very inefficient.
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Hi Peter
1) Good idea, I intended selecting some kind of pot that would be bigger than a throw away container. Waxing the mixing cups or container for easy clean up, that will definitely make my life a little easier. Great tip Peter.
The silicone will not make a permanent seal between the lid and the pot, it will act as a gasket. There are special types of silicone for this purpose.
2) You're absolutely right, the machine will deflect more than what it does right now, and even more in reality. And yes, the stiffer the better, but if the end result is overkill, then I don't necessarily need to add carbon strands into the composite (unless I get a free source to these)...
3) The cast iron plate is not what gives the z-axis most of it's stiffness. That would be the clamp. The cast iron plate will be precision milled with a 0.01 mm flatness and the linear rails will be mounted to it. I'm not sure just how precise my mill can mill parts flat, since I don't have a surface plate to verify it. I'll buy a straight edge and test this to see weather or not I could mill the plate flat myself, but until then I'll use a flat milled cast iron plate to mount the linear rails of the z-axis to. By carbon fiber swarf I mean carbon fiber strands. Some fiber strands mixed with cutoffs just looks a bit like swarf to me.
Thank you for explaining the difference/properties between woven and stitched cloth. I will use stitched cloth.
I've contacted every company in Denmark somewhat near me, that delivers or uses carbon fiber, for carbon cutoffs.
(the z-axis will also have two screws for the clamp at the top)
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Re: WillMill - HSM Benchtop Composite Mill
Hi Will- The composite industry talks about short fibres say <50mm milled fibres <5mm long fibres all the rest. Woven cloth, stitched cloth, multiaxial cloth. I still don't see why you want to use cast iron but so be it. Peter
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Re: WillMill - HSM Benchtop Composite Mill
Hi Peter I have not found any source of flat milled composite slabs. But you right, and it annoys me a little too. I'll make the saddle and z-axis "backplate" of composites too then. I hope my mill will make them flat enough, but if they end up with a flatness of 0.03 mm, then so be it.
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Re: WillMill - HSM Benchtop Composite Mill
Hi Will - Given your resources you can only do what you can do with your mill. So probably time to give your mill an overhaul in prep for the new mill build. Sharpen your tools before you do a job is always a good plan. In your objectives it says "experience casting composites" so CI is in conflict with that objective. Plus this is a design exercise. There is no value in making a compromise in this project. You do the best you can on paper and at the end you may have to make compromises due to practical or economic reasons. But right now there is no point in compromising. Design the best composite mill you can then figure out how to realise it.
The practical side of infusion of parts is still a huge learning curve and your months away from that unless you start parallel projects... such as making a slab of G10... who will be your composite supplier of wax, fabric, vac bags, tacky tape, breather cloth etc etc??
Peter
https://www.professionalplastics.com/contact they have a european distributor. G10 or FR4 should be readily available but you can make it yourself now you have a vac pump.
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Hi Will and others - I have been designing a small mill and have come back to looking at gantries. As usual the issue is square gantries lozenge (change to a diamond shape ie shear deflection). In the Maximus thread I determined that the corners needed thickening or to use a diagonal brace. Others have filled their tubes with EG, placed internal webs etc. This has pointed to the fact that gantries are shear dominant and are not strictly beams in flexure. This is called short beam behavior. Short beam theory is very different from long beam theory. FE takes this into account mostly but large shear deflection is best looked at via non linear solvers not linear solvers. But that's getting technical.
So I was looking at a 10kW laser video yesterday and noticed the gantry was triangular (flat bottomed isosceles, point upward) and this brought me back to the brace I had used in Maximus. But what if I just left off the back square bit so the loads went directly to the bearing corners vs around the back of the square section? (called shear flow or shear lag) So I modelled a simple gantry this morning SQ vs TRi and tri wins hands down. SQ 0.62mm TRI 0.45mm nearly 30% stiffer. The question is how do I build this shape in aluminium plate???? Always a hurdle to new things perhaps its time to make a mould...
The gantry modelled is aluminium 6mm thick 1100mm wide. The tool plate is 200mm wide and 25mm thick with a 200mm cantilver. The SHS gantry is 150x150x6mm The tri is 150mm high and 150mm wide and 6mm thick. The tri is lighter and stiffer, win win...
Peter
Will maybe do something similar in F360 and confirm the result?
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Hi Peter - Well my resources doesn't have to be a limiting factor... The idea of buying a cast iron plate was to find a supplier that could also grind it or make it flat with a precision mill. Your're right Peter, I need to remember that this is a design exercise, and that building the machine will a project for another time. Designing the machine already requires a few projects, like measuring the stiffness of the composites I'll be able to build in my garage (I'm really exited about testing my own composite one day!). I'll design the dream machine, and make compromises when the time comes to build the machine. I've spent most of the day learning about some of the imperial threads used for sealing connections to connect fittings and hoses to. I've also made a little list of things to buy and contacted some sellers on Alibaba to buy a whole bag of barb tee fittings... I've mostly been looking at consumer suppliers, but when i took a look at professionalplastics.com I was blown away. What a wide selection of materials. I just contacted them regarding potential suppliers or warehouses in EU.
Triangles, of course! Why have I never considered a triangle? Great idea Peter. I can confirm the results. I made a quick search for carbon triangles, and found this interesting trapezoidal tube which left me wondering how strong it would be compared to a square. It turns out this shape has about 30% less deflection compared to a square. http://www.dodge.com.tw/rw_products_6e6086ab.htm
Profile: 50x80mm - 400mm long.
Load: 700N in the middle of the front face 40 mm below the lowest point of the gantry.
Weight: Both profiles weight the same +- 100 g.
Square deflection: 0.004931 mm
trapezoidal deflection: 0.003433 mm
Interesting... If I increase the beam length from 400mm to 700mm the trapezoidal profile is only around 20% stronger instead of being 30% stronger. When I think about the beding forces it makes sense. The trapezoidal shape that I stimulated is not perfected for it's job, so it could potentially be stronger... Trying to think of any scenarios where a trapezoidal tube would be practical or useful compared to other shapes, but I can only think of the "cool" aesthetic of the angled faces.
A right triangle will be more practical, but I was just wondering how much of a difference the trapezoidal shape would have compared to a triangle.
Regards William
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Hello everyone - As mentioned the other day I contacted Jingjiang City Jianken High-Speed Electricmotor Co., Ltd. about a CAD model for their JGD-85 2.2 kW ER-20 spindle. After I few messages I had gotten my hands on a drawing of the spindle with some information that I thought someone could benefit from - so I'll post a picture of the drawing here.
The spindle has a housing that protects and holds everything together. I was wondering what the wall thickness of the housing was for FEA purposes, and the drawing states that the wall of the housing has a thickness of 2.5 mm, but this doesn't seem right to me?? Does typical "consumer" HF spindles only have a wall thickness of 2.5 mm? - or has something gone wrong in their engineering department? With cutting loads and vibrations I would have though that the spindle housing would be at least be 4 mm thick.
- William
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Re: WillMill - HSM Benchtop Composite Mill
Hi William - a triangle is a fixed geometry. It can't change shape. So the more the trap moves to being a rombus the more compliant it becomes. Even the trap is more compliant then a tri. Even in FE we use square mesh to model various things because it can lozenge and twist. If we use triangular mesh it can "lock", say if we are doing a plastic flow problem and we can't get to a solution. Using square mesh allows the material to flow and allows the solver to keep solving... Peter
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Re: WillMill - HSM Benchtop Composite Mill
Hi Will - You have reverted to saying "stronger" vs correctly saying stiffer or more rigid. Peter