Does polishing one or both surfaces reduce metal to metal friction?


I have some Flitz metal polish that I am using to polish the internals of a trigger and there seems to be two ideas of thought on which has less friction, so I thought I'd post to the metalworking group and see if anyone here has an opinion about it.

In a metal to metal contact where one piece of metal rubs against another piece of metal, is it smoother to polish both metal surfaces, or just one metal surface. Some oil or grease would be used as a lubricant.

Some people think that both surfaces should be polished very smooth.

Other people think that one surface polished very smooth and the other surface left alone is better because they think that two smooth surfaces have a tendency to stick and catch on each other much more than one smooth, one rough...


SA Dev

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SA Development
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Ok now, I don't know much about metal, but my common sense would say that if you have one rough sde and 1 smooth side, the rough side would just scratch it up and undo your polishing. Besides, if you polsh both sides and use grease, the parts don't come into direct contact with each other anyway. Sinc, Gothfox

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Here is what little I know, from being an amatuer machinist and a control system's engineer who has to push mechanical engineers around:

There are three major types of friction, and a gazillion different materials, so both camps are probably right. You should choose a solution that works best for your particular application.

Running friction is easy. Coulombic (dry) friction is the amount of frictional force your parts will experience once they're moving. It's a constant, and in the absense of liquid lubrication it's the only running friction you'll see. Viscous drag is a frictional force that comes from your lubricant, it goes up with speed and lubricant viscosity.

Stiction is the amount of force required to get the parts moving in the first place, and can range from nonexistant to many times greater than coulombic friction. Material scientists are still studying it, and they still don't understand it very well. It's difficult to model and dang hard to compensate for in a control system. It appears to be a function of just about everything, and seems to involve real (albeit weak) molecular bonds that develop between the two pieces.

If you want to reduce running friction (coulombic friction and viscous drag) then you want two smooth pieces made out of dissimilar metals. You also want to use some fairly low-viscocity lubricant consistent with the use (in a gun that probably means fairly thick grease, by the way).

If you want to reduce stiction then two smooth pieces in intimate contact may be the worst thing you could do -- this may be why some folk want one piece rough, to reduce the amount of surface area in contact, and hence the amount of adhesion. If I were to use the "one rough piece" approach I'd do something like use the piece as cast (or as knurled) with a single light pass to make the tops of all the little knobbies flat with respect to each other.

You can do a lot with the right lubricant - lubricants with molybdenum, graphite or (AFAIK) sulfer are good things to use to reduce stiction, because they prevent the molecular bonding that it rises from. You can also control stiction to a great extent by careful choice of materials

-- but be aware that many "low friction" materials only reduce running friction, and it's often not the _amount_ of stiction that really kills you but the _ratio_ between the stiction and the running friction. In automatic control systems (and perhaps human-in-the-loop ones as well) it's often best to use materials and lubricants that allow a higher running friction if it means reducing stiction.

If you wanted to make the ultimate friction-free trigger mechanism you'd eliminate all sliding contact, and either run hardened steel shafts in bronze or if you're really nuts get a whole bunch of miniature ball bearings, one for each pivot point.

Stiction can be a real bitch in a control system, whether its automatic with some poor little microprocessor trying to make things work, or if there's some human in the loop, squeezing a trigger with no tactile feedback until the thing lets loose with a great big "BANG"!

Reply to
Tim Wescott

Running hardened steel shafts in bronze (or even ball bearings, for that matter) do not eliminate sliding contact, they just reduce it.

But I don't think the OP was concerned about the pivot points in his trigger group so much as he was concerned about the lineal sliding of the sear.

Actually, in the sear, you want some static friction as that is part of the "break" of the trigger. But once you overcome that, you want your dynamic friction to be minimal.

Yes, both sliding surfaces should be polished. As for lubrication, do not use grease or oil on the sear. Use a dry lubricant (like Kano Laboratories' Molyfilm) that will not hold dirt or other contaminants.

- Michael

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I agree. Though I tend to simply use BreakFree or TriFlow on the sears. The only weapons I worry about having too light a trigger pull are the small bore match guns and the iron monsters.

The street guns all have intentionally firmish trigger pulls, which is a result of springs, proper sear geometry and so forth. They are glass smooth, but firm.


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Polish both but don't stone. Changing the geometry on your sear should not be attempted no matter how tempting. Just a little polish and a dry lube and keep it clean. For best results take it to a gunsmith. It depends on what the gun is for and how you like your sears. Every gun I ever had I examined the mechanics and at least removed all burrs and polished.

Reply to
Tom Gardner

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