Gear reduction understanding

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22 May 2022 23:11 #243580 by Aldenflorio
I have built several plasma tables. I’ve use direct drive though, I have never used a gear reduction or stepper reduction box. Though I have been told they give better quality and accuracy. I would like to incorporate them into my next build. Though I’m not sure I understand fully what they do and how they improve the quality when using a stepper motor.

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22 May 2022 23:41 #243582 by spumco
Replied by spumco on topic Gear reduction understanding
(Simplistic answer)
Gear reduction:
  • Increases the stepper motor torque transferred to the gantry & head
  • Increases mechanical resolution
  • Increases acceleration
  • Increases "smoothness" (8x microstepping will become 40x with a 1:5 reduction system)
  • Increases backlash
  • Increases inertia (both rotational and basic system weight)
  • Decreases top speed of gantry & head
(Complicated answer)
  • Reduction systems can be great, or horrible.  It depends on a number of factors:
  • Motor torque curves
  • Motor top speed
  • Reduction system quality (backlash, sloppy, springy belts)
  • Reduction system type (belt, planetary, spur, harmonic, cycloidal, etc.)
  • Gantry/head weight

For consideration:
Figure out the maximum cutting speed you anticipate - really thin aluminum perhaps - then add 10% to that number.  That's the max speed you need.  Anything faster is a nice-to-have for rapids (repositioning between cuts).

My experience with plasma cutting is that acceleration is king.  Top speed doesn't matter as long as you can reach the programmed cut speed.  After that high acceleration will permit very fast direction changes, leading to improved corners and smaller corner radii able to be cut cleanly.

 

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23 May 2022 00:08 #243583 by rodw
Replied by rodw on topic Gear reduction understanding
Another answer. Assumes rack and pinion with approx 30mm pinion diameter:
Nema 34 5:1 reduction on each side of the gantry,
Nema 23/24 3:1 reduction on X axis across the gantry.
You should also not have the stepper moving in its resonance zone with the reduction

These were designed at a cut speed of 10.24 m/min max cut speed.

Motor selection and voltage are pretty important to get max performance.

Acceleration should be at least  3 m/s/s, you can get to 5000 m/s/s with steppers but you need a very very rigid table to handle the inertia! eg accel to 2200mm/min cut speed will be  30 milliseconds.
 

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23 May 2022 22:32 #243651 by Aldenflorio
so if you plan on using a hypertherm45 xp it states that the recommended speed for 1/10” aluminum would be 240in/min or 7890mm/min. Therefore if you want a machine that can maintain that you should the top speed should be 264in/min or 8,679mm/min. I have only used stepper motors, NEMA23 570OZ/IN to be exact. My rack and gear has always been 20 Degree Pressure Angle, Round Bore, 20 Pitch, 15 Hardened Teeth. Like I said I’ve always done directs drive. I haven’t had an issue with quality at all. Or speed. Would a stepper reduction box really make a positive difference for me?

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23 May 2022 23:42 - 24 May 2022 00:24 #243656 by snowgoer540

so if you plan on using a hypertherm45 xp it states that the recommended speed for 1/10” aluminum would be 240in/min or 7890mm/min. Therefore if you want a machine that can maintain that you should the top speed should be 264in/min or 8,679mm/min. I have only used stepper motors, NEMA23 570OZ/IN to be exact. My rack and gear has always been 20 Degree Pressure Angle, Round Bore, 20 Pitch, 15 Hardened Teeth. Like I said I’ve always done directs drive. I haven’t had an issue with quality at all. Or speed. Would a stepper reduction box really make a positive difference for me?


FWIW, I run the exact same setup, stepper size, rack and pinion, etc. I too have had very positive results. I've also considered doing a reduction (mainly due to people here saying it's necessary), to the point where I designed it, but have yet to create it. I've been kind of waiting for something to break, or provide more indication that the current setup is lacking something. I have also considered that "I dont know what I'm missing", but it goes hand in hand with "if it aint broke dont fix it".

If you decide to go the reduction route, please share your results, I'd be very interested to hear from someone who is running the exact mechanical setup I am.

Also, acceleration gets talked about a lot, but I'd say that keeping the gantry weight to a minimum is equally important.
Last edit: 24 May 2022 00:24 by snowgoer540.

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23 May 2022 23:53 #243658 by tommylight

Would a stepper reduction box really make a positive difference for me?

Not really, if everything is working as you expect.
Get us some pictures of the cuts done on some thicker material.

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24 May 2022 10:53 #243673 by rodw
Replied by rodw on topic Gear reduction understanding
Tommy, thats not what you said to me on this thread!
forum.linuxcnc.org/30-cnc-machines/31509...pinion-drive?start=0

This is a very useful thread I think notwithstanding the extra work I was given from it.

On page 2, the  discussion on gearing starts.

Note that on the feedback given I did not try the direct drive because I knew the ideal travel was about 1" per rev with a range of 15-30mm
We've since done a lot of engineering design which confirmed this.
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24 May 2022 14:13 #243686 by tommylight

Tommy, thats not what you said to me on this thread!
forum.linuxcnc.org/30-cnc-machines/31509...pinion-drive?start=0

I am fully aware of that, but he insists it works for him, so why bother?

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24 May 2022 23:38 #243732 by phillc54
Interesting topic.
It would be interesting to see some data that shows the optimal distance per revolution and why, rather than anecdotal evidence. When I built my table I saw the inch per revolution figure tossed round but due to constraints at the time (and laziness) I ended up using GT3 belts at 48mm per revolution. I thought I may add gearing at a later date but the cut quality to me appears good and I haven't yet seen a reason to change. 

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25 May 2022 00:47 #243737 by spumco
Replied by spumco on topic Gear reduction understanding

Interesting topic.
It would be interesting to see some data that shows the optimal distance per revolution and why, rather than anecdotal evidence. When I built my table I saw the inch per revolution figure tossed round but due to constraints at the time (and laziness) I ended up using GT3 belts at 48mm per revolution. I thought I may add gearing at a later date but the cut quality to me appears good and I haven't yet seen a reason to change. 
 


I don't have charts or torque curves to post, but on the two big tables I built I had two variables to play with: pinion diameter and reducer ratio. 

To answer your 'why?' question, my objective was to have have the motor torque start to fall off around 500ipm, ensuring that at any reasonable cut speed they would have full torque, plus quite a bit for rapids as it's painful to watch a gantry creep along for 12 feet at 200ipm.

Servos had a top rpm of 5k, and the torque started dropping off at about 3500rpm (acording to datasheet)

In addition, one of the tables has an interchangable router spindle and the owner wanted an electronic & mechanical resolution of <0.001".

Drive system for both is a home-brew version of the Bell-Everman Loop Track arrangement.  Wound up with a 15T pinion (0.947" pitch dia) & 10:1 planetary reducers on all four drive motors (for commonality of spare parts).  This gave an effective distance per motor rev of 0.2975", and a theoretical top speed of ~1000ipm.  Servo drives were programmed to be just under the 200kHz input frequency at top speed.  The Bell-Everman system can be treated like a rack and pinion for speed/rpm calculations.

The gantries can hit around 1100ipm and the heads can go a bit faster.  Long-term testing at full speed revealed the need for braking resistors on the gantry motors if I left the speeds way up and both owners declined the added expense.  They're both happy with the tables throttled down to 800ipm with fairly aggressive acceleration left.

As has been noted multiple times: if cut quality is acceptable, don't change anything.

In my case, however, both owners told me "I want a Hot Rod" and they got what they asked for.  The gantries are fairly light & stiff, heads are light, and the drive system is remarkably stiff & backlash-free.  When programmed aggressively, they're pretty frightening to watch - like oversized pick-and-place machines.
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