Sinking

[Gene_Olson]

sinking metals like copper and silver involves stretching the metal out.
this is done various ways,
pounding over a cavity (in the air)
pounding into pitch
pounding into rubber or wood

pounding with hard (steel) or soft hammers (lead, plastic)

In this process the metal is stretched out but it seldom stretches uniformly. This is especially pronounced as the metal gets thinner and you work with soft hammers.

Some small spots are stronger than others and tend to act as nucleuses for local crystal growth/stability, the areas between them start to stretch more. Pretty soon the metal starts to look like orange peel, one step away from a mud flat with all those cracks around the seed/strong spots .

Soft hammers are great because they let you develop large relatively smooth areas with a minimum of work marks but they allow the crystal growth from the nuclear areas to go unchecked. The solution to this is that every once in a while, when you see the surface start to orange peel you need to planish the area smooth. This means smooth hammers and stakes. The strong portions will all be thick so when you gently pound the surface smooth you crack their crystal structure and make the entire surface more uniform again.

Yeah, I gnu that.
But thanks to Jack daSilva for reminding me.

"Planish when you see orange peel," he said.


All the rest of the above rambling
is mine, my mind as it raced thru, "Why?"

And you may note that it agrees with Wray's description of steel getting softer and more workable if you ewheel/hammer/planish it smooth.


Gene

[raferguson]

Sounds like good practice to me, hammer for a while, then wheel to see whether you have the shape you want, then hammer for a while.

Would using a hard hammer on rubber be less likely to form those thin spots or orange peel? I have my helve set up that way, but I know that many people use sandbags, and some people like the UHMW hammers.

Richard

[Gene_Olson]

Richard,

I think the key is that the hard steel hammer/wheel against hard anvil is crushing the strong spots. Any time you stretch over air or sand or soft backing you are allowing the structure to stretch as it wishes at least on one side and when you use a soft hammer, both sides.

This unequal stretching of the surface means that if you don't planish before you anneal, you will have a mesh of cracks waiting to happen all wrapped around thicker spots, now annealled and soft but thicker and stronger than the already stretched areas around them. In order to sink/stretch it further you will need to plannish it back to a uniform state. Of course the planishing itself can be part of the stretching/sinking process too.

G.

[ralph]

And you may note that it agrees with Wray's description of steel getting softer and more workable if you ewheel/hammer/planish it smooth.

Wray's theory works for a couple of reasons.

First, it gets rid of the walnuts formed by hammer blows. (your orange peel must have smaller bumps than my walnuts :lol: ) Each of those represents a strengthening formation in the steel much as short bead roll adds strength to a steel sheet.

Second, I think the rolling process also imparts some of the stress relief we used to get when we flex rolled steel for stamping in presses. Waaaay back when I was a kid (mid 60's) when a lift of steel was performing poorly we would pull it and send it to the flex rolls. The blank would roll through a series of 'S' curves and straightening rolls. 9 times out of 10 the steel would then run well. The process was also used on 'old' steel (30 days out of the mill was considered old) to 'loosen it up. We installed flex rolls on a couple of our blanking presses in line after the straighteners and looping pits.
They were abandoned shortly after installation because of dirt issues. :roll:

I thought I'd look up flex rolling to see if it was still in use. A google search turned up the Ohio State University Sheet Metal Forming and Stamping Glossary which has this definition for flex rolling:
Passing sheets through a flex roll unit to minimize yield-point elongation in order to reduce the tendency for stretcher strains to appear during forming.

Yield point elongation may very well be the your orange peel. We called them "Luder lines" they were the points in a panel were the stretch showed in the surface. You could count on splits in that area if steps weren't taken (improved die condition or "softer" steel) to improve them.

BTW the link to the OSU glossary is:
http://nsmwww.eng.ohio-state.edu/Stamping_Glossary/index.html

Fascinating reading. Lots of circular references too.:lol:
Remember, Stamping and Metal Forming have little in common except for the use of steel. Stamping gets the shape all in one hit. We get shape with thousands of hits.