Repairing a Drive Roller

Before I forget, I want to thank John Bernard, my friend who lives just a few miles up the road from me.


Without his help and guidance, I would not have gained the knowledge I am able to present here.


Okay, so we know where the skew forms.  This post will explain how to fix it.  


The first step in the process is to remove the pulleys from the drive roller.  While you're at it, you might want to first order a new set to replace them with.


My recommendation is to use a 3/8" pulley with deeper teeth in place of the existing set.  This will require a new stepper pulley as well as a new belt.


The reason I recommend the larger pulley is because the fine teeth on the existing ones can clog up with MDF dust as you get into cutting harder material than just foam.  The existing pulleys can easily jump a tooth because of the shallow grooves in them.


I get my pulleys from this website https://sdp-si.com/eStore/

The pulley we want is an XL series pulley.  You will want to pick up a 12" and 13" belt for them.  You can probably just get the 13" belt, but I ordered two sizes just in case one was a little short.


The large pulley is part number A 6L 3-28DF03708  and runs $5.38 each.
The stepper pulley is replaced by a larger one, part number A 6Z 3-12DF03708 which sells for $6.71.  I would recommend a smaller OD version, but the two smaller ones under this one are out of stock.


The 13" belt is part number A 6B 3-065037 and is $5.78


Okay, so now that you have the parts ordered, cutting the old pulley off the drive roller is the next step.


I made some simple V blocks to hold the roller and used a diamond blade cuter in my Dremel tool to quickly cut the old pulley off the roller as I manually rotated the drive roller in the V block.


The next step is to measure the actual amount of slop in your Hex Shaft.


Using clamps, clamp the roller to a table top and use a dial indicator (available from Harbor Freight for about $12) to get a reading when the hex shaft is at its lowest point.


With your fingers, push up on the hex shaft till it touches the top of the bearing race.  Measure the distance traveled.  In my case it was .015.


Next, you need to shim the hex shaft with a toothpick or sliver of wood or carbon fiber to split the difference (.0075).


Mix up some JB Weld Kwick (4 minute Epoxy) and put a small amount in between the hex shaft and the bearing race with a toothpick.  Don't touch anything until this has setup, then do the opposite end the same way.


When both pieces have been completed, mix up some JB Weld Original and go around the entire outside bearing race filling in between the hex shaft and the race.  Let this cure for 24 hours without disturbing it!  Your Hex Shaft should now be centered and will be used to ensure the pulley that is attached in the next steps will be centered as well.

The picture below shows the V block and a dial indicator used to measure the slop in the Hex Shaft.

When the new pulley arrives, you will use JB Weld to reattach it, but first you will need to cut the center out of the pulley so that it fits over the end of the drive roller.  Again, the trusty Dremel tool comes in handy for this.  I put a 1/16" mill bit from my Phlatprinter bit kit into the Dremel tool and just slowly carve out the center.  Be careful!  You may want to lock the pulley down in a vice (protected with a rag or rubber of course) so that it does not slip while you mill it out.


You will need a set of digital calipers for the next step.  Lowes has them for about $30.

Once you have the pulley prepared, clean the surface of the drive roller and make sure it is flat.  Sand it down if necessary.  Mix up two parts evenly of JB Weld Original Epoxy.  Spread it on the back side of the pulley and place it on the end of the roller where you removed the old pulley.  


Set the calipers to 1.095" and place one edge on one face of the Hex Shaft.  The other end of the caliper should fit over the edge of the pulley.  If not, position it so it does.  Then rotate the caliper and hex shaft around the edge of the pulley, several times, adjusting the pulley in place so that when done, all sides of the pulley are accurately spaced the same distance.


When you have the pulley centered well, stand the roller on end where it can set undisturbed for the next 24 hours.  Allow the epoxy to fully cure before putting back in your Phlatprinter \\\.


Now that the hex shaft is locked to the roller, you will only need two lock collars to hold the roller in place.  You will not need any spacers, just position the roller so that the inside of the pulley is just past the MDF and align the new stepper pulley so that the belt centers on them and lock down the hex shaft with the two lock collars.


Your X axis calibration will have to be changed.  With the new pulleys, your calibration values will decrease by about half, in the 500 range.


In follow up posts, I will explain how to calibrate your X axis, but you should already know how to do that by now I would expect.