Selecting Rack pitch

[m_c]

I've started looking into suitable racks + pinions for the plasma cutter i've designed, and was wondering if anyone has any recommendations. I was thinking about 2MOD or 12DP, as the design involves the gantry running on the racks, with no other support.
My main reason for asking is that leadscrew come in suitable pitch (ie. 10tpi), which make working out the distance moved fairly easy given you know the number of turns. Whereas with gear racks, the figures aren't as easy to work with.

Is it a case of pick whatever gear pitch you think suitable, and then work everything out from there?

Or is there commonly used pitches, to make calculations easier?

[metlmunchr]

Moray, a pinion can be spring loaded into a rack somewhat to help counteract the inherent separation forces occurring due to the gear pressure angle, but you can't let the pinion be the sole support for the gantry and have the full weight of the gantry jamming the pinion into the rack and get any kind of satisfactory operation whatsoever. The tooth engagement depth will be way past the design amount, and the gears will literally eat one another until they get all the clearance they want. The R&P in a plasma cutter has to run dry because of all the flying gritty debris from the cut. My guess would be if you build a machine in this manner and put it into a test loop where the bridge is traveling continuously and the gearing is dry, simulating whats going on when cutting, your rack and pinion will both be substantially destroyed before 4 hours is up. The motion, right from the start, will be so rough as to be unusable.

[Al_The_Man]

In the past I have used 12dp rack 20deg pressure angle 1inch face , this has a slightly higher higher load capacity than the 14.5 pressure angle. The final counts per unit of movement depends on the # of teeth of the spur gear and the gear/belt reduction used so I would not worry about tailoring the rack so it works out to anything even etc as most software allows the entry of any fractional ratio counts per unit of movement.
Al

[m_c]

metlmunchr - you have a very valid point. I hadn't really thought about the effect of the rack + pinion meshing. I'll have to have another think about other possible gantry supports.

Al - thanks for the reply. I'll just use what ever suits me best (ie. wht can be found at the best price).

Thanks
moray