Why do Morse Tapers stick?

[sophiecentaur]

I have a mini milling machine (Sealey SM2502 aamof) with the tools held in an MT3. If I don't tighten it up well with the drawbar, the tools sometimes work their way out and the tool can rattle.
If I do it up tight enough to avoid that, it is very hard to release it and I use a rod with a soft end (not the draw bar because I don't want to hurt the thread) and give it a hard whack to release. This can't do the bearings any good and I want to do something about it.
I have tried model engineering forums but the opinions tend to be based on personal experience - although the guys want to help. I wonder if anyone reading this will have any informed ideas about the problem.
I wondered about buying a suitable reamer but they are expensive and the internal taper could be hardened.
Just posting this on the off chance that someone else has had similar problems.

 

[Twigg]

Do you see any scratches or other bad signs on the tool shanks?

 

[Baluncore]

Steep wedges and tapers are self-releasing, they need a drawbar to keep them in place while being used. Long thin wedges and tapers are self holding without any draw bar, they tend to stick.
So what is that critical included angle that makes the difference?
It is the arctangent( friction coefficient ).

If the friction is steel on steel you can change the coefficient by lubrication, or by inserting a thin layer of paper between the taper and the socket.

To get a better grip when clamping steel in a steel vice or chuck, a pack of uniform thickness paper (cigarette papers?) can be used, because steel-paper has a greater friction coefficient, and two layers have the same FC as one layer.

Your stuck taper problem arises because the self-locking MT-3 is being used with a drawbar and is being over-tightened. That is probably because it is oily when assembled, but tightens as it is used which expels the oil, which makes disassembly difficult.

At the end of the job, do not leave the taper engaged with the drawbar under tension. It will tighten and lock as the temperature changes and the oil migrates overnight, or by next year.

The cure is to clean and dry both surfaces of dirt and oil, if possible blow the socket out with compressed air before assembly. Use a truncated cone of clean paper between the steel tapered surfaces, to grip better with less tension on the drawbar. Dirt will bed into the paper, slight spots of oil will be absorbed by the paper. The tapered surfaces will not be damaged and you can avoid regrinding the taper. Paper also averages the surface imperfections due to earlier use.

A taper expands under axial pressure from either end. It shrinks under tension. That is why the drawbar tension pulls it in too well, and it is then so hard to remove. It is often small side forces that release a self-locking taper, such as using a face cutter. You may be able to attach a cap so the drawbar can press gently down when the drawbar is screwed out a couple of turns. Then gentle side-forces on the tool, applied by hand without a hammer, will break the taper and cause it to walk, then jump out, while still being safely restrained.

 

[Twigg]

The cure is to clean and dry both surfaces of dirt and oil

I would add, be sure to put some oil back inside the taper on the spindle nose before you store the mill for a long time. If you live in a rust prone climate, that is.

 

[sophiecentaur]

Your stuck taper problem arises because the self-locking MT-3 is being used with a drawbar and is being over-tightened.

Thanks for you input (and the other guys too). I get the theory and I've tried various approaches. With the draw bar at finger tight, I have actually had the taper rotate inside the quill. Also, the draw bar can come loose - which may not, in itself mean the taper can be loose, I guess.

The taper is clean and not at all rusty. It feels and looks (mirror / torch ) unscratched but by no means shiny. I've washed it out with WD40 and scrubbed it vigorously with a cloth on a stick. Shining a small laser along the surface doesn't reveal any visible shadows but the 'optics' of that arrangement are not very sensitive. I get the same effect with various MT3 tools; even recent purchases so I imagine it could be the quill that's the problem.

Solution: none so far and I don't want the expense and hassle of replacing the quill assembly. The temptation to go for a belt drive conversion and a better motor would be too great. The thing would be out of action for months!

I'm just in the process of making up a pair of wedges to go between the bottom of the quill and the top of tools. It's only mild steel but it won't be 'bent' by its action - just could be gouged a bit. It's nice to use the mill to 'mend itself' with.

I will report back.

 

[sophiecentaur]

I will report back.

I used some gash steel plate to make a double wedge and, with an additional spacer, found that not too much force seemed necessary to release the taper by tapping with opposing hammers (no net vertical force on bearings). It's much less eventful than whacking with a hammer as the wedge has a small angle a couple of mm in 80mm.
Downside was masses of steel chips and some splinters in my fingers. but that's what you get with milling. Gimme aluminium.

I am cleaning, oiling and wiping more carefully now.

PS I measured the tapers on the two opposing wedges and they're about 3.5 degrees each. That means the hammers only need to provide about 12% of the force that a bash on the top needs and it two taps are more or less balanced
A different extra U is needed to match the clearance for each different toolholder.

 

[NTL2009]

Take this with large grains of salt, I have no direct experience with this, but I wonder if, after cleaning, applying something like fine valve grinding compound to the shank, and running that loosely inside the socket would help to clear out any irregularities in the mating surfaces?

 

[Baluncore]

... after cleaning, applying something like fine valve grinding compound to the shank, and running that loosely inside the socket would help to clear out any irregularities in the mating surfaces?

Given time, temperature changes and drawbolt tension, a polished surface will be pulled in and will stick better than the original honed surface. If the polished taper starts to spin, it will spin more freely with less lubricant. A cold taper pulled into a hot quill will shrink on.

A paper sleeve conforms to, and accepts all irregularities as many small teeth. Steel-paper-steel has a higher friction coefficient than steel-steel. Paper allows air to enter the gap during separation, and absorbs liquid oil that would otherwise lubricate the surfaces.

 

[Twigg]

Take this with large grains of salt, I have no direct experience with this, but I wonder if, after cleaning, applying something like fine valve grinding compound to the shank, and running that loosely inside the socket would help to clear out any irregularities in the mating surfaces?

Typically, you only turn to abrasives as a last resort. After all, you can't put the metal back. Also, which tool shank would you use as the grinding surface? No offense to the OP, but given the intended market of the mill in question, I'm guessing the OP's tools aren't going to all have perfect shanks, and that means the ground internal taper would inherit some of the defects of the tool shank. (Seriously, no offense to you OP. The only "mill" to my name is a vise attachment for a mini-lathe that doesn't even develop enough clamping force to dust off a whisker of 12L14. I get the appeal of small tools, even when they end up making you cuss a lot! )

@sophiecentaur I'm having a hard time visualizing where the wedge goes. If it's not too much trouble, could you post a picture of it in action? Thanks! Just to be sure, you never apply the wedge when the drawbar is threaded tight, correct? That's the only way I can imagine it doing harm.

 

[sophiecentaur]

Paper allows air to enter the gap during separation, and absorbs liquid oil that would otherwise lubricate the surfaces.

I'll try that. I have some old textbooks or perhaps a bible with suitable paper. Only the most expensive books these days seem to have decent paper. Ah - ciggy papers are what I need.

I'm having a hard time visualizing where the wedge goes.

The oval hole between the two halves fits around the too shaft between the spindle and the top of the tool. (Not usually a drill chuck) There's only a few mm rise available but the extra U works for the collet holder. (see high tech pencil arrow) It works better and better as the faces polish up with use.

applying something like fine valve grinding compound to the shank,

I did consider it but with 'marking Blue' I couldn't see any particular high spots. Perhaps the fit is too damn good. There are many sources of movement on a cheap lathe like this one; the X and Y dovetails always need nipping up when I use a fly cutter and the ER25 collets need to be socked up really tight Plus the vise on the work, of course. But I now have ground a fly cutting tool so that it works acceptably on steel. Getting the face angles right is another learning process. I do love my little milling machine - it's almost a real tool.

Just to be sure, you never apply the wedge when the drawbar is threaded tight, correct?

Haha!
OR without the draw bar as a safety net! So many opportunities to do damage on these machines. Teaching safe workshop practice to sloppy kids must be a nightmare.

 

[Baluncore]

I'll try that. I have some old textbooks or perhaps a bible with suitable paper. Only the most expensive books these days seem to have decent paper. Ah - ciggy papers are what I need.

Regular 80 GSM has controlled thickness for use in photocopiers and laser printers. Honestly, 80 gsm would be too good for the quill in that mill, newspaper would work, (I have one like it).

After doing the job, examine the finish on the paper, it will carry a print of the quill and tool tapered surfaces.

Measure the paper thickness before and after with a micrometer. Variation in the final crush thickness will give you an idea of alignment tolerance.

 

[sophiecentaur]

it will carry a print of the quill and tool tapered surfaces.

I see where you're coming from. I do get the same effect with at least four different tapers and rotating them in the quill doesn't produce any noticeable tight orientation angles. That seems to imply the quill could be at fault and any mis shaping has circular symmetry (A circular bulge?).
My wedges will make that particular experiment more convenient.
If I were prepared to sacrifice a taper, I could always take a new one and gradually shorten it to find where the sticking occurs.
However, the wedges are so convenient that I may leave things as they are for a while - but I can easily do the paper thing - strange I haven't come across the idea elsewhere on model engineering forums. It would appeal to DIYers.

 

[Dullard]

I have (Sherline) a mini-mill and mini-lathe. I initially had the same concerns with Morse Tapers. I consulted some actual machinists and was assured that some 'encouragement' was usually required to un-seat a properly seated taper. My solution was to fabricate a suitably sized (very small) brass hammer for that operation.

 

[Baluncore]

I see where you're coming from. I do get the same effect with at least four different tapers and rotating them in the quill doesn't produce any noticeable tight orientation angles.

Fundamentally, a MT sticks because it has a gentle taper and you leave it to bed in with tension on the drawbar. A MT spins when you need it to stick because it is lubricated metal-metal, and/or subjected to side forces with insufficient drawbar tension.

The single layer of paper is NOT there to identify imperfections so they can be ground out. The paper is there to increase friction while transferring torque to the tool. Obviously it also requires some drawbar tension to work, more with interrupted cutting. As I said earlier, the paper reduces lubrication and complies with imperfections in both surfaces, so you don't have to dress the surfaces. Instead, you use those imperfections to advantage.

The other advantage of paper is that it allows the surfaces to separate later when side forces are applied with a slight drawbar push, which is what you intend your wedges to do.

If I do it up tight enough to avoid that, it is very hard to release it and I use a rod with a soft end (not the draw bar because I don't want to hurt the thread) and give it a hard whack to release. This can't do the bearings any good and I want to do something about it.

I had a stuck taper in one of those mills, but I needed different bars with different threads for differently threaded tapers. I made my own special drawbars from threaded rod for that mill.

I removed some thread at the bottom of the threaded rod, so the rod would seat hard against the very bottom of the hole in the taper. I used a free nut on the threaded rod to pull the taper into place. I topped the rod with a cap nut, clear of the free nut.

For a stuck taper, removal is then a two step process.
Step 1. Loosen the draw nut, turn the cap nut to bottom the rod in the taper. Disengage the vertical feed mechanism if possible, so you don't break it. Hit the cap nut with a big hammer, gently, increasing until it ejects the stuck taper. That will not damage the thread since the rod is bottomed in the hole.
Step 2. Lift the rod gently to seat the taper again, sufficiently to loosen the rod from the bottom of the hole by turning the cap nut. Tap the cap nut gently to release the taper again. Then you can unscrew the taper from the rod by hand.

 

[sophiecentaur]

Tap the cap nut gently to release the taper again. Then you can unscrew the taper from the rod by hand.

I can't understand, from your description, how I could apply that to my situation. There is nowhere at the top of the spindle where a downwards force can be countered. The 'gentle tap' would need to be the same as in my present situation. (I used a separate drift with a bronze end which involved no damage to the draw bar thread.)
In any case, there is a very 'gentle' tap required on my wedges and the only force is tension along the taper so it's a win - win.
I will look around for some corroboration about your paper method. It does sound interesting.

 

[NTL2009]

... A paper sleeve conforms to, and accepts all irregularities as many small teeth. Steel-paper-steel has a higher friction coefficient than steel-steel. Paper allows air to enter the gap during separation, and absorbs liquid oil that would otherwise lubricate the surfaces.

I don't have any machinery with a spindle like this, but I'm going to tuck that information into my memory banks. It could come in useful in some other situation someday. Not something I would have thought of. Very interesting. Thanks.

 

[Baluncore]

I will look around for some corroboration about your paper method. It does sound interesting.

Just try it. It is called experimentation. Subvert the dominant paradigm.
When metal parts are bolted together, even when there is no seal required, why do you think they often use a paper gasket ?

Very interesting. Thanks.

Paper also helps when holding steel in the steel jaws of a vise or a chuck.

 

[256bits]

Regular 80 GSM has controlled thickness for use in photocopiers and laser printers. Honestly, 80 gsm would be too good for the quill in that mill, newspaper would work, (I have one like it).

After doing the job, examine the finish on the paper, it will carry a print of the quill and tool tapered surfaces.

Measure the paper thickness before and after with a micrometer. Variation in the final crush thickness will give you an idea of alignment tolerance.

Never heard about the paper before.
Seems as if parchment paper should be OK - thin and heat resistant.
Maybe baking cup paper could be worth a try.

As an aside,
The collet on my dremel ( not a dremel but a takeoff ) sticks the shaft and does not release.
I have to bang the tool into the collet to release.

 

[Baluncore]

Maybe baking cup paper could be worth a try.

I would expect baking or parchment paper to be a high temperature wax paper that will not absorb oil, and that will have a low friction coefficient.
I believe the best paper will absorb any surface oil, embed any grit, and have a high friction coefficient.

 

[Tom.G]

Never heard about the paper before.
Seems as if parchment paper should be OK - thin and heat resistant.
Maybe baking cup paper could be worth a try.

As an aside,
The collet on my dremel ( not a dremel but a takeoff ) sticks the shaft and does not release.
I have to bang the tool into the collet to release.

For the dremel-style tools, don't insert the tool all the way into the collet (chuck). They sure don't like to be pulled out but they will push in rather easily, thereby breaking the seizure and falling out!

(at least it works a charm on a real Dremel, just finger pressure)

Cheers,
Tom

 

[256bits]

For the dremel-style tools, don't insert the tool all the way into the collet (chuck). They sure don't like to be pulled out but they will push in rather easily, thereby breaking the seizure and falling out!

(at least it works a charm on a real Dremel, just finger pressure)

Cheers,
Tom

That is what I do do.
Good advice for anyone using these tools.
Just wondering why the tool is hard to pull out, but easy to press in, for release.

 

[Tom.G]

Just wondering why the tool is hard to pull out, but easy to press in, for release.

Probably an incarnation of the Chinese Finger Trap. (Google it)

 

[OCR]

.
Chinese Finger Trap . . . .
.

 

[256bits]

Ingenious @Tom.G and @OCR
Chinese finger trap.
Thanks.