Ball screw size/pitch?

[jakk100]

Ball screw size/pitch?
What dia and pitch ball screw size is suggested for a cnc router 60”x40”x8”?
Ball screw or precision ACME lead screw?
I want to be able to hold +/-.001.
Any suggestions would be appreciated.
Thanks,
Jakk

[wjbzone]

Size will depend on the weight you are trying to move and how fast you want to move it.

I found this formula to estimate linear force developed using a screw / motor combination:

1. Convert ballscrew lead to equivalent pully radius:
example:
Lead =0.200
0.200 / (2pi) = 0.200 / 6.283 = 0.0318" (equliv. radius pully)

2. Convert motor torque into linear force:
example:
Torque = 100 Oz.-In.
100oz-in / .0318 in = 3144 oz. (linear force)
3144 / 16 = 196 lb (linear force)
196 * 0.8 (efficency for ballscrew) = 157 lb linear force.

If this formula is correct, a 100oz-in motor and a 0.200 screw can push 157 pounds force.

The force you need to push depends on weight of the gantry, friction of the bearings, and cutting forces. (not to mention acceleration).

Hope this helps.
Bill

[ger21]

Go to

http://www.nookindustries.com

Download the appropriate .pdf catalog (s), for ballscrews or acme screws. They list the force required to move a 1 lb load for each of their screws. For example, with my 1/2-4 acme, to lift 1 lb I need .064 in-lbs. Multiply by 16 = 1.024 oz-in to move 1 lb. So my 250 oz steppers should be able to safely give me 175 - 200 lbs even at a few hundred rpm as the torque falls off. And 300 rpm should be 75 inches per minute. We'll see when I'm done, but I'm hoping it works out like this.


Gerry

[HomeCNC]

The screw is going to be very expensive if you are wanting to hold .001 tolerance over that long of distance.

[HuFlungDung]

Accuracy and repeatability are not exactly the same thing. You might be able to broaden your requirements, if you can settle for the machine being less accurate, but more repeatable. If your screws are mounted in "tight mounts", so that you never get unexpected slop from reversals, if your slideways are properly positioned relative to the driving screw, if your slides are of sufficient bearing length to prevent jamming and stiction, then the machine should be fairly repeatable no matter what screws you use, providing that you always machine in the same direction.

Linear scale feedback is the best way to get high accuracy from low grade screws, however, low grade screws usually have a lot of backlash, which can make your machine "hunt for position" quite a lot.

Scales also cost a lot.

So, this boils down to spending the cash to get high grade ballscrews if you want fewer headaches down the road, IMO.

[jakk100]

Thanks to all for their replies and patients with my questions. There will be more to follow I’m sure!
So using Bills formula, same values he used except the servos I am considering have 600 oz peak torque.
I would get 943 lbs linear force. Is that correct?
As far as holding tolerance, wont the software be able to dial most backlash and ball screw lead error out?
And wont that improve accuracy and repeatability? ( I’m guessing)
I would like to rapid at 600 IPM.
CNC router 60”x40”x8”. I think I’ll use 1” ball screw with 5 TPI on all aixs.
Will use encoders with 250 CPR.
I agree with HuFlungDung, “…spending the cash to get high grade ballscrews if you want fewer headaches down the road, IMO.”
Your thoughts would be greatly appreciated.
Thanks,
Jakk

[HuFlungDung]

Jakk100,

Yes, a 600 oz-in motor is a pretty good size, common on a lot of knee mill retrofits. You should be able to break off a 3/4" cutter with it

Your controller software typically can be set up to make a backlash compensation. If it is really fancy, it may have a table of adjustments for so and so much adjustment at such and such a position.

Having said that, there are some limitations to backlash compensation, such as "You can't fool a climb milling cutter", and believe me, you will want to climb mill.

Also, backlash comp is tricky to make work really smoothly during arc cuts through the quadrant lines, where a reversal must occur, and the comp must be applied. This interrupts the cut with a slight pause and leaves a mark.

[jcc3inc]

Dear Sir:
You ought to consider using rack and pinion drives for that size machine. As the screw length increases, you approach the "critical speed" for a given diameter screw where whip of the screw becomes a factor. Larger diameters (more $$$$) have a higher critical speed.
I have used rack and pinion drives on CNC plasma cutting machines which have high speeds, low backlash, and tolerate dirty conditions. Backlash can be held low, dual side synchronization can be accomplished mechanically, and sufficient drive force can be obtained, all at reasonable price. Most large commercial routers use rack and pinion drives.
Regards,
Jack C.