Direct or geared reduction to threaded drive ? (stepper)

[andy_ck87028]

I've been trying to work out if there are distinct benefits on having either a geared reduction between stepper motor and threaded drive vs a direct drive arrangement (let it be either acme or ball screw so friction doesn't distract the argument)

My reckoning is that optimal mechanical advantage can all be managed through combined choice of thread pitch and size of motor. Basically I have concluded that gearing a stepper to the threaded rod achieves nothing of tangible value.

I can see an argument that reducing the risk of whipping could justify having a geared reduction therefore enabling the screw to have fewer turns per inch but couldn't you achieve the same just by having a more powerful motor?

I can also see an argument that having a geared arrangement using pulleys and belt overcomes coupling alignment issues but that is surely an argument of convenience rather than mechanical advantage.

I appreciate that I might be over simplifying things and making some gross assumptions but what are other people's opinions on this subject?

Andy

[NC Cams]

You are overcomplicating your analysis.

Our CNC Bridgeport is belt driven at 2:1 with a 0.200"/rev pitch ball screw. With our cam master milling program, we can mill to within 0.0001".

If you could have a motor strong enough to drive at the speed you want AND with the torque you want AND the resolution you want, that would be perfect. However, that isn't going to happen. Motors cog, torque curves aren't what you want or where your wan them to be and so on and so forth.

To get around/minimize the ineveitable "issues", gearing (IE: belt drive) reduces the cogging effect the motor has at the part and/or your trade speed for torque.

THe other issue is packaging. A motor hooked directly to the screw does not package well. Imagine a decent sized motor coupled dirctly to the Y axis of a mill and use your imagination..... By tucking it down and out of the way, it just packages better.

A well engineed 2:1 timing drive will work just fine and the concept is WELL proven in hundreds of applications. I think your trying to separate the spilled salt from spilled pepper looking at it the way you are....

BTW, we do NOT have a tensioner in our drive - it is strictly a 2:1 cogged belt. Keep the center distance short and properly spaced for the pitch of the belt you choose, leaving some space for belt tensioning by moving the motor in slotted holes, and you'll be fine.

Besides, the more pulleys and belts you run to get from the motor to the ultimately driven member puts more inherent slop/hysterisis and complexity potential into the drive mechanism.

[ger21]

Originally Posted by NC Cams A well engineed 2:1 timing drive will work just fine and the concept is WELL proven in hundreds of applications.

Provided your talking about a mill and not a router that would be run at much higher speeds.

[andy_ck87028]

You are overcomplicating your analysis.

That's me ... always has been!

Provided your talking about a mill and not a router that would be run at much higher speeds.

I'm talking router

Andy

[Bruggles]

It definatly does help to not have motors sticking out of your router/mill, if you can use a belt and stick the motor under the table, that can be nice.