Sprockets

[ArtF]

Bruce:

Looking at the code, Im thinking it may be me. Im triple checking.. Ill let you know soon..

Art

[ArtF]

Hi Bruce:

  Ive run through the math, and I did find the numbers were reporting a bit wrong  on the screen for root depth,
Ive fixed that, but otherwise the numbers appear to be correct. I also checked them against known formulations
from the table at this link
http://www.gearseds.com/files/design_dr ... cket_5.pdf

  If you use the sample in this pfd for example, you'll find Gearotics drawing identical.

  All I can think of is that the tool  may have been just a touch smaller than entered in CamBams table,
it would take only a very small wear to add up over 60 teeth..

    Take a read through the link, let me know if you see anything you figure I may have missed,
but the formulas on that page are the ones I use. They dont really conform to normal pitch
calculations for spurs , but they do produce the example results to within .001 on all my checks.

  I may be missing something Bruce, Ive been doing this long enough to know that sometimes
the obvious can fool me, but I do have several reports of good sprockets cut too, so Im stumped
as to what, other than a tool size error could cause it.

Art

[bruce]

So I tried to make this sprocket again.
I had ordered a commercial sprocket from Amazon for ANSI 25 chain and 60 teeth. I received it today and
measured it's thickness which was about .090 inches and fit the chain perfectly (It better!), then I placed this
sprocket over the one I had made a couple of days ago. Wow it was from eyeball measuring almost a perfect match
with no change in pitch I could discern.
So I surmised (incorrectly) that it was the thickness being the issue so I found some stock a little thinner and machined
another sprocket. Well same issue, it was like the pitch changed but comparing it to the commercial one looked good.

I then measured the root to root diameter and on the one I just machined was 4.655 but the commercial one measured
4.635! My end mill measures .125 inches so I am not sure why the root to root is off but what i plan to do now is
tell CamBam that my cutter diameter is .115 inches but use the .125 inch cutter. Should that not reduce the root to root diameter?
Maybe there is a better way to make this change?

[ArtF]

Bruce:

  This is exactly why its hard to cut a sprocket, most people wont believe how
much the slightest variation in tool size can make it not tooth. The higher the number of teeth
the harder it is. any error is multiplied by 60 in your case for example.

  Yes, Id do exactly as you suggest, its why I use a scale factor of 98-99% in 3d printing pulleys..

  If I might suggest though, just use a piece of 1/4 plywood or even masonite and machine a test
whenever you do a pulley or sprocket, adjust from there. Its easy , fast and cheap to
do tests , and I highly recommend it.

  Thank you for telling us how its happening, doesnt surprise me much you cant tell them
apart easily. Im willing to bet your next one accepts the chain easily, usually, subtracting
just a bit form the tool is enough to make it work..

Art

[bruce]

Third time is a charm! (plus lots help from the forum). :D
I re-did the G-code via CamBam and as was suggested in that forum I put in a -0.01 inches for
roughing clearance and that worked. I now measure root to root diameter of 4.633 inches and the
chain rides the sprocket no problem.
So next time I will pay close attention to the root to root diameter as shown on the Gearotic screen
for DXF.  ;D

[ArtF]

Bruce,

Great!. And good advice to have on here, GM puts out exact sizing as it sees it and leaves no
clearances. I do always advise using cheap easy material for testing sprocket and pulley code,
most have no idea how picky a sprocket or pulley can be, but error multiplied by teeth can
be a terrible thing.. :)

Glad to hear she's running for you..

Art