Burnishing

[darryl]

I wonder- has anybody here burnished a turned surface to further prepare it for use? Perhaps you turn a shaft to about a thou or two large, then you burnish it with rollers to knock down the high spots and bring it to size.

Seems it would improve the wear life of a plain sleeve bearing, and it could facilitate the mounting of a press-on ball or roller bearing. It potentially could become shinier- probably depending on how smooth and precise the rollers are.

I usually do a careful dressing with a file to level the surface a bit more- and now I wonder about burnishing instead, or in addition to.

[RMinMN]

I'm not the expert but I think the order of operations is turn, grind, then burnish. I don't think you will be able to just burnish to size.

[J Harp]

I once worked at a plant which made auto, truck, and tractor parts. We used Cogsdill roll burnishers to rework cast iron parts with undersized bores. These were right off the Acme Gridley automatic lathes, no grinding, usually one pass with the burnisher would allow the plug gauge to enter.

[Mcgyver]

I do it all the time, at least in the watchmakers style....which may not be what you are expecting. Burnishing in watch or clock work is SOP, pivots are always burnished to a mirror finish (or should be). you turn to a couple of tenths, harden, temper to blue then burnish. Basically a burnisher is made by taking a flat piece of lapped hardened steel and drawing it across stones or fine emery. The burnisher would be the proportions of a long thin file and the action across the abrasive is perpendicular to the length

In theory a burnisher is supposed to work material over but not remove it, so the microscopic grooves formed by the abrasive do have some cutting action, but its so minor that using oil you get just the slightest hint of particles of grey in the oil. One might argue this is really an exceptionally fine cutting tool, but a burnisher what its called horology and it works. It should be noted that material removal is almost nil. You're probably taking some of the peaks off and also folding material over, but you're not taking say a thou off this way, at least not unless you sat for ages. otoh burnishing with a rigidly held tool using say a bearing race and a wee bit of pressure doesn't seem to work when I've tried it, not sure if it momentarily digs or causes material on the surface to flake off because its pressing in, but i've been able to get a great finish that way.

[reggie_obe]

Would that be similar to the processes of steeling a chef's knife?

[Mcgyver]

Would that be similar to the processes of steeling a chef's knife?

hmmm, I don't know much about steels but I'd say no as the steel I have has the grooves parallel to the length so it puts serrations in the knife, where as a burnisher (watchmakers) they're perpendicular. They are also extremely fine, you almost can't see them especially on a fine burnisher

[BCRider]

Would that be similar to the processes of steeling a chef's knife?

The grooves on the steels I've seen are more like a longitudinal file. Also diamond steels are becoming quite popular from what I've seen in the cutlery stores with lots of kitchen knives. And those would certainly abrade material off the edges.

[cameron]

The only knife steels I've used have been very old, and if they were files, they would have been worn out years ago.

I don't think they are meant to cut, but the fine grooves reduce the area of contact between the knife and the steel, increasing the unit pressure. I believe they work by drawing out the edge and thinning it. This is somewhat similar to the way scythe blades were sometimes sharpened, by hammering the edge out on a small, slightly domed anvil ( I've never had any success in sharpening a scythe that way!).

I haven't seen any evidence of "filings" from a knife steel, but of course if the edge is drawn out thin and worked back and forth, the wire edge can break off as a fine sliver.

Edit: We speak of "grooves" in a knife steel, but of course, they are ridges.

[RWO]

Burnishing ball bearing fits on electric motor shafts is common. Bores are sometimes burnished by forcing a hardened steel or carbide ball through. The process is called "ballizing".

RWO

[Illinoyance]

Rail car axles are burnished before the wheels are pressed on.

[fixerdave]

I burnish the edges of scrapers for woodworking, basically taking a filed-flat surface and rolling a hook on it. Just a hardened rod drawn down the edge at an angle. That's how you make a scraper.

How would a HSM go about doing it on a shaft... say before pressing on that wheel? Knurler with the wheels replaced by something without teeth? Maybe just some appropriately sized bearing assemblies?

I wonder how burnishing would actually work with a press fit? I can see it being used to get a more precise size, there's no depth of cut issue... work hardening I guess. But, if it's basically taking away the bumps, what's going to give when stuff is pressed together? If it's an interference fit, something has to give, right? Temp diff for assembly? Would the results grip better, in the way that gage blocks ring together, or worse because there's less texture to grab? Just curious.

Seems like a good way to work-harden a surface though, if I needed some wear resistance on a shaft but didn't want to heat-treat. Something to remember when that day comes. Might try it one day to see if it works.

David...

[MattiJ]

I burnish the edges of scrapers for woodworking, basically taking a filed-flat surface and rolling a hook on it. Just a hardened rod drawn down the edge at an angle. That's how you make a scraper.

How would a HSM go about doing it on a shaft... say before pressing on that wheel? Knurler with the wheels replaced by something without teeth? Maybe just some appropriately sized bearing assemblies?

I wonder how burnishing would actually work with a press fit? I can see it being used to get a more precise size, there's no depth of cut issue... work hardening I guess. But, if it's basically taking away the bumps, what's going to give when stuff is pressed together? If it's an interference fit, something has to give, right? Temp diff for assembly? Would the results grip better, in the way that gage blocks ring together, or worse because there's less texture to grab? Just curious.

.

Commercial shaft OD burnishers look like wheel with rather sharp corner radius, probably to get enough high surface pressure.




What "gives" in press fit? Part with hole stretches and shaft is squeezed thinner. (after the peaks are flattened)
If the parts are "stout" and you assemble them using temperature differences the smooth burnished parts would grip better as there is less "give" in flattening the peaks of surface and resulting clamping force is greater.
Steel doesn't compress much in volume so the force in interference fitting is much larger when the fit is trying to "squeeze" the entire shaft "out" of the hole, instead of just flattening the peaks.

[Rich Carlstedt]

This is what we used at work

https://cogsdill.com/products/burnishing-tools/

incredible finishes

For OD work, I built a home made "Super Finisher"

Rich

[Mcgyver]

For OD work, I built a home made "Super Finisher"

I bet I'm not the only one who'd like to hear more - can you tell us/show us more about it?

[rickyb]

I wonder- has anybody here burnished a turned surface to further prepare it for use? Perhaps you turn a shaft to about a thou or two large, then you burnish it with rollers to knock down the high spots and bring it to size.

Seems it would improve the wear life of a plain sleeve bearing, and it could facilitate the mounting of a press-on ball or roller bearing. It potentially could become shinier- probably depending on how smooth and precise the rollers are.

I usually do a careful dressing with a file to level the surface a bit more- and now I wonder about burnishing instead, or in addition to.

As a rule, burnishing is not for sizing. It is a process which smoothes the surface finish without affecting size. Certain geometries can lend themselves to a size change if wall thickness is thin or the surface finish is extremely rough. The forces on the rollers are high enough to flatten a surface so plastic deformation of the entire section of a thin part would not be unexpected.