Hammer Hardening Cast Brass

I need to hammer harden a cast brass rod to make bushes. I have never done this before and I could do with some advice. Should I hold it on an anvil and hammer along the length of the rod, or hold it upright in the vice and hammer the end? Thanks, David

Seems a rather strange requirement. Metals work-harden by being distorted, so your hammering needs to locally alter the shape of the crystal structure over the whole bulk of the rod, this presumably means a lot of hammering. In practice I'd suspect the result would be rather uneven, with most effect being near the outer skin. Are you sure that this is needed? Bushes don't usually need to be particularly hard.

Why don't you use phosphor bronze for a bearing which will outlast brass and can be cast if nessesary.

David

Following ancient instructions maybe? Not sure hardening the whole rod was how it was done in the good old days, I think you cut a blank rather bigger than the bush and bash that before machining. One of my old books mentions the method - I'll see if I can find it.

These days more usual to use Phosphor Bronze, Oilite or a roller bearing I guess.

Dave

Well it's certainly possible. Just youtube "work hardening bronze", bladesmiths do it all the time. And most brasses should work harden as well.

Here's a fun demonstration from Finland: https://www.youtube.com/watch?v=AqNHwc8MGTM

For a thin walled bushing it should be possible to just work the top layer, but you'll have to do your own tests to see how deep you get. The real question is how to get predictable results. One approach would be to over-harden it somewhat and then temper it back somewhat.

I was always under the impression that cast brass was preferred to CZ121 bar for bushes as the structure had the ability to hold more lubricant. I have been using my casting risers for sometime now for bushes and haven't hade any problems (perhaps I shouldn't have said that) so far.

Cast brass, unless very rapidly chilled, will have grown large crystal grains, which is why it is softer. This was typically preferred for eg clock plates presumably because it is soft for the holes to work with hard steel pivots.
Drawn or rolled brass will tend to have had the grains broken down by the stresses so becomes harder. However to remove such stresses it would normally be stress relieved by heating to avoid warping etc when machined and this works by allowing new large grains to grow - back to soft. Mostly preferred condition.
So to make a bit of this regular brass harder to act as a spring it can be hammered to break down the grains. Still pretty weak springs.
Not heard of it being done to rod as it would kind of defeat the object of using cast rather than drawn material.

BTW I can't think of a mechanism for it holding more lubricant unless it is such a crappy product it is porous.

If you are making bushings, the work hardening needs to be in the hole where the pivot goes to be of any use. So I think you will struggle to do that by hammering, even of a blank.

The normal way of work hardening the bearing surfaces of a clock bush is to finish off with a smoothing broach which burnishes the inner bearing surface.

Surely the burnisher that is used on clock bushes is to provide a friction free surface for its mating shaft which has also been burnished to a fine finish?

Found my book with a sections on Bearing Brasses but it's all about fitting, not making them. Amusing to discover things dismissed as "old-fashioned" in a 90 year old book, but it would have been nice if it had answered the question!

Hammering to harden cast-brass for use in a bearing sounds likely to me, but I think it's 19th Century technology. Brass was one of the first bearing metals, but it's not ideal. Fairly quickly given a white-metal coat.

However, before white-metal, hammering Brass would improve the hardness, and the metal would last longer in a plain bearing. I imagine the hammering was done to a lump before the bearing was machined, much as spanner blanks are drop-forged today.

Judging by what the fitters had to do with them, brass bearings weren't precision made. Lots of filing off high spots found with red-lead, before super-careful scraping. Apparently common to scrape slightly too much off, causing the bearing to fail prematurely. White-metal is described as a huge improvement, not least because it can be melted off and done again.

Also described disparagingly are early octagonal brass bearing holders. They made life difficult because the brass had to be carefully fitted into them as well as to the shaft. Time consuming and easy to get wrong!

David hasn't explained how big the bush is, or what it's for. Fitting as described above is of large bearings in textile mills etc. I use small brass bushes (up to 15mm) quite a lot, and simply make them from brass rod, which is already hardened. Not heavily loaded, but do David's need to be.

Dave