Diameter, Pitch, Number of Teeth, etc. on Spur gear?

[SlimJimmy]

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.?

[Robin Hewitt]

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

[massajamesb]

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

[Perp]

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]

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.

[David Bord]

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

David

[massajamesb]

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]

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.

[Perp]

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

[Robin Hewitt]

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

[TheNotchJohnson]

Hello. I am very new with cnc machines. I am building a 24" x 36" cnc machine out of aluminum stock. Linear motion will be done on ball bearings driven by a gear rack and spur. the motor kit that i am using will be from this link. http://www.probotix.com/3_axis_stepp...or_driver_kit/

my goal is to cut cast aluminum parts. the only problem is im very confused on what ratios to use when i choose my gear rack and spur. I am getting mine from mcasters but there are so many choices and i am not comfortable to make this desicion on my own. can anyone that knows more about this advice me on the proper set up, thank you.

[Torchhead]

the ratios (number of teeth) are of less concern than the DIAMETER of the "spur" (Pinion gear). The linear speed of the pinon is PI (3.1416) times the DP of the pinon gear.

Here is some quick math:

A 1" pinon moves 3.14 inches per rev...in essence a 3:1 STEP UP. That buys you 3 times the speed (about 1800 IPM off a direct coupled stepper) but costs you 2/3's of your torque and resolution. Since you start out with .005 per step resolution that carves it down to .015 per step. Coupled with the other errors (backlash, flex, etc) your accuracy will be less than .030

I know it's everybody's goal to build the cheapest table they can, but if you end up with a machine that won't do it's intended job then you have wasted your money.

The drive kit you listed is not a bad value at that price but beware that those drives are based on an older chip that is limited in speed and current and has no input isolation. It also offers none of the features you will need for plasma and you are on your own to deal with the noise and ground loop issues.

TOM CAUDLE
www.CandCNC.com