Thread: Pressure Fitted pins

We are all sitting in the restore Chat right now wondering about this????

Could ya explain further or post a drawing????

What excatly are you pressure fitting together????

One of my hobbies involves tight tolerance friction-fits between dissimilar materials, using composite constructions with natural materials, so this kind of topic attracts my attention. 1/16 =.0625 or thereabouts

My Mitutoyo digital micrometer suggests that 1/16 drill is .06165 inch. 5/64 mic'd .0775 and 1/16 brass rod .06265. (these are not of high tolerance manufacture- just regular stuff)

the run-out/wobble of my egg=beater hand drill is .00000000011001{not really but you have to include some tolerance based on tools used. what did i overlook, ignore or misconstrue?

At some low temp point your rod will fracture under pressure, right? and the typical scale materials would not respond well, so you can only cool within the malleable range, and ignore any cooling effect upon scale material, which could be temporary or permanent.. not sure.

...At razor scale thicknesses. Bone will not compress very much without splitting. Horn somewhat more agreeable. Wood is very agreeable- but can readily change dimension through moisture movement.
as well your fit must be tight enough to resist the flex of opening and closing.

so how much will brass rod contract?

You're asking about pressing the pins into the wood, horn or plastic. If it were metal scales and metal pins it might work but the metal would need to be steel. Everything else is too soft and unstable. The wood, horn, plastic and aluminum world grow and shrink with mild temperature changes. A press fit is only a couple of .0001s inch. Drilling would not produce a hole size close enough and the pins would need to be finished to a very close tolerance. Google metal dowel pins and press fit. Tool and Die Making 101.

You may be better off gluing the pins in place with epoxy. In the end the peened pin is best bang for the effort. They have la track record with proven results over a 100 yrs or more.

To work correctly, the interference fit should be with the scales, allowing the razor to pivot on the rivet and lock into the holding medium - the scales. Since the scales are significantly softer than the razor, wear will loosen that fit and continue to increase that looseness with continued wear on the softest medium.
Good luck, but I can't see it working well.

If my education serves, every metal has a specific temperature-deformation constant, denoted alpha. This is linear within the elastic range, which on the cold end of things is pretty far. You will not shatter something unless you drop it's temperature from 350 to -100 in a second or something. If it is an even controlled drop it will only shatter if it is very very cold, and stressed. On the hot end of things linear deformation ends when the material becomes maleable, closer to it's melting point. Kevint I think you are thinking of pressure as in lots of pressure. A pressure fit doesn't involve much deformation or stress, it relies on friction. Have you every tried to stick something the "perfect" size into a hole? It will go but its pain in the ass to get in and out. Same concept, but with a little added squeeze. Plus once the fit is formed their are cohesive/stiction forces as well (recall from lapping a hone...).

HOWEVER, JoeD and Bruce you guys has burst my bubble. The softer mediums won't hold up as well. Though I'm going to disagree on aluminum. I have done A LOT of press fits into aluminum, including a catapult swing arm (press fit steel rod into steel bearings, then the steel bearings into aluminum mounts- which was subjected to MASSIVE forces... think a 250psi pneumatic rod as the driving force on a second class lever, operating nearly normal to the pivot, which was press fit, and the pivot had a cross sectional area of .25pi(~.8) in^2) and that held up for around.... I think it was 500 shots before the lever arm (steel) began to crack and had to be replaced, at which point we created new fittings because the old ones were too hard to get apart...)

I'm wondering if the material was very dense and work hardened before hand to relieve stress if it would work...

I'm thinking I should try this on my solid scale (glass or mineral) idea...

No, I do not believe I am thinking of a lot of pressure.

but i am only considering natural scale material or maybe a pretty plastic

I don't foresee many scale materials cracking under .005" displacement... Open and close a pair of scales, see how much they flex. Flexing creates greater stress than simple compression, as it puts one edge in compression and the other in tension, and exerts a shear between them... and I would be willing to bet that in flexing they compress/stretch more than .005" displacement at outside faces. Try and check this with your calipers please.

Anyway. FWIW I am still going to try and run tests on various materials, pine, oak, walnut veneer (if veneer holds up solid will), and I can cut samples of I think ash, cedar and osage orange (my bet is on osage orange... very hard very dense. We use it for making fenceposts. Only wood I've ever seen dull a chain saw in five cuts), steel (not sure of the thickness I'll have), plexiglass/lexan, guinea fowl bone (porous flying bird), chicken bone (porous flightless bird), lamb bone (dense bone), possibly kudu horn (if we have a mucked up one lying around, and warthog tusk (if we have one that my dad will let me play with, usually they go towards knife handles and bottle openers and carvings) and possibly glass, mineral samples and stone samples if my dad has kept his masonry bits sharp.

I'm having trouble mentally gauging what .005 displacement is like. I have never been able to measure the difference between cracking/breaking a workpiece, and not. But I do know how it feels in the gut.

the greater stress of flexing is what the pin must resist, and it is that force which will work the assembly loose. There just isn't enough material there to provide the friction required. I could be exactly wrong, as flex and amount of force required to keep the scales tight on the tang can be controlled with scale thickness and wedge design.

if you take that osage and dry it to 5-7% moisture content- then drill and press fit the pin. Keep the scales clamped to the tang and allow the wood to swell to daily shaver mc will likely work better-

actually, since the flexing of the scales puts them in compression/tension along their length, this increases the shear (perpendicular) force on the pin, there by increasing normal force and increasing friction. Correct? F(s)=mu(s)*N

I was just saying that the material can flex enough without breaking. I am more concerned with JoeD and Bruce's point that the scale material is much softer than the pin and that it will actually give- not break- but stretch/deform plasticly/wear down and loosen the press fit.

Why couldn't you just press fit into the washers? Would the washers give at all? Maybe a 1/8" (.125) rod would work in a #4 microfastener (.118 ID) washer?

Curious...