The incredible (non-)shrinking holes

[Dan Cordon]

Forgive the late reply. I was away from the computer this weekend.

The only reason I brought this up was based on an experience one of the
senior design teams had this last semester. They had a plate
12"x12"x3/4" with a few 1/2" holes in it near the center they were going
to press some dowels in to. It was a very tight interference fit and the
smaller arbor press they had wouldn't quite do it. So, they heated up
the area directly around the hole with acetylene and tried again.
However, this time they couldn't even get the chamfered part of the
dowel to go in. Instead they decided to freeze the dowels and press them
in, which worked fine. Later just to experiment, they poured nitrogen
around the hole and were able to push room temperature pins in by hand.
Side note: there was a betting pool on the results from this experiment.
Even our journeyman machinist guessed wrong on this, as did nearly 90%
of everyone involved. I wasn't in on the pool, but would have likely
guessed wrong as well.

The main thing that makes this case special is the geometry and where
heat is applied. In the cases that have been mentioned, these were more
like 'rings' than plates with holes in them. The other main difference
is that in most cases the whole thing was being heated up, not just the
zone directly around the hole. If a whole piece (with a hole in it) is
heated up, the whole piece (including the hole) will in fact get larger.
For VAG content....on the old aircooled flat 4's, you generally freeze
the crank and put the cam drive gear in the oven before assembling the
two. They simply slide together. Getting them apart is a different beast
:o)

But if the edges of a piece remain at normal temperature, while just a
zone around a hole gets warmed, then the hole can shrink. Try
this.....take a piece of sheet metal that's at least 8" in each
direction. Use a propane torch to heat one spot in the center. Does the
whole plate grow? No, just the heated spot bulges out.

Another way to think of it is surface area. As the molecules heat up,
they do need more space between each other. And they make this space in
the easiest way possible. In the case of sheet metal, a small dome is
created on the flat plate -- surface area increases. It's easier to move
material out of plane than to strain all the molecules in the rest of
the cool plate. In the case of the hole in a big plate, it's easier to
move in to the hole than strain the rest of the material.

It was also mentioned that for the molecules to move apart, the
circumference would have to get larger. This is true for the molecules
around the perimeter of the hole, but as the hole gets larger, this
makes the molecules in the radial direction get *closer* together. Also,
if the circumference of the hole gets bigger, then the surface area of
the whole part will get smaller. The team doing the experiment deduced
that the edge of the hole was becoming sinusoidal, allowing all of the
molecules to increase their distance from one another, and increase
surface area.

Anyway, the only reason I mentioned it is because of the experiment the
students ran, and how this may effect us car nuts. The only application
I can see where this would be a problem is O2 sensors in a cast
manifold. However in those cases I still use heat because as Larry Leung
mentioned, it does help to break the bonds of corrosion. Hondas
manifolds in particular seem to stick to O2 sensors like nothing else.
On my civic I was not able to break it loose with a 1/2" drive. Heated
all I wanted with acetylene but it still wouldn't budge.  After letting
it cool I was able to get it out with a regular end wrench!

In nearly every case, heat is helpful for unsticking things, or making
circles larger. Especially since we usually heat the whole bugger up. I
just wanted to bring awareness to one odd case (which we rarely see on
our cars).

--
Dan Cordon
Mechanical Engineer - Engine Research Facility
University of Idaho