Author Topic: Rotary Axis Variations - Belt/Gear vs direct  (Read 90 times)

ThothLoki

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Rotary Axis Variations - Belt/Gear vs direct
« on: August 02, 2017, 07:47:13 AM »
I have seem a lot of different variations of a rotary axis (or 4th axis) on this site, youtube, google, instructables and thingiverse.

My question is this, for our laser engravers, does it really matter if the rotary is driven by a belt/gear or direct drive from the motor?

Can someone smarter than me explain why you would want to choose one over the other? I can see the idea with belt/gears for possible adding more torque or protecting the motor shaft from getting bent. I don't know if that matters with what I plan on doing. I am not trying to reinvent the wheel (well, maybe I am) but just want to understand WHY.
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mrehmus

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Re: Rotary Axis Variations - Belt/Gear vs direct
« Reply #1 on: August 02, 2017, 09:08:42 AM »
If the object to be engraved is symmetrical around the long axis, the only problem with a direct drive from a stepper would be overcoming inertia. That could be a problem, for example, with the new BCL grayscale capability because they use velocity to change burned density. If one is rotating drinking straws, no problem but if one were rotating a baseball bat, it would probably be a problem. Having a reduction setup to increase torque would almost be mandatory I think. That said, I am not certain you wouldn't need a larger drive circuit matching a larger stepper to rotate the object.

If the object were not symmetrical around the long axis, then a reduction setup would almost certainly be required for more than a drinking straw . . . you get the idea.
mrehmus
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ThothLoki

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Re: Rotary Axis Variations - Belt/Gear vs direct
« Reply #2 on: August 02, 2017, 09:30:39 AM »
I think so.

The largest and heaviest item I am thinking of turning is a wine bottle.
I did make this: https://www.thingiverse.com/thing:1731833
but was thinking of making something like this: https://www.thingiverse.com/thing:1156932 but altering it to make a quick disconnect so I could change it out to a lathe chuck setup as well.

Thoughts?
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mrehmus

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Re: Rotary Axis Variations - Belt/Gear vs direct
« Reply #3 on: August 02, 2017, 10:29:53 AM »
I suppose either of those would work but trying to get them exactly to size so the rotation would be accurate will be very difficult and they will be subject to wear. But grabbing a bottle in any other fashion would be problematic I think. The accuracy problem is probably not going to be the issue it would be on a CNC mill. Nobody will notice if the label is off by a few steps.

Gears have a backlash (well, inexpensive setups do), belts do not which is why our lasers are belt-driven. I built my 6: ratio 4th axis with a belt drive for just this reason.
mrehmus
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ThothLoki

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Re: Rotary Axis Variations - Belt/Gear vs direct
« Reply #4 on: August 02, 2017, 10:37:41 AM »
So, let me get this straight just to see if I am picking up what you are putting down:

1. For a "roller" system, it is best to have a belt/gear ratio setup (preferably belt for backlash) to help with torque and inertia.
2. For a "lathe chuck" system, a direct drive (connect the chuck directly to the motor) would be acceptable.

I am not trying to make one intended for a CNC mill or router, just laser.

I am wondering if I should just make a direct drive with lathe chuck with some adjustable-height roller wheel stands for the other side of the bottle
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mrehmus

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Re: Rotary Axis Variations - Belt/Gear vs direct
« Reply #5 on: August 02, 2017, 10:47:56 AM »
I don't think the direct drive will work well with an NEMA 17 motor that can be driven by the typical low-cost laser unit. Most affordable lathe chucks have some run out so probably not a whole lot better set up than the rollers. And the far side of the bottle will have to be supported anyway. The problem with a chuck and roller setup is that the bottle sizes change and unless you want to make a height-adjustable roller for the far end, the bottle surface will not be parallel to the laser travel.

The simplest mechanical setup will be the rollers although you will have to calculate diameters for each different bottle size in any case.
mrehmus
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ThothLoki

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Re: Rotary Axis Variations - Belt/Gear vs direct
« Reply #6 on: August 02, 2017, 11:01:09 AM »
Thanks. Sorry, that was what I meant by the adjustable-height roller stands. That would be my support for the far end.

Honestly, I think I might have about 6-8 designs done before I figure out what I like best and what best works for me. I will just need to get my A5 up and running again so I can start testing 2 at a time.
Laser: A5 2000mw with L7 Board / A3 2500mw PWM with Mana SE -- All using T2Laser
3D Printer: Monoprice Maker Select v2.1
Image Resizer Applications (All OS): http://benboxlaser.us/index.php/topic,1434.msg17297.html#msg17297

mrehmus

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Re: Rotary Axis Variations - Belt/Gear vs direct
« Reply #7 on: August 02, 2017, 04:13:27 PM »
How about supporting the bottle with an air bed?  :o
mrehmus
editor, Model Engine Builder magazine
Elekslaser A3 2.5 watt via Banggood
BCL
Qidi X-one 3D printer
Machine Shop with manual and CNC machines