Dremel Runout

[ajayre]

Very new to CNC and started cutting some tests in pine yesterday. I'm using a Dremel 300 with end mills from Drill Bit City.

I seem to have some runout however. If I rotate the collet nut by hand I can see the mill wobbling. I measured it with a dial indicator and it was 0.004". However I can't see the collet nut wobbling. The mill is 1.45mm (0.0571") and cuts holes that are perhaps 20% larger than this.

I tried another end mill (from the same place, same size) and obtained the same result.

How can I tell if it is the mill, collet or dremel that is causing the problem? And what can I do to fix it?

thanks, Andy

[CarveOne]

Try another collet with the same bit. Collets may have to be hand selected to get one that runs true, assuming that the Dremel shaft is running true. Push the bit all the way into the collet then raise it a little before tightening the collet nut. The bit should not be tight against the bottom of the collet when tightened as it can make the bit go off center and even loosen up while running under load.

CarveOne

[gandalf1]

Is the cutter shank the same diameter as the collet bore? If it is 1.45 mm diameter the chances of the collet holding an odd size like that are a bit remote, even a 1.5 mm or a 1/16 would not hold the cutter truly.
John.

[ajayre]

It's a 1/8" shank and a 1/8" collet.

I bought another collet set and it's the same. I made sure the bottom of the bit is not against the bottom of the collet.

As I turn it with my fingers, it really seems as if the collet nut is NOT off center, but the bit obviously is.

Any ideas on what to try next? Is it possible to get collets for Dremels from other sources?

thanks, Andy

[gandalf1]

since the collet and cutter match for diameter and you have tried more than one I would begin to suspect the bore the collet sits in is eccentric. Can you gently push a straight piece of som,ething a good fit into the bore the collet sits in and check that for running out? Sadly no advice regarding where to buy collets that fit a Dremel since I'm in the UK. From some thousdands of miles away it seems that either the bore for the collet is eccentric of maybe too large in diameter.
John.

[ajayre]

I inserted an 11/64" drill bit into the shaft of the dremel. No collet and no collet nut. It's a nice, tight fit. If I then turn the shaft by hand and watch the end of the drill bit, I don't see any wobble.

To eliminate the end mill as the problem I inserted a 1/8" ball nose bit that came with the dremel. It also has an obvious wobble.

Andy

[gandalf1]

Hi Andy,
All I can think of now is that the collet is loose in the spindle bore and is only being located by the spindle end and inside of the cap nut.
Midnight here now so I'm off to bed.
John.

[Larry83301]

Try rotating the end mill up to 90deg in the collet and see if this helps. If the collet is off center this should cause the end mill to cut a larger or smaller hole depending on where the cutting edge is in relation to the offset in the collet.

[ajayre]

I'll give that a try, thanks!

Andy