[fannlow921]

Hi I'm pretty new to the servo side of CNC, however, I have built at least two working CNC using steppers and linuxCNC.

I need some help to actually size up SERVO + DRIVERS + CONTROLLERS or as a KIT if anyone knows any?

any help will be greatly appreciated, the current machine built is as follows:

its a 5-axis machine, with the 4th & 5th (2-axis spindle head) using timing gear of 20 teeth and a 4" (100mm) slew bearing with ext. gear.
each of the 4th and 5th axis, being driven by a NEMA 24 stepper with uStep @ 10 to give higher precision at rotations, but also finer resolution and OK torque. the 4th and 5th axis can be directly driven by the NEMA 24 stepper, if space is a concern.

the Y-axis uses 2 servo, one on each side.
Its a Gantry system, where the top moves, and the top weights in a max at 125kg (full aluminium extrusion + Hiwin linear rails + PMI ball screw + servos).
so each servo on the Y-axis needs to move say around 70kg ~ 80kg max of weight.

the X-axis uses 1 servo.
It moves the aluminium Z-frame, the spindle, and the 4th and 5th axis head along, weighing in about 70kg max (full aluminium extrusion + Hiwin Linear Rails + PMI ball screw + servo + 4th & 5th head + Spindle)

the Z-axis uses 1 servo.
it moves up & down, carrying with it a max weight of 50kg (full aluminium extrusion + 4th & 5th axis + spindle)

Friction coefficion based on PMI manual is 0>0.005>1.
Lead & Pitch = 10mm since its single start ball screw (otherwise pitch = lead x number of starts)

All axes run on Hiwin Linear Rails which says will reduce weight by 40% if lubricated regularly (once a month) within 2~4 years (100kg will require 60kg of force pushing)

Machine travel speed unloaded (not-cutting) for all axes should be around 40,000mm/min or 4,000rpm (using velocity/lead)
Cutting speed should be around 25,000mm/min ~ 30,000mm/min (2,500rpm ~ 3,000rpm).
Servo should be capable of achieving these speed and cost effective.

I know there are thrust to be calculated, but this is where I get lost...
lots of servo out there and manufacturers with sizing software, and all returns different results...

really need some help here, if anyone can offer any help ?
need the servo size vs price, reliability, what I can really use?
need amplifers or drives or control board? someone told me about some other board? totally confused...

i thought SERVO is just the motor (with encoders), a driver + controller then connect to the computer?
pretty much like a stepper system? or am I wrong about it?

did some search online, but points to nothing much...

looking for any help available
thanks !

[ger21]

Define cost effective?

How quickly do you want to accelerate? That's what determines the size you need. What size are the ballscrews, and how long are they? Can they spin at 4,000 rpm without whipping?
Commercial machines that use ballscrews and those kinds of speeds would probably use closer to a 50mm pitch.

[fannlow921]

Cost effective by any means is about not oversizing the needed servo.

I havent have an idea for the acceleration, but I would think anything under 0.5sec to 1sec should be doable?

Any suggestions here?

Ballscrew for Y-axis is 25mm dia. & 10ft long.

Ballscrew for X & Z axis is 20mm dia. & 5ft long.

All ballscrews are on fixed-to-supported.
Fixed is the rear end, supported in the front, going into a rigid coupler that joins the servo.
> I wrote this opposite... Fixed is the front, where a rigid coupler + servo goes, and supported is at the rear...

No gears used.

Let me check how much rpm the ballscrews
Will take before whipping.

Good point mentioned!
I didn't even check if it will whip!!

Thank you so much!

[fannlow921]

just found the PMI manual book....how do I upload it to share with everyone here?

PMI-GROUP// PMI PRECISION MOTION INDUSTRIES, INC. - Support - CAD download

I have selected Ballscrews > English > download > page 260, has alot of information on FA series.


and then.. i went to check the invoice... it should be this one..
http://www.pmi-amt.com/products/pmi/pmi_ssvw.htm

rolled ballscrew, not sure what that means, but its called this...

not sure if the RPM on this page points to
3240rpm for 25mm & 1100rpm for 20mm?

if anyone can verify this for me to confirm my doubts?


thanks again!

[fannlow921]

Attachment 213440

From the data pulled out and sourced from the catalog, it says safely around
1,500~2,000 for dm.n <50,000

From the attachemnt that I have included.
Left side is catalog guide.
Right side is excel theory calculation.

However, I did see a major difference, its either my calculations are wrong...
or they have made changes to the catalog and updated it.

on the image I uploaded, it says:

dia. 25mm - lead 5 (max RPM 3,800 ish)
dia. 20mm - lead 5 (max RPM 3,000 ish)

so I assume using lead 10 for dia 25 and 20, the 3,000 RPM and 3,800 RPM is a safety RPM without breaking the ballscrew.

let me know if I'm reading this correctly?
There is distance below as well, but I ain't sure how to read it...>.<"

Y-axis = 2800mm
X-axis = 1400mm
Z-axis = 1400mm

[mmoe]

The quick math on a 10mm lead ball screw at 3000rpms would be that you will have a maximum of 30,000 mm/min possible. At 25mm diameter, my unscientific experience with various machines is that's too fast for the long axis which should probably run closer to 1500rpms, and 3000rpms may be too fast for the short axis as well. Most machines running that kind of speed are using 50mm dia. ballscrews with a minimum 20mm lead on the long axis and many on the short axis as well. The difference between a 50/20 ballscrew and a 25/10 ballscrew is night and day for resisting whipping. Some machines like that will use a 25/10 ballscrew on the short axis, but they don't usually run more than about 10,000mm/min at rapids. Just basing all that on what I've seen working on production machines. I'm sure they are a little conservative when they design them, but that's the way I typically see them.

Are you planning to stick with LinuxCNC?

[fannlow921]

In that case, I will be looking for servo at max 3000rpm, running from 1500rpm and up to see if I get whipping.

Yes, sticking with linuxCnC.

What servo size would you recommend?

[mike_Kilroy]

Ballscrew for Y-axis is 25mm dia. & 10ft long.

Ballscrew for X & Z axis is 20mm dia. & 5ft long.

You be guessing too much and getting really really wrong info.....

Your Y screw will begin to whip at approx. 400rpm max!

Your X&Z at 1000rpm max.

And these HIGH speeds are if you indeed have thrust bearing on 1 end, which you did not make clear (at least to me so far). Figure a lot less - see chart - don't guess - if you have simple/simple single bering each end.

Get engineering data instead of "I heard it on the internet so it must be true" data.

For instance see chart for max speed OF ANY SCREW IRREGARDLESS OF PITCH on page 201 of Thomson screw catalog:
http://www.thomsonlinear.com/downloa...lines_cten.pdf
It is a simple function of LENGTH, diameter, and bearing arrangement. Period.

So do not figure on any 3000rpm on any of these.

Now that you know your max screw speeds, you can pick your pitch to give you the linear speed you want. Picking pitch of 10mm/rev or 50mm/rev cuz someone said it on the internet is not right way to do it.

You asked what a 'rolled screw' was: a screw can be machined, ground, or rolled. Each as its pros and cons. rolled screws are about the lowest cost and thus lowest accuracy also; typically .005-.010" per foot as opposed to more expensive ground screws at .001-.002"/foot accuracy. Rolled is made just that way: shove hot rod thru a threaded mold while turning it and it comes out other side with threads pushed into it. cool it, ship it.

Beware Y & Y' dual axis and make sure your linux cnc can handle a dual drive axis; most will not be able to. You may get by with a drive that can do master/slave (aka electronic gearing) but it will have more and more error the faster you go - which may be ok.

I strongly suggest:
MOTIONEERING® | Kollmorgen
for motor & screw sizing. May take you 1-2 hours to get comfy with it but it will prevent a lot of the "I heard it on the internet so it must be true" misdirection.