Stepper size question

[Thinwater]

I am almost done with my homemmade cnc router. it uses thk rails and 1/10 1/2" acme rods to drive it. I made one drive nut out of brass but it has more backlash than I want. I made another out of UHMW and it has about 1/2 as much as the brass. The machine is only 20" x 12" but is getting very heavy very fast. I think I over did the X and Z by using large chunks of solid aluminum for the bodys of the parts.

I made the mounts for a nema 23 size motor. I want to use this for engraving some of the products I make and will use mach 3 to drive it and comp for backlash.

I am trying to deturmine how big the steppers need to be for it. I want it fairly fast and know the acme limits this some. I am thinking more power can overcome the screws resistance but how big?

Also any recomendations on full stepper kits with drive and wiring are welcome.

This is the build thread for the machine http://www.cnczone.com/forums/diy-cn...4_plans-2.html

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[OCNC]

I am trying to determine how big the steppers need to be for it. I want it fairly fast and know the acme limits this some. I am thinking more power can overcome the screws resistance but how big?

I think you're going to find that the 10 TPI acme screw gives you way more resolution than you can take advantage of. For example with a 10 micro step drive (2000 pulses per revolution) the theoretical resolution is 0.00005" (.1/2000). Stepper motor torque is high at low rpm and diminishes rapidly as rpm increases. I use 600 rpm as my target maximum motor speed. At 600 rpm you're only getting 60 ipm feed rate with the 10 TPI screws. I use a target feed rate of between 100 and 200 ipm minimum for routing wood. (I don't know what you would want as a target feed rate for engraving metal.) To get the target feed rate of 100-200 ipm (actually 120-240 ipm) I would choose a screw with a lead of between .2 and .4 inches per revolution.

Chris

[louieatienza]

I'd say it ultimately would depend on the spindle you're using. The Sherline and Taig mills both use 1/2"-10, single start screws. And even with the heavy carraige assembly they typically use smallish (135-150in-oz) steppers. I've seen those mills retrofitted with 50 adn 100 watt servos, with 3:1 belt reduction.

While the rapids are not high, I don't think the machne is so large that there would be a great benefit from having high rapids. Also, you probably don't need high feeds if your spindle speed is not that high either. So in this particular case it would be advantageous to have the finer-pitched leadscrews.

As for backlash, 1/2"-10 is a pretty common thread and anti-backlash nuts are easy to find. DumpsterCNC makes a very affordable one; Thomson and Kerk both make different grades but they are more expensive.