This article, video, and pictures documents a mod I and John Bernard made to my Phlatprinter in which I substitute the grip tape covered conveyor drive rollers for rubber neoprene PVC rollers.

This mod was not an easy mod but with help from John Bernard and his excellent machine shop and table top CNC machine, we were able to pull it off.

I wanted to have a set of rollers with rubber neoprene to protect the Balsa and acrylics that I will be cutting. Grip tape is very abrasive and leaves marks in Depron and Balsa and I wanted something that would still hold the material without slipping, yet not leave marks on the finished product.

I also wanted to increase the pulley capacity to handle larger loads. I replaced my X axis stepper with a bit larger one I had on hand and wanted to use a 3/8" belt with deeper grooves in the pulleys for this mod.


The first hurdle is to find a PVC pipe that has the least amount of runout. The 5' length of 1 1/2" thick wall PVC pipe was hand picked by checking it with a digital caliper. I went thru 4 or 5 samples before finding one with a .004" runout.

Next I purchased a sheet of 3/4" MDF which John Bernard used to cut pulley and bearing blocks to fit into the inside of the PVC tubes.

Ace Hardware had some 1/2" flanged bearings and steel rod which were used to support the PVC.

The first set of pulleys I ordered were plastic nylon with aluminum inserts. Drilling them out proved to be difficult,

We broke one of the pulleys trying to get the aluminum hub out so I ordered steel pulleys to replace the larger roller pulleys.

The steel pulleys were quite a bit more expensive and much much harder to drill out. If it were not for the diamond tipped end mill that John had, we would never have been able to do it.

I think next time, I would go with the nylon/aluminum inserts.

End caps were cut from the MDF to fit the bearings into and to attach the drive pulley. Thin CA was applied to the MDF after cutting to harden it.

New adjuster arms were cutout to accomdate the 1/2" steel rod that the drive roller rides on. The opening for existing hex shaft was enlarged by sanding out a small amount of MDF using a Dremel stylus with sandpaper covered bit.

Ballpark Material cost for one roller:
5' length of PVC (enough for 2 rollers) - $2.83
2 Bearings $4.49 each ($8.98)
1 Steel Rod $8.99
1 large steel pulley $18.41 (plastic is $8.20)
3 MDF (3/4" ) cutouts $.50 each

Material Cost totaled $38.30 for a 2 Steel Pulley Version Roller, $28.05 for plastic.

Matching 3/8" drive belt and plastic stepper pulley ran another $11.60

Neoprene Rubber strips were around $35 for enough to cover two rollers.

Total cost for two rollers plus pulley, belt and stepper pulley plus covering - ~$123 plus taxes and shipping for the steel version, ~$75 plus taxes and shipping for plastic version.

The video shows some of the tools I cut out of MDF to measure runout on the drive rollers and the procedure I used to check the accuracy of the rollers.


Attached files

 

Using a carrier for Depron is a good idea regardless. It will prevent the grip taped rollers from damaging the foam provided you don't have the upper pressure rollers treated as well.

I did not put the grip tape all the way across on the upper pressure rollers. I just put two inch strips along the outer edges for gripping larger sheets and left the center section open for balsa and smaller type of material.

I am gluing in the pulley blocks on my PVC rollers this morning and will be reassembling everything one more time.

The interesting thing about the skew is that when I put in the rubber coated rollers, my skew changed directions. I had not glued in the pulley blocks at the time because I wasn't sure if the rubber coating would work and wanted to be able to swap out another PVC tube easily.

Now that I know the rubber will grip the material well, I am going ahead with the permanent mounting.

Should know the results later today.

 

Well, I have good news and bad...

First off, It was a beautiful day in Florida, so I took my RC sailboat out to get away from computers and CNC and just clear my mind. Sometimes getting away from a problem gives you a whole different perspective when you come back to it.

So when I got home, I went to the shop and setup my drive rollers again, making sure everything was as accurate as I could determine.

I ran my usual test and this time the skew was worse than ever!

Rechecked my calibration and noticed my acceleration value had gotten changed to 200! I thought sure that was it. Set it back to 35. Ran test again. Still not right.

Slowed the feed rate down to a crawl.... That's when I noticed it! I spotted one of the reasons, if not the reason, why we are having skew issues.

Now for a little background first. My new rollers use a 3/8" wide belt and deeper grooved pulleys. This accentuates the problem and had I not used them, I probably would never have been able to see what was going on.... The pulleys and belts on the phlatprinter have a larger number of teeth and are more shallow which makes observing the problem much more difficult.

The rollers are setup per your instructions with the high sides of each roller set together. This of course introduces slack in the belt tension when the high side rotates 180 degrees. The more eccentric the rollers, the more slack you will have.

When we reverse direction on the low side when belt tension is slack, one pulley or the other, depending on the direction we switch to, will actually start driving the roller while the other roller is still sitting still waiting for the slack to be taken out. When the slack is taken out, the second roller begins moving.

Since one roller is moving and one is fixed during this period of time however brief, the material is being pushed over the idle roller which is possibly grabbing the material forcing skew. This is conjecture on my part, but a reasonable assumption.

By holding my finger on the belt near the top of the rollers, I was able to force the slack out of the belt so that both pulleys began turning at the same time.

While I am not sure if this will cure the problem all together, it does seem to have merit. I am thinking we may need to come up with a mod that applies some spring loaded tension on the belt.

This also explains why when I removed the front pressure roller a few weeks ago and let just one drive roller do the work, my skew disappeared. I reported that back at the time, but assumed it was the eccentricity of the rollers that was at fault and never associated it with belt tension.

It also explains why I have been able to nearly reduce the skew to zero at times only to find it back again after having the rollers out.

At one time I set the high sides of the each roller 180 degrees from each other and had the best results thus far. Putting both high sides on the same side, introduces more slack and hence, more skew.

Having rubber coated rollers allowed the slippage to become more noticeable. The grip tape on the normal rollers helps to obscure the problem which is probably why it has been so difficult to find.

Now to figure out how to spring tension the belt...

 

I think this is what is going on. The picture should help visualize it.

Belt slack occurs along the top between the two drive rollers when the high spot passes under the belt.

The belt was taught as the roller was being driven clockwise and the high spot passed along the top. As it continues, the high spot rotates to the bottom creating slack in the belt.

As the stepper reverses direction, the left pulley starts its counter rotation, but now there is slack in the belt between the stepper and the right pulley since the right pulley has loosened it due to the eccentric shape.

The stepper continues to rotate taking out the slack until the belt is once again taught along the top and then the right pulley is once again engaged.

Anyhow, this is what I believe I observed earlier in the day. I also believe that if a spring could be fashioned somehow to apply tension along the top of the belt between the pulleys, the slippage would be eliminated.

However, there is not any material to fasten the spring to since there is a cutout in the MDF in that area.

Any ideas on how to accomplish this? Attached files

 

I haven't started mine yet, so I don't know how much space is available, but can we put a spring tensioned idler pulley there? They do it in cars very successfully.

 

I can't believe you did it to me, Flash. You put your last post up while I was still typing.

As I've said, I didn't start mine yet, so excuse the ignorance, but can a idler be put on the lid, where the rollers with the shrink-wrap are, and when it is down, it pushes on the belt. It will have to be spring tensioned, just like the rollers.

 

I have been looking at the lid solution myself. Thought maybe a curved piece of spring wire (flat piece of spring steel) fastened at one end might work, but that area of the top is open. Would have to have a new top made to do that, but I think it would be the best solution.

On the other hand, I decided to tighten my belt again, this time on the low side so that there will be stress on the belt but no slack.

That proved to be a BIG help.

I changed my acceleration to 2 in the USB controller settings for the X axis and ran my test at 200 ipm air cut moving a 3/4" section of Blue foam. This setting let the X axis slow way down before reversing and being as my rollers don't have the grip power that the grip tape provides, I don't mind having it slow down if it can reduce slippage.

The test simply runs the X axis 12 inches back and forth over and over. After about 10 passes, I sent the gantry home and measured the skew. It was about .030 on the Y and .050 on the X. Certainly acceptable for 99% of anything I will be doing although I would still like to see even better accuracy but without precision drive rollers, this is probably about as good as I am going to be able to get it.

Tomorrow I will start running different material thru it to see if I can move phenolic, plastic, MDF etc without introducing more skew.

 

Flash, just to make sure I understand you well, you still have a high point in the rollers which are the new rollers as seen above? Wasn't the mod supposed to fix the high point problem? In the video you said it's only about 7 thou.

When you've mentioned originally that you are going to make the mod I remembered that someone else (Shaun?) made a mod on the MK I using aluminum rollers, for just the same reason, the wobbling. Might it be better to use that rather than the PVC in light of the wobbling you still have?

 

Yes, the mod was two fold... reduce wobble and make a rubber coated roller.

Aluminum tube would be much better. I just could not find any readily. The best I could do on the PVC was .004 runout on the outside, but there was also runout on the inside and added in were runout on the MDF cutouts so I still ended up with around .011 runout overall.

So in the end, I did not fare much better than the conveyor rollers.

 

A spring type tensioner would work well if the belt only went one way like an automotive type does, but since it reverses travels i think that the flexion and deflexion would cause a spring type tensioner to not work properly.

 

At the moment, I have both rollers oriented such that the high spots are on the same orientation, but orienting them so the high spots are 180 out like I had it once before reduces the overall slackness.

It is just that you double the number of times that the slack occurs even though you halve the distance.

Today I want to try recalibrating the X axis again since I have not done so with this setup. Then I want to run some actual material cuts to see if that introduces any additional error.

I think the spring tensioner will work because it worked when I used my fingers to provide the tension that a spring would normally provide.

However, I am getting reasonable results now by simply tightening down very tight on the belt when the rollers are at its most slack belt tension. I know this is not good. Something has to give and I am not sure at the moment what that is doing to the drive train.

 

Still using the PVC rollers at the moment. I need to pick up some grip tape to cover the roller you sent me and try it out.

I made a set of Vblocks using 1/4" MDF to test runout. Also a little bracket for mounting my dial indicator.

I still have -.010 X and Y skew on a 12" pass done 3 times with the belt tightened, but it must have loosened somewhat because I can still depress the belt about an eighth or more of an inch with my fingers.

Coincidently, .010 to .011 of inch is how much runout I have in the rollers...

I went ahead and ran the circles test file you posted in another thread.

The circles are still a bit out of round even though the ruler cutout measures exactly 4.04" on both X and Y directions.

Attached files vblock.skp (78.6 KB)Â

 

There are a few skew related threads out there, but this one seems most populated and larry and Mark seem to be taking the problem head on! I wish I had some more ideas but unfortunately I've run out!

If there is anything I can try/tests to run I would be glad to help out!

- Jeff