Question for the Mech.E's on CTE and interference fits

[WinterDeere]

Simple problem of a BeCu (eg. C17300) or aluminum (6061-T6) dowel pin pressed into a blind hole in an aluminum (6061-T6) block: how will the fit or withdrawal force change with temperature?

This part will be carrying a reasonably high current density, and so maintaining a tight fit is important to avoid excessive heating and oxidation... which begets more heating and more oxidation, runaway scenario.

It's relatively easy to calculate the change in diameter of the pin and the hole with temperature, but what is less obvious to me is how temperature cycling will affect the fit over time. If the interference increases with temperature, is there a probability the parts could become loose upon cooling?

We normally knurl such parts, and have never had trouble, but in this case the pin diameter is only Ø.020. That may be too small to reliably knurl, or at least with any reasonable control over how far we're raising the material. We can have these pins made to ±.0003" for just a few dollars each in low volume, which should be tight enough for a reasonably-controlled interference fit, at least by the uneducated rules of thumb always used by this EE.

Temperatures are not extreme in this case, something like 0 to 120°C, and probably nearly uniform between pin and part into which it is pressed. All parts will receive hexavalent-free chromium conversion coating, which I know is not sufficient for passing salt-spray tests much above 70°C, but has always worked well enough in our non-corrosive environments at these temperatures.

 

[MAX-24-Dean]

Depends on how much heat dissapating material is on the back side of the blind hole? If sufficient to sink the heat away from the interference connection and the current is low frequency, then I would go with the 6061-T6 as both parts would expand and contract similiarly being the same property materials.
Have fun with the small pin diameter!

 

[oldtimer 66]

You could use a friction enhancer on the assembly. There is a " PTS " solution that has 5 micron ceramic particles suspended in a colloidal solution of alcohol. It would be applied before assembly. It leaves a slight haze on the components after application. The parts need to be cleaned/ no oils or contaminants. This would need to be heat shrink operation. Both parts to be T6 as per Max-24-Dean.
I need to question the pin size. .020 dia. and plus or minus .0003 tenths. You had mentioned knurling before and that to me implied a press fit operation. With the +- .0003 tolerance and a .020 dia. this is no press operation. Then there is the surface finish consideration if it is a press fit. Confirm that pin size, maybe .200 dia.?

 

[WinterDeere]

Thanks, guys!

Yes, pin size is .020", not 0.200". It's actually a common size in my industry, and not nearly our smallest, but this is the first time I've ever looked into the possibility of press-fitting them. Usually, they're captured in a sprung tempered BeCu socket, but there are some other issues with using that method in this particular design.

The ±.0003" is a number I can't easily change myself, since the Ø.020 pins are made elsewhere, and that's the manufacturer's standard tolerance on the part, after plating (Ni/Au). It's very possible that alone makes this job infeasible, unless I have my own pins made at much higher cost (due to reduced volume). But do keep in mind .0003/.020 is only ±1.5% variation, and the base material is only 6161, pretty forgiving in terms of interference fit materials.

It might be possible to knurl a Ø.020" pin, I've honestly never looked into it. We use that method (spline knurling) when pressing pins from one manufacturer into sockets made by another manufacturer, to allow more forgiveness in the relative sizing between two manufacturers, but I've never tried it myself on any pin under Ø.120".

I have some .036 +.0003/-.0000 core pins and a .036 +.0000/-.0002 reamer on order at the moment, just to test and play. This would yield an interference of .0000 to .0005, which wouldn't be too awful far from sizing my holes at .0195 to .0197 with the standard .020±.0003 pins.

Although the core pins are M2 steel and not BeCu, my thinking is that an experiment with a fixed weight pulling on the pins while temperature cycling the assembly might give me some idea if there's any hope of the pins staying tight in use. If that doesn't fail, I could set up the more difficult and expensive experiment of repeating with the real parts.

 

[dstig1]

You ever actually try pressing a pin that soft into something before? That may be the first real challenge, especially on a 1/2mm pin...

 

[MAX-24-Dean]

I have never seen knurling completed on this small of a diameter.

I would think the softness of the Ni/Au plating would help in the knurling process. You may also run the risk of losing the corrosion protection if the knurling penetrates the plating.
The other variable is if you can get a good air seal when you do the interference fit the corrosion problem goes away.
Haven't done any lab work for 30 years in this field and not up to date on new tech advances. So I'm just thinking out loud here.

 

[WinterDeere]

You ever actually try pressing a pin that soft into something before? That may be the first real challenge, especially on a 1/2mm pin...

Yes. More than 10,000 times per year!

BeCu is actually relatively stiff, but on these small sizes I would completely capture the pin in a pressing die, such that only the pressed length + .010" - .020" is exposed.

 

[WinterDeere]

I have never seen knurling completed on this small of a diameter.

I would think the softness of the Ni/Au plating would help in the knurling process. You may also run the risk of losing the corrosion protection if the knurling penetrates the plating.
The other variable is if you can get a good air seal when you do the interference fit the corrosion problem goes away.
Haven't done any lab work for 30 years in this field and not up to date on new tech advances. So I'm just thinking out loud here.

The plating is only maybe 30 micro-inches thick, so a knurling depth of even .002 the knurling depth is maybe 65x deeper than the plating thickness. The knurling is usually done before plating, at least on the .120 to .300 sizes where we more often use knurling and press fit.

Side note: I'm working the alternate / traditional solution in parallel with this one, where we use folded sheetmetal sprung receptacle contacts to interface between the pin and base part, similar to the female contacts in any small round electrical connector. There are some space and performance problems with the sprung contact, which would be more easily resolved if I dropped down a pin size (we can do down to Ø.009"). But the sprung contact already limits our thermal conductivity between the parts, which hurts our power handling a bit, and the smaller diameter pins have more power loss (even more self-heating). The pressed solution has the potential to relieve all of these issues, if it can be made to work.

 

[dstig1]

Yes. More than 10,000 times per year!

BeCu is actually relatively stiff, but on these small sizes I would completely capture the pin in a pressing die, such that only the pressed length + .010" - .020" is exposed.

I was thinking the only way to do it would be to support the pin all around in a die while you pressed it in with a hardened steel pin. Seems like you got there a long time ago... We use BeCu for some mold components for the improved heat transfer at times (Moldmax) and that stuff is quite strong but the alum you mentioned is not so much...

You temp range is so small that I cannot imagine there being issues related to differential thermal expansion in a metal.