[Bigtoy302]

Ok, so I am getting ready to build a plasma table and I Dont know what rack and pinion I should use. I have read 20* 20 pitch and also 14.5* 16 pitch. What is better? From mcmaster the 20* racks are $22 cheaper than the 14.5* for six feet. Also how small of a pinion can I use on eaither rack? For the 20* the only 3/8" bore one is 15 tooth .75 PD.

[Greolt]

20 degree pressure angle.

I think 18 tooth is considered minimum, with 20 preferred.

Greg

[Jcar]

Bigtoy302,

Either will work. I used 16 pitch, 14 1/2 degree pressure angle racks. Initially I felt that 20 pitch was a little too small. After having used the 16 pitch, I think the 20 pitch would have been more than adequate. According to Boston Gear, a 20 degree pressure angle is stronger, while a 14 1/2 degree pressure angle is smoother running and has less backlash. They also state that, in order to minimize undercutting, the minimum number of teeth for a 20 degree pressure angle is 13 and the minimum number of teeth for 14 1/2 degree pressure angle is 16. I used 14 tooth pinions because I don't believe that the weaker tooth profile of an undercut pinion gear is an issue with the loads involved in plasma cutting. Take a look at pages 140 and 141 of the following link.

http://www.bostongear.com/pdf/gear_theory.pdf

Here's a link with more info on undercutting. Take a look at the animation in the upper right corner.
http://www.drgears.com/gearterms/terms/undercut.htm

[Greolt]

Bigtoy302,

According to Boston Gear, a 20 degree pressure angle is stronger, while a 14 1/2 degree pressure angle is smoother running and has less backlash.

That is interesting info. I'd always read that 20 degree was better. Going on that 14.5 would be a better choice in this application.

Greg

[Mike Nash]

By far the best pricing I found on rack was here http://www.stdsteel.com/gr_stock.htm and I had zero problems with the purchase. But you have to ask for pricing and shipping and send a check, no credit cards. I have heard they have gone to per piece cost on shipping based on two shipping zones lately. I bought a gear blank from McMaster that's 12" long that I intend to make my own pinions from. Lot's cheaper and I can get the 1/2" bore that I need that way (I sure love my 12x36 lathe!.) I went with the 20DP and 20 tooth pinion but have yet to build. I've gotta finish the mill first.