MT2 collet slipping

[David HaynesParticipant @davidhaynes53962]

Hi, after using the chuck for milling in my ML7 lathe I have now got bought a set of MT2 collets in the hope of improving the finish and operability. I am using a new end mill, have a rigid setup and have wiped any traces of oil from all four interface surfaces of the morse taper and tool. However I still experience intermittent tool slipping during milling. I am trying to keep my cut as shallow as possible, but still this occurs. Tool is 3/8″ four flute endmill and speed is about 800rpm. The drawbar has progressively been tightened to a high amount and I am concerned that any more tightening may cause damage. I understand that use of a different type of collet may be a solution, but having just bought the new MT2 set, I am reticent to have to get another one when I believe this can be made to work. Many thanks, Dave

[David HaynesParticipant @davidhaynes53962]

Advice sought on preventing tool slip

[DustyParticipant @dusty]

David

I can only think of two things that could be wrong. First is that the collet is oversize at the back where it fits into the taper, if this is the case you will never get it to close properly. Ideally the collet should be slightly oversize at the front. The second thing is the drawbar, make sure that it will pull the collet up tight and not jamb on the thread. If you remove the collet and drawbar from the lathe screw the collet right onto the drawbar, then measure that the length from the front of the collet to the shoulder on the drawbar is less than the length of the lathe spindle. I have assumed that you are not using a metric collet for an imperial cutter.

[FunnyturnParticipant @funnyturn]

Can you clarify whether slippage is at the MT2 taper or is it the end mill slipping in the collet?

[Martin WParticipant @martinw]

Hi

 

This might be a silly question but is the collet the right size for the end mill. MT collets, if I am correct, are only intended to grip to size i.e. 3/8 collet to 3/8 milling cutter, if the size isn’t correct then they will not grip uniformly over the clamping area. ER collets,on the other hand, will grip down over a limited range of about 1mm or 40 thou from their max size and so can accommodate various size cutters while maintaining a good grip.

 

Just a thought.

 

Regards

 

Martin

[David HaynesParticipant @davidhaynes53962]

Thanks for your responses. The collet I have been using is a 3/8″ collet and the tool a 3/8″ four flute end mill. When the collet goes into the the arbor taper, there is the same positve wedge that I would expect from the tailstock taper. The cutter fits snugly into the collet and the tool need a little push with something soft, like a piece of wood, to slide it in. With the drawbar, I have used an all thread bar, so I simply screw the drawbar into the collet first, about 5 turns but stopping one thread before it is screwed tight, then place a large washer of appropriate bore over the drawbar and a nut tightens the drawbar. Most of the time the milling works okay but when there is a bit of a challenge to the cut, the cutter starts to turn in the collet. Regards, Dave

[John HaineParticipant @johnhaine32865]

I have heard of Myford pattern collets slipping when used in a milling machine as well.

 

Two thoughts. One, don’t take such challenging cuts! You might also try a very slight amount of lubrication on the taper, it may be that slight roughness between the taper and the collet is preventing it exerting maximum clamping force because friction prevents it pulling in completely. I have also seen that Morse tapers can get stuck if over-tightened – a bit of lube would help prevent that but also perhaps make it happen! Possibly worth a try though.

[mgjParticipant @mgj]

Surely ordinary 2 MT collets were never designed to be used as milling cutter holders.

 

I am told that ER collets were, but milling collet chucks are of different design. Are you sure it is not a case of the wrong tool for the job. Purely personally , I wouldn’t dream of using a standard work holding collet for milling. I have milling chucks with their specialist collets for that

 

Even the little baby FC3 cutters have a drive screw to stop the cutter rotating under load, or moving forwards into the cut.

 

There is also a very big difference between the way an ER collet grips and a standard MT collet grips. The MT collet effectively grips at (more or less) the point of closure – look how they are ground. ER collets are double tapered, and grip at both back and front of the collet. This stops the job or tool wiggling for a start, because it is held along its length, which is is not reallyy in a standard collet, and provides a hell of a lot of grip for a second.

 

I might just modify my procedures?

[NEIL SMITH 1Participant @neilsmith1]

My Clausing model 8520 milling machine has a #2MT spindle and the cutters from 1/8 to 1/2 dia. are held in a collet,I have never had one come loose in the past 30 years!and it has had far more use than any hobby only machine.To remove a cutter requires a sharp blow with a copper hammer and a block of hard wood between table and quill.There are pictures and information about these very well made ,versatile and accurate machines on Google.I t would be hard to find a machine of this size that would be more suited to model engineering than the 8520.

[mgjParticipant @mgj]

I’m sure its a wonderful machine – and you have been holding all your cutters in a plain 2MTcollet in the spindle nose? Not a 2MT milling type collet chuck.

 

If thats so then you are a much braver man than I because milling chucks are designed to take heavy side and vibration loads, and above all to lock the cutter axially so it doesn’t walk into the cut. A plain MT collet isn’t designed to do those things, as the Op has found. .

 

So all credit to you if it has worked, but I think one would hesitate to recomend that as normal practise, particularly as 2MT is not really a milling taper, though it is commonly used. It doesn’t have an awful lot of diameter for rigidity (see the damping ring on the Clarkson Autolock), and it is self locking (deliberately so), which can make it very awkward to remove. Which is why most tapers designed for milling (like R8 and the various Ints) use a drive dog for the drive and use the taper simply for location, and so they are self freeing.

 

Personally, were I David Haynes I would put that 3/8 collet in the Myford and clock up a piece of plain bar, in the full expectation of finding, after bits had slipped about inside etc, that it was no longer true, and therefore fit only for scrap. If it was true, I’d breathe a sigh of relief, and spend the money I had saved on a proper milling chuck like an Autolock which has a proper extractor/damping ring , with something like a Pozilock (which is plenty stiff in R8) as second best.

[Martin WParticipant @martinw]

Hi

 

I have used an ER 25 chuck and collets to hold mills up to 12 mm with no problem. However as mjg says the grip on ER collets is different from a standard MT collet and what’s more its more forgiving regarding variations in shank sizes, i.e. it is designed to grip a range of sizes from each collets max to a slightly smaller diameter.

 

That said I would have thought with gentle cuts even using an MT style collet would grip a shank that is to size for that collet though with precious little latitude for variation on size. If the cutter is not secure then I wonder if the shank size of the cutter is accurate, whether the bore of the collet is true to size and parallel or if the collet truly matches the bore of the lathe spindle or if the spindle bore has been bruised which could interfere with the collet closing uniformly. I assume that the collet and spindle bore have been cleaned thoroughly.

 

A few random thoughts after a glass or two of vino tinto.

 

Hope you get it sorted on way or another.

 

Regards

 

Martin

 

 

Edited By Martin W on 07/02/2011 23:44:16

[Gordon WParticipant @gordonw]

Just a thought, which may have been covered already ,- Clean and lightly oil the taper. Check the slots are clean, and properly cut, incomplete slots not unknown.

[Ian S CParticipant @iansc]

Dave, I note that you are using “All Thread” as a draw bar with a washer under the nut for tightening, how thick is the washer? When I use a draw bar in a similar situation, the washer I use is over 1/4″ thick so that it does’nt flex, mind you my mandrel has a no.,5 MT and a large bore with the collet in a 5 to 3 adapter. Ian S C

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, and some threaded rod is low tensile which can streach (see HTS Bolts thread) use 8.8 grade at least.

 

Regards Nick.

[Ian S CParticipant @iansc]

Apart from one draw bolt that came with my mill, all mine are home made. Two bolts with the head cut off are welded together leaving a thread at each end, one end to fit the tooling, the other end, what ever bolt that comes out of the junk box that I happen to have 2 nuts. First on is a washer, then a nut, run that a wee way down, put the other nut on and weld it in place. I don’t use HT bolts, I perfer not to use too much tension, and I’v never had a slipping taper. Ian S C

[Keith LongParticipant @keithlong89920]

Gentlemen – before we get too hung up on what material the drawbar is made from – it doesn’t matter. Unless the drawbar is fouling something in the spindle ANY tension applied by the nut being tightened is taken by the collet regardless of whether the drawbar is mild steel, high tensile or wet spaghetti! The only difference that material will make is in the load you can apply to the collet before you strip the thread on the drawbar in the collet or break the drawbar. If the collet isn’t gripping then assuming that you haven’t run out of thread on the drawbar the problem is in the collet and it’s interface with the spindle bore and the cutter.

Keith

[John HaineParticipant @johnhaine32865]

Just a point about comparing collets. I mainly use R8 collets directly in the bore of my VMB (never seen an R8 collet chuck as such!). Though the drive to the collet is via a key, the cutter is held in the collet itself without any other aid such as you might find in an Autolock chuck. And the taper on R8 is significantly greater which means that the mechanical advantage is less than the rather gradual taper on a Morse collet. I have never had a cutter move in R8, and as the system is widely used on machines like Bridgeports I would guess that few other people have either. So what’s the difference between holding say a 3/8 cutter in a Morse collet or an R8 collet? And I should add that the R8 collets I bought were J&L’s cheapest so they can’t be much better made than the standard MT2 drawbar types.

 

As for drawbars, AFAIK there is little difference in Young’s modulus between different grades of steel, though they have different hardnesses and yield strengths. So whether you use HTS or studding won’t make much difference as long as you don’t approach the elastic limit of the material, and if you do then probably the collet will stick immovably! When I bought my VMB there was a choice of MT3 or R8 nose, but Myford strongly recommended R8 because of the danger of MT3 sticking.

[StovepipeParticipant @stovepipe]

Might a newbie pose a possibly silly question here – there’s no chance that the drawbar had cross-threaded the collet, is there ? Just a thought.

 

Dennis

[David HaynesParticipant @davidhaynes53962]

Hi again folks,

As this rumbles on with lots of interesting discussion, I am still looking for a solution. I have just tried out a witness marker on the cutting tool, but I am using layout blue, which is the nearest I have to the correct product. My layout blue often rubs off when I don’t want it to but, despite sitting very snugly in the associated collet, none of the marking is witnessing when I push the cutter into the collet. Looking down the mandrel taper, I am fairly convinced it is in good condition. I have cleaned the tool of oil etc and I have tried a drop of slide oil on the MT2 taper in the hope that it will help it slide in better and thus clamp the tool better, still with no luck; I have even tried only pushing the tool bit half way along the collet with the thought that this will allow more pivot in collet. Still no luck, the tool still rotates even when a lot of torque has been applied to the draw bar. So many people have said they have had no problems in using MT2 collets for milling on a lathe, so I am sure the MT2 solution must be out there somewhere!

 

Oh, and the drawbar can be screwed in easliy by hand!

With thanks,
Dave

[joegibParticipant @joegib]

Hi David,

 

Can you tell us a bit about the origin of your kit, please:

 

1. Collets — Are these from a reputable British/American/European source or somewhere else? I only ask because there was some controversy a while ago about Chinese ER collets purporting to be Imperial actually being re-labelled Metric collets. Now that’s not too serious given that ER collets are designed to flex anyway. But if they’re pulling the same stroke with fixed-size MT collets i.e. labelling the nearest Metric equivalent as a fractional Imperial size, that would be a different story!

 

2. Cutter — Likewise, is this from a reputable source? I measured a number of my cutters (from makers such as Dormer/Clarkson/Osborne etc.) and the shanks very closely match their nominal size — none were oversize, the largest error being 2/10th of a thou undersize. Have you actually measured the shank of your cutter with a mike?

 

No offence intended but you’ll appreciate it’s a bit hard to diagnose the problem without the opportunity for hands-on testing!

 

Joe

[Anonymous]

Slightly OT, but the set screw in an R8 taper spindle prevents the collet turning when tightening the drawbar; it is not intended to be used as a drive key. In most Bridgeport mills I have seen, including mine, it has been removed, or sheared off. I have never had a collet or chuck slip in the R8 taper despite no set screw.

 

Regards,

 

Andrew

[NEIL SMITH 1Participant @neilsmith1]

I don’t beeelieve it,i.e.That a company such as Clausing(and others)would produce a milling machine with a set of collets that were incapable of holding a cutter securely,and may even require a brave man to use them! Unless of course MJG knows something that they didn’t? I don’t believe that either.Andrew J. is correct about the R8 keyway,it is the taper that does the holding,I too have used a mill without the pin with no trouble at all. Yours grippingley V.Meldrew.

[JonParticipant @jon]

Quite agree with the R8 not needing the pin and never had a cutter slip even when putting a cut on.

For that matter never had an MT3 collet and any cutter slip except buying from one outlet up north Wales, chinese of course.

We wont mention all the ER and similar types not able to hold the cutter.

 

Meanwhile we await Joes 1st question, confident problem arises there or MT2 taper isnt correct.

 

[BusterParticipant @buster]

i have already given an answer that might at least get your job done, forget the collet for now and use a larger collet to carry the cutter with a grub screw, these questions get me so annoyed when they go on and on, ask yourself what would industry do if the line was down, someone with common sense would change to tool or the collet, i dont mean to be rude but trying to track down 1/10th thou with your gear at this stage is not helping you to move on with your project, time for that later when the pressure is off. I have home made 2 morse tapers to hold cutters with grub screws and they work fine

[Gordon WParticipant @gordonw]

I’ve just read an old MEM ,a retired professional machinist recommends using a piece of newspaper wrapped round the Morse taper, without overlap, claims to never have a slipping taper. I agree with David, make the holder from soft ended tapers.

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