Size and pitch of leadscrews

[transtar]

Hi all:

I am building a Cnc Rounter machine of the following specifications:

Type: Fixed bed with moving gantry
Travel: 600mm x 350mm x 75mm

Anyone can advise me on the following
- Size and pitch of lead / ball screw capable of driving this machine.
- Size of stepper motor compatiable to the above leadscrew.

Thanks

[ger21]

It depends on how much the machine will weigh (the moving components), how fast do you want to cut, how fast do you want to rapid....

A lot of people start out with 1/2-10 acme and 200-250oz motors. That's the cheapest way to get started.

[BobWarfield]

Look at it from 2 directions--your travel rates and your accuracy.

A step motor develops its max torque in the 900-1200 rpm range. Let's leave aside the drive ratio for a second and think just about the leadscrew specs. Let's figure 1000 rpm to be in the middle of the torque band. Now you can look at your travel speed:

1000 rpm / leadscrew threads per inch = inches per minute of travel

So, a 10 tpi leadscrew will run 100 inches/minute direct drive.

Now let's look at the accuracy of positioning. Most step motors will give you 200 steps/revolution (forget microstepping for the moment). So, you can position your leadscrew to 1/200th of a revolution. A 10 tpi leadscrew can then be positioned to:

1/10 inch per revolution / 200 steps = 1/2000th = 0.0005"

Using a spreadsheet and these two formulas you can work out what your performance will be for various leadscrews. You will want to add in the ability to use a belt drive reduction in order to increase torque and accuracy by gearing down. A 2:1 ratio will double your torque, halve the travel speed, and double your accuracy.

Best,

BW

[fyffe555]

A step motor develops its max torque in the 900-1200 rpm range. Let's leave aside the drive ratio for a second and think just about the leadscrew specs. Let's figure 1000 rpm to be in the middle of the torque band.

Got any torque curves for steppers with torque like this? all steppers I've seen have max torque at or just off stall. After 300 rpm or so they're less than 1/4 rated torque..

[BobWarfield]

Yep, torque in fact is not the right term--"Power" is a better one. Since power is what we are calling on the motor to produce, not holding torque, that's how we want to look at it.

Max power is found in that rpm band for step motors because power is torque times speed, and so max power will be were the curve starts to drop off, which is where the 900-1200 rpm figures come in. FWIW, I got those numbers from a Mariss F. post somewhere, but you can see the logic by looking at those curves. Of course a servo will give you a much broader curve, but let's hang in there on the step example. Most step motor torque curves go out to about 1000 rpm give or take and then start to fall off. Here is a typical example:

http://www.usdigital.com/products/ms...que_curves.gif

One other thing you need to bring in here is the "20x voltage" to a good driver like a GeckoDrive to get the motor to perform. That's going to be key for getting that torque to hang in there to as high a speed as possible. Check the Gecko Drive Yahoo group for more information on this under the DC Supply design information.

There are many other factors, but this kind of thing should get you to the back of envelope ballpark for evaluating leadscrew pitches for various purposes.

Best,

BW

[ViperTX]

Okay...if you aren't confused by now....then just select a ballscrew with approximately 5 turns per inch and you will be okay!