Run from here.

[Scott Pancheau]

I have a Pulley blank cut to size for L type pulley. Is it possible to change the start location to be the center of the pulley so the machining location will be concentric to what I have ?
Thanks,
Scott

[Scott Pancheau]

OK, I went back to the tutorial and paid attention to the instructions on settings and found the place to set the origin to be at the center of the pulley rather than the lower left corner. Now I am wondering if anyone has had trouble with L pulley (timing belt). I just machined a 49 tooth and the spacing of the lugs is off by I think about 1 tooth on the full diameter. The belt can be forced into the lugs for about a third of the diameter, but get farther off as you go around. I used a .125 mill bit.

[ArtF]

Hi Scott:

The only time I had that happen to me was the zeroing was slightly off. Its very particular as to the zero being exactly the blank radius specified.
Even the hairiest bit off makes it do exactly that, the fit becomes off over less than half a roll around the teeth.


Art

[Scott Pancheau]

Thank you Art. I will go back to the setup and see if I specified that wrong. I know I previously did a 14 tooth and it seemed exact other than using a .25 mill which was too large for correct profile fit.

[ArtF]

Scott:

Be very very picky about tool zero to the blank. Any error is multiples basically by 6.28 times over the circumference, so it doesn't take much on a tooth of only a few mm's of size to make the belt slowly mismatch. If the belt gets too short as you roll it on, the zero was high, if the belt bunches up, the zero was too low..

Art

[Scott Pancheau]

Thanks again Art, I have been pondering how I can do the set up wrong. My assumption has been that once I enter that I want to start from the center of the pulley, all of your math takes over and all movements should then be based on the very center of the 0 start position. The only input I thought I could alter is the size and depth of the mill bit. If I use too large of a bit then the profile will not be fully defined. The pulley I made was exactly round within 1 thou. I was able to deepen the slots slightly and then run the outer diameter a couple times against an occilating belt sander to get the fit pretty good. Can you further explain how to check for proper set up in this regard.
Thanks,
Scott

[ArtF]

Scott:

I've always zeroed to the top of the blank, but your right, zeroing to the actual zero should be fine.
I'm not sure if its the bit wear or the fact that pulleys are way more sensitive than gears in terms of size, but belt
pulleys have always made me get very picky about the bit as well as the zeroing. I always mention zero first
as that's usually the culprit, but bit wear can also make that kind of impact.
A gear is easy, one tooth matches another and if its a touch sloppy, wont make much impact, but a belt will
be tight or loose with less than a mm off in 1/4 turn. Numerically, this is a tight spec. Take a 3" pully, 75mm roughly,
If due to to zero or wear we're .2 mm off, that will make a pulley tight at a few teeth from start.

BUT...

Which tooth type did you use? Ill take a look at the numbers to make sure Im putting out what's called for.
Pulley specs are usually secret. They obfuscate them on purpose and many of mine were sent to me by people
who had access to the specs. The GT for example is a concatenation of several arcs ( up to 5 or 6) in space for each
tooth, this makes for tight specs and some uncertainty, though I have reports on almost all of them from makers
who have made them so I'm pretty sure of the numbers at this point. I also have not made every type myself
for verification, so if my numbers ARE off, I apologize in advance.

The easiest way I think to check this is for you to measure the belt itself over the run of 1 pulleys # of teeth.
Help me check it, if its a 20 tooth pulley, measure 20 teeth on the belt as tightly as you can, let me know the
type and belt length of one round, lets crunch those numbers and see what they show us,
might help the next guy. If the specs suffer from some obfuscation I missed on the pulley, Ill massage them
to match your belt's length figures to make it more accurate. In the end, the belt length must match that
zero circumference at top of tooth or the numbers must be off.

Glad to hear grinding them out made it fit, but let me know the type and tooth count and belt length
for that tooth count, Ill run some numbers to see if it shows me anything obvious.

Art

[Scott Pancheau]

The longest I was able to measure without cutting the belt was for 23 lugs measured out to out at the base of the lugs. That measurement is 8.4 inches. The belt is a D and D Syncro Power Belt, 270 L. This belt fits the factory made 270 L pulleys I have perfectly. Would there be any chance that because the number of teeth do not divide evenly into 360 that the trailing decimals end up causing a slight variance? Thanks for your replies and input Art.
Scott

[ArtF]

Scott:

No, number of teeth won't matter, the error is probably belt thickness. The pitch line for a timing belt is not in the pulley, but at midpoint of the thin part of the belt. Ill crunch that and see how it fits and let you know what I see.

Art

[ArtF]

Hi Scott:

Ive run the numbers, and they seem to agree on screen with the spec I can see on https://www.pfeiferindustries.com/docum ... d%20OD.pdf

This pdf lists the pulley diameters, root and pitch diameters of the various pulleys, and a type L comes in at the same diameter and pitch as
the spec. of the L type at 49 teeth. What did the end diameter of your pulley come out as when cut? (This isnt to say perhaps the tooth spacing isnt too tight by a touch or something, but the end diameter and pitch diameter calculations' appear correct. )

Can you see anything that looks unusual that might show where any error lies? The actual outside diameter should probably tell me where
its screwed up.

Art

[Scott Pancheau]

Unfortunately I only made one and I reduced the diameter per our previous conversation so won't help to measure now. I used a new 1/8" mill bit, but don't know if it is accurate to spec. I think the problem is on my end. I previously made a 28 tooth pulley with a .25 mill and that was too large of a bit for the complete profile to be defined, but the belt wrapped around it with a good fit. If I get time this coming weekend I will try another tooth count and see what comes out. If it is wrong I will measure the diameter to report back. I will then tell the setup the bit is slightly smaller and see if that makes it come closer to fitting?
Thanks,
Scott

[ArtF]

Thanks Scott:

I appreciate your patience on it. Pulleys are one thing than can be a bitch to make, the intuitive thought is they are as easy as a gear,
but as you've found, they can be pretty picky, more so as the manufactures often lie a bit on spec just to make sure the secrets are safe.
:) . Each time someone reports a problem I analyse to be sure as I can be of accuracy. The last proven change was a couple years back
so I think things are mostly good, but I still like to check .


Art