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IndustryArena Forum > MetalWorking Machines > CNC "do-it-yourself" > Scrapheap Challenge, cost-effective alu/steel CNC mill
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  1. #1
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    Scrapheap Challenge, cost-effective alu/steel CNC mill

    Heya,

    Been lurking for a little bit and would appreciate some feedback and ideas on a somewhat uncommon "lifting X-axis" gantry mill. Here is a very rough mockup from Fusion360, it's still missing a lot of detail but hopefully it's enough for discussion. General theme is to use scrapmetal and old "leftovers" as much as possible.





    Goals:
    - Ability to machine parts out of aluminium and steel, softer materials aren't of a big interest to me at the moment
    - Not expecting stellar accuracy, but hoping for something like 0,05-0,1mm on the finished part
    - I am quite limited on width in my garage, but depth space isn't a problem. Hence one reason for this somewhat deep Y-axis
    - Travels of approx 300x400x400

    Frame materials:
    - Not pictured, but for a subframe and legs I have various 5mm and 10mm RHS available
    - For the lifting X-axis "gantry" I could use either 45mm plate steel or 200x100x10 RHS
    - For the "table top" that the Y-axis linear rails attach to I have part of an old cast iron paper cutter table, it's about 12mm cast iron with reinforcement ribs
    - For the Z-axis uprights I have sourced 220x120x10mm RHS, for the beam between the towers I can use for example 180x180x8mm RHS
    - As a fixture plate I was planning to use a 300x600mm cut of 45mm steel plate with a grid of threaded holes. Mild overkill I guess, but reasonably priced T-slot tables are pretty hard to source here in Finland
    - Welded table frame, but would use bolted joints in the critical places (Z-axis uprights to base frame etc) to allow shimming/tramming

    Spindle:
    - I'm aiming for sub-10k rpm so will probably end up with a bt30/iso30 spindle. I have a couple sketchy options lined up, either an aliexpress special bt30 as pictured or I might try to resurrect an old worn out Fanuc Robodrill spindle
    - The spindle housing is very much TBD as it will need to follow the spindle. Can be 200x200x12 RHS or something bolted together from 45mm plate steel

    Linear motion:
    - Planning to use LinuxCNC with a Ethernet Mesa card
    - I have 6m of THK HSV25 linear rails with some carts, was planning on using it at least for for X axis as it only sees the weight of the spindle housing, motor and spindle. Could probably also use it for the Y-axis too? I have some Hiwin HGR30 carts so might source some rails to match those for the Z-axis, as that probably sees the most stress having to carry the whole X-axis + spindle parts.
    - Planning on using closed loop steppers
    - Ballscrews are a bit of a question mark, plan B is to just go with the Aliexpress special "C7" rolled ballscrews, but still seeing if I could dumpster dive something better accuracy wise

    Questions/concerns:
    - I guess the Z-axis uprights could use quite a bit of gusseting to prevent Z-axis "nod" ?
    - Is 45mm steel plate overkill as the X-axis cross-beam? The other option is carving out 200x100x10 RHS to run the ballscrew inside the RHS. This feels like a pretty elegant design assembly wise, but I guess 10mm wall thickness might not be rigid enough for steel milling?
    - Could I get away with lifting the X-axis with one central ballscrew on the Z-axis, connected to the beam between the uprights? Or is it asking for racking if the spindle is near one of the extremes on the X-axis? Currently I have drawn two ballscrews on the insides of the Z-axis uprights, but simpler is of course better

  2. #2
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    Re: Scrapheap Challenge, cost-effective alu/steel CNC mill

    Two questions;why such a huge Z travel range and have you found the Mesa card you will require?I ask the first because you need to take into account the practical limits to which you can work on a piece without the collet or the rest of the machine fouling a protruberance.The second is related to a seeming shortage of suitable Mesa cards that would do the job.Which isn't to say that others couldn't work,but you may find information very scarce.It may also help your design if you progress it to the point where the steppers are present.

  3. #3
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    Re: Scrapheap Challenge, cost-effective alu/steel CNC mill

    Hi OOK - The lifting gantry for 400mm Z is not required. Lifting gantries do have issues with clearance for tall jobs. So think that thru as you actually don't have 400mm Z travel on many jobs. Since your using Fusion you can do a simulation and figure out the machine stiffness. Compare different configurations against the machines static stiffness. This is a clear way to make decisions. The design is not as substantial as a commercial CNC with the same envelope. For steel cutting you are going the right way with a 30 size spindle. You have the rails mounted in the middle of the sides of the hollow sections. This will deflect and vibrate. They need to be over to the side as far as possible to stop the middle bit hammocking or oil canning. Or tack weld a thick strip on top of the hollow where the rail goes to improve the local stiffness. Lifting gantries are not uncommon in very large machines, but they still have a Z axis assembly. They call the lift the W axis. The columns will need cross bracing to prevent them from vibrating. Same as the bridge at the top, you have it mounted central, it needs to be over at the side so its stiffer. Or use the same size hollow so the sides align. May sound picky but to produce a very stiff machine every bit of geometry and bracing helps. I'd use two drives for the columns if I had to pick. But I'd go a conventional Z axis in this case. I prefer designs with static beds. Then there are no drives in the muck and the part does not move so you can optimise the drives better as they do not have to account for the moving part mass. Something like the Mori M1 configuration is good.

    You have to think about how you are going to machine the rail lands. As drawn if you weld the columns to the bed its unlikely you will be able to machine the rail foundations. Usually the columns bolt to lands on the bed so that the bed is machinable and the columns feet are machinable to square and true. Welding distorts the sections and flats don't stay flat etc. So you have to work thru how you are going to achieve flat and parallel surfaces. The usual and best approach is stress relieve the weldments then final machine...

    Look at dynomotion machine controllers vs MESA. I think they will provide much better motion control. There is a thread in the controller forum. Keep at it.

    edit - do not cut slots on hollow sections, if you do you have just removed most of its stiffness. Peter

    https://www.store.dynomotion.com/pag...EaAuxdEALw_wcB

  4. #4
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    Re: Scrapheap Challenge, cost-effective alu/steel CNC mill

    Quote Originally Posted by routalot View Post
    Two questions;why such a huge Z travel range and have you found the Mesa card you will require?I ask the first because you need to take into account the practical limits to which you can work on a piece without the collet or the rest of the machine fouling a protruberance.The second is related to a seeming shortage of suitable Mesa cards that would do the job.Which isn't to say that others couldn't work,but you may find information very scarce.It may also help your design if you progress it to the point where the steppers are present.
    Heya, I was looking at something like an MESA 7i76E but I did notice and read that they have been having poor availability lately? 7i96S could get me started but I guess I might run out of I/O pretty fast depending on what kind of control panel etc I end up with. This is my second DIY machine build so I've learned to set pretty realistic expectations timeline wise, I'm fully expecting for this to take many months or even years. Still hoping to get a barebones version within a reasonable timeline though

    Regarding the tall Z, I guess the whole gantry beam lowering has it's own clearance drawbacks compared to a smaller "Z-sled". My reasoning for the exaggerated Z height are two-fold. I work a lot with car parts so I would like to be able to fit something like an intake manifold or a cylinder head under the gantry. From what I gathered the fixed gantry builds usually face challenges with keeping the Z-slide rigid, and they are the least supported when they are fully extended down to work on thin sheet work which seems like a shame. Secondly it seems like 90% of builds either fall into C-frame or fixed gantry categories, in all of my hobby projects I have a personal preference of sticking to the other 10% and trying to figure out things that haven't already been done a thousand times over on Youtube

    Quote Originally Posted by peteeng View Post
    Hi OOK - The lifting gantry for 400mm Z is not required. Lifting gantries do have issues with clearance for tall jobs. So think that thru as you actually don't have 400mm Z travel on many jobs. Since your using Fusion you can do a simulation and figure out the machine stiffness. Compare different configurations against the machines static stiffness. This is a clear way to make decisions. The design is not as substantial as a commercial CNC with the same envelope. For steel cutting you are going the right way with a 30 size spindle. You have the rails mounted in the middle of the sides of the hollow sections. This will deflect and vibrate. They need to be over to the side as far as possible to stop the middle bit hammocking or oil canning. Or tack weld a thick strip on top of the hollow where the rail goes to improve the local stiffness. Lifting gantries are not uncommon in very large machines, but they still have a Z axis assembly. They call the lift the W axis. The columns will need cross bracing to prevent them from vibrating. Same as the bridge at the top, you have it mounted central, it needs to be over at the side so its stiffer. Or use the same size hollow so the sides align. May sound picky but to produce a very stiff machine every bit of geometry and bracing helps. I'd use two drives for the columns if I had to pick. But I'd go a conventional Z axis in this case. I prefer designs with static beds. Then there are no drives in the muck and the part does not move so you can optimise the drives better as they do not have to account for the moving part mass. Something like the Mori M1 configuration is good.

    You have to think about how you are going to machine the rail lands. As drawn if you weld the columns to the bed its unlikely you will be able to machine the rail foundations. Usually the columns bolt to lands on the bed so that the bed is machinable and the columns feet are machinable to square and true. Welding distorts the sections and flats don't stay flat etc. So you have to work thru how you are going to achieve flat and parallel surfaces. The usual and best approach is stress relieve the weldments then final machine...

    Look at dynomotion machine controllers vs MESA. I think they will provide much better motion control. There is a thread in the controller forum. Keep at it.

    edit - do not cut slots on hollow sections, if you do you have just removed most of its stiffness. Peter

    https://www.store.dynomotion.com/pag...EaAuxdEALw_wcB
    Hi Pete,

    Re: Z-axis rail positioning and weakness of uprights, good points. I originally wanted a bigger and thicker beam like 300x100x12 ideally, but this seems very rare so will have to do with the 220x120 unless I stumble onto some later. On my first machine I metal epoxied and bolted some C45 key steel strips to fit my linear rails, would a bolted flat plate on the front face of the RHS columns help similarly?

    Re: machining, in my own shop I have a magnetic drill, mig welder, lathe and a smallish RF-30 manual mill with approx 450x180mm travels. I also have a small metal shop nearby that have a Bridgeport VMC with approx X1000 Y500, although I have not confirmed yet if their pricing will be compatible with my budget.

    My very rough and incomplete thinking for the fabrication and assembly would be:

    1. Drill and bolt the Y linear rails to the paper cutter table (bed). It has quite a bit of surface rust, but beneath it seems to have a planed surface from the factory so it should be quite close to level already (besides 50 years of potential wear and standing outside in someones yard for a few years...). I did some very rough measuring with a straight edge and feeler gauges and even before rust removal it seemed within ~0.1-0.2mm. With it's current 1200x480 dimensions it doesn't fit in any machine shops nearby that I'm aware of, so will probably just try to remove the rust and see how bad it looks after that. Possibly do that straight edge imprinting with something like DWH 310 FL leveling compound that the german youtubers like to do, although that still has it's own challenges with keeping the linear rail mounting surfaces parallel.

    2. Rough mill the column and x-axis RHS faces and ends on my poor RF-30, possibly send them to the machine shop next door to take a quick skim pass to clean up all the small (or big) alignment errors that would come from using an undersized machine and flipping the workpiece etc.

    2. Bolt together the Z columns with the bridge(??), since columns welded to the bridge don't fit in the neighbor shops machines for final planing I'm pretty sure.

    4. I was going to weld a quite heavy skeleton frame under the bed consisting of a 300x200x10 and 200x100x10 RHS (mainly because I happen to already have a planed 300x200 RHS from my previous machine). I would then bolt both the bed and the uprights to this skeleton frame somehow, probably with the columns actually sitting next to the bed at one end, on top of the 300x200x10 RHS. This probably requires a picture to explain better, and it's probably not ideal either to make the bed and column alignment easy.

    I have made some preliminary inquiries about thermal stress relieving in my area, but it's not looking too promising so far. So it's likely that I'll just have to stick to welding as little as possible and leaving bolted joints that can in theory be shimmed into alignment (probably at the cost of sanity). In theory if all my lego pieces are square and planed, it should be possible to bolt them somewhat into alignment...

    I did run into lukahr's topic last night where he was looking into a ram style machine like that Mori M1, definitely gives some food for thought. Although it goes back to my original problem where I prefer a "deep Y" machine over one that needs a lot of space X width wise.

    Thanks for your comments guys, will think some more about it and get back to you with an updated plan.

  5. #5
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    Re: Scrapheap Challenge, cost-effective alu/steel CNC mill

    Hi Ook - The Z axis stiffness is always the crux of a design no matter what the configuration is. The rising gantry design fixes some of this but creates other issues. These need to be worked thru for your application. Correctly designed bolted connections are just as good as welded connections. I do a lot of bolted connection analysis in mining equipment and as long as you have high preloaded bolts in the right spots they are just as efficient as a weld for machine applications. So design your parts to be made on mills accurately and then bolt them together accurately. I'd stay away from using thick self levelling epoxy. Fine for "setting" connections but to self level epoxy has to be 5-6mm thick and this is not much better than a piece of rubber between objects. Pursue the stress relief it will be worth it if you weld. I think the Mori config would be better than your #1 design a) syncronising two axes can be hard that's why things like the Mori are all single controls for a single axis. Getting two columns square and parallel can be tricky b) The spindle and its drive will be large and heavy and take up a lot of real estate, the large Z axis ram takes care of that. Having designed many, many machines and built a few I suggest you start at the Z axis and work outward. If you start at the machine base and work towards the spindle you will inevitably run out of space and make a wimpy Z axis. So make the Z axis correctly and work outwards. This may end up bigger than you expect but then that's the conundrum. Mills for steel require rigidity and there's only a couple of ways to get it. Commercial mills of your size are huge for a reason. But keep at it the forum will jump on any mistakes... Peter

  6. #6
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    Re: Scrapheap Challenge, cost-effective alu/steel CNC mill

    Hi,
    a large Z axis travel is VERY desirable if you ever want a fourth or fifth axis.

    The travels on my machine are 350mm x 350mm x 350mm. I very seldom have any problem with X or Y travels, but the number of times when I cannot fit the part 'under' the Z axis....and especially
    with the four axis (trunnion) and fifth axis fitted.

    Despite the very obvious challenges in designing and build a 'tall' Z axis, I think for the usability of the machine you must try for the most travel you can.

    Craig

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    Re: Scrapheap Challenge, cost-effective alu/steel CNC mill

    Hi Joe,
    Another good case for "overkilling" the Z travel, I have already been watching various DIY fourth axis videos so it would probably be an inevitable development down the road.



    Some "quick fixes" to my earlier draft, added 11mm (random number assuming 12mm planed flat bar) of plate sandwiched under the Z-axis rails and a quick sketch to illustrate my rambling about the RHS bedframe. I guess that whole subframe could be a prime case for an UHPC or similar casting, but not sure I want to take that direction quite yet. Another weak spot I see is that big 300x200 hollow section under the Z uprights, it will see the whole weight of the uprights + x-axis + spindle which could easily reach the 150-200kg range. Would an EG or UHPC fill inside the tube help with compressive strength? Intuitively it would, but from what I've gathered EG and UHPC fills often aren't as beneficial as one would think.

    Please ignore the spindle and spindle housing area for now, it should get clarified as soon as I get my hands on the worn out Robodrill spindle and I'll make a decision if I try to restore and retrofit it or not.

  8. #8
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    Re: Scrapheap Challenge, cost-effective alu/steel CNC mill

    Instead of the lifting gantry, which as Pete points out introduces as many problems as it solves, consider using a shorter Z axis mounted more conventionally, and making headroom for the 4th axis or trunnion assembly by mounting it at a lower level. The tool never needs to reach around underneath the part to cut it, so just having access to the top half is sufficient, Just cut out the table where it's convenient to do so, and position the assembly so that the center of the rotary axis is approximately at bed height.
    Andrew Werby
    Website

  9. #9
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    Re: Scrapheap Challenge, cost-effective alu/steel CNC mill

    Hi,
    I see that you have the spindle mounted on an RHS section, presumably so you have room for a belt and tensioner back to the motor.

    May I suggest that if you get a direct drive spindle or a motorised spindle then you do not need the extra 'block' and thereby reduce the torque on the gantry very considerably.
    Sneaking the center-line of the spindle as close as you possibly can to the plane formed by the bearing cars thereby decreasing the torsional moment will pay big dividends in terms of stiffness.

    Chinese made motorised ATC spindles of the 3kW to 10kW class are as cheap as I've ever seen them. They must be close to selling at cost, or even below. If that is the case then
    get in soon before various Western governments apply tariffs to them.

    I bought a new Dake spindle (3.5kW (S1), 3.4Nm (S1), 10,000rpm rated, 40,000rpm max, HSK32E tool interface) including 10 HSK32E-to-ER20 toolholders off EBay for $2000USD including shipping.
    Has been a major revelation how much ATC improves your CNCing! Torque is still too low to be considered a genuine 'steel capable' spindle, but I've been using it on steel with 6mm and 8mm tools with very good
    results.

    On the basis of the success of this new spindle I'm now dreaming of an HSD ES332 spindle, that's 13kW and 10.3Nm (S1) with an HSK 40E tool interface.

    I disagree with Awerby, go for as much Z travel as you can so you can put a fourth axis direct down on the bed and still have enough room for the part, workholding and tooling.
    A lifting gantry design is probably the best for maximising Z axis travel. Z axis stiffness equates (largely) to torsional stiffness of the gantry and bending stiffness of the uprights both of which can be made arbitrarily stiff
    with big enough sections of steel.

  10. #10
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    Re: Scrapheap Challenge, cost-effective alu/steel CNC mill

    Hi OOK - One solution for the z axis ht issue is to use a bolster machine base. Then you can have two or three bolsters to adjust the bed ht. when needed. This keeps the Z ht small and the machine simple.

    You are better off using thicker steel sections then adding concrete. Steel is 7800kg/m3 and concrete is 2300kg/m3 so adding a small amount of steel gives the extra weight if your chasing wt. Plus the steel is in the load path and it contributes to more geometric inertia. Plus, concrete does not stick to steel over time so it eventually will uncouple from the steel and be ineffective. You can get expanding concretes for this purpose but that will create other issues I think. Peter

  11. #11
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    Re: Scrapheap Challenge, cost-effective alu/steel CNC mill

    Hi,
    that's not a bad idea.

    I always knew that when I built my machine that in time I would also have a fourth, and then a fifth axis. Consequently I always knew that I was going to be 'Z challenged'.
    My machine is a column machine with a cast iron axis bed that bolts to the column, so I made the column 300mm longer so I could unbolt the axis bed and shift the whole axis upwards.

    To date I tend to fit the four/five axis when I require it, then remove it to take it back to three axis for normal work. Being able to shift the Z axis upward to regain lost travel is my sloution,
    not unlike peteeng's idea. I would prefer just a much longer Z axis.....but then I'd prefer an F16 as weekend transport as well, and I'm no more likely to get that either!!

    Craig

  12. #12
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    Re: Scrapheap Challenge, cost-effective alu/steel CNC mill

    Hi,
    the stiffness of the Z axis is dependent on the torsional stiffness of the gantry. But surely the gantry is just as stiff with long uprights as short ones? The gantry is itself exactly the same length.

    The intersection of the uprights with the base must be perpendicular and rigid. With longer uprights the gantry will in effect exert a greater torque on that intersection, but not hugely so. The torque
    is proportional to the length of the uprights.

    The stiffness of the uprights is inversely proportional to the cube of the length, so is sensitive to upright length......but the lengthwise bending stiffness of an RHS is optimised and generally very good. I personally would make the uprights
    as long as I deemed necessary to secure the travel I want, and then concern my self with making them sufficiently rigid. The last thing I'd want to happen is to build this machine at big cost in terms of money and time,
    only to find I cannot use a fourth axis because I did not think that far enough ahead to allow the travel I need. Too bloody late once its built!

    Craig

  13. #13
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    Re: Scrapheap Challenge, cost-effective alu/steel CNC mill

    Hi OOK - I think if you shrunk the CMX Mori series it would give you everything you want.

    https://youtu.be/kCaWOs4Z3BU

    Peter

    https://youtu.be/aENcuG6mhKc the DMV series has a bolster table, so it can be swapped for rotaries etc good movie of the inside of the DMV

  14. #14
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    Re: Scrapheap Challenge, cost-effective alu/steel CNC mill

    Quote Originally Posted by joeavaerage View Post
    Hi,
    I see that you have the spindle mounted on an RHS section, presumably so you have room for a belt and tensioner back to the motor.

    May I suggest that if you get a direct drive spindle or a motorised spindle then you do not need the extra 'block' and thereby reduce the torque on the gantry very considerably.
    Sneaking the center-line of the spindle as close as you possibly can to the plane formed by the bearing cars thereby decreasing the torsional moment will pay big dividends in terms of stiffness.

    Chinese made motorised ATC spindles of the 3kW to 10kW class are as cheap as I've ever seen them. They must be close to selling at cost, or even below. If that is the case then
    get in soon before various Western governments apply tariffs to them.

    I bought a new Dake spindle (3.5kW (S1), 3.4Nm (S1), 10,000rpm rated, 40,000rpm max, HSK32E tool interface) including 10 HSK32E-to-ER20 toolholders off EBay for $2000USD including shipping.
    Has been a major revelation how much ATC improves your CNCing! Torque is still too low to be considered a genuine 'steel capable' spindle, but I've been using it on steel with 6mm and 8mm tools with very good
    results.

    On the basis of the success of this new spindle I'm now dreaming of an HSD ES332 spindle, that's 13kW and 10.3Nm (S1) with an HSK 40E tool interface.

    I disagree with Awerby, go for as much Z travel as you can so you can put a fourth axis direct down on the bed and still have enough room for the part, workholding and tooling.
    A lifting gantry design is probably the best for maximising Z axis travel. Z axis stiffness equates (largely) to torsional stiffness of the gantry and bending stiffness of the uprights both of which can be made arbitrarily stiff
    with big enough sections of steel.
    I also noticed those quite tempting 6-8k BT30 spindles from China, some are even down to 600€ for just the spindle which is tempting indeed. It seems I will get my hands on a 10k Robodrill spindle but more on that later.

    Quote Originally Posted by peteeng View Post
    Hi OOK - One solution for the z axis ht issue is to use a bolster machine base. Then you can have two or three bolsters to adjust the bed ht. when needed. This keeps the Z ht small and the machine simple.

    You are better off using thicker steel sections then adding concrete. Steel is 7800kg/m3 and concrete is 2300kg/m3 so adding a small amount of steel gives the extra weight if your chasing wt. Plus the steel is in the load path and it contributes to more geometric inertia. Plus, concrete does not stick to steel over time so it eventually will uncouple from the steel and be ineffective. You can get expanding concretes for this purpose but that will create other issues I think. Peter
    Bolster is a very good point, that actually negates a lot of my worries about a more traditional Z-axis with the spindle on a small sled. I'll steal some inspiration from the Mori's and make a draft with a fixed fixed gantry and we'll see how it looks

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    Re: Scrapheap Challenge, cost-effective alu/steel CNC mill

    Hi Ook - By the way a "fixed" gantry is a bridge or a portal. Some mill makers like Mori call them double column mills. It's an oxymoron to say fixed gantry. Gantries always move. IMHO anyway Looking fwd to Mark2 design...I'm working on a boat project at the moment and we are up to version 12. From experience I think you'll get close around #15 maybe #20.... Peter

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    Re: Scrapheap Challenge, cost-effective alu/steel CNC mill

    I think the idea of a machine with a low level bed and then using an accurate spacer to raise the job to the spindle would be an excellent plan.One of these would be the sort of thing and you could locate custom fixtures on whichever is the most suitable top surface.https://www.bwmachinetools.co.uk/lis...engineers-cube

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    Re: Scrapheap Challenge, cost-effective alu/steel CNC mill

    Hi,
    now here's a novel idea.....just make the Z axis travel big enough to do the projected work, including a fourth axis if that is a likelihood in the future.

    What is the aversion to having decent Z axis travel? The lifting gantry design is, I would guess, the best design for that to happen....so why not?

    As I posted earlier my Z axis travel is 350mm. That came about because that was the C5 ballscrew that I could get affordably at the time. If I'd had my choice,
    and certainly, if I were to repeat the design, I'd want 500mm to 600mm travel. My machine is useable as is, but would be better, or at least more convenient, if I had more Z axis travel.

    How about you guys proposing some bolster or block arrangement post some pictures of how you got around the limitation of reduced Z axis travel?

    Craig

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    Re: Scrapheap Challenge, cost-effective alu/steel CNC mill



    I already have this picking up dust on a shelf so the bolster idea is definitely worth exploring.

    Regarding the Mori machines that Pete linked, the DMV has a cool traveling ram but you can see the footprint of the rams "X-axis" is rather large compared to the table area to get the linear rails far enough apart:



    I probably like the CMX a bit more, clever ram turret with nested linear rails and the footprint of the machine is more rectangular which I would prefer. You would probably want to keep the Y-axis travel reasonably short on a machine like this:


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    Re: Scrapheap Challenge, cost-effective alu/steel CNC mill

    Hi OOK - The larger the bearing spacing the better. It will always be an issue; maximising the working envelope and minimising the machine footprint. It's the battle that all designs go through. I would make the Z, the height you want and design a machine to suit. The best machines are designed to fulfil an application. Then it can be optimised to its mission. General purpose machines are always compromised somehow as its mission is furry so it will have a strong suite and weak areas. Particularly when it comes to its envelope. We always want it as big as possible. Bigger is better they say etc. The real trick is to make a machine that is balanced in its cost/performance and utility. It's taken me several years to find suppliers of suitable parts so nearly each part is at its best cost to performance ratio for its quality (at a hobby/maker DIY) level. There are many ways to skin the Z cat. Knee mills have solved the Z issue neatly for over 100 years for instance.

    Design the best machine you can with what you have, that's the start and don't buy anything until the design is cooked really really well. Otherwise you will end up with more over run stuff. I have had two goes at using up old stuff and ended up with more stuff. Seems its par for the course.

    Think about what you want to do in future eg a rotary - and design it now and do it now. This sort of project takes a lot of time and energy so you may as well drag the future closer to you now. The future becomes NOW real fast then the machine is outdated (in your head). The idea that it can be upgraded into the future is false. Once the basic design and build is done its DNA is set and it's a battle to improve the structure. So make the structure right from the biggening. I think your going down the right path via the forum theres a lot of support and knowledge here.... Keep at it. Peter

  20. #20
    Join Date
    Jul 2018
    Posts
    6841

    Re: Scrapheap Challenge, cost-effective alu/steel CNC mill

    Hi OOk - I have been helping a young designer with a large mill design and he has used the CMX wide stance with the Z axis slipping between the Y axis and on paper its super stiff. He's machine is a 5 axis machine for engraving vehicle tyre moulds, its up to around 7-8 tonnes now. But I like Mori machines they have lots of good structural features. The CMX has a 90deg saddle and I now use that on all my machines. Has made bearing access easy and its really stiff. This is Frankenrouter - one attempt at using up parts...Peter

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