interference fit on small brass parts

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
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megoodsen writes:

I think you mean a force fit, as given in Table 9 of _Machinery's Handbook_, "ANSI Standard Force and Shrink Fits" in the chapter on dimensioning. Not an "interference fit", which is for locating things, not holding them, at least in ANSI and ISO terminology.
For sizes 0.24 to 0.40 inches and Class FN 1, the hole is oversize by 0 to 0.4 thou, the shaft by 0.5 to 0.74 thou, yielding an inteference of 0.1 to 0.75 thou. For Class FN 5 the values are hole 0 to +0.9, shaft +1.4 to +2.0, inteference is then 0.5 to 2.0.
This two-page table gives the values for every possible class and size.
Have you considered knurling the shaft instead of force fitting? Doesn't require holding such difficult tolerances on the machining.
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megoodsen wrote:

If the parts really are that small, and really do have to stay together, and they really do have to be electrically connected, what's wrong with solder? (Uprate to silver solder or brazing if necessary, but I doubt it will be.)
If these are the mating surfaces of much larger parts then soldering can get to be awkward sometimes (although not necessarily, jewelers know lots of tricks!)
Tim.
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Will the lead in soft solder interfer with the plating process?
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Chuck Sherwood wrote:

It might. But plumbers and jewelers do a lot of similar stuff too, day in and day out, so I'm surprised that he thinks that what he's doing must be new or different. My inclination is to do what all the other guys do, and when someone insists that what they're doing is necessarily different or superior I generally leave it up to them to tell me how rather than trying to outmaneuver them!
Tim.
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I believe he's asking "what do the other guys do." Just because he is assuming it needs to be press fit doesn't mean he knows all other options are out. He might not know of other options. Where did you get the feeling that he thinks he's doing something new or different? Sounds like you're the one doing the insisting this time.
Eide
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wrote:

Consistent press fit will require pretty close tolerances on the parts. Can you peen or stake the part in place? That's how contacts are often made.
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Why must it be cold press fit? A few degrees chill and a few degrees hot will make for an easy assembly process and will lock in the parts forever if you hold to Machinery's formulas mentioned above.
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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
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Nice post, Dave. Informative and insightful!
Bob Swinney

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Nice to see you here again Dave.
Surface finsish is a big part of this of course. If one has turning marks a thousanth deep on a part, 50 percent duty cycle on the surface, that effective increases the clearance by about a half thou for the press-up fit.
Also the kind of lube used in assembly plays a large role in how much force is required. I've seen a very interesting paper that discusses the press fit forces to install, remove, and then install again, railroad wheels on axles. The forces (for the same interference) are highly dependent on the kind of lube used.
I seem to recall a certain german motorcyle manufacturer specifying venison tallow for their crank press-up assemblies.
Jim
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