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Cold hobbing in tool steel


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#1 Tom Maringer

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Posted 29 July 2012 - 12:47 PM

I just wanted to open a thread devoted to the subject of cold hobbing in tool steel, specifically as a method for forming the working faces of punches and dies. As I had a blacksmithing background I will admit to being somewhat shocked and aghast at the idea of doing significant amounts of cold work on tool steel.

To be clear, the word hobbing has two distinct usages in machine work. One is "gear hobbing" which is essentially a milling process with rotary control of the workpiece. Die hobbing is the pressing of hardened tool steel into annealed tool steel to make an impression that will become a die-face after that tool has been hardened.

So did you ever wonder how the US mint manages to make all those dimes look EXACTLY the same? They're all press-hobbed clones of a master die! It turns out that annealed tool steel will press with just the same crisp detail as soft metals... and if you are just a bit judicious it will not crack the steel! So for instance... let's say you'd like to make a die with a person's face on it. As most people know, it's quite difficult to carve a face IN REVERSE into a die, and have the impression come out looking hubman. So instead you carve the hub! You carve a positive in steel, harden it, then press it into the die steel to form the reverse impression. Now you have a die AND a master hub. That hub goes on the shelf and becomes part of your collection, and can possibly be used again whenever you can use that same image.

Here's an example of the process... I wanted to do a coin with a raven on it. So I got out the old birding field guide and found a decent raven picture, then carved it on the end of a piece of 3/4" diameter 0-1 rod. That becomes the hub. After heat treating I pressed it (hobbed it) into another piece of steel that was to become the die. After that pressing the metal is distorted, so it goes on the lathe to be trued up. Then I added a rock for the raven to sit on, a rim, and inscriptions. Finally the silver coin is struck from the die.

Attached Thumbnails

  • RavenHub01.jpg
  • RavenHub02.jpg
  • minidalerDie02.jpg
  • NewDaleRaven01.jpg


#2 saign charlestein

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Posted 30 July 2012 - 02:06 AM

that's pretty cool, Thanks for sharing the process. What size press do yo need to do this?

#3 Tom Maringer

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Posted 30 July 2012 - 07:49 AM

that's pretty cool, Thanks for sharing the process. What size press do yo need to do this?


A little one like that (that's about penny size) can easily be done with around 30-40 tons, like the press I'm leaning on in the photo. For a full silver-dollar size (1.5") you'll need right around 200 tons! That's the bear of it. Cold annealed tool steel will move... but it's stiff and moves only when you push hard enough... and you can only go so far without cracking. If you must go further then at least one stress relieving step must be added... 1150f for 1 hour.

Edited by Tom Maringer, 30 July 2012 - 07:50 AM.


#4 Rich Waugh

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Posted 30 July 2012 - 05:48 PM

That was ice tutorial on the hobbing process, Tom. Thanks! I do the same process for making touchmarks for my own work. Fortunately, mu touchmarks a smaller than a penny so I can do the hobbing with my #5 fly press - just barely. On one touchmark stamp that was slightly larger I did have to resort to hot-sinking the hob into the die, but it still worked fine for my purposes. For coining the distortion would be unacceptable, of course. I like doing my touchmarks this way because I can produce them as needed without worrying that they'll change over successive iterations. Also, I have the option of doing one further sinking from the die to a new positive, to yield a reverse stamp.

Thanks again for the tutorial - you do excellent wor!
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#5 knots43

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Posted 31 July 2012 - 04:02 AM

Tom this is a bit off topic except for the fact that I would like to give this process a try with my old Waterburry Farrell screw press, and have no idea of how to determine it's tonage rating. Anyway sorry for the detour. However if you have access to a rating method, that would be extremely interesting and helpful. 2 3/4' diameter screw, 2 start screw with 1 1/2 " lead. Any hint will be appreciated.

#6 Tom Maringer

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Posted 31 July 2012 - 10:34 AM

It's very subjective! The only way I have been able to "calibrate" my sense of tonnage is because I have the CMC hydraulic coining press where you set the desired tonnage with a dial pointer. I then compare the result against what a screw press will do. For instance... my Waterbury Farrell 150 ton knuckle press balks at anything over about 100 tons (comparative). I think it's got worn bearing sleeves and some slap-back in the dog clutch.

The two-lead screw and 1.5" pitch on your press pretty much tells you that it was designed for embossing metal... probably things like small signs and pressed metal interior ceiling trim panels. It would be perfect for that. The power tends to be delivered efficiently through a quarter or half inch of depth.

A press for coining is going to be three or four leads, and a much steeper pitch. The power dumps very quickly over a very short distance... as you would need for a coin. When you hit bottom the press should bounce back up, not lock down.

Edited by Tom Maringer, 31 July 2012 - 10:36 AM.


#7 crquack

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Posted 31 July 2012 - 12:08 PM

Tom this is a bit off topic except for the fact that I would like to give this process a try with my old Waterburry Farrell screw press, and have no idea of how to determine it's tonage rating. Anyway sorry for the detour. However if you have access to a rating method, that would be extremely interesting and helpful. 2 3/4' diameter screw, 2 start screw with 1 1/2 " lead. Any hint will be appreciated.


http://chestofbooks....and-Stamps.html

This may be of some help but I suspect there are limitations: The formula ignores friction.

#8 knots43

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Posted 31 July 2012 - 10:56 PM

http://chestofbooks....and-Stamps.html

This may be of some help but I suspect there are limitations: The formula ignores friction.



Thanks. This should at least give a ballpark estimate.




#9 Duckworth

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Posted 08 October 2012 - 11:00 PM

This is an interesting use of the fly press, or in your case FLY PRESS.. The touchmark thing I like too.

#10 Tom Maringer

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Posted 09 October 2012 - 08:57 AM

http://chestofbooks....and-Stamps.html
This may be of some help but I suspect there are limitations: The formula ignores friction.

Friction is the least of the issues, Much more importantly it ignores momentum. This formula is for a single-lead screw press of the bookbinder's type... or a vise. But fly-presses use steeply multi-lead screws and depend HUGELY on the momentum of the flywheel (or weighted arm) for applied force. In my experience all tonnage ratings on mechanical presses are simply estimates. Only a hydraulic press can actually read an applied tonnage. The only way I have of estimating comparisons between presses is to do a strike on one press and then move the dies to the hydraulic and try different settings until it's about the same.

Using this method my (supposedly) 150 ton Waterbury Farrel knuckle press does not even come close... actual striking force is more like 100 tons. If I try to push it further I start smoking six-gang drive belts. Same with the screw presses... the 40 ton is really a 30 etc.

General rule... use a bigger press. If you try to run a press close to its rating, you'll probably break it within two years. Use it at 60% to 75% capacity and it will run forever.

#11 Randy McDaniel

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Posted 20 December 2012 - 08:52 PM

Great information. I'm doing basicly the same thing but doing it hot and using my 60 ton hydraulic forging press. Biggest problem I have is making the positives, only because I'm not used to working that small. Any tips for working that small? What are you using for your carving?


Here's the pirate coins I make:


PirateCoin7Sm.jpg


And the tooling to make the die:


PirateToolingSm.jpg


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