Using my asm911aadc2.65v servo motor to make timing belt pulleys

Hi folks, I have an idea to machine my own timing belt toothed pulleys using my Asm911aadc2.65v stepper motor and driver combo. I will have a timing belt sourced and will gauge the pitch of the teeth on the belt in order to determine the corresponding pitch on the pulley. I am going to model a pulley in Rhino once I have sourced the belt to determine all the correct measurements. I would then create a rectangle and place that over the top dead center of the pulley and machine a slot. Then I will turn a pulley blank on the metal lathe I have access to and mount the pulley blank onto the stepper motor shaft. Then I will mount the stepper motor in a bracket attached to my CNC bed. I need to rotate the stepper a determined amount of degrees and minutes to allow for the correct pitch of the teeth. Does anyone know how many degrees of rotation per pulse this motor has? Its basically the old PRT Alpha set up. I am essentially making a course indexer which will make a fine indexer via these pulleys. The multiplier value of 1273.2395 in combination with the software rotates the motor 360 degrees with what input value? Any help with this would be greatly appreciated. I'm doing this because I like to make my own things but mostly because I can't seem to source the correct pulleys to make my reduction box.

Most steppers have 200 steps per rotation (1.8 degrees), but not sure about this one. That is a pretty coarse resolution if your number of teeth is not a straight fraction of 200. Do not rely on microstepping because it is not completely linear. I would also be concerned about the forces involved. The holding torque of this motor may be sufficient but within the step angle, the stepper behaves like a spring and rather small forces can move it substantially.

I have machined timing belt pulleys from bamboo for my own machine but used dxf outlines I found on the Internet with a simple profile cut. two 3/4" slices are stacked for the required width including flanges. These are 8mm HTD timing belts and the radii are big enough to be machined with an 1/8" endmill. The pulleys are now 4 years old and moderately used. Other than getting a little black they show no signs of wear. Not too surprising since the bamboo (or wood as well) is much harder than the belt rubber.

https://lh3.googleusercontent.com/-I...4/P1020812.JPG

Jon...
the 1273.2395 step resolution (steps per inch) is based on 500 steps per motor rotation and a 20 tooth pinion as the number of steps to move a linear inch. For use as a 1:1 rotary you would use 500/360=1.3889. (steps per degree)

That said, G is correct, I would not use that motor to machine the pulley. Instead I would machine a hole into the table and bolt the blank down then machine the pulley in that fashion. You will have much better holding torque and can machine faster that way.

That's a nice looking pulley G, what type of motors do you have on your bot?
Thanks for the advice to you and Gary. I will think about this further, maybe a clamp placed on the pulley blank during machining and then removal for rotation?

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Originally Posted by jon

That's a nice looking pulley G, what type of motors do you have on your bot?....

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Thanks Jon, but I must admit I am a tolerated guest here not having a Shopbot. This is the synchronizing belt system for the two x-axis drives of my homebuilt 31" x 34" x 8" machine. The motors are two low inductance NEMA34 steppers with 470 in-oz holding torque.

If you are looking for pulley CAD files, try SDP/SI. They have files of all possible pulley types and I used them to make my pulleys. The HTD ( or similar derived belt types) are easy to machine with an end mill because of the rounded tooth shape. But as mentioned for an 1/8" bit you need min 8mm tooth spacing. Otherwise it is really simple once you get the dxf into VcarvePro.

Edit, forgot to mention...to get the dxf file I imported the 3-d cad file into my CAD system, sliced through the middle and exported the cross-section as dxf.

Jon...
You are missing my point and/or not doing the math. Assuming yours is SB OEM programming at eighth steps. You will use 11.11 as your unit value. IF you look at my chart below you will see that the step resolution at 2" diameter is 1 1/2 thousandths, barely acceptable and almost 5 thousandths at 6inches. This is the smallest amount that your proposed setup could move. Couple that with the low holding torque of an eighth step 1:1 setup in the second chart and you will see that along with loss of accuracy, you most assuredly would have to clamp the part.

I assume you are planning this because no one makes a pulley large enough for your design. As you complete your design, remember you will need to adhere to a minimum tooth engagement in the smaller pulley. To achieve this you will need to increase center to center distance, this longer distance will result in belt induced backlash, rendering your "big number" reduction useless.

Well I don't mind that you don't have a bot, I have respect that you made your own CNC! Also very nice 'life hack' on the gear dxf download, I'll be sure to try it out and then should purchase my belts from them to compel sate them for the loss of sale on the pulleys...

Ahhh, thanks for the clarification there Gary, no I had not done the math. 0.05" is totally unacceptable. I am in the speculation phase of my design. I suppose I will follow yours and G's advice of milling my pulleys in the x and y orientation. I would likely make the same pulleys again but with the completed indexer as I had originally planned. I have a small amount of play in my y axis due to the old design of the PRT chassis.

http://www.talkshopbot.com/forum/att...id=28689&stc=1http://www.talkshopbot.com/forum/att...id=28690&stc=1
Thanks to Gary Campbell and G Burkhardt I think I have my reduction box figured out for my indexer build. This reducer will give me a 25:1 reduction with 0.0556 degrees per step and a resolution of 0.004 inches on the surface of an 8 inch diameter cylinder. Unless I am mistaken of course. Oddly the source for the gears did not seem to stock the 8mm pitch belts, so I had to source them elsewhere. I used the cad model for a 56 tooth pulley to build my 70 tooth pulley models but still needed to adjust the pressure angle of the teeth. I will be having a Haas mill make the gears for me so that I don't have to rely on the shopbot for that, although it probably could do a decent job of it. I also added tensioners to the pulley system in order to allow for a smaller box and increased tooth contact on the smaller pulley.

Jon...
I think the 25:1 will work well for your 8" diameter work. My numbers do not agree with yours tho. Using the OEM programming from the ASM911 I get the numbers in the attached chart. (with DIP switch set to 1000 use Eighth steps, set to 500 use Quarter)

With 34.72 steps per degree I get 1/34.72=0.028802 degrees per step. I also show .00025 step resolution @ 1/8 steps and .0005 @ 1/4

I am all about the project, but are you aware that ShopBot sells a 25:1 Rotary motor with a planetary gearbox? There are probably a good number of them that could be acquired used on this forum.