Split cotters

Hi Derek, Guys,

Whilst the formula are basically correct I just add a couple of MM to the clamp screw diameter. However one thing that you do need to be aware of in cast iron. Split collets exert a great deal of pressure and whilst very effective at clamping that pressure has to be contained. So take very great care that the wall thickness directly opposite to the split collet is adequate to contain the pressure.

In Steel it isn't a problem, but I've seen castings broken by excessive tightening of a spilt collet ! One way round this is to use a small/smaller diameter material for the collet. Virtually all mine are in steel, I've only one in cast iron and one in aluminium.

HTH.

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So … Given that D/2 and d/2 are ‘inevitable’ that’s actually a tolerance of 15 thou on a dimension of 30 thou

MichaelG.

Edited By Michael Gilligan on 24/06/2021 17:10:52

If you get the 2 bores too close together and the bolt won't go in don't despair, turn the centre of the bolt down to root diameter of the thread, won't weaken it and it might then go in. Don't ask how I found this out!

Of course you have to put the cotter and bolt in before the main shaft

Edited By duncan webster on 24/06/2021 18:58:49

For what it’s worth … This appears to be the total extent of what [in that Duplex article] is specifically relevant to the subject of this thread.

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MichaelG.

Stating the obvious. George Thomas is largely aiming at a scribe,dot punch and drill audience Hemmingway are advocating a co-ordinate machined fixture produced on a vertical mill which is very likely to have a DRO in morst cases.

The curved pads ensure that the clamped bar is not marked. The critical dimension is actuallt the gap between the two pads when closed. Half the angle between the two contact points of the pads and the centre of the clamped bar will be the minimum effective taper which must be at least the minimum for a self releasing taper. If G is the gap and D is the diameter of the bar then the taper is arcsine (G/2)/D unless I've muddled my maths.

The clearence value of 20 or 30 thou does comeinto it.

regards Martin

Yup, you've muddled the maths, it is arcsine (G/D). This site suggests that the magic number is 7 degrees PrcMac

and as sin(7) = 0.122 so you won't go far wrong with G = D/8, I'd make it a bit bigger just to be safe.

Ah I was taking the 'gap' to be the distance between the two pads so I divided it by 2 to get the distance from the mid point to the start of the contact point or the oposite side of the triangle. But then I am not sure how tapers are generally defined so I went for half angle although I didn't say so.

I also mistyped the last bit, the point of which was that the clearance of 20 or 30 thou is irrelevent (I meant to type "does not come into it".)

G=D/8 sounds good to me with a taper target of greater than 7 degrees.

cheers Duncan

regards Martin

Martin & Duncan

It’s not worth worrying about … but I find that analysis difficult to grasp

Surely there are more variables involved in analysing the vectors

Where exactly are you assuming that either ‘locking’ or ‘self-releasing’ takes place ?

… is the diameter of the two ‘half-nuts’ [to use Duplex’s terminology] irrelevant ?

MichaelG.

My thinking was that the curved section of the pad forms a taper that increses along it's length and is at it's minimum at the pad bolts inner edge. The taper is actually the Tangent to the clamped shaft at the point of interest and so is only defined by the gap and the radius of the clamped bar. (I see now where my maths went wrong I used the Diameter not the radius, note to self don't do maths at bedtime).

Once the angle has been found one must just ensure that it is greater than the critical angle for the transition between self holding and self locking, given in most books as 7 degrees.

You are more than likely correct in saying there is more to taper locking as far as analysis is concerned but that is 'built in' when the transition angle of 7 degrees is taken from the literature.

The diameter of the two half nuts or pads is irrelevant as this only determines where the taper 'leaves off' at the outer end of the pad and, as this must be a greater taper angle, is of no concern.

Duncans G=D/8 certainly gets you where you want to be and if there is any tendency to fail to unlock, facing a fraction off the inner ends of the pads will easily correct it.

regards Martin

Edited By Martin Kyte on 25/06/2021 10:31:21

Edited By Martin Kyte on 25/06/2021 10:32:34

Thanks for the response, Martin

… I shall mull over that

MichaelG.

Hi Guys,

The clamping angle varies very little for a given collet diameter and ultimately depends upon the diameter of the clamped part.

This drawing of a 12 mm cotter with an M6 clamping screw shows the angles for a 20 mm diameter and a 50 mm diameter shaft. The blue lines representing the collet, whilst the red lines the diameter of the clamped item. There is barely 7 degrees between them. A larger collet diameter will only make a small difference. Hardly a locking taper.

As I mentioned earlier the sharp edge on the scallop edge will cause binding and should be removed. I few swipes with a stone is enough.

I think the angle of concern is right at the inner end of the collet, not the 'average' as drawn by JB. Whether it is a real concern I don't know, I've never managed to get one to jam, but I do have to give the quill lock on my Centec a whack wth the ball of my hand sometimes

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I will sit back and let the discussion continue … but I admit that I am bewildered by your closing comment, Martin

Anything ‘inboard’ of the lines of contact can, as I see it, have no effect on the clamping or locking

… all that ‘facing a fraction off the inner ends of the pads’ can do is increase the gap between them.

Do please correct me if I have misunderstood your comment.

MichaelG.

Hi Guys,

Michael is right, facing off the inside ends would only tend to reduce or remove the burrs that the saw or parting blade left. FWIW I just use the bandsaw to cut the collet across its centre. This usually leaves a 30 thou gap in the middle. Occasionally I face the inner ends if the cut is a bit rough.

? How about considering a split collet clamping a square cross section !

The inner end of the pad (the gap if you like) defines the taper at that point. Removing a small portion increases the minimum taper angle and makes the pad self releasing.

regards martin

Edited By Martin Kyte on 25/06/2021 15:22:57

Edited By Martin Kyte on 25/06/2021 15:27:24

Hi Guys,

Michael is right, facing off the inside ends would only tend to reduce or remove the burrs that the saw or parting blade left. FWIW I just use the bandsaw to cut the collet across its centre. This usually leaves a 30 thou gap in the middle. Occasionally I face the inner ends if the cut is a bit rough.

? How about considering a split collet clamping a square cross section !

It's not a collet it's a cotter.

regards martin

Edited By Martin Kyte on 25/06/2021 15:27:00

Edited By Martin Kyte on 25/06/2021 15:27:47

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Sorry but I still don’t get it … The pads each make contact at a line ‘outside’ that area; so your increased taper cannot have any effect.

MichaelG.

The pads have curved contact surfaces that match the main bar. There is no line contact but two matched curved surfaces as shown in Duplex's sketch (which you posted). Increasing the gap increases tha angle of the smallest part of the taper.

If the gap was almost zero then the taper angle would be very near to zero too and the pads would wedge in place.

regards Martin

I must get a drawing package, it would be so much simpler than just explaining in text.

Quite right, my error !

Since it wont let me edit my original post, error corrected.

I do understand where you are going Martin, but the cotter wont work if there is no gap between the halves. So the risk of any locking is non existent. As far a CAD package is concerned, I use Qcad. The community edition is free but a perpetual licence is only £30.

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Sorry Martin but I continue to disagree or misunderstand

The pads should be turned to the same radius of curvature as the hole in the block [bored in-situ] which should be a sliding fit on the bar [so perhaps one thou of radial clearance] … therefore, when the pads are clamped there is only line contact between each of them and the bar.

In the Duplex drawing, these lines would be mere points, because they run into the page.

MichaelG.

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Edit: __ if they exactly matched the bar radius then things wouldn’t slide, and there would be no need for a clamp

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Busy doing microscopy at the moment, but I will try doing an exaggerated sketch later.

Edited By Michael Gilligan on 25/06/2021 17:58:14