[blitz355]

I have a machine that is about 50% built using 80/20, 3060 gantry etc... Pretty beefy (to me).
the machine footprint is about 42x60 w/ a cutting area of about 30x48. The y axis (dual ballscrew) will have 60" ballscrews on each side. 20mm 5 or 10mm pitch.
Electronics are up in the air right now, I'm between a 4 axis g540 kit from cnc router parts and possibly up to a smoothstepper+gecko/leadshine combo. Maybe servo if I can find a 'cheaper' option.

Basically I want a machine that can do it all, I want to be able to machine aluminum. (yes I know this isn't a large machining center)

I also want to be able to machine foam/wood (3d) for casting or mold making (probably wont happen a ton!)

The majority of the work I actually do currently w/ my 1st machine is smaller aluminum engravings.

So my wants (whether they are unrealistic or not) need a balanced result to accommodate both. By wanting to do 3d work in softer materials, that typically warrants faster speeds, I'm sabotaging the fact that I want to also do aluminum, typically lower speed requiring greater torque.

I have ran through countless threads discussing the pros and cons of 5mm vs 10mm pitch screws. Part of me says 10mm would be best, but the epic part of me wants the sub .005" resolution

With a ballscrew critical speed of about 1200rpm the best i could hope for out of 5mm pitch would be ~200ipm rapids and double that for a 10mm pitch. If 700+ rpm on a stepper system is easily attainable, I lean towards a 5mm pitch system for the torque and resolution advantages

What is your take on this predicament, I think I've made up my mind, but like to hear others thoughts (the decision is maddening).

[ger21]

By wanting to do 3d work in softer materials, that typically warrants faster speeds, I'm sabotaging the fact that I want to also do aluminum, typically lower speed requiring greater torque.

You don't need more torque to cut aluminum. Your largest torque requirements are when the machine is accelerating up to speed.

You want to use screws with a 10mm pitch.

[blitz355]

You don't need more torque to cut aluminum. Your largest torque requirements are when the machine is accelerating up to speed.

You want to use screws with a 10mm pitch.

Hi ger,
Thanks for the reply, what is your opinion on the reduced resolution by going with 10mm pitch? Is your thinking that the theoretical res. Of 10mm about .002" vs .001"(5mm) negligible once the machine is built, with the sure to happen flex any machine will inherently have?

[ger21]

With steppers, you can typically count on resolution down to 1/2 step, which is just under .001" with a 10mm pitch screw.

I didn't read you entire first post before, but you'd find that 700 rpm with a 5mm pitch screw will be painfully slow, at only about 140 ipm. Imo, you want a machine capable of at least 300ipm.

[Jon.N.CNC]

Hi ger,
Thanks for the reply, what is your opinion on the reduced resolution by going with 10mm pitch? Is your thinking that the theoretical res. Of 10mm about .002" vs .001"(5mm) negligible once the machine is built, with the sure to happen flex any machine will inherently have?

Sorry kinda just jumping in on this conversation as it relates to a decision i just made to replace my 5mm pitch screw with 10. The 5 was just far too slow, especially with this Chinese spindle which seems to loose torque below 4000rpm making welding instead of cutting soft grades of aluminum inevetable. So you could say min spindle rpm is a governing factor to pitch also.

The resolution you speak of is motor accuracy not so much screw accuracy as screws are either c7, c5 or c3 pitch is irrelevant as the accuracy is the same for all the pitches of the screws grade. there is no doubling effect of resolution as its simply a one time payment, 0.001" accuracy of 5mm is the same as 0.001" accuracy of 10mm. Otherwise if you got 0.001" error every 5mm over a meter it would be out 0.05" and that would be no good to anybody. Lol

so i suppose the most accurate arrangement with a good level of speed would be a 10mm pitch c3 with a planatary gearbox.

I'm just going to be running a 10mm pitch c7 direct drive with nema 34 as I mostly cut wood and plastics so it will likely be more than surfice for what I'm doing but let's see a c7 ballscrew has an accuracy of 0.001968504" which is infact less accurate than a stepper motor so no improvement on accuracy would be made using a planatary gearbox so the accuracy being lower than the motor makes the motors accuracy irrelevant and the screws accuracy supersedes. but for me 0.05mm res is all im looking for.

Now a c5 accuracy is 0.00070866" so you would get an improvement with gearing upto 2:1, and c3 accuracy is 0.00031496" which you would require a gearing of 3.175:1 to make use of the screws accuracy. Without gearing the screws accuracy being greater than the motors becomes irrelevant and the motors accuracy supersedes.

Alternatively micro stepping will improve the accuracy of the motor, just not quite as accurately as gearing I believe.

The other thing pitch has an halving effect on as you go up from 5 to 10 is holding torque, I suppose this is where gearing over microstepping comes out on top. Or both.

Hope this helps, and I hope I didn't sound condescending as I have a tendency of doing I'm told, if so it wasn't intensional.

[moparx12]

I know this is an old thread, but I have a question about back-driving. All else equal, 10 pitch ball screws are easier to back-drive than 5 pitch. Do you suppose there would ever be a problem with the weight of the z axis? Would it be OK to run 10 pitch all around? I'm not planning to use ACME screws anywhere.

[ger21]

Do you suppose there would ever be a problem with the weight of the z axis?

The only time backdriving is an issue ios when you turn the machine off. This can be an issue with 5mm pitch ballscrews as well.

[awerby]

You can't depend on ball screws to keep your spindle from dropping to the table when you cut the power. If that's an issue for you, the best solution I've found is providing one or more gas struts to counterbalance the weight.

[moparx12]

Hmm. A gas strut works better than springs because its force is independent of position? should strut be sized to have force equal to the weight of the sliding part of the z?

[awerby]

Yes, around that weight, anyway. I've tried constant-force springs too (they work sort of like tape measures), but they weren't up to the number of Z cycles required in 3D carving and broke.

[moparx12]

After digging into what some of you guys have done to counter this problem, I came across an electromagnetic brake that can be installed on dual shaft steppers and thought that might be the simplest way to go. Other than added circuitry, is there a reason why an electric brake might not work here?

[awerby]

Sure, that would work, as long as it was configured to power up when everything else powers down.

[109jb]

When powered off, you can just short one or both of the coils on the stepper motor and it will lock the stepper enough to prevent the z from falling. You would need to prevent then from being shorted when the driver is powered though.