reaming to a shrink fit size

[Jelly_Sheffield]

That’s why I sometimes find an odd sized reamer or imperial if I’m lucky to pre ream the pilot
On the Devlieg if critical and I’m nearly always doing production I have a small boring head preset just under size normally using a slot drill as the bar

Production is by far where reamers make the most sense, IMO.

There's a lot of fiddly setup to get a workflow that gets the best out of them, but once you have that workflow they will do multiples in seconds.

For one-offs I prefer to use lapping, honing or ID grinding if surface finish is critical, or straight boring taking finishing passes with a carbide shank bar, and positive geometry, small nose radius insert if it's about absolute circularity.

 

[graffian]

This is a really good point, aluminium has more than double the thermal expansion of steel or cast iron...

So in a hot environment like an engine, the fit of an aluminium pulley should be tighter than a steel one, or ideally calculated based on the size at operating temperature.


To walk OP through the thought process:

If we assume that the shaft is 12mm, and want to have a "Forced Fit" because of heavy load and high operating temperature.

We need to achieve a Shaft Basis fit of U7/h6 (see these Fit Tables and this Useful Calculator)

The U7 tolerance says that the hole should be between 26 and 44 microns (0.001" to 0.0017") undersized (i.e. if the shaft is 12mm, the hole is 11.966mm to 11.974mm)

The h6 tolerance says that the shaft should be between 0 and 11 microns (0" to 0.0004") undersized (but because we're measuring the shaft not making it, this just means we need a very accurate measurement).

That gives an interference of between 44 and 15 microns

We also know that a good clearance fit (H6/h6) has a clearance of 29 microns.

So to reliably shrink fit the part, we need to expand it by at least 73 microns (0.0028").

Aluminium has a thermal expansion coefficient of 21×10^-6 m/m°C (handy table Here).

So for every 1°c we heat our 12mm bore, the bore will expand by 0.25 microns (formulae Here).

Going back to how much we need to expand the part, 73÷0.25 gives us a temperature difference of 292°C.

Assuming the workshop is 20°C, that means shooting for a part temperature of 310°C to 330°C to just slide it on freely, then shrink to a very permanent fit.

I use figures I learned in school physics for linear expansion.

Aluminium ~ 23x10^-6 and steel ~ 11x10^-6, ok these are for linear expansion but it works well enough. If you want exact coeffs for the material look for the metal spec.
From the ones I have looked up I would say steel is never far from 11e-6, but aluminium can vary quite a bit.

I [used to ]press fit lots of motorbike stems into yokes. So will give that as an example. I usually re-arange this to find the stem size I need to achieve the fit I want in the
existing hole.
I work on the fact I am going to heat the ali bit around 130°C and cool the steel bit to -18°C[freezer temp]. I am not allowing for ambient temp but you have to. I am aiming for
something like a ~3.5 thou fit. I want the stem to drop into the yoke before I can bring the press down[or the metals cool/heat each other to much] so 4 thou or more is what I really want.

For a 30mm stem the size change will be:
30mm stem, heated 130° and coef 23x10^-6; so size of hole when heated will be 30mm + 30mm x 23x10^-6 x 130
If you have a calculator that is 30*[1+23e-6x130]
which = 30.089mm
.089 is 3.5 thou, which is how much fit I want, but i want it to go together so I need a little more.

To work out the change in size of the bit that went in the freezer is the same as above, but times the coeficient by -18[-18° in freezer] the steel is going
to shrink by something like half a thou.

Long time since I have changed water pump bearings, I just pressed them off and on cold. They are nothing like as tight as a stem.
I would try some 638 loctite on the pulley/pump you have. It glues loose parts in solid. It won't get hot enough for it to turn to goo will it?
I change the size of reamers by running a tungsten along the cutting edge or packing a fag paper into a flute.

 

[pressbrake1]

Production is by far where reamers make the most sense, IMO.

There's a lot of fiddly setup to get a workflow that gets the best out of them, but once you have that workflow they will do multiples in seconds.

For one-offs I prefer to use lapping, honing or ID grinding if surface finish is critical, or straight boring taking finishing passes with a carbide shank bar, and positive geometry, small nose radius insert if it's about absolute circularity.

A nice jig grinder would be nice!

 

[Jelly_Sheffield]

A nice jig grinder would be nice!

That would do the trick nicely!

 

[lotus 30]

Thank you all some brilliant replies the more I learn the more I dont know. Its more than just drill a hole most out there do not realise how clever these things are.
Kindest Andy

 

[DAPPH]

Morning all
I appreciate the reamer needs grinding down a touch to achieve what I am after. Just interested to know why this happens. these are Coventry Climax water pumps I heat the std pulley and it comes of nicely. I machine from castings the Automotive pulley which are much bigger for road race use. now my machine reamer I have will go through the std hole size easily. when I ream the casting after a lot of work it is a useless fit on the water pump shaft it just pushes on no heat required and as you can imagine will spin on the shaft when in use. My question is what is happening why is the reamer taking to much metal away when it slips through the std pully which is a shrink fit. Ok I appreciate the reamer needs reducing in size that is a little beyond my skill set to do accurately


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A tapered adjustable reamer will solve your problem , set it for an interference / heat shrink fit

 

[oldbikedavey]

Would an expanding reamer have any role to play , given that they can be supported on their rear centre ?