MIlling cutter pulling out of collet

Another thing to look at if other suggestions haven't identified the source of your problem: my first ever R8 collet, bought from a reputable supplier, was fine out of the box on its first use. On its second use I couldn't get it to close quite firmly enough onto the cutter.

A careful look along the full length of the collet's slots revealed an inch-long curly strand of metal hiding away in one of the slots that was still attached to the collet at its upper end. It had clearly not been machined away as it should have been by the manufacturer during the cutting of the slots. On its second use it must have moved position to interfere with the proper closing of the collet. Once removed, everything worked as it should have done.

Edited By Bill Phinn on 04/05/2021 11:03:41

The other thing to check is that your drawbar is not running out of thread and tightening into the collet rather than pulling it up tight.

Thanks for even more thoughts!

Jason - I'm not sure what you mean by 'out of thread'. The drawbar screws down about 40mm into the collet. Having said that some collets are still sticky at the end. I keep losening the setscrew in the spindle (which appears to be the issue - or more correctly R8 collets which have a keyway that's not to spec). Do you think this could be a contributor?

Iain

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I’m afraid that revelation adds emphasis to my earlier suggestion:

“May I suggest that you inspect both the cutter and the collet ?”

Did you do ‘goods-inward inspection’ or just put it into use ?

I’m not casting aspersions about any particular supplier

[ ... I don’t know where you bought it, or at what price. ]

MichaelG.

Iain, if you have 20mm of thread on the end of the drawbar but you need to pull the R8 collet up 25mm to close it properly then you will run out of thread. More likely to occur with an oversize collet or undersize shank. Washers under the top of the drawbar would be a simple solution.

Martin C

Thanks both.

Michael: I've not actually measured the bore of the new collet, but 10mm end mills (not reground) and edge finders fit in nice and snugly and don't drop out (unlike in it's predecessor) so I'm content that this is reasonably sized.

Martin: There's plenty of thread on the drawbar. There's more thread on the drawbar than it sticks up by when a collet is inserted and the drawbar fully unscrewed.

I will, however double check that.

I've ordered a torque wrench so I can apply some science to this, but will need to make an adapter between a 3/8ths socket and the 12mm square spanner nut thing on the top of the drawbar.

Iain

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Sorry to labour this, Iain ... but ... going back to your opening post :

”MIlling cutter is a 10mm carbide running at full pelt (2200 rpm) - the cutter is a re-ground 'pro' cutter - possibly a little undersize because of it.”

Are you saying that the shank is ‘possibly a little undersize’ ?

... does it have the same subjective fit as the others ?

MichaelG.

Looking at r8 collets, they seem to only have 1" of thread in them or less so if you have more drawbar than that sticking out the top before engaging any threads then you have your problem. I can't find any that have 40mm of thread due to the collet slots running up close, so it could be the short slots are too stiff to grip properly.

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Although showing a Morse socket, not a collet ... Post #10 here: **LINK**

https://www.practicalmachinist.com/vb/bridgeport-and-hardinge-mills-and-lathes/problem-new-r8-tooling-threaded-rod-192636/

... is a useful reference.

MichaelG.

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Edited By Michael Gilligan on 05/05/2021 14:31:50

I like the drawbar to hold at 2 diameters, 7/8" deep in the tooling, and since the museums Tom Senior R8 spindle does not have the internal pin, all the tooling gets a 7/16 UNF tap run down it, the threads cleaned and lubricated with moly grease. That maximises the drawbar holding power. As well as not holding properly, an undersize tool in an R8 collet will cause the whole thing to close up more than it should and be pulled deeper into the spindle. That risks bottoming out the drawbar as well.

Thanks (as usual) for all the feedback.

I've checked the cutter I've been using. This shaft measures around 0.01 mm ( 2 tenths) undersized (9.99mm) though it's possible that the flutes are a tiny bit smaller - not easy to measure.

It is a tight sliding fit in the collet which measures (best I can with calipers) pretty spot on 10mm.

Good news I thought. I cleaned both with denatured alcohol and mounted them in the quill.

I've got a new torque wrench but had to make suitable socket to go over the 12mm drawbar. I would have happily bought one but couldn't find such a thing.

The socket was constructed from some mild steel bar drilled one end at 9.5mm and milled to 12mm the other and then squared up with 2mm carbide endmills. I had several approaches to this each of which lost me one end mill. In the end I was able to take 2mm deep cuts going round the square and it worked. For various reasons the socket is a bit of a mess, so no photos!

I applied what I believe to be around 28Nm of force and proceeded to mill the same piece.

10mm deep. I started at 1 mm depth of cut, but got brave and ended up with 2mm depth of cut. I cut 3 stripes at 2mm (6mm in total) conventional milling and then 3 more 2mm stripes climb milling.

As best I can tell there is was no movement between cuts.

I'm not sure if you can tell from the photo, but there is almost no ridge to the finger touch on the top. Scraping a finger nail there is the slightest hesitation. In short this is about as good as it gets.

So it would seem that my earlier error was simply not to tighten the drawbar enough.

Having said that 28 Nm felt like a LOT at the end of the torque wrench and it was VERY hard to undo with the 6 inch spanner I normally use.

Is my general solution just to get a longer spanner? I'm not particularly keen on messing with a torque wrench to set tools.

Iain

I use er25 collets more often than R8 and have a pair of spanners with extensions to make them about 10" long. The drawbar spanner is about 7" long but as I mentioned, the threads in the tooling and drawbar are free running and lubricated with moly grease. I would tend to tighten R8 collets more than the rest of the tooling though. Having some tightening done with a torque wrench, you will know how tight to go with ordinary spanners, make them the same length as the TR.

Edited By old mart on 09/05/2021 17:25:16

I have had cutters walk out of an ER 32 collet chuck.When I have to cut to a positive depth I use a Posiloc chuck and sometimes a Claire chuck and neither give any trouble.I still use Er collets for most work.

Frank