Self Gauging Tapers for Shrink and Interference Fits

[Clive FosterParticipant @clivefoster55965]

Shrink and interference fits are reliable and theoretically easy to do but, as a has been frequently discussed, getting practice and theory to align in the home workshop can be hard.

Realistically very few of us have the equipment to accurately and reliably measure to the tolerances needed for known shrink fits. Getting something to grip isn't too hard but the actual hoop stress, and therefore grip achieved, is somewhat in the lap of the gods. It doesn't help that, as Fulmen said in a recent post on a related thread, our parts are usually small so rapid heat loss and the concomitant tendency to self ream part of the joint if cooling is too rapid as the fit is made adds uncertainty. Small parts with small wall thicknesses tend to stretch too.

It's important that stresses in the material don't drive it past the elastic limit when a shrink fit cools. Again not easy for most of us as not only don't we have the maths and metallurgical knowledge to properly evaluate things the exact provenance, specification and heat treatment of the materials we have to hand tend to be uncertain. Given this it's risky to aim for high levels of interference fits.

For the inexperienced attempting anything seriously stronger than loctite is likely to be unsafe.

Self gauging tapers are an effective and relatively easy way of reliably achieving a known interference fit.

The two components are made with shallow tapers such that when offered up cold the male taper only goes part away into the female taper yet when pushed fully home the desired interference fit is achieved.

Usual practice is to make the taper over the full length of one side of the part equal to the desired interference so when offered up cold the tapers engage over half their length. Obviously the total interference possible over the whole length of the taper parts is double the desired fit but as the tapers start off half way home the actual fit corresponds to what is needed.

There are various relatively simple methods of creating matching male and female tapers from one set up on a lathe. Mine have taper turning attachments and camlock chucks enabling safe reverse running so dead easy for me. With screw on chucks you need to use an inverted tool for one side. If you don't have taper attachment it's wise to arrange some form of screw driven adjustment so as to accurately set the topslide.

Best to use a test bar as a reference and use a decent amount of travel. Even when using the topslide a couple of inches or more should be safe making it relatively easy to set a taper very close to that theoretically desired.

As this isn't something you do every day its advisable to draw things out first time through as the small offsets can make it confusing to see whats going on. It's also worth tabulating the adjustments needed to bring the tapers to the final mating lengths.

Due to the small taper angle the longitudinal adjustment needed to take half or quarter of a thou off to get the initial engagement right is usefully large. If you are using the topslide the error resulting from moving along the taper rather that along the bed will be inconsequential.

The method is quite tolerant of small errors. Half engagement is what you are aiming for but 1/3 rd or 2/3 rds will be good enough. In practice its quite difficult to end up with a bigger error given the usual modicum of extra care needed for the precise bits.

I first encountered this on a Velocette Thruxton camshaft drive. Some dinkle brain had pushed cams and camwheel together incorrectly giving the wrong timing. Pushing the pair apart and getting it back right was satisfyingly easy. Due to the tapers a hand push before pressing ensured things were inline.

Accurate alignment of parallel fits can be tricky.

Clive

Edited By Clive Foster on 21/07/2023 09:44:13

[Clive FosterParticipant @clivefoster55965]

[not done it yetParticipant @notdoneityet]

I like that idea/method. Even though I’ve not yet broken anything by shrink fitting.

Boring/reaming provides a parallel bore and careful turning and finishing is generally no great problem for sizing the inserted part. I would never try to bore to fit the inserted part (well, not if I could possibly avoid it). I would normally leave the inserted part long, so I can basically make it ‘go-no go’ at the end to determine the oversize measurement.

The angle of the taper will determine the interference after shrinking the parts together, of course.

The one problem may be the final overall length – dependent on how far the inserted piece enters the taper?

I make my taper pins to go with taper reamers for small diameters – usually so they can be removed, if needed (much preferable to roll-pins!), but fitted ‘hot and cold’ could make taper pins really permanent (flush and almost invisible).

I know (or am fairly certain) MT2 joints can be difficult to part with only a few degrees difference when fitted and tightened. I use a straight-cut taper reamer for setting the angle of the taper as it is easy to measure/set the taper at precisely centre height.

How small is/are the angle(s) you are suggesting?

I shall certainly keep this in mind if/when the next shrink fit is required.

[Clive FosterParticipant @clivefoster55965]

NDI

I'm glad you like the idea.

Overall length isn't an issue because you always push the tapers fully home. Small errors in relative taper diameter merely alter the actual interference ached slightly. In practice it's far easier to get very close to what you intend that it is with a standard parallel fit.

To my mind the great thing is you can see exactly what's going on as you bring things to final size and adjustments are feasibly measurable.

Who can guarantee to take 1/2 or 1/4 thou off parallel shaft or hole? Even with the angled topslide trick and a lathe in first class order.

In principle the final cuts for self gauging tapers are effectively an extreme example of the angled topslide trick.

Worst point is that gauging and setting final diameter by longitudinal movement along the taper makes the relationship between cut and diameter a bit unintuitive. Which is why drawing out and calculating first is a good idea.

The angles simply fall out of the length of the fit / taper. Basically during set up the DIT movement as you traverse along the test bar needs to correspond to the fit. So if I want 1 thou of interference over a 1/2" long part I'd set-up for 8 thou over 4 inches using the taper attachment. If you are using the topslide 4 thou over 2 inches is probably practical.

Which is sensible magnitude of measurement. Even you only have a Unique indicator and an old Drummond.

Clive

 

Edited By Clive Foster on 21/07/2023 11:49:40

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