A milling anomaly

Hi, I thought some might find this unusal situation of interest.

When I began my machining career some forty odd years ago one of the very early things I was shown was the correct way in how to place a cutter in an autolock screw type milling chuck. I have contiued to do that since safe in the knowlege that any cutting forces that were trying to 'pull' the cutter out of the chuck due to the helix would infact screw the cutter tighter into it's thread and subsquently push the collet into the nose piece tightening itself in the process - a state of equilibrium if you like. In all that time I have never seen a cutter move outwards from such a chuck. In fact until yesterday I would have said it was impossible but yesterday I had it happen not once but twice!

I was step milling a concave radius in cast iron using a fresh long series Clarkson 1/4 dia ball nose slot drill. Step over was 2mm at the depth required from an initial 10.7 mm depth. At around 1000 rpm the cutter was chattering - it sounded as if it was almost plucking at the material but did not appear to being stressed. I was climb milling to ease the pressure on the cutter and the surface finish was not great but I put that down to the long series cutter and the longish side cut with that fair amount of overhang I had. Feed was by hand and steady for the situation - ie not being forced or rushed

On a cut on the hidden side the noise slowly increased so stopped only to find the cutter had pulled into the work and was loose in the collet. The nose piece itself had tightened substantially on the chuck but the cutter was removed by hand. I put this down to possible carelessness on my part in putting the cutter in and not tightening sufficiently so reset every thing paying particular attention to that and continued in the hope that I would be able to use the part if the score was not too deep. Obviously approched this the second time with caution but within another cut on the other side and back again for the next on the hidden side the same thing occured again but with a vengeance. - this is the result

It wasn't until I took it out of the vise to photograph that I noticed the run off (bottom corner) and began to think this was definitely due to cutter deflection and vibration but it didn't explain how a cutter can unscrew itself from an auto lock type chuck and penetrate the work - yes, the quill was very firmly clamped and no, the head did not move.

I reset everything again and continued but this time milling conventionally - a slight improvement but no pull out - until that is the cutter snapped - this wasn't my bloody day. I found a standard cutter I didn't know I had which performed much better and managed to clean it up considerably so part saved.

But the mystery remains - how does a threaded cutter pull itself into the work from a screw in collet chuck.

If I had't witnessed this nyself I would have said it was not possible but the proof was right there.

Any thoughts?

Hope that's of interest - Tug.

Hi Tug, Check that the small centre that the threaded end of the cutter locates into, is still in the chuck. Also have a look to see if the threaded length on the failed cutter is shorter than expected. Both faults will stop the locking action from taking effect.

I've had that happen without my realising it until I saw the steel swarf also contained cast-iron granules... from the table. (The work had either a space or a steel pad below it in that area.)

I was down-miliing and assumed I had failed to screw the cutter into the Autolock collet. That was probably so, but your experience does makes me wonder if I had, but the cutter had unscrewed itself.

How though? Vibration? One would assume any snatching should tend to tighten the cutter, whether down- or climb- cutting; but is the cutter snatching very, very slightly then recovering enough for elasticity to spring it back down-thread?

Just as well you spotted it happening while there was still enough material to remove for the worst of the bite to disappear.

Could it be a combination of undersized cutter thread and worn collet thread allowing the cutter to pull through? Would it be worth checking how well the cutter screws into the collet.

Let's assume the cutter thread is the same as the collet Ie not a metric diameter sitting in an imperial collet.

The threads on Clarkson cutters seem a bit odd to me, they are 20TPI on the imperial ones which is not unexpected, but from memory the metric ones are also 20TPI.

Phil

A 1/4" cutter is pretty small, as well as being long series, and could easily bend. Try a 1/2" cutter? 1000rpm also sounds rather slow.

Hmm, is it 'cos you are climb milling ? ..is it possible that as / if you slow the feed down the helix is being 'pushed' by the side of the cut and 'unscrewing' the cutter? Was the collet nut tight or just 'spun-up' by hand..?

Edited By DiogenesII on 24/02/2021 18:09:33

You were climb milling when the problem occurred, but that should not have been the total cause. Check that the vise has not moved, and also the backlash on the Y axis. The small cutter might have bent, as already mentioned.

" only to find the cutter had pulled into the work and was loose in the collet. The nose piece itself had tightened substantially on the chuck but the cutter was removed by hand."

So assuming you seated the collet in the stops within the chuck and wound the cutter in on it's thread till it stopped turning then tightened the collet nut there is no way the sentence in bold could have happened.

On my Vertex Osbourne clone the cutter can't wind right through the collet as the cutter runs out of thread, except if I fail to engage the collet properly (the Osbourne is a bit like a D cutter) Swarf maker may have the answer or the collet wasn't engaged in the chuck till some time after cutting commenced which gave it all that slack to rattle and rotate around in doing up the chuck nut in the process. There may be some burrs inside now which could be why it happened twice.

Hi Tug,

I once had an apprentice who was convinced the Knee had come up on the Bridgeport he was using. The root cause of the increased depth of cut was the Quill had moved due to the Quill Lock not being properly applied and the enthusiasm of my apprentice to remove metal.

The Torsional cutting forces on the cutter are no different, irrespective of whether the cutter is being used conventionally, or climb milling. The cutting loads can be greater in the latter case, due to the tendency of the cutter taking a larger bite.

Ball ended cutters always complain especially the smaller ones.

Regards

Gray,

Edited By Graham Meek on 24/02/2021 19:17:24

Strange things happen with vibration! I have seen on an engine the worm gear driving the oil pump and distributor shaft, which had designed in end float of around 0.1", shift against the expected direction due to torsonals. This meant that the power to drive the pump and distributor was being overcome by this vibration. It only did this at two small areas of engine speed bandwidth. It manifested itself as a step change in the ignition angle.

On the job in hand here I would have thought a more substantial ball nosed end mill would have been more appropriate, 1/4" is a little 'flexy' and was probably a constituent to the vibration 'chatter' it was generating. I always took heed of an old engineer that said "remember boy, sound and its generation is lost energy"!

Glad that you managed to salvage the job at the end of the day though.

Martin

I have a lot of ball nose cutters some long series ,aquired for free from a toolmaker or very cheap,I have found that they are very prone to chatter and will break like a carrot particularly the long series type,they are not suited to long roughing out runs ,they work better finishing radii in mould tools or simiar work ,and its best to avoid cutting too deep on the side of work ,long series versions tend to be used when working in the bottom of deep pockets.Speed at a 1000 rpm for 1/4 dia is about right ,the Clarkson calculator used to recomend 1 inch dia cutters on cast iron run at 240 rpm.one of those facts that stuck in my memory,Rough out with end mills finish off with ball nose. I just cannot understand why a Clarkson cutter unscrewed unless it was due to high speed vibration,acting like an impact electric drill working in reverse. The 20 TPI thread was part of the Clarkson patent, when I was in the drawing office and drawing up a special cutter which would go out to a special ist cutter maker the invoice would have a charge added to the tool price of one shilling which was a royalty paid to Clarksons for the use of their patent ,the 20 tpi thread,there was no metric version.I have found a another use for these ball end cutters,on the lathe,clamp them in the tool post and use one edge to produce very accurate radii in the corners of turned work. The opposite tool,the radius end mill cut very good external rads.

Hi Tug, well my two pennyworth -- it should not and cannot happen !!! but it did and I'm sure you will have checked my thoughts but they are that there are 3 possibilities where it could. 1, the centre in the cutter has failed/broken 2, the thread on the cutter has stripped 3, the thread in the collet has stripped. Any of these would prevent the auto tightening feature from working. Plus as you said the nut was extra tight as it would be due to cutting forces but you could remove the cutter by hand so the collet was not gripping -- both of which point to one of the above failures.

Hope this may help

John

Edited By JohnF on 24/02/2021 19:49:05

The 1/4" cutter would not have fitted in a 6mm collet, so that is not the reason, although the other way round would be a disaster. We have not had a reply yet from Ramon regarding the condition of the centre inside the holder which may have failed,or the condition of the threaded end of the cutter. I had a spare centre when I had a Clarkson Autolock, but have changed to Osborn Titanic II and haven't come across any for that.

Edited By old mart on 24/02/2021 20:19:54

Hello guys - first off many thanks for all those interested enough to comment - all much appreciated.

This wasn't so much how do I prevent this problem as to one of interest. I did think immediately that 'Swarf Maker' may have had the answer in that the point had gone inside the chuck but no, not so. It is not of 'Clarkson' quality but never the less has proved perfect for my needs since bought ( about five years or so) Recently it has had a lot of use mainly on 10, 12 and 16 mm shanks. This is the first time I have used the smallest collet however - usually I use an FC3 cutter in a holder but as the cutter was a screw shank went for this method of drive instead. I am totally satisfied that there is nothing wrong with the chuck, the collet threads are fine and a close fit on the cutter (now deceased) and that the cutter was of good quality. Of Clarkson make, probably a hang over from working days and thus well over twenty years old so made in the UK and laying in the drawer waiting a use for it.

I am well familiar with fitting cutters to this type of chuck and whilst I considered I may have been a bit hasty the first time through familiarity I certainly wasn't the second.

Dave Halford says there is no way this could have happened if the cutter had been screwed home and he collet tightened accordingly - I totally agree David, in fact I phoned a good friend of mine who has a lifetime of machining experience. "What would you say if I said I've just had a cutter pull itself into the work from an Autolock type chuck" I asked him - "Not possible" was his instant reply and were it the question had been levied at me is exactly the same answer I would have given

I have to say I can only conclude it must have been due to vibration - I still can't reconcile myself to the fact however that in order to mill itself into the work piece so deeply the cutter had to unscrew itself outwards in direct opposition to the cutting forces trying to screw it back in.

I called this an anomaly which it is but maybe that should have been engima!

Everything is fine though, the small divots left after final cuts will fill with JBW and not be seen and the part is now finish machined with the steam ways ready to fit.

Alls well that ends well - happy days eh !

My thanks again for the response - I thought that it would be of interest if a bit mystifying.

Regards - Tug

Glad to hear the holder has not been damaged , but worrying for anyone using the threaded system as similar could happen to any of us. Maybe some strange resonance.

Tug,

Glad you have sorted the job but as you say, still a mystery. If nothing else it will make those of use with Clarkson Autolock chucks take a moment to double check our milling cutter fitting and tightness in our chucks.

Colin

Hi Tug,

I was mulling this problem over during my usual bowl of porridge this morning. In the Osborn and Dormer versions the centre cone is a top hat section bush. Which sits in a ground bore, the brim of the top hat stopping any further inward movement.

On an R8 version of this collet chuck which came with an Oriental milling machine I purchased. The top hat bush had been replaced by a cone pointed grub screw, locked by a further dog point grub screw.

The hammer blows from the ball end cutter would be trying to undo such a grub screw set up. It would only take a miniscule movement of the grubscrew assembly to release the grip on this type of collet holder.

Even though this would produce a loose cutter, it would not totally loosen the grub screw assembly.

It would be interesting to know if there is an Allen key socket at the bottom of the drawbar hole?

Regards

Gray,

I've had this problem with smaller diameter screwed-shank cutters too.

I think that due to the small diameter, there is simply not enough torque generated during a milling cut to keep the collet screwed tight to the cutter and this is exacerbated by the vibration of the cutting action. The problem is far worse if the cutter shank is below size or the collet is a loose fit.

My solution is to leave the collet carrier slightly loose, screw the cutter in hand tight, and then nip-up the carrier until it simultaneously clamps the cutter and fully seats on its registers.

Martin.