Please could you clarify as I have never heard of Thompson collets. The most common Autolock collet system was made by Clarkson.

If it is Clarkson collets you are speaking of, the topic has been discussed in the past on this forum, so a search would bring forth the most common views.

incorrect post

Edited By Tony Pratt 1 on 13/08/2021 19:34:32

For rigidity and security of cutter I prefer the Clarkson (Vertex copy in my case) to ER collets. I have had a cutter pull down into the work previously with ER. Due to the thread and stop system in Clarkson this doesn't happen.

Both systems are clearly fit for the purpose of holding cutters, the ER system can handle cutters and material so can be useful for a work holding chuck as the collets have a working range rather than a single size with the Clarkson. A few care points with the ER should avoid the cutter moving axially or running out. For cutters it is best to use a collet that is the nominal shank size at the largest size of the collet rather than have to close down to the smallest size. It is essential to snap the collet into the closing nut before inserting the cutter and assembly to the chuck body. The recommended torque for tightening the nut may surprise you how high it is. As ER is often used on CNC machines then the cutter moving would be disastrous, I suspect anyone experiencing cutter movement has not assembled the chuck as recommended or as many of us buy budget priced versions they may not offer full performance. I suspect the availability of screwed shank cutters will decline as the professional users have largely retired Clarkson type chucks. One of the pitfalls of the Clarkson was that if the chuck was assembled as recommended then it was possible for the cutter to self tighten very slightly which could alter the axial position so a school of thought developed to leave a tiny gap between the nose and the body and finally tighten to positively locate the cutter on the centre point in the chuck body. If you have ever wondered why small Clarkson cutters are often found with the screwed end broken then it is because the gap technique has been used and excessively tightened with too large a gap. It is not really practical to tighten the cutter by hand to match the torque encountered when machining, you are trying to screw a cutter in using the very sharp cutter edges which are usually wrapped in a rag and tightened using a bare hand.

Mike

I would say screwed shank cutters have already considerably declined in availability. Take a look as something like the MSC catalogue and of the 40 odd pages of milling cutter sonly about 3 are screwed shank. Similar with Dormer, their cataloge shows maybe 80 ranges of milling cutters and only 2 or 3 are screwed and they are tee/dovtail ones.

The hobby and small end CNC users may use ER for a lot of work but industry has moved on to other methods particularly shrink fit which hold better and allow for longer reach and smaller body diameter which is needed for 3+2 and full 5 Axis machining.

So without saying what you want to do with the collets it's hard to say which is better.

This thing about cutters moving in Clarkson Autolock chucks. I don't get it. If the end of the cutter is hard up against the male centre in the body of the chuck surely the only way it can move is to rotate. This rotation, in my book, then rotates the closing ring and tightens the collet fingers. I don't see how the cutter can be drawn into the chuck.

If however the clocing ring was assemble tight against the shoulder of the chuck body then I suppose the cutter could start shy of the male centre which would give conditions suitable for draw in of the cutter. Thats why leaving a small gap is the way to use the chuck.

regards Martin

I've long been skeptical of folk claiming that a properly inserted screwed shank cutter in a Clarkson / Osborne / Posilock holder is capable of moving forwards a measurable distance under cut.

Properly Inserted being the operative words.

Assuming a gap is not left between cutter and centre I suspect this can only occur if step 4 of the instructions:-

"take the Autolock spanner and give a final tighten of the sleeve" is omitted.

Of course the folk who are too darn lazy to read the instruction and too stupid to understand how the thing works will also omit the equally important inspection parameters at a) and b).

The whole point of a screwed shank is that if the cutting torque overcomes the collet grip so that the cutter turns in the collet the collet itself is forced forwards until the grip is increased by enough to stop it turning. Not by much. Has anyone actually seen a Clarkson collet or screwed shank cutter shank damaged or scuffed by rotation? I haven't.

I have broken the end off a 1/4" end mill by blindly following the leave a gap instructions from a man who really should have known better. But that one appeared to have simply slid straight back. After that I went looking for the instructions as my Posilock set had none.

If anyone cares to dust off their maths its relatively easy to calculate the grip forces generated. Compare them to those given by an ER or native spindle collet too whilst you are at it. The results might surprise you. (I long ago decided never to do maths unless I'm being paid or need it for a "me job".)

The final bump with the spanner seats the nose so the body is, effectively, solid and ensures the cutter is pressed firmly against the centre point. It probably also tightens the collet a fraction. Tightening the sleeve against the flange obviously makes a great deal of difference to rigidity and grip. One reason why ER collets need such ungodly high tightening torques is that the nut is loose on the threads and therefore the assembly is inherently less stiff.

It would be interesting to make some tests before and after cutting with a tool setter.

Clive

Edited By Clive Foster on 15/08/2021 10:04:33

Yes if it's too loose but it still cannot move axially if it's up against the male centre in the body.

regards Martin

Fair enough, I've heard so many leave a gap don't leave a gap instructions it gets confusing. If Clarkson say don't then don't. My point was that if the cutter is hard up against the male centre it cannot move axially only rotate. I believe that the chuck design is intended to increase clamping force in the event of it being needed (cutting forces being high enough to rotate the cutter slightly) not a mechanism to initially tighten the chuck. That should be done by the operator.

regards Martin

Clarkson collets will be only for the stated size.

ER collets can accommodate a small range of sizes, usually 1 mm, so that Metric or Imperal sizes can be accommodate. And are widely available.

Your choice, depending on budget and space consideration..

Howard

Do not write off the Clarkson altogether. I use my Er most of the time. But you will find the Autolock HSS cutters in the used job lot section of ebay etc, quite cheaply. Which i did. I have a good array of different sizes that all fit into 3 collets.