MechMate CNC Router Forum

MechMate CNC Router Forum (
-   Driving Mechanisms: Rack/pinion, gears, screws, belts & chains (
-   -   Why must the gear teeth be so small or fine? (

driller Wed 03 October 2007 06:16

Why must the gear teeth be so small or fine?
This thread started eleswhere . . .

. . . . . . . drive gears that are selected to have the tooth count be equal to their pitch are often tight and a little rough in operation. using 20 pitch gears and a 20 tooth spur gear might be a little rough. going to a 30 or 32 or so will be noticeably smoother. There is no gain by going any larger.


stefanv Thu 04 October 2007 06:34

Wait a moment,... Smoother catches my attention. I still have to order my rack and pinion. What are we saying here? Is it better to go with 32 gear and 30 rack? What's the logic behind this? Does anyone have some real-life experience with this setup?


Gerald D Thu 04 October 2007 07:34

What is a "30 rack"?

astarguy Thu 04 October 2007 19:06

i think he means a rack with 30 teeth per inch.

Gerald D Thu 04 October 2007 20:45

Driller, it was your comment "using 20 pitch gears and a 20 tooth spur gear might be a little rough. going to a 30 or 32 or so will be noticeably smoother." that led to the confusion. Can you help to clear it up please? Are you talking pitch, tooth count, or both?

driller Fri 05 October 2007 06:10

Sorry for the confusion.

the simple answer is more teeth of the spur gear. same pressure angle as the rack, same dp as the rack.

The suggestion of using 20 pitch as the standard looks like it has been done sucessfully by a few. don't change what works !

a 20 pitch rack is where one starts.

if you are not familiar with gears, they are designed by the diameter of the spur gear and how many teeth are on a perfect 1.00000 inch circle.

There is a little more to an actual gear, but the dp or Diametrical Pitch is based on that exact 1 inch.

so, a 20 dp rack will mate with a 20 dp spur gear. using the formula for how far apart the teeth can be, one can make a gear with 20 teeth, or 21 teeth, or 30 teeth or even larger. A larger diameter means more teeth. if anyone has seen the back of a lathe, you will see how many sizes one can make. but they all have the same dp or tooth spacing.

back to my origional comment. a 20 dp rack and a 20 tooth gear. That gear will move a little rough because it is tight. if one uses a 20dp gear with 24 or 28 or some larger number, the gear itself will move smoother on the rack.

There is also a factor called pressure angle. you must use a gear with the same pressure angle as the rack. if you think of them as families, then 20dp and 14.5pa would be in one family. in that family, you can get spur gears with tooth counts that start low and go really high. all will worth together.

but a 20dp (same) and 20 pressure angle (different) are in a different family and will not work between families.

I am not suggesting you use something different than what has already been used by others, except to consider that a spur gear with more teeth from the same family, will run smoother on the track.

Hope that clears up the confusion.


Gerald D Fri 05 October 2007 12:22

Dave, you are causing lots of confusion. The reason for the small gear size on a direct driven motor is to get TORQUE. Bigger is smoother, yes, but they don't give the torque to drive the machine.

driller Fri 05 October 2007 16:30

Sorry for any confusion.

Torque comes from the gear ratio, The stepper gear-head has many more gears, so getting the final drive gear is just one on the whole gear train.

And, smoothness means less resistance from mechanical fits.


Richards Sat 06 October 2007 07:57

I use a 20-tooth spur gear on my Shopbot PRT-Alpha. The machine originally came with direct drive motors (no-gearbox). The pitch diameter of that spur gear is 1-inch, so each time the stepper motor's shaft turns one revolution, the axis moves 1 X PI or 3.14-inches. If I were using a PK299 motor and Gecko stepper drivers on my machine, that would mean that each step of the stepper motor would move the axis 3.14 / 2000 = 0.00157-inches. If I changed the spur gears to 30-tooth gears, the new gears would have a pitch diameter of 1.5-inches (30 / 20 = 1.5). So, PI X 1.5 = 4.71 inches per shaft rotation, or 4.71 / 2000 = 0.00235-inches per step. With Mach 3, I could easily adjust the steps-per-unit setting for either size spur-gear (626.9426 for the 20-tooth or 425.5319 for the 30-tooth).

But, as Gerald pointed out, torque is also involved, which is the main reason that I've stayed with the 20-tooth gears on my machine. However, almost all of the machines that I've seen that use either a 3.6:1 or 7.2:1 gearbox use the 30-tooth gears. On those machines, the 30-tooth gear gives smoother operation and much longer spur-gear life. So, bottom line: For a non-geared motor, I would use 20-tooth spur gears. For a geared motor, I would use 20-tooth, 25-tooth or 30-tooth gears (all gears on the machine the same of course) depending on what was available, with a preference for the 30-tooth gear.

Gerald D Sat 06 October 2007 08:52

Gents, lets not forget this is (was) Stefan's thread. He said at the beginning he has motors "KL34H280-45-4A from Kelling (620 oz 4V 4.5Amp)". If he wants a good balance between smoothness and torque, and wall thickness for the grubscrew, tooth strength, etc. . . . . . the recommendation for him is firmly the DP20 pitch system, with 20 teeth on the pinion.

driller Sat 06 October 2007 21:29


As Stefan has mentioned, he has a direct drive stepper.

As I and at least one other have mentioned on this list, a 32 tooth final gear is smoother and therefore offers less resistance (more delivered power)

Can you offer how a intermediate gear train of any sort (belts pulleys, gears, etc) requires a final gear and cannot be designed to accommodate a final gear ?

A gear that has only a few teeth with interfere with the teeth on the mating rack. therefore the manufacturer will undercut the teeth to create more clearance. Spur gears of 20PA will automatically be undercut at 18 teeth. Undercutting removes some strength and compromises life of the gear and rack.

the manufacture recognizes this as a problem and recommends using larger tooth counts. 20 is at the threshold of having problems.

here is a quote as it seems not many people are familiar with properly selecting gears.

Anybody who has no experience with gears and gear selection will find this confusing, so if you are not interested in engineering and a technical discussion, jump to the end to see how this post turns out.

My comment was not to confuse, only to offer a helping hand to someone who is asking for help.

I am only offering advise to people who want to make it work better.

Further reading. Stock Drive Products,
Here is a link to their technical page

This will be my last post on this subject unless someone wants to discuss technical points.


Gerald D Sat 06 October 2007 23:46

Dave, what would be your choice of pinion for a direct drive motor? Not a theoretical choice - something readily available from a catalog. Remember, we want enough cutting force, so the pitch diameter must be around 1". The tooth must be fairly strong. It must not clog up with sawdust. It must not jump out when overloaded. It must be able to take a fair amount of wear before it strips. Racks must be readily available to match your choice of pinion. The rack must not be much narrower than about 0.5" for wear and ease of mounting.

"I am only offering advise to people who want to make it work better." Offer some catalog part numbers and I'll discuss it from there . . . . .

This is a forum for people committed to building the MechMate and having problems with instructions and sourcing materials. I don't want it to become a second CNCzone where general engineering is discussed at length.

Richards Sun 07 October 2007 08:25

McMaster-Carr has a variety of spur gears. The 20-tooth part number is 5172T12 (catalog page 1013, $13.67 each). The 25-tooth part number is 5172T13 (catalog page 1013, $14.23 each). The 30-tooth part number is 5172T14 (catalog page 1013, $15.62 each). All three gears have a face-width of 1/2-inch and a plain 1/2-inch bore. You would have to drill and tap them for set/grub screws. Further down the catalog page is their listing for 20-pitch steel gear racks. The racks range in price from $15.00 for a two-foot length to $40.60 for a six-foot length. Of course, racks can be butted together to achieve any length.

I've bought a number of 20-tooth gears from them in the past that I've bored out to 14mm to fit the larger shaft of the motors that I use. Drilling and tapping the gears for set screws is a bit of a bother, but it only takes a few minutes. I tapped mine for 8/32 sized screws which are much too small. The screws loosen even when smeared with loctite compound. But, I consider the spur gears to have an expected life of about one year, so in a few months when I replace them, I'll tap the new gears for 1/4-24 set-screws.

In the future, I might try using the 30-tooth gears, just to see how they work, but the 20-tooth gears that I've been using since July of 2004 have worked perfectly for me. They've been abused and neglected, but they've never caused problems. Most of the time I only clean them with a wire brush dipped in solvent every three or four months. By that time the dirt build-up is substantial; however, the parts that I've run before and after clean-up have been identical. The only reason that I'm even considering using 30-tooth gears is that I have 3:1 belt drives on the machine that increase the torque 3X. But, since I actually use that machine regularly, I really don't want to turn it into a test bench. The 20-tooth gears have given me no reason to experiment.

driller Sun 07 October 2007 09:11

The rack suggested is the best for a number or reasons. probably the best reasons are that it is available and meets all the needs of the machine and is a good price.

The Oriental motors with the internal gear reduction along with the 20 tooth gear is a good choice because it works. A 30 tooth gear could be used and would offer a few benefits. longer life, less wear and increased smoothness.

Going to a 30 tooth gear would increase machine speed and decrease torque.

But, if one is using a standard stepper motor and will be making their own reduction with pulleys, then they have the ability to pick the best set-up. In that case, one has the ability to design the best drive that will fit their needs. The origional question was regarding a standard motor and using common parts.

For the record, I would say that using a gear-head motor is the easiest path as there is nothing else to do.

If one is making their own drive using timing pulleys, then they have a bit more work, but also more more options.

Both ways will work.


eloid Wed 27 February 2008 20:03

Why must teeth be so small or fine?
After doing some more research i found out i need a rack with 3/4 x 3/4 face(for mounting needs) there for limiting my search to stock items from for racks

the pitch available are 12 and 16. the question here is which is the better size
and why?... looking AT the 16,12 both are available in pitch diameter of 1 inch... does this change the calculation
i dont think so, please explain?

Gerald D Wed 27 February 2008 21:56

Eloid, a 12 pitch gear with a diameter of 1" will have 12 teeth.

A tooth count of less than 20 makes for a rough running gear. That is just the way it is. Make the rack too big and the pitch number too small, it gets rough.

All times are GMT -6. The time now is 21:05.

Powered by vBulletin® Version 3.8.3
Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.