Timing Belt Pulleys and Belts
Timing belts are strong, flexible, and lose almost no movement to slippage. We used XL timing belts with 0.2" pitch - 77" long (and 3/8" wide to match the pulleys). This seems to work fine - we thought about testing the MXL belts with a 0.08" pitch, but didn't see the need since there was no noticeable play in the system for our purposes, and there was a wider belt selection. We used a fairly large timing belt pulley since that has a big impact on the final distance (the circumference) - it's about 1.5" in diameter - the largest we easily found with the 1/4" shaft size we were using. With a bigger pulley, the range would increase, but the system is ultimately limited by the precision of the potentiometer, so a much bigger belt may not work as well - certainly less positioning precision. We used 1/4" shafts throughout for simplicity - the same as the ten-turn potentiometer shaft.
We got our timing belts and pulleys from McMaster-Carr (but they are available elsewhere
part # 6484K454 Trapezoidal Tooth Neoprene Timing Belt .200" Pitch, Trade Sz 770XL, 77" Outer Circle, 3/8" Wide
part # 57105K21 Acetal Pulley for XL-Series Timing-Belt for 1/4" & 3/8" Belt Width, 1.63" OD, 22 TeethBearings and Collars
For the timing belt pulley bearings, make sure to get the extended inner ring ones so they don't rub against the shaft collars. You could also use regular ones with small washers on the inner ring. We used flanged ones to make mounting easier.
We got our timing belts and pulleys from McMaster-Carr (but they are available elsewhere):
part # 6462K12 Type 303 SS Set Screw Shaft Collar 1/4" Bore, 1/2" Outside Diameter, 9/32" Width
part # 57155K337 Miniature Precision SS Ball Bearing - ABEC-5 Flanged Shield, Extended Inner Ring, .25" ID, .5" OD
After building these, we noticed that Home Depot has ball bearings for patio doors, and these may work almost as well at a much lower price. Rather than mounting the bearing in a hole, you could put a couple bolts right through the outer plastic ring and bolt it right to an L bracket.Potentiometer
Most servos use a 5K potentiometer, so we got one of those. Note that these are 10 turn potentiometers per the intro. The 1/4" shaft couplers for the potentiometer to the drive shaft connection are available at many places (McMaster-Carr, ServoCity, and Jameco all have them). The potentiometer could be connected to the shaft on the other side of the timing belt pulley from the servo, but in our case we extended it to the same side of the other pulley just as a simple means to keep the mechanics on one side of the device.
We got our potentiometers at Jameco: part # 183548 - they have a tolerance of +/- 5%. Amazon has them too.
We also saw some at Digi-Key with +/- 0.2% - part # M-22E10-0502K-ND - we may try these at some point to see if they have any finer resolution.
Note: with heavy use on the site, the potentiometers started wearing and freezing up, so we have ordered ETI Systems MH22B series hybrid potentiometers (Mouser #882-MH22B-10-5K). Hybrid potentiometers use a layer of conductive plastic over the wire windings, so they are potentially more accurate and they last a lot longer - 10 million turns vs. 1 million turns for the wirewound ones. It's almost definite that the issue is in the slight wobble of the shafts, so we used a piece of plastic tubing to connect them rather than the rigid shaft couplers - that has worked for years now.Servos and Servo Hubs
We started with fairly standard servos - Hitec HS-425BBs
(57 oz. in. and 0.16 sec to 60 degrees @6v). In testing, we had a standard servo driving a shorter timing belt, and got about 1.5' of movement in about 4 seconds. Not bad, and the servo was powerful enough to move it. But we opted for more speed, and upgraded to higher speed servos - Hitec HS-6965HBs
(111 oz. in, and 0.08 sec to 60 degrees @6v). The newer ones were twice as fast, and much more powerful as a bonus, though that wasn't required. They are also digital, so they are programmable and all, but they do whine a lot more due to the higher frequency motor control.
(June 2009) See notes on the servo and last page, but for the X axis, we are currently using aPololu 3A Motor Controller with Feedback
and a 12VDC 250RPM DC Gearhead Motor
. The board from Pololu works just like a servo control board, and we already have the external potentiometer.
To connect the servos to the shafts, we used Servo to Shaft Couplers from ServoCity (http://www.servocity.com/html/servo_to_shaft_couplers.html) - part # HSA250. As far as we know, those couplers are fairly unique to Servo City.Drawer Glides and Misc Hardware
Mechanically, this is a simple system - we used wood and various metals in the prototypes, and they all worked fine.
For the linear motion, we used Accuride full extension drawer glides from Home Depot. We bolted pairs of them (top to bottom) to give a longer travel. You can buy longer drawer glides, but they get expensive fast, so bolting two together works well at a low cost. One disadvantage with drawer glides is that they extend out past the machine when in use. Also, with two shorter glides screwed together, they tend to dip slightly when fully extended. We used 24" ones for the parallel glides and 20" ones for the single track. Both were fine since we had about 38" of travel. We may switch to linear bearings and shafts at some point if we can find lower cost ones.
We got the 1/4" steel shaft (zinc plated), the 1x4 and 1x6 wood and various aluminum and steel angles from Home Depot. All of the small bolts used in the project were #6 size, and the wood was screwed with drywall screws and pan head screws. We also used some electrical conduit boxes for the plastic shape (see below), but this is totally optional.