So im looking to acquire my spur gears and racks, but im a little lost as to what size I should get. It will be a direct drive set up for now. Ive heard the common is 24 pitch but what about number of teeth, diameter, etc.?

Have a google for diametrical pitch, 24dp is quite fine, metric gear pitches are given as a Modulus.

I would look into getting a spur gear with as few teeth as possible, if you are direct driving it.
A 20 tooth, 20 pitch, 20 degree PA would be the best bet. Much smaller and you will run into issues with the teeth undercutting.
www.mcmaster.com has the rack and spur gear

Massajamesb, for the Gear and rack you recomended above, at what gantry weight/servo power/speed does the chance of striping teeth start to be a concern? or is this ever an issue at all? I am looking around 100 lbs gantry with Tom C's 850 oz servos(dual drive) and around 10:1 reduction for a plasma table I am building.

Thanks
Doug

Massajamesb, for the Gear and rack you recomended above, at what gantry weight/servo power/speed does the chance of striping teeth start to be a concern? or is this ever an issue at all? I am looking around 100 lbs gantry with Tom C's 850 oz servos(dual drive) and around 10:1 reduction for a plasma table I am building.

Thanks
Doug

Honestly, I need to weigh my gantry, but it is a heavy steel gantry. The gears/rack I am using are pretty heavy duty, and I have noticed no wear so far, and that is a lot of use:)
I would imagine if you had a 300 + pound gantry, and a really poor bearing system, then stripping teeth would be a concern. Other than that, I don't see any gear stripping problem, but then again, that is one of those subjects I am a bit ignorant on :)
I would think since there are no cutting forces, and the gantry is rolling smoothly on bearings, wear would be minimal at best.
Hope that helps a bit.

Are you using direct drive on your system? If so, are you happy with the accuracy?

David

No, I have a 15 tooth pulley on the motor, and a 40 tooth pulley on the 20 tooth driven gear. Roughly a 1:2.66 ratio. I do not recommend direct driving your machine. The downsides outweigh the upsides.
I am quite happy with the accuracy, although I am using plasma.
I can honestly say that the machine is way more accurate than the process I use.

Massajamesb, for the Gear and rack you recomended above, at what gantry weight/servo power/speed does the chance of striping teeth start to be a concern? or is this ever an issue at all? I am looking around 100 lbs gantry with Tom C's 850 oz servos(dual drive) and around 10:1 reduction for a plasma table I am building.

Thanks
Doug

Perp, I weighed my gantry (roughly), and altogether, gantry, carriage, torch, Z axis, motors, motor mounts yadda yadda, everything mounted on that axis, comes out to right around 80 pounds.
The thing to remember is that your spur gear doesn't really see that weight directly, as the weight of the gantry is actually on your bearings. What the spur gear "sees" is the force needed to move the gantry forward/backward, and then come to a sudden stop when neccesary.
This is not enough force to strip the spur gears that most of us are using, as long as your gantry is able to move smoothly.

Thank you for your efforts. I do think all of the great info you guys have shared will help me (and countless others) build better machines than we ever could on our own.

Soon I will begin a build log of my own. I have a computer and all of the electronics, now its time to start buying the steel and linear motion components.

Thanks again
Doug

There is no escaping the simple fact that 1 pound force will accelerate a mass of 32.16 pounds at a lowly 1 ft/s/s.

Even monster sized motors can't give instant acceleration and constant feed rates can only be approximated.

So, how low can our accelerations be before the cut becomes compromised?

I just switched table design to racks to eliminate twist, and discovered pi had suddenly sneaked in to the equasion.

Confidence truly is that feeling you get just before you understand the problem :)