What taper angle to use?

Hi All,

I am in the middle of roughing out a design which calls for a taper to be used to a) centre the two objects together and b) to allow them to be firmly locked together when some force is applied (so that they will not twist relative to each other).
When the force is removed, I need the two parts to be free to move relative to each other - so a self releasing taper.

So my question to the knowledgable out there is what taper angle should I be looking at? Obviously greater than that used for MT, maybe less than the ER collet angles...?
The diameter is roughly 80mm max and the taper length about 15mm, materials will be something known as XC38 over here, equivalent to the UK's EN8 apparently.

Thanks,

Simon

Simon,

It's worth having a look at this

In summary, he American machine tool industry settled upon a taper of 3.5 inches per foot as a reliably Self Releasing Taper. ... Anything shallower than that is likely to lock under some condition.

MichaelG.

Simon

15° to 20° full angle seems to be a fairly typical range for this sort of thing. CAT / ISO tooling uses 7 in 24 slope on the side which is a little over 8° half angle to do pretty much the same job as you require so thats probably as good as any.

The shallower the slope the greater the locking capability and the less likely it is to self release. There is apragmatic relationship between the tangent of the slope angle and the material coefficient of friction which approximately defines the self locking capability. Details are in one of the books iin my library but darned if I can recall which.

Clive

PS Mike types faster!

Edited By Clive Foster on 11/06/2014 22:39:21

If you really want to work the theory to death ... try this paper.

... Time for bed, I think.

MichaelG.

Michael,

I can only look in awe at your collection of literature at your fingertips. However both you and Clive confirm my empirical 'touchy feely' inclination to go for 8°, or 16° included - roughly the same as 3.5" per foot and the same as ER collets.

Thanks again,

Simon

My horizontal mill uses INT40 tooling, but it certainly isn't reliably self-releasing. May be I'm over-tightening the drawbar.

Regards,

Andrew

.

Near perfect surface finish ... the same as your Slip Gauges ?

MichaelG.

My Shizouka VRG mill is has an Int 40 taper in the spindle, When I first got it I used to really tighten the draw bar. However I have found this is quite unnecessary, I just take up the slack then give the spanner a further push... and that is enough. I have never had a tool work loose. To change tools I just untighten the bar about one turn then a light tap with a small plastic faced mallet and the tool drops down ready to finish unscrewing the drawbar and changing for another.

What is important is to keep the taper clean. As a habit I wipe the taper with my hand before inserting it. I guess that puts a small amount of oil on the taper, otherwise I leave it dry.

Regards
John

Hey Clive, did you go to Redruth School of Mines? If you did you would be thinking back to that time or else you have some connection with geotechnic engineering!

Slopes, tangents and friction - The tangent of the angle of slope equates to the mu value for the material - the problem is ( I think?) you would need to grind the materials up and pile them in a heap..... try searching for "angle of repose" and it will all come back!

Mark

At my first job there lots of stub mandrels most threaded ,some plain expanding mandrel type these were used for holding lens holders ,locking rings,eyepiece tubes ,for microscopes, so that they could be used on a variety of lathes ,each lathe had a steel "nose" which screwed onto the lathe spindle and it had a tapered bore of 15 degrees and each mandrel was tapered to suit ,so we had an interchangeable work holding system some 50 odd years ago, I was told that 15 degrees was selected as it was just on the point of taper holding firm but not self locking, the mandrels were locked in posn with a drawbar.