[Mystichrome]

Hey guys, was hoping some of the experienced people could help me with some problems I'm having trying bore pulleys for my belt drive conversion. I have 4 - XL Timing Pulleys I purchased from econobelt that I'm trying to open up the bore on. I have 2 with .5"bores that I'm trying to open up to .853" and two pulleys(one .375" bore and one .5" bore) that I'm trying to open up to a .669" bore. This is actually my second attempt as the first attempt appears to have been a disaster. So I ordered a new set of pulleys and want to be sure I get it right this time. I don't have a lathe, so the finish boring will be done with a a boring head on my G0704.

The problem I had with the 1st attempt appears to be that the bore and OD are not concentric so I'm getting woble and having the belt thrown off. I first verified this by using calipers to measure the distance between the center bore and a pulley tooth in various positions. I forgot to write down my results, but there were discrepancies. To further verify, one by one I checked each pulley by using an interapid .0005" indicator in a spindle indicator holder. I had each pulley in the vice and indicated off the center bore making table adustments until I had about .0004" runout on the indicator when rotating the indicator (this is one procedure for centering a hole, correct?). I then repositioned the indicator to read from the OD. On the worst pulley I got .012 of runout on the OD. All of them showed .005" or worse.

So my first question is, am I correctly checking the concentricity of the ID and OD? In relation to this question, another problem I'm having is indicating off the teeth of the pulley. Depending on where on the tooth the indicator is positioned, the reading can vary up to .003". What I mean is that if I start to sweep the indicator across the tooth, the measurement can vary .003" depending on it's left-right position on the tooth. My experience thus far seems to be that the highest point on the tooth seems to be at the right edge. My instinct is to take the indicator reading at the highest point on the tooth. Is this the way to go, or should I perhaps be indicating of the valleys between teeth? I'm not sure the valley's are wide enough to have the indicator tip resting on a flat spot and not being cradled by the sides of the adjacent teeth. Using the method I described above(centering on the center bore then indicating off the teeth), I checked the new pulleys from econobelt and found .0015"-.003" difference in concentricity between the ID and OD(based on using the method I previously described). What is acceptable concentricity to achieve a smooth rotating pulley?

My procedure for boring the pulleys last time was this:
1. Center pulley to spindle by indicating off the bore using an indicator holder mounted in the spindle and adjusting until I had .0005" or less runout.

2. Using circular interpolation to open the bore's to within .03" of final desired bore size.

3. Using boring head to make several passes to final bore size.

Could the circular interpolation have been my problem? Should I have allowed for more material removal by the boring head? Would using a drill bit(as opposed to interpolation) be a better option for getting me close enough to use the boring head to finish the bore? I didn't have any drill bits over .5" which is why I went with circular interpolation, but I'm not opposed to buying them if that's going to be a better option.

For my last question, the reason I have 4 pulleys is that I'm going for a 2-speed setup. For this, I need to mount the pulleys together in pairs. One smaller pulley on top of a larger pulley. For my previous attempt, I indicated off the bore on each pulley to center the spindle and ran a program to drill 4 holes. Same program for each pulley. However after all the concentricity checks I've ran, I've determined the two pulley are not mounted together concentric to each other either. Any suggestions for ensuring concentricity when mounting one pulley to the other? Any and all feedback to my questions is greatly appreciated guys. I'm still very much a noob and learning as I go so I rely heavily on what I read about on the forums. Thanks in advance.

-Kevin

[691175002]

I suspect that backlash/poor repeatability means that after your circular interpolation, you return the table to the (0,0) position but it actually ends up somewhere like (0.002, 0.002) and you end up finish boring in the wrong location.

Indicate to the original bore like you do in step 1 then completely lock the table to ensure the spindle and pulley remain concentric. From there, drill then bore.


If you want to pocket out the hole first with an endmill, you will need some way of verifying that you are returning to the correct position. You could rig up a pair of dial indicators but that is a lot of work and if you bump them its over.

[109jb]

Since you have to do it on the mill, I would make a simple fixture to do it. You mentioned circular interpolation, so I guess the mill you are using is cnc capable. Anyway, Take a block of aluminum and clamp it to the table. Then mill a pocket into the block that the pulley OD fits into nicely. put the pulley in and use a couple clamp blocks to hold it down. Now you can run your operations. If you want the bore to be really close, then mill the pocket in the fixture leaving some material and then take it to size with the boring head. Then without moving the table you can bore the ID after putting the pulley on the fixture and it will be concentric.

[jid2]

Forget referencing the OD.

Align to the bore with your indicator. Dont move anything. If you cant drill to rough the holes you need to do it all with the boring head.

I found its not called boring for nothing.

[Mystichrome]

Thanks guys for your responses.

691175002: I suspect you are right about it not returning to 0,0 even though it believes it has. On one of the pulleys I did re-indicate the center hole after running the circular interpolation. It was difficult to do because circular interpolation doesn't make perfect circles and though not visually noticeable looking at the hole created, the indicator was all of over the place. I'm running backlash compensation but it could be that there is some issue with my axis movement that is making circular interpolation worse.

109jb: That's not a bad idea, but considering that each pulley has a different OD, that means making 4 fixtures. If that's what it takes I'm not opposed to it, but if there's another way to go about it, I'd like to explore that first. I left some material last time for the boring head passes, but maybe I needed to leave more since circular interpolation isn't perfect. That's another question I have, how much material should I leave for boring after using circular Interpolation? I thought I had read you should leave about .02" for boring, trying to play it safe, I left around .03". Although now I'm thinking, I never checked the bore size after Interpolating the hole, so maybe it cut the hole larger than it should have. That coupled with the hole being out or round from interpolating, maybe the bore was then off center when I finished it with the boring head.

jid2: My concern with ignoring the OD is that I've determined that from the factory the Pulley's have .0015"-.003" discrepancy in the concentricity between ID and OD. The belt drive will be spinning at 6000rpm on the top end, I'm worried that discrepancy will cause vibration. I don't know though, I'm still very new at all this, I'm just assuming you want it as close to spot on as possible. That's why I'd like to get some info on acceptable tolerances for these kinds of things.

I'm going to pick up some larger drill bits and try opening up the bore with drill bits rather than interpolation and see if that's any help. I'm still open to suggestions though. Thanks again guys.

[109jb]

109jb: That's not a bad idea, but considering that each pulley has a different OD, that means making 4 fixtures.

One fixture with 4 successively deeper pockets.

[loply]

Forget the circular interpolation, just use a boring head the whole way.

Remember to check the alignment of the bore to the spindle AFTER you lock the table axis, and remember to lock the Z-slide too.

Also, check the nod and tram of your head, because if they're out at all your pulley will run badly.

Lastly, how are you holding the part down? Are you sure it isn't moving? If you've just clamped it to some parallels with toe clamps or suchlike you may be surprised how easily it can move. Try holding it in a chuck and clamping the chuck down instead, because you can fit more clamps on that way.