[Woodtek]

good day to all,

I'm also in the process of building my first cnc router.
I do however have a question regarding gear reduction on the stepper motors. The norm seems to be a small gear/pulley on the motor and a larger gear/pulley on the lead screw. This arrangement provides a certain speed : torque ratio, since most of the lead screws are acme, multi-start threads.
Now, my lead screws will be metric, M12 * 1.75mm. In order to keep the motor speed within the max torque limits, I was wondering what the effect would be if I reverse the normal arrangement, i.e small gear on the lead screw and the large gear on the motor. This will produce more speed at lower motor rpm's, but ..........?

Could this arrangement work, or would I damage my motors/controller or something else?

Any help will be appreciated.

Greetings from Dark Africa.

[louieatienza]

There's no problem "gearing up" a stepper as far as electronics are concerned. The only problem I see is the potential for your leadnuts to seize up or wear prematurely. At the very least it would require constant lubrication. Plus I think it would be more cost effective to just purchase multi-start screws with inexpensive coupling direct to the motor.

[Woodtek]

Ah, at last some useful info. The problem being that them multi-start lead screws and nuts are next to impossible to source in Namibia. Taking the current exchange rate into account, it's also extremely expensive to import.
That's the main reason why I wanted the info. I hear your statement about premature wear and lubrication, and I appreciate that.
In the mean time I think I will go that way until monetary sources allows the import of the proper stuff.

Thanks a million for replying.

Greetings from Dark Africa

[awerby]

Since steppers tend to lose torque as their speed increases, the gearing you're talking about makes sense to me. I don't think you'll damage anything, but make sure the screw doesn't get a chance to "whip", which is possible for skinny screws run at high rpms.

[Woodtek]

Since steppers tend to lose torque as their speed increases, the gearing you're talking about makes sense to me. I don't think you'll damage anything, but make sure the screw doesn't get a chance to "whip", which is possible for skinny screws run at high rpms.

Thanks Andrew.

I am aware of the whipping. The plans that I'm using actually makes provision for that since they propose an even smaller lead screw.
I'm not planning to step up the gearing that much. Thinking around 1:3 or so. I'm going to use 305 oz-in steppers, so I reckon they are more than capable.
I really appreciate your input and advice. By the way, great website. Not boring like mine. Had to design it myself. Check it at Woodtek. I'm also planning to use Deskproto as well as VCarve Pro.

Greetings from Dark Africa.

[louieatienza]

Steppers don't just lose torque as speed increases, the torque remains more or less constant until the motor goes from being current driven to voltage driven. This is because the drve sets the max current to the stepper. The voltage more or less determines what that takes place, and more or less proportional to the voltage supplied.

Being you have a relatively fine pitch even weth gearing up, I would use a smaller stepper, like 150in-oz, or even less, 8 wire stepper wired bipolar parallel. Should give you good torque for a wider speed range than larger steppers.

[mengineer101]

We used Helix Linear Helix Linear Technologies on a recent project and they offer a big selection of screw leads and ship pretty quickly. I think they can machine the screws as well.

[hub]

Here is some info on steppers speeds & torque:
Speed - Torque Curves for Stepper Motors

The 1.75mm pitch is very small... And if it's a regular threaded rod, it (& nuts) will wear out very quickly (tried it in my very first machine attempt, not a good idea...).