Nice post, Dave. Informative and insightful!

Bob Swinney
"Dave Baker" Dave wrote in message
om...

megoodsen wrote in message
oups.com...
Hi,

I'm drawing up some small parts that need to be cold press fit
together.

They are made from brass, and the basic diameter of the two faces being
pressed together is 7mm (0.275"), mating face length (depth of mate) is
5mm (0.2").
I need to press the parts together when both cold, and I need them to
be securely mated.
Afterwards they are to be either gold or silver plated.
There isn't a great deal of force being applied to them afterwards, but
what force there will be will be in extraction direction. I would guess
maximum extraction force to be 3 or 4 pounds.

Can someone advise me the amount the rod (actually thick walled
tube-all relatively speaking of course) needs to be oversize to be the
right fit into the hole.

I need electrical contact too, so I won't want to be using any
compounds.

Is there a reference website that I could extract this information from
in future, so save this post occuring all the time. (I already found
loads of similar posts, but none like my application).

thanks

A lot depends on how sturdy the parts are. If the bar is solid and the
hole
in a large piece of metal then the fit won't have to be as great because
the
parts won't compress/expand so easily. Thin tube in another thin tube
would
be an entirely different problem. You say the male part is thick walled
tube
but not how thick and nothing about the female part. This sort of problem
comes up all the time in engines. Valve guides and valve seat inserts into
cylinder heads, bronze bushes for the little end of conrods for example so
I
have a reasonable experience of similar things.

Gut feel on your application without the full information to hand is that
about 0.5 thou would be sufficient. Extraction force isn't much different
to
the force required to fit the parts and it takes a fair old pressure to
get
even small parts like that together at a 0.5 thou fit. They aren't going
to
come apart again with three or four pounds force.

I'd say the closest similar thing, albeit scaled up somewhat, in an engine
is a steel valve seat insert going into an aluminium cylinder head.
Anywhere
from 30mm to 45mm diameter on the engines I work with, maybe 4mm to 5mm
wall
thickness and about 6mm to 8mm fitted depth. The fit needs to be large to
cope with thermal expansion of the head compared to the insert though when
the engine is running. 3 to 4.5 thou fit is the range I use depending on
insert diameter and that takes a good old hammering to get the insert in.
Usually 20 sharp taps with a claw hammer and I can gauge the fit I've got
to
very fine tolerances by just how many taps it takes. Much better than a
press which gives you no feel or feedback. Anything that goes in too
easily
would get redone although it's never happened yet. If engines didn't get
hot
then 1.5 to 2 thou would be plenty and that would scale back down to about
0.5 thou on your smaller parts.

What you have to remember though is that accuracy of machining and
measurement (and even surface finish) make a huge difference to the actual
fit you end up with compared to your target fit on such small parts. Half
a
thou out when you're aiming for a 20 thou interference fit on a 10"
flywheel
ring gear is nothing. Half a thou out when you're aiming for half a thou
means anywhere from zero to double what you really wanted. Reamers in that
size range tend to cut anywhere from nominal to three or four tenths large
so you probably need to make a couple of go-nogo gauges in 0.0002"
increments from scrap bar to find out what size hole you are actually
getting. Then turn/polish the tube to suit. Try a test piece at 0.5 thou
fit
and see if it holds in well and if not go up a tad on the tube o/d.
--
Dave Baker