One can buy a set of reamers with exact diameters (say 0.25") but also sets which are 0.001" over-sized and 0.001" under-sized. I can see the reason for the 0.001" over - this will give a sliding fit. The under-sized reamer is supposed to be for a press fit but isn't a hole of 0.25" a press fit for an identical part anyway? Does this depend on material? To get the press fit with the undersized hole does one need to do other manoeuvres e.g. heat up the parts etc.?
A press fit actually is an interference fit. You can find the interference amount for a given diameter in handbooks. If you heat the female part and/or chill the male part, you have either a shrink fit, or temporary impotence and a frustrated partner.
A .250 reamed hole will accept a .250" hardened and ground dowel pin using a small arbor press or just some serious hand pressure. Apply any reasonable torque to the resulting assembly and it will spin. The .249" hole in something like aluminum or stronger will be press fit, can take as much torque as the shaft will handle. I need to temper the last statement in that the strength of the materials shows up: something with a low yield point will never tighten up much.
The assembly can be done with heat although a .250 rod is a lousy example: it is so small in relation to the surface area that it loses heat VERY rapidly. A 2.50" hole and shaft is a different story.
For that I ream most of the hole to the size of the pin and leave a short press-fit section at the far end. I follow the book recommendations if I have the right reamers, but if not I don't let the full-sized drill break completely through and use the remaining tapered rim for the press fit. Another way to adjust the press fit of an undersized drilled hole is to tap it.
When you don't need a hardened pin it's a lot easier to ream the hole with the reamer you have and turn the pin to fit. You can lightly knurl the pin to make it larger, or roll a coarse file over it, use Loctite, or as a last resort hammer it.
I leave the press-in area relatively rough so compressed ridges have a place to flow. I cut or file a very slight taper on the end of the pin and press it a little ways in with the intended pressure, put it back in the lathe and set the bit to shave a chip (or dust) just past the end of the press marks. If it won't recenter, file it and use the feel of a micrometer as a gage.
You can make a drill bit enlarge a slightly undersize hole to size nearly as well as a reamer by snugging up the quill lock so it doesn't jump and feeding slowly. Measure the drill bit's diameter right at the point, the shank may be smaller. With care and a good chuck I can drill within a thousandth of the measurement. Any runout makes the hole larger.
Stuff like that is for non-critical home shop jobs on a limited budget. When you build something and use it yourself, you can get away with a lot more. You know what's weak, keep an eye on it, and can fix it. It becomes a grey area on a free repairs for friends. I'm likely to admit I don't have the right tools rather than do something questionable that could break and hurt them.
They DO sell dowel pins with the same tolerances, .001 under and .001 over, so for your example hole you could ream the mating hole exactly .
250" and use a .249" dowel pin, if you ever wanted to take the assembly apart again. Clamp together, drill and ream the mating halves .249", separate and ream one half .250". It just depends on the design and what you want to do with it. Mostly for jig and fixture folks.
On Mon, 09 Mar 2009 21:48:54 -0500, RoyJ wrote: (top posting obsessively fixed)
What sort of diameters do you use?
I know that shrink fits are routinely used on some wrist pins for automobile engines, with a hot rod, a room-temperature pin and piston, and a light (or no) press fit.
I've also been told that this is a way to sleeve out a flathead V-8 engine -- you go ahead and bore out the cylinders until you're into the water jackets or have at least seriously thinned the walls, then you drop chilled cast iron sleeves into the heated block.
The jig and fixture folks have milling machines accurate enough to make multiple dowel pins all align. They rarely let me use them.
Yuo can still locate with dowel pins if you have simpler machinery but you need to make adjustments. As Stan noted, drill and ream them one at a time without moving the table. This can be difficult on a drill press or mill-drill with limited quill travel unless you cut the reamers short. A lathe with a faceplate is very good for precisely locating and boring holes although the process is slow.
If one dowel pin locates a hole and a second locates a slot, then the distance between the pins isn't critical. Kinematically this is as good as two dowel pins in holes, maybe better if you have to bang the pieces together, but it will wear faster.
If some of the pins end up slanted, drill through so you can punch them out to unlock and separate the parts.
Boy! Lots of good input so far. Just a note of caution about reaming softer materials for a press fit: Over time, some softer materials must flow away from the junction, I think. We do a fair amount of pressing steel pins into alumimum bubs that really take a beating all their life. Take a look at tensile strength tables for various aluminum alloys and you will see vast differences. I think one would need to proceed pretty cautiously when designing the proper fits for the proper applications. This is why the over and under AND special "decimal" reamers are available.
First off -- the dowel pin may well be a few tenths undersized for free sliding in a precise hole.
Second -- depending on how firm a grip you need, you may want more interference. A precise sized dowel pin in a precise sized reamed hole may well be just a thumb strength press fit. Not counting what may happen with temperature changes if the hole and the pin are different materials. What fits at one temperature may be a sliding fit at another, and something requiring a hydraulic press to install at yet another. :-)
Well, the terms have been established for over 100 years and they're pretty universal in manufacturing. An interference fit is a press fit in which the male part is dimensionally larger (by a very small amount) than the female part. It's done at room temperature. A part that's fitted for interference by heating of the female and/or chilling of the male is called a shrink fit. A slip fit is one in which the male is dimensionally smaller than the female part, also by a very small amount. These don't often slip in place, actually. They usually require a little pressing.