Hi again Kevin:
I have only ever hobbed one part myself in soft copper, so I am pretty inexperienced but I think there are some general principles you can use when you imagine what might or might not work.
First, is that the part volume does not change; all you do is displace the material around and your part is the same mass when it's finished as it was when you started.
So when you confine your part you need to allow enough room for the material to flow wherever it needs to in order to create the new shape but still confine it enough so the material has to flow where it might not want to so easily.
When all the available space in the confinement box is taken up, the pressure will escalate rapidly until something fails, and you can use this pressure curve to tell you when the metal is fully displaced.
Confining your part can be done in several ways:
- You could put it in a box
- You could make your blank big enough that the material thickness creates its own box.
You can see this at work when you do a simple letter or number stamp, the material flows locally up around the sides of the punch but the block retains its shape if it's big enough, (except for the raised metal immediately around the punch).
The second thing is flow, and you need to encourage it wherever possible.
I can think of a few ways; you may think of others too.
First is the quality and lay of the polish.
Finer is better.
Along flow lines is better than across them.
Next, radii are better than sharp corners.
Dissimilar materials are better than similar.
Tall vertical walls are bad
Displacing material into a cavity is harder than displacing it around a punch.
Lubricants are better than no lubricants but there are caveats like no combustible ones if you're displacing into a cavity (You will get "dieseling" in the cavity if your lubricant is combustible).
Shapes that trap material are bad, so a crosshatched surface is not as good as ones with parallel lines that material can flow along.
Small projected area requires lower pressure than large.
Ductile materials are easier than stiffer ones
The third principle is that you have to be able to get the parts apart again when you're done:
- Undercuts are bad
- Vertical walls are bad
- No means to pry or jack the parts apart is bad.
I believe all of this and everything else is empirical; you get better at it with experience, with observation and with imagination.
You appear to have all those qualities well in hand, so I'd keep doing what you're doing, try new things and imagine what's going to happen when you do.
To help you visualize, get yourself a big gob of Silly Putty and put it into a strong clear polycarbonate box so you can try out some things you might be unsure of and just watch what happens.
Get some dye too and make strands of different colours so you can watch the flow, or get some plasticene to play with when you're unsure if your design will work.
You'll be the GO-TO guy before you know it...in fact you probably already are!!
Cheers
Marcus
Implant Mechanix • Design & Innovation > HOME
www.vancouverwireedm.com
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