Gear reduction understanding

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25 May 2022 09:57 #243772 by rodw
Replied by rodw 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. 

There is not really a definitive rule but if you do the engineering, as spumco says both gear ratio and pinion diameter are both different tuning parameters. If you want to get to the moon in your spaceship or build a hot rod, you need to maximise performance. I worked with Marshall, an engineer in the US while he developed a stepper motor design model over 3 months. The model was then tested on my table using LAM Technologies drivers. The main benchmarks we reviewed was the motor voltage required (some motors only needed 30 volts), the amps required to achieve the desired performance (some motors only needed half their rated amperage) and the temperature rise to make sure we did not melt the insulation. But then you also needed to review the torque curve to see if the selected motor had enough headroom for your rapids. This is where higher voltage than what is required helps.

So changing the pinion diameter and the gear reduction makes a difference. We tried various combinations from 60mm pinions and gearing ratios from 10:1 and down to get the best results so we could achieve 30 m/min rapids, 10.24 m/min max cut speed and 5 m/sec/sec acceleration (approx 0.5G). This means we accelerate from a stand still to 10.24 m/min in 30 milliseconds.

From all of this, we found that my seat of the pants design was close to optimal when benchmarked with the model. Whilst there was a case to change the reducers from the existing 5:1 and 3:1, it would have involved pretty major mods to the gantry ends so we just went with it as it met the model design. We also found that every motor was different so it really is not possible to generalise  design parameters.

I was able to get my X axis up to 60 m/min rapids and 0.8G accelleration (8000 mm/sec/sec) but this was too much for the NEMA34's on the heavier gantry so we settled on 5m/sec/sec and 36 m/min  rapids. At this level of performance from a stepper, Lam's boost technology is mandatory.

So whilst Phil's 48mm pinion circumference is probably OK, but it is likely suboptimal A 1:1 gear ratio would fall far short of our design parameters as it would not be able to harness the massive low down torque steppers produce to achieve the desired acceleration profile.

This video attempts to outline what we were looking at 
www.loom.com/share/64ef8fb135ad47bcaf006761e222b076
Remember Acceleration is king with plasma.
 

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25 May 2022 10:12 #243776 by phillc54

So whilst Phil's 48mm pinion circumference is probably OK, but it is likely suboptimal A 1:1 gear ratio would fall far short of our design parameters as it would not be able to harness the massive low down torque steppers produce to achieve the desired acceleration profile.


I understand the relationship between voltage, accelleration an velocity, I also undserstand that changing pinions and gear ratios are tuning parameters.

What I don't understand is the 1" per revolution "rule". I guess it is just a rule of thumb because I haven't seen any supporting data.

I assume my table being "high geared" does utilise the high torque range of a stepper which is at low rpm. I can get 0.8G accelleration and 80M/Min velocity so it can't be that bad. I only use 0.3G and 30M/Min because I don't see any benefit from higher values.

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25 May 2022 10:23 #243778 by rodw
Replied by rodw on topic Gear reduction understanding
Personally, I think the "1 inch rule" came from Tom Caudle at CandCNC.
Jim Colt's rule of thumb was 0.3G for acceleration. Any more than that and the inertia really shakes things around so the table and gantry needs to be very stiff and probably bolted down.
I never shared my dial indicator tests for a reason :)

The other factor to consider in the design is are you outside of the resonance zone at cut speeds? Gearing helps here.

Hypertherm talk about centripetal radius limits. Your acceleration determines the minimum arc radius you can negotiate without slowing down. Faster acceleration will give crisper corners.

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25 May 2022 11:38 #243785 by tommylight
Use a TB6560 drive with microstepping, to much vibrations for direct drive.
Use a TB67S109 (ebay TB6600) drive with microstepping, it is much smoother.
Use a LAM Technologies drive with microstepping, very smooth as it can do 256000 or more micro steps.
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Servos are a different beast-
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The 1 inch/turn rule is old, it should be used as a rough estimate, anything within reason will work with minor differences.
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Homework for those all:
-do a move back and forth on one of the axis while lowering the velocity till it hits resonant frequency and the machine starts vibrating,
-do the same with the other axis,
-try to avoid those speeds while cutting
-problem solved.

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25 May 2022 13:49 #243795 by spumco
Replied by spumco on topic Gear reduction understanding

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The 1 inch/turn rule is old, it should be used as a rough estimate, anything within reason will work with minor differences.
-

 


What is the "1 inch /turn rule"?

Is that a rule of thumb that the designer should target 1" of linear motion per motor revolution? 

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25 May 2022 14:49 #243797 by Aldenflorio
Well, at the rate of which this conversation has escalated I no longer have anything to offer. All of this is over my head. I need to do more research and learn better about motors, voltage, torque, and all things involved with that.

Here is where I stand on plasma CNCs: I have built two, the first one with the idea of starting a business and not having enough money to buy a 15k plasma table. It worked great and I learned so much. After making it though I knew I could do so much better. So I sold it. Well upon trying to sell it I was commissioned to build another table. So I did, it was much better then my first but I still know it could improve. So before I offer to make any other table I would like to perfectly fine tune and research all the little things that would improve my table design. I love building plasma tables. Im a welder by trade.

just figured I’d give some background

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25 May 2022 16:24 #243811 by tommylight

Is that a rule of thumb that the designer should target 1" of linear motion per motor revolution? 

[/quote]
It somehow became a rule at the time when microstepping was very expensive and there was no "resonant frequency compensation" of any kind in the drives, so with reduction steppers would turn faster hence lower vibrations.
Try to run a direct driven machine with no microstepping (drives set to 1/1), it is a terrible noise ! :)
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25 May 2022 16:25 #243813 by spumco
Replied by spumco on topic Gear reduction understanding

Well, at the rate of which this conversation has escalated I no longer have anything to offer. All of this is over my head. I need to do more research and learn better about motors, voltage, torque, and all things involved with that.


 


Everyone has something to offer - even it's just anecdotal evidence of something not working as desired.

In your case, you had/have a perfectly acceptable plasma table without a reduction system.  If you know, more or less, what the smallest corner radius you could cut (at high speed) with acceptable quality is, then let everyone here know and we'll all be better informed.

For example:
Gantry estimated weight - XYZkg
Motors & driver voltage - Nema34, 1200oz-in, 80Vdc
Pinion diameter - XYZmm
Smallest acceptable corner radius:  XYZmm @ (insert metric nonsense) per minute on 1mm (steel/aluminum)

You can certainly contribute something like that.  Remember, most of the 'above your head' stuff is theoretical.
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13 Jun 2022 10:24 #245063 by samforsey12345
Take this with a pinch of salt, as I have only built my own machine and have relatively little experience with plasma cutters.

After having decided on using stepper motors, my go to thought when deciding on gearing, was what precision do I want per step of the stepper motor.

Assuming 200steps/motor revolution and 25mm per rev, that gives you a resolution of 0.125mm per step. That's a fairly precise machine.

That's just how I approached the problem, although I went for slightly greater precision per step myself. I guess in a simplistic sense the rule of thumb works. It does seem to put the stepper motors in a decent range of operation.

Cheers
Sam
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