toocool006 wrote: ↑
Wed Jan 23, 2019 11:13 am
This is really interesting. Is it possible that lapping (if done accurately) would bring some microscopic increase to the flatness of the surfaces in question?
I'm finding it hard to follow the argument that a marred surface has the same coefficient of friction - my sens is that there would be some amount of mico-binding vs a polished surface. Why isn't that so?
If you can make two surfaces truly flat to a molecular level, they actually stick together. Gauge blocks machinists use are actually pretty hard to get apart if you "ring" them together.
No doubt I'm forgetting something about the way friction and surface finish work out, but if you read the textbooks and other reference material they all have the same note.
*edit* I misspoke. I'm remembering something I read about perforated washers after investigating the advantages of CRK's design in the Sebenza
It's not accurate to say that "surface finish doesn't matter." Obviously, a badly-machined surface is going to cause more friction. But why?
Evenness is important. You don't want to machine in a way that will leave ridges that can interlock, but if nothing is binding then you're just left with the friction properties of the material.
So long as the peak, load-bearing surfaces are even and spread the same load over the same area, the amount of load per area over the surface of the washer is the same no matter what you try to do. If you cut holes in your washer, or add ribs, or do anything to reduce the contact surface, you're increasing the amount of force borne by those contact points and your design feature is cancelled.
Dirt and uneven ridges or bumps will obviously change the dynamics of the washer when it's moving, but once you get the washer flat it's hard to increase its efficiency. Once the surface has an even finish and there are no unusually high peaks on either side, you're just getting more and more microscopic peaks to bear the weight, which means more and more surface area rubbing.
Contrast polishing a slightly-uneven washer (with one point that's thicker than the rest of the washer) with compound to sanding that same slightly-uneven washer with 400-grit paper to get an even thickness. Making the surface area even means you're actually changing the contact area that rubs when you tighten the pivot. Polishing the washer can only very slightly improve the characteristics of the contact patch that's already rubbing.
In my actual experience with knives, the washers are rarely cupped or un-polished. They are, fairly often, bent enough to make two "sides" (e.g. 3 o'clock and 9 o'clock if you think of the washer as a clock face) bear more load on one side (and of course the opposite pattern on the other side.) The surface finish is almost always rougher on the knife blade, but the fact that the finishing lines are parallel on the knife (or, better yet, stonewashed) means it doesn't cause many problems. I feel that the knife is smoother if I can flatten the washer, but I know I'm almost always getting the same result with a good cleaning.
Also note that if you use any kind of lubrication, the effect of surface finish is basically nil. The residue left when your lube dries out is far more important than surface finish in that case.