[Thunza]

Hello, here is my Z axis design,

I'm thought it would be better to use ground steel plate so I can reduce the distance the spindle extends from the gantry. I plan to use 8mm ground steel plate with some thin side plate to add stiffness. I haven't built a cnc before though so have doubts if ground steel plate will be flat enough compared to aluminium tooling plate. I have done a bit of hand scraping in the past and don't mind spending time if the steel isn't good enough.

The supplier on one website states their ground steel plate is under 1mm per 1 meter, my Z plate is about 30cm long so 0.3mm would be the worst if I got a poor piece.
Maybe i'm over estimating the importance of saving around 12mm extension from the y gantry if i did use say 20mm aluminum. I intend to use the cnc for hardwood, perhaps a bit of nonferrous metal too. I'd like to be able to machine things 8cm high at most.

Any thoughts?
Thanks

[peteeng]

Hi Thunza - 8mm is a bit thin for a Z plate by itself especially if you want to cut aluminium. Adding the flanges is a good strategy but yours need to be much deeper. 25mm thick steel is common and 30mm aluminium is also for a flat Z plate. Make the flanges much much deeper and it will work well. The images attached show one of my machines. The tool plate is equivalent to about 60mm thick steel by having the deep flanges... Its 3mm steel so the middle is 6mm thick the flanges are about 40mm each side. The two parts are epoxied together to create one part. Peter

[joeavaerage]

Hi,
I agree with peteeng, 8mm is a joke, 20mm min and 32mm preference. Steel is just so much stiffer than aluminum, and cheaper too.
Get some 32mm plate plasma or laser cut then take it to a tool maker and have them put it on the surface grinder.....easy. I got three pieces of
350mm x350mm x 20mm surface ground for $50, dropped it off at lunchtime, and picked it up that evening....easy, flat and parallel to better than 0.01mm.

Craig

[Thunza]

Thanks a lot for your input.

I imagined 8mm steel is equal to 24mm aluminium if it's 3 times stiffer. I guess the extra distance between the aluminium faces mean it might not work quite as simply as that.

Peeteng, its interesting you were able to go to just 6mm with the large flanges. Did you use that machine for just wood?


So deeper flanges and thicker plate if I can't increase the flanges height. Hopefully it won't get too heavy if I were the increase plate thickness, I bought a set of 4nm Nema 23 motors to power the cnc. I suppose the plate itself isn't as large as some cnc's are, it will be 30 x 14cm if you don't count the little end section where the motor mounts.

[joeavaerage]

Hi,

I imagined 8mm steel is equal to 24mm aluminium if it's 3 times stiffer. I guess the extra distance between the aluminium faces mean it might not work quite as simply as that.

No, that is not correct. It might surprise you that I calculated, or rather used Fusion's FEA tool to calculate, the equivalent thickness of steel and granite. Granite has about the same
stiffness as aluminum, so the comparison will be close. A 32mm thick steel section has the same stiffness as a 50mm thick granite section despite steel being three times stiffer than
granite.

I would expect, say 16mm steel, to be approximately equal to 24mm aluminum.

If you do the momentum equation for your axis its highly probable that the mass of the solid plate is likely to be less significant than the rotating ballscrew. Again
hard to believe but the physics dictate that the momentum is often dominated by the ballscrew.

Craig

[Thunza]

I presume I could place the flanges to they come out instead of inwards like in this drawing? I would add a front brace too as the spindle carriage wont go past the end of the plate.

The only issue might be it could make things fiddly with tramming and so on

[joeavaerage]

Hi,
I think you are making the part more complicated than it need be. If you used a thick solid plate, be it steel or aluminum would be by far and away the cheapest and
easiest solution. Flanges add manufacturing complexity.

Give us the details of the Z axis, namely the ballscrew diameter, ballscrew pitch, ballscrew length and then we can calculate the momentum equation which will likely show that the
increased mass of the solid plate is insignificant.

Just as an example: my new build mill has cast iron beds of 115kg each, but for the calculation I allowed the axis mass of 150kg. The ballcresws are 32mm diameter, 5mm pitch and 700mm long. The servo
has a first moment of inertia of 1.1 x 10^-4 kg.m². The momentum calculation shows that 80% of the momentum is in the rotating ballscrew alone, 12.5% in the armature of the servo
and the remaining 7.5% in the linear axis mass. So despite the axis weighing 150kg its still a minor contributor to the momentum. Thus if I decided to add a whole bunch of mass to the axis, say a trunnion table
and fifth axis, the extra mass is STILL a minor to modest contributor to the overall momentum.

Give us the details of the axis and we'll see just how important, or not, the mass is.

Craig

[Thunza]

Hi,
I think you are making the part more complicated than it need be. If you used a thick solid plate, be it steel or aluminum would be by far and away the cheapest and
easiest solution. Flanges add manufacturing complexity.

Give us the details of the Z axis, namely the ballscrew diameter, ballscrew pitch, ballscrew length and then we can calculate the momentum equation which will likely show that the
increased mass of the solid plate is insignificant.

Just as an example: my new build mill has cast iron beds of 115kg each, but for the calculation I allowed the axis mass of 150kg. The ballcresws are 32mm diameter, 5mm pitch and 700mm long. The servo
has a first moment of inertia of 1.1 x 10^-4 kg.m². The momentum calculation shows that 80% of the momentum is in the rotating ballscrew alone, 12.5% in the armature of the servo
and the remaining 7.5% in the linear axis mass. So despite the axis weighing 150kg its still a minor contributor to the momentum. Thus if I decided to add a whole bunch of mass to the axis, say a trunnion table
and fifth axis, the extra mass is STILL a minor to modest contributor to the overall momentum.

Give us the details of the axis and we'll see just how important, or not, the mass is.

Craig

Thanks Graig. My inexperience probably makes me want to be over cautious. Its probably worth saying the nonferrous metals would be more of a bonus than anything essential, speed would not be a real concert cutting them either.

The z axis details are SFU1605 ballscrew so 16mm diameter 5mm pitch, 30cm long, it will use a 4nm nema 23 motor. The spindle is a chinese 2.2k one.

[peteeng]

Hi Thunza - 1) no 8mm is not equal to 24mm as its 3x stiffer material see calc attached. The rigidity of a beam is equal to its material stiffness x its geometric inertia. Use 5mm pitch ballscrew for Z and it will lift 461kg so don't worry about Z weight. 2) Yes the I section Z plate easily cuts aluminium. Its the stiffest plate I have made on any of my machines. Would not go any "thinner" into the future 3) 24mm thick AL is equivalent to 17mm thick steel and its lighter. Peter

[Thunza]

I'm surprised granite is only about as stiff as aluminium given granite isn't at all ductile.

"Would not go any "thinner" into the future 3) 24mm thick AL is equivalent to 17mm thick steel and its lighter. Peter"

I felt this might be true but didn't realize just how much of an effect the geometry has, even though they're both just plates. I'm glad I asked before going ahead with my plan.

Thanks to both of you. I feel I can continue the design and buy parts now.

[peteeng]

Hi Thunza - Make the flanges thick and the Z plate thick - as thick as you can. You have other hurdles to clear. Are you welding this? Bolting? brazing etc. There maybe a commercial channel in alum or steel? Mine are laser cut and bent. You also have to consider if you are going to machine the rail foundations. Then there is access to bolts on the saddle cars, you have many more issues to resolve in this area. You have the drive nut under the spindle clamp. This will be a pain to maintain as you will have to remove the spindle to check or undo the drive bolts etc. Don't bury any bolt heads!! Is the spindle 2.2kW if so consider downsizing it to 1.5kW or even 800W. A 2.2kW is a big beefy spindle and not needed for wood or aluminium. The size of thr 2.2 will drive many areas to be oversize. If your trying to keep things compact go with aq 800W water cooled spindle... The only reason to go with a 2.2kW spindle is to get a ER20 collet so you can run 1/2" tools. Thats serious cutting territory and your machine won't match its capabilities... Peter

edit - ductility and material stiffness are not related. Some of the stiffest materials we have are ceramics and they fail at <1% strain. Metals are ductile because they have grains that are held together by friction just like lego blocks so they can slip along each other when stretched. Crystalline structures have no slip are very strong and stiff.

speed of machine cutting metals is nearly irrelevant. The machine still has to be stiff enough cutting slow or fast. maximise the geometry of every part, chase stiffness with every part. What is the work envelope you are aiming at?

[Thunza]

Hi Thunza - Make the flanges thick and the Z plate thick - as thick as you can. You have other hurdles to clear. Are you welding this? Bolting? brazing etc. There maybe a commercial channel in alum or steel? Mine are laser cut and bent. You also have to consider if you are going to machine the rail foundations. Then there is access to bolts on the saddle cars, you have many more issues to resolve in this area. You have the drive nut under the spindle clamp. This will be a pain to maintain as you will have to remove the spindle to check or undo the drive bolts etc. Don't bury any bolt heads!! Is the spindle 2.2kW if so consider downsizing it to 1.5kW or even 800W. A 2.2kW is a big beefy spindle and not needed for wood or aluminium. The size of thr 2.2 will drive many areas to be oversize. If your trying to keep things compact go with aq 800W water cooled spindle... The only reason to go with a 2.2kW spindle is to get a ER20 collet so you can run 1/2" tools. Thats serious cutting territory and your machine won't match its capabilities... Peter

edit - ductility and material stiffness are not related. Some of the stiffest materials we have are ceramics and they fail at <1% strain. Metals are ductile because they have grains that are held together by friction just like lego blocks so they can slip along each other when stretched. Crystalline structures have no slip are very strong and stiff.

speed of machine cutting metals is nearly irrelevant. The machine still has to be stiff enough cutting slow or fast. maximise the geometry of every part, chase stiffness with every part. What is the work envelope you are aiming at?

Hi Peter thanks for sharing your knowledge. My initial intention is to use it for musical instruments like Harps, perhaps the odd bit of furniture and artistic pieces too. Part of my reason for going down this route is by enclosing the cnc and using it instead of other equipment I hope can reduce my exposure to dust which had been giving me some trouble because of allergies, I work in an old garage without a main door, keeping my extraction ducted to outside.

You are probably right about the spindle size, my routers which are lower KW are capable, I think I liked the idea of a ER20 collect for having larger diameter tool bits, enabling deeper cuts with more rigidity. Apart from carving 3d shapes I can't see the need to cut through more than 5cm. Spindles aren't too expensive and I could sell the old one so swapping is feasible.

As far as frame goes I plan use box steel, bolting combined with metalized epoxy, perhaps roughly following this sort of method. I have looked into buying a stick welder but having zero experience and hearing that they can wrap the steel. I would only be able to power a welder capable for doing 3mm corner joints too, with bolting I can go thicker.

Machine frame - MadVac CNC

I'd like to level the top rails with epoxy probably followed up with scraping.

[peteeng]

Hi Thunza - For your application I'd stay away from metal nearly all together. Frankie is mainly F17 formply which is stiff and damp. I have not run it yet but it feels very stiff and I'm sure its strong from building a machine from MDF and having tested that thoroughly for a couple of years. Made lots of guitar parts for people. The form ply is significantly better.. Look up the Frankenrouter thread... Since you are heading for an enclosure I would integrate the "walls" as the enclosure sides and extend them up to create the roof. I would not use the 2.2kW spindle. If tool size is an issue use a 1.5kW with an ER16 collet. This will give you 10mm tools which can be quite long. The size of the 2.2kW will flow into the rest of the machine making it bigger than needed. If its bigger, then it won't be as stiff and you will chase your tail... The size of the Z axis parts (like the spindle clamp) ultimately drives the machine footprint. You will be surprised as you flesh this out how the machine will grow and grow.

If you use metal frames bolt them together and if the connection is permanent, then epoxy the joint. This will be 95% as stiff as welded and damper. This way the joint can be adjusted until correct then set with the epoxy. Good luck with epoxy levelling and scraping I would not go near that that can of worms and wombat holes... For instance in steel structures design, the welded damping coefficient is zero and the bolted connection is 2% which is quite damp...

Hi Craig - You use the word "momentum" to describe "inertia" please stop, this is annoying lay use of momentum is to describe something that wants to carry on moving. Engineering momentum is mass x velocity. Momentum is conserved in a system and we use it to calculate various things like collison velocities. Momentum - Wikipedia

What you are describing is "inertia" and inertia is the objects RESISTANCE to motion. You do use this correctly in spots. So the inertia of the screws and masses have to be overcome by the motor. Geometric inertia is the members resistance to bending for instance... I have sleep inertia sometimes and don't want to get out of bed...Sleep inertia - Wikipedia Thunza keep at it... Peter

[Thunza]

Thanks again Peter.

I'm still looking into the epoxy leveling but can't see much alternative without paying a lot of have the rail supports machined.

I'll definitely consider your recommendation to downsize. I don't think i'll need a huge layout in terms of width, so i might be okay with as little as 40cm. It's good to hear you are confident about the screw and epoxy joining method. There do seem to be some 1500w spindles with a ER20 collect, I noticed if theyre watercooled they are only about .5kg less weight though

[joeavaerage]

Hi,

You use the word "momentum" to describe "inertia" please stop, this is annoying

Don't be a nit picking jerk.....I now your Aussie but that's no excuse for this crap. I too did a degree in Engineering and that's based on a Physics education that precedes that for some years, if there is anyone
who objects to the terms I use its you....then get used to it.

Craig

[peteeng]

Hi Thunza - For your application if you get straight extrusions or steel foundations they are close enough. Even plywood foundations can be lapped true enough. Epoxy has to be at least 6mm thick to self level and epoxy is effectively rubber, why mount your rail on 6mm of rubber? Peter

[Thunza]

Hi Thunza - For your application if you get straight extrusions or steel foundations they are close enough. Even plywood foundations can be lapped true enough. Epoxy has to be at least 6mm thick to self level and epoxy is effectively rubber, why mount your rail on 6mm of rubber? Peter

I did find the idea a little odd as i know epoxy isn't all that tough, maybe a little harder than rubber but I definitely see your point. I imagine perhaps with the weight being spread over the rail and 4 carriages perhaps that makes up for the epoxy not being that hard? Just a speculation for me though.

I did have an idea, maybe you could level with epoxy then router away material so you only have a very small amount of epoxy left in height. As a router would be referencing off the flat epoxy surface it would preserve the shape. Well hats my theory but I'm not entirely sure it would work. Perhaps having too little epoxy could be problematic if it gave way, deforming more than the surrounding hard metal


When you suggest lapping instead is that something that's feasible to do yourself? I've used an angle grinder and granite plate marked with blue.. stuff to flatten a bobbin sander table in the past successfully so I imagine the practice would be the same for leveling the rail supports, the only thing is getting a wide and long enough flat reference, I think other people also use engineers levels for checking coplanarness.

[joeavaerage]

Hi.
I agree with peteeng, standard extrusions and hollow sections are usually straight enough for most purposes.

Some years ago, before I started with CNC I made a 'Trammel' mill, which relied on a 1.5m length of 50 x 50 x 5 RHS steel on which ran a shuttle which
in turn carried a router. I used it for making wing cores for model aeroplanes. Clearly any deviation from straight would cause an inaccuracy in the wing cores.

I did not have much in the way of measuring equipment at that time, certainly not a granite surface plate for which the measurement almost demanded, and
the best I could measure was that the piece of steel which is just as supplied, was better than 0.05mm from straight. I concluded that the material was good enough to use
as is, without be straightened or machined or anything.

50 x 50 x 6 RHS in Christchurch is $37.00/m (NZD)....and that's retail, if I bought a full length, 8m, it would be cheaper again.

Once you start welding it then you'd really need to stress relieve or it will be significantly bent, but if you don't weld, but bolt and/or bond then you'd probably not have to straighten or
machine anything. Even if you do want to get it machined flat, then; when still in short, say 1m or less lengths and not assembled in any way you could get then milled or ground
for bugger all. The biggest surface grinder in Christchurch, at least to my knowledge, has a capacity of 1.8m.

Steel is still by far and away the cheapest and stiffest material for engineering projects. Don't make things over complicated or try to use exotic materials or techniques when a simple
and well thought out design in steel will do the job every bit as well for a small fraction of the cost. That means you've got money left over for a real good spindle, tooling, coolant/dust extraction
or what-have-you.

Craig

[joeavaerage]

Hi,
on an unrelated matter just purchased (EBay) an Atlanta Drives low lash (<1 arc min), high torque (120Nm) servo reduction box for a fifth axis for my mill. Its new old stock for
$226USD shipped to my shipping agent in Portland Oregon.

I have another one, again Atlanta Drives, slightly larger (400Nm, <2 arc min), for the fourth axis/trunnion, and have had for a while.I have just finished fitting that drive with a 750W Delta servo, and its working great.

Craig

[Thunza]

Steel box section is what I'd like to go with, I'll.

I was under the impression for mounting hiwin rails rails you need an extremely flat surface or the bearing wont run smoothly. My end product tolerance will be much lower than what many people making metal parts need, perhaps this is what you are partlysuggesting?

Thanks