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Which is better Thompson or er collets

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terry callaghan13/08/2021 17:22:25
237 forum posts
10 photos

Hi, which is better the Thompson type auto lock collets or the spring type er collets. I have always used the spring type collets, but with the machine I purchased was a set of Thompson collets and Chuck. Anyone have any views on which is best and why

DC31k13/08/2021 17:46:32
571 forum posts
1 photos

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.

Steviegtr13/08/2021 18:09:18
2242 forum posts
311 photos

If it is the Clarkson then i have both the ER & those. I find both good. The Clarkson is a very rigid setup. Someone on another thread remarked that the mill cutter can ride up a thou or 2 on initial plunge. I have never found this to be the case. However i have never needed that accuracy. If you did then of course you could plunge into a piece of scrap with the Z ,to make sure it is in place before setting a zero etc.


terry callaghan13/08/2021 18:21:26
237 forum posts
10 photos

My bad it is of course clarkson. Thanks for reply’s

SillyOldDuffer13/08/2021 18:52:52
7550 forum posts
1680 photos

Can't comment on Clarkson holders from experience, but one advantage to us of the ER system is it can be used for both work holding and tool holding. The same ER32 collet set is used on my mill, lathe, and rotary table (with a Stephenson Block.)

For what it's worth, not much in my workshop, ER collets seem to rule on CNC systems, I guess this is because ER suits automatic tool changers: no need for the mechanism to faff about with a screw thread. They also have wide gripping range, plus the lighter weight/smaller diameter supports high speed Carbide, 30,000rpm woof, woof!


Tony Pratt 113/08/2021 19:24:41
1692 forum posts
8 photos

incorrect post

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

old mart13/08/2021 19:31:18
3345 forum posts
208 photos

I use both, but using the threaded milling cutters has one disadvantage, most are HSS. I do have a few threaded shank cutters with inlaid carbide cutting edges. I believe that a thread on the shank of a solid carbide cutter would not be strong enough to be viable. The er system can hold both HSS and solid carbide.

Clarkson, Acramil and Osborn are three makers which can hold the threaded cutters by default, er can also hold them as long as the threaded end can pass through the collet body.

Richard Millington14/08/2021 07:39:36
35 forum posts

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.

Martin Connelly14/08/2021 08:54:29
1889 forum posts
203 photos

I have both ER and Vertex Posilock. ER is quick and convenient and can use plain shank cutters as well as threaded shank and depending on the size in use can hold larger diameters. The Posilock for threaded cutters has some advantages in terms of rigidity and repeatable positioning of the cutter. I have added the thread and a centre hole for use with the Posilock to a number of other items of tooling such as a Mini Hogger set, a 2 insert Ø16 milling cutter and a parallel Ø16 shank ER collet holder so I can hold drills up Ø10mm.

Downsides are ER has more potential to slip and is less rigid and can be slightly out of position if the collets have been distorted in use from closing down too much. Posilock is more restrictive in the length of the tooling setup, you can't loosen it off and slide the bit up or down to suit requirements. The Posilock system I have is limited to Ø16 shanks so I can't use it for Rotabroach cutters with a Ø19 shank. There is a more plain shank tooling choice than threaded shank tooling choice (as mentioned you don't tend to see threaded shank solid carbide cutters).

It is useful to have both but if you had to choose one for flexibility in setup I would go with ER, for rigidity of the tooling I would go with Posilock or Autolock.

Martin C

Mike Poole14/08/2021 09:44:31
3071 forum posts
72 photos

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.


SillyOldDuffer14/08/2021 10:09:56
7550 forum posts
1680 photos

Ways to abuse an ER Collet!

  • Fail to insert the collet correctly into the holder. (ArcEuro's description is good)
  • Don't tighten the collet enough (the professionals apply a lot of torque.)
  • Force oversize items into the collet. (Causes permanent damage.)
  • Don't use the full length of the collet to grip the item (Collapses the far end)
  • Consistently use the collet clamped fully down at the minimum end. (encourages failure to tighten enough)
  • Fail to understand ER Collets are replaceable. (Industry chuck them out ruthlessly so beware second-hand 'bargains'.)


Edited By SillyOldDuffer on 14/08/2021 11:25:54

JasonB14/08/2021 10:15:52
21435 forum posts
2448 photos
1 articles

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.

old mart14/08/2021 20:42:03
3345 forum posts
208 photos

There was a time when every company which ran Bridgeports used the screwed shank tooling, I have a box of about 1000 blunt cutters left over when I weeded out the sharper ones for the museum. The posilock type of collets are still available new, they are the type that the Osborn Titanic II uses.

Martin Kyte14/08/2021 22:29:12
2558 forum posts
45 photos

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

Andrew Johnston14/08/2021 22:46:55
6264 forum posts
677 photos

Posted by Martin Kyte on 14/08/2021 22:29:12:

Thats why leaving a small gap is the way to use the chuck.

Not according to Clarkson - see Section 4a:

Autolock Instructions


Clive Foster15/08/2021 10:01:03
2835 forum posts
103 photos

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.


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

Zan15/08/2021 10:06:21
282 forum posts
19 photos

You beat me! The cutter is locked when it hits the work piece causing it to rotate more slowly than the holder, thus screwing the collet taper out and locking against the cutter shank thus stopping further rpm drag

Martin Kyte15/08/2021 10:10:04
2558 forum posts
45 photos
Posted by Zan on 15/08/2021 10:06:21:

You beat me! The cutter is locked when it hits the work piece causing it to rotate more slowly than the holder, thus screwing the collet taper out and locking against the cutter shank thus stopping further rpm drag

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

Andrew Johnston15/08/2021 10:33:57
6264 forum posts
677 photos

Posted by Clive Foster on 15/08/2021 10:01:03:

I have broken the end off a 1/4" end mill by blindly following the leave a gap instructions........

Me too, after that I followed the manufacturer instructions.


Martin Kyte15/08/2021 11:15:02
2558 forum posts
45 photos
Posted by Andrew Johnston on 15/08/2021 10:33:57:

Posted by Clive Foster on 15/08/2021 10:01:03:

I have broken the end off a 1/4" end mill by blindly following the leave a gap instructions........

Me too, after that I followed the manufacturer instructions.


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

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