14.5 vs 20 deg pitch rack

[wcarrothers1]

Well I'm dreaming up a new machine and looking at the different rack choices at mcmaster and all..

Anyway new machine will be a 10x5' machine. Current machine is 4x8 with rack drive on the long axis using 14.5 deg gears.. Was thinking about going to 20 for the larger machine. But looking at the gear selection it seems they have less choice far as gear sizes in a given hub size. Thinking I might switch a few gears after the fact to get my speeds right only to be limited far as what I can do.

Probably would be using the 3/4" face with racks. Would be using 2 racks on each side for the long axis, 1 rack on Y and 1 on each Z. I think the 14.5 in that size would hold up and work well given my experience using the 1/2" face width rack on my first machine.

But figured perhaps some one who might be using 20 deg rack might chime in and convince me that 20 is way better (maybe better back lash as well or something else.. then the 14.5)

Rack gears will be driven by servos through 10:1 thomson planetarys..

This will be a suspended gantry design (like my other machine.. except using proper (LARGE) star rails in all directions. Pic of my old machine here: http://users.ameritech.net/watchdogf...ineDetails.htm

Thanks for any

[HuFlungDung]

20 degree pressure angle is a bit thicker through the root of the tooth, and the tips are a bit pointier. You might gain some advantage if you are using the absolute smallest tooth number pinion available, because the 20° should be slightly less undercut (for a given tooth number) than 14.5°. I would probably choose whichever system gave me the most flexibility to design whatever I needed for ratios and which cost the least money.

[wcarrothers1]

Right now if I used your logic getting the rack from mcmaster it's about a wash far as cost goes probably. But the 14 pitch gives me a lot more flexibility far as ratios go.. Seeing I'll be using 10:1 planetarys I'll be using pretty large pinions. (probably 2" perhaps) so I can get my rapids up a bit. right now I can only rapid ~290ipm on my large machine with the 1" pinions I had on from it's initial build. Have some slightly larger pinions I could put on but have not had time to mess with it.

b.

[ger21]

14.5° Is apparently smoother running than 20°, but 20° is stronger. This is what i got out of Boston Gear's literature.
As far as purchasing rack, check out Moore Gear. They're supplying rack to a lot of guys building joe's hybrid machine.

[wcarrothers1]

Thanks for the link ger.. I'll check them out. Hopefully they are less then mcmaster.. I know I think it was quality transmission products seemed very expencive..

Will probably go with 14 pitch.. Really don't want it to be noisyer then it has to be.

b.

[ahren]

The 20 pitch racks that people are using with Joes / CNC Router Parts machines are really not very noisy -- I haven't had any complaints. You can check out a video of a machine with these racks here and see what you think of the audio:

http://www.cnczone.com/forums/showth...t=96416&page=4

Best regards,

Ahren
www.cncrouterparts.com

[wcarrothers1]

All the noise there is from the stepers. Didn't think 20 would be much louder. I use servos to be quieter to,'
b.

[ger21]

To me, cutting noise drowns out all other noise, so drive system noise isn't really a factor. The only time a quiet drive system is nice, is when you're not cutting anything. Otherwise, it doesn't matter.

[wcarrothers1]

Hehe.. As always a good point ger..

b.