What MOD rack size to use?

**Norgmonster** · 07-06-2016

Originally Posted by komatias

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:

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?

Click image for larger version.

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Sorry if the image is small.

**Clive S** · 07-06-2016

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

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

Originally Posted by JAZZCNC

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

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

Originally Posted by Norgmonster

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.