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View Full Version : What MOD rack size to use?



marbles
06-06-2016, 01:18 PM
I'm about to start a second machine build. A 3m x 2m for general materials, wood, polystyrene and occasional plasma. Sort of multipurpose.

I'm going to upgrade from T5 belt to rack and pinion but i'm not sure what is the smallest rack size I can get away with is? This machine will get used maybe once a week but for several hours at a time. I'll be gearing between the stepper and pinion as well.

Any suggestions or examples anyone?

m_c
07-06-2016, 12:26 AM
Having just looked at the actual size diagrams in my HPC gears catalogue, I'd probably pick something in the 2-3MOD range.

The smaller you go, the more noticeable any contamination will be, and the larger you go, the more the non-linear motion from straight cut spur gears become.

magicniner
07-06-2016, 10:15 AM
Marbles, if you're looking at their catalogue, HPC gears do some nice anti-backlash gears BTW ;-)

m_c
07-06-2016, 10:38 AM
Anti-backlash gears don't solve the non-linear issue though. You'd have to use helical rack/gears for that ;-)

komatias
07-06-2016, 10:59 AM
Anti-backlash gears don't solve the non-linear issue though. You'd have to use helical rack/gears for that ;-)

Having recently designed a machine with rack and pinion, where the requirement was speed as opposed to smooth running/accuracy, I would suggest you go with the smallest economical one you can. Nonlinearity will be a real concern with anything larger than 2.

To avoid contamination, mount the rack, with the teeth pointing down

For a visual comparison:

http://www.newpages2u.com/userfiles/17926/image/gear.jpg

marbles
07-06-2016, 11:37 AM
Thanks for all thoughts :adoration:

I'm beginning to think that if I was just cutting large blocks of foam or using as plasma i'd stick to a belt drive system because of the speed and accuracy. Not entirely sure why more people don't use belt. I'm guessing its just that its less well known as a tech and maybe is not so good over long distances.

However as I do wish to cut timber and make up a 3m x axis R&P and has history so is probably the way to go. How about helical rack/gears and probably 2.0 MOD with a gear ratio to suit?

What ratio required to suit high speed roughing cuts in polystyrene and and also give some repeat accuracy in timber is the next thing to sort out.

magicniner
07-06-2016, 11:42 AM
Anti-backlash gears don't solve the non-linear issue though. You'd have to use helical rack/gears for that ;-)

Yes, it's kind of in the name ;-)

m_c
07-06-2016, 11:54 AM
The issue with belts over longer distance, is you get into resonance/flex issues. You can minimise it by lots of tension, but then the design of what they're tensioned between becomes critical, and you have to make sure everything under tension is strong enough.

There is that system that is essentially a rack the belt sits/locates in, with the belt looping up and over idler and drive pulleys, which eliminates having a long unsupported belt, however from what I remember it was more expensive than other options.

It's like most things, you have to weigh up the pros/cons of each method, and decide what's the best compromise for you.

m_c
07-06-2016, 12:08 PM
Yes, it's kind of in the name ;-)

And they're not really a good solution for large CNC machines. Any anti-backlash gears I've looked at, the backlash torque is never high enough that the forces on CNC won't be able to over power it.
A quick check of the HTC catalogue I've got, lists the torque for a 50t 2 MOD gear at 2.5Nm. I'm pretty sure a decent cut could manage to overcome that, and on high speed machines, I'm sure the accelerations involved could manage to do the same.

And that's before you consider the additional friction they'd introduce into the system, which depending on the machine, may or may not have any significant effect.

That's not to say they wouldn't help, but you have to weigh up the pros/cons of using them.

Norgmonster
07-06-2016, 06:23 PM
Having recently designed a machine with rack and pinion, where the requirement was speed as opposed to smooth running/accuracy, I would suggest you go with the smallest economical one you can. Nonlinearity will be a real concern with anything larger than 2.

To avoid contamination, mount the rack, with the teeth pointing down

For a visual comparison:

http://www.newpages2u.com/userfiles/17926/image/gear.jpg



I'm planing a R&P router myself and would like to ask two things?? What do you mean by contamination? And, When you say mount the rack, do you mean something like this?

18583

Sorry if the image is small.

Clive S
07-06-2016, 07:17 PM
I'm planing a R&P router myself and would like to ask two things?? What do you mean by contamination? And, When you say mount the rack, do you mean something like this? Contamination means dirt or swarf getting on to the teeth that is why he said mount them with the teeth facing down Like in the image.

JAZZCNC
07-06-2016, 08:57 PM
MOD 1 is perfectly fine for R&P machine and most common size used at DIY level. Certainly no need to go above MOD 1.5.
I've got industrial built 10 x 5 Router with very heavy gantry which is running MOD1.5.

marbles
07-06-2016, 10:30 PM
MOD 1 is perfectly fine for R&P machine and most common size used at DIY level. Certainly no need to go above MOD 1.5.
I've got industrial built 10 x 5 Router with very heavy gantry which is running MOD1.5.

Totally fine to go with MOD 1 in that case, quite a lot cheaper than 2.0 as well..

Norgmonster
17-06-2016, 07:29 PM
Sorry to chime in but i was wondering if the type of MOD (1 or 1.5) you use is decided by how heavy the gantry ends up being? If so then what is the limit?

m_c
17-06-2016, 10:00 PM
Sorry to chime in but i was wondering if the type of MOD (1 or 1.5) you use is decided by how heavy the gantry ends up being? If so then what is the limit?

The deciding factor is cutting and acceleration forces, and if the chosen MOD size can handle the required torque/forces.
As part of that you also have to consider tooth engagement, as a larger pinion will have more teeth engaged at any given time so can handle more torque than a smaller pinion.

Norgmonster
17-06-2016, 10:05 PM
Thanks for this info. I'm still working on the designs for my CNC Router so when i'm ready can i ask again?

:-)