Rack pitch and angle

[3Dsigns]

I'm in the process of building a 4x8 table with servos, racks, square rails, and 80/20 aluminum.
I plan to mount my racks with teeth facing sideways on x axis and down on the y (long) axis.

For optimum accuracy, what pitch a pressure angle do I need for this rig?
Thanks for helping

[HuFlungDung]

Pressure angle does not have much to do with accuracy. However 20°PA lends itself to a stronger tooth because the root is thicker, and for this same reason, pinions with small numbers of teeth will have less undercut when they are manufactured.

[3Dsigns]

Thanks for the quick response!

So how many teeth would you recommend?
I intend to direct drive it, is this a good Idea?
I think the motor shaft is 1/2''. What size spur would I need?
I would like to get about 800ipm rapids out of this thing.
'Thanks

[HuFlungDung]

I'm not sure if its a good idea. There are some pitfalls. First off, a lot of torque is required on the motor because one rotation of the pinion is quite a long distance. You might need a motor with gearhead to get the max rpm down to 300 to 400 rpm on the pinion.

Secondly backlash is a problem. When you use commercial rack and gears, extra clearance is cut into both gears. The pinion is designed to run at a specific center distance to the rack. If you force it in tight, then this makes the gear run rough because the tip of the teeth is causing some interference and won't go in and out of mesh cleanly. So, you actually need a dual pinion, with one half rotated slightly with respect to the other one, so that it contacts the unloaded side of the rack tooth, and this prevents backlash, yet permits smooth rolling motion.

As to size, I dunno, there are formulas to be consulted I guess, depending on the mass you will be moving. I'd probably go with something between 8DP to 12 DP just from a survivability standpoint. You don't want to bend a tooth in minor crashes.

Short pieces of rack can be pieced together accurately. Take a third piece and temporarily lay it over the spliced joint. Then drill and pin the other two pieces. Make the joint at the bottom gullet between two adjacent teeth.

If I were considering this sort of drive, I think maybe I'd look at using timing belt and pulleys. You could come up with a method of fastening a timing belt to your gantry to serve as the rack. You would position the drive pulley out past one end enough so that travel of the gantry is not restricted by the belt going over a pulley. The advantage of the timing belt is quiet operation, almost backlash free, and a good 180° belt wrap around the driver pulley makes a strong drive, without much tendency for anything to ever lift or jump out of mesh.

[3Dsigns]

Thanks HFD,

I'm going to put this on the backburner for awhile....
I found a great deal on a nice Shopbot with vacuum holdown, digitizer probe, and a boxfull of bits. Couldn't pass it up. It will be a slow machine but will get me started. I'm saving the parts I bought, so far, for the DIY machine and the information. I do intend to build it later.
Thanks for the help!

[kuna2]

hi

i have a question plesae tell me the answer
when i post the program , i couldnot get the G41,G42 code .is there any way i can get the g41 and g42 when i am posting one file.

[HuFlungDung]

Kuna,
That's not a thread hijack, that's called a thread nuke