FieryFurnace

punching/slitting holes

Recommended Posts

Someone aske me to expain why, when punching or slitting a hammer eye, you end up with a "plug" that comes out of the hole.
I could have answered in the original thread, but I figured by starting a separate thread, I might add to the ease of others finding this info in later years.

What is slitting and what is punching?

Slitting is using a chisel to cut a hole in the middle of a metal bar (whether a tiny hammer blank or a 20 foot section of rail cap.) Usually these slits are used in conjuction with a drift used to open and then shape the hole. Hence the common technique, "slit and drift."

Punching can refer to several types of holes used for different purposes. In this case I am referring to punching a rectangular slot for the purpose of being able to open and drift it into a round, square, or rectangular hole, just like in the slit and drift technique. In any form, square, round, or rectangular, punching remains the same.


The type of punch I use for the hammer eye and for regular bar stock, is the punch and technique taught by Mr. Brian Brazeal. For thicker stock, a v-shaped punch is used, as this is easier to drive into the material. In thinner material, you can't use this shape, so a straight-cut slot-punch is used.

Here is the v-shaped slot-punch! Side view!
DSC08724.jpg

Straight on view!
DSC08726.jpg

Here is the plug that this tool punches out. It is shaped just like the end of the punch!
DSC08730.jpg
DSC08731.jpg

A flat cut punch looks like this on the wide side.
DSC08727.jpg

On the thin side notice that there is a slight and very blunt bevel ground into the edge. This edge helps it move through the material easier than just a flat abrupt punch.
DSC08729.jpg

Like the V-punch, the flat punch makes a slug that is the same shape as the working edge!


Alright, so let's look at some cross section drawings to see what exactly is going on when you use a punch and when you use a chisel.

Here is what a slot punch does (whether a V-shaped or a flat punch, it works the same,) as it goes through the metal. This is the first side as you drive the punch in.
DSC08732.jpg

You'll go a good 75% of the way or more. Flip the stock, line it up, and go in the other side.
DSC08733.jpg


If we take our stock and look at what the punch is doing on the thin side, this is what it looks like. The dotted lines shows where the material inside the hole, shears and falls out, giving you the plug. This leaves a clean, smooth hole from both sides, even if the slot-punch is slightly off alignment.
DSC08734.jpg

The reason you get a plug is, as you drive the punch, most of the material gets pushed away to the side. A small amount of material gets pushed in front of the blade. The steep angle grind on the punch, shears the metal instead of cutting it out.


What is the advantage of this? Let's look at some cross section drawings of the same hole, slit with a chisel instead of being punched.
My drawings show a round blade chisel but it works the same with a flat one as well.

First pass.
DSC08735.jpg

Second pass.
DSC08736.jpg

Now notice where we are now. There isn't any material to support the material being cut. From this point on, the material in the path of the chisel blade is not going to cut, it's going to stretch. Without something to support the material being cut, it will not cut.


Here is an end view of how this looks!
DSC08737.jpg

As the material stretches and your blade makes it through, you end up with this. I'm sure everyone has ended up with rags on the metal due to chisel cutting.
DSC08738.jpg

If you are good at cutting, the rag is simply a sharp edge that needs to be forged down. If you aren't good at cutting you'll have to grind the rag off, or you'll end up with a "cold shut." A cold shut is a little pinched piece of metal that gets hammered into the material. As an example of a cold shut, take a piece of paper and put it on a bit of muddy dirt. Then jump on it. It's been pushed down into the mud, but it isn't part of the mud.

Well you will have a really hard time grind this bur out of the inside of your hole. When you drift your hole, the bur will get pushed into the side-walls of the hole and form a tiny cold shut crack. See the next picture.
DSC08739.jpg




This isn't anything I've made up. Try both techniques and see which one consistantly gives you a good clean hole! If you can consistantly get a rag-free, smooth, clean hole using a chisel......more power too ya!

This is not a technique argument thread! I've used both techniques, and I find that I can consistantly get a good clean hole with the slot punch, so that is the technique I use.

Share this post


Link to post
Share on other sites

Thanks for posting that Dave. There are numerous threads on this subject, but you've posted a very clear, concise and easily understood version. Thanks Mate.....

Share this post


Link to post
Share on other sites

Thanks for the post. I agree with you 100%. I have experimented with many slitters over the years and when I saw the technique you described it was a light bulb moment. I am actually having a hammer in today and we will be making slitters and drifts. I am planning on correcting my verbiage many times today to try and say slit punch as that is probably more correct. I have been using the Brian Brazeal type of hand punches and they work well. The only one I tend to have some trials with is the square drift on the diamond. I get more distortion than I like and am messing with the slot punch size to get a better fit.

Share this post


Link to post
Share on other sites

Thanks folks! I'm always talking to / demoing to people who don't know what hammers are, much less, anvils, forges, or blacksmiths. I tend to over describe everything so that someone who doesn't even know what a blacksmith is, can, in a matter of a few minutes, understand exactly what's going on when I'm demonstrating.

I've not had much success with diagonal slitting/drifting. I've used the undersized and then true sized opening technique, and it helps but I still don't like the way it looks. However, I haven't done it more than a handful of times.

Share this post


Link to post
Share on other sites

yea buddy!makes sense now. I was trying to figger out "how in Sam Hill" you could drive that thru and punch out a slug
but I did not know you made 2 cuts at it. I woulda wore myself out by tryin to drive all the way thru. many thanks for taking your time to lay this out

Share this post


Link to post
Share on other sites

Brian,
I don't understand what you mean by a diagonal slot opening drift.
Thanks,

It is a drift that is forged in such a way that it opens up a slot that you have punched to a square hole diagonally to the rod or bar that you punched a slot into. I can't post any pictures, but it is posted in a thread called " Slitter Geometry".

Share this post


Link to post
Share on other sites

Brian I was trying the slot punch being just a little shorter than the diagonal length of the square hole. I made the punch a little too long and ended up with a really ugly hole. The reason I was trying to punch it so close to the finished size was to prevent bar growth in the length. If I use an undersized diagonal drift then and on sized square then I will get some growth. Since I can find no other way to do it I will just have to determine the growth and compensate in the layout. it works so well on the square on the square and round holes I was hoping I could get better results on the diagonal. Oh well it is still a lot better than the drill and drift method that thins and stretches the bar that I had used for years.

Share this post


Link to post
Share on other sites

Something that helps with a slit chisel is continuous sharpening on the bottom and around on the sides as well. This is shown in Schwarzkopf, "Plain and Ornamental Forging," page 186. Schwarzkopf shows a straight cutting edge on the bottom with radiused corners, but it could also be rounding cutting edge or angled to a central "point." Sharpening all around helps to provent a pronounced wedge shape as you would get if slit with a cold chisel shape.

Schwarzkopf can be seen via Google.

Share this post


Link to post
Share on other sites

I saw in a recent video by Brian that this also applies to center punches for clean round holes. Would you reccomend this for all punching of holes? (and would it be worthwhile to start making center punches instead of flat sided punches?) Thanks!
~rf

Share this post


Link to post
Share on other sites

Ridgeway forge, it is already in "Slitter Geometry", a thread started back in 2009. It covers thin and thick stock and different shaped holes. Try all the different ways out, and report back. I've done that, Dave's done that, and a few people I've shared it with have done the same.

"Knowledge comes from experience." Einstein

Share this post


Link to post
Share on other sites

Thank you, Brian. I will try it out and see what works best for me.
(just for curiosity's sake, would an inwardly concaved punch be of any use? Thinking of all the different shapes made me consider that one. I believe it would act like drilling it out, removing a lot of material, mixed with a cicular chisel, leaving behind a rag.)

Share this post


Link to post
Share on other sites

Thank you, Brian. I will try it out and see what works best for me.
(just for curiosity's sake, would an inwardly concaved punch be of any use? Thinking of all the different shapes made me consider that one. I believe it would act like drilling it out, removing a lot of material, mixed with a cicular chisel, leaving behind a rag.)


A concave punch would drive easily until it seated fully. Then the metal trapped beneath the punch would have no where to go.
With a pointed punch, the material that doesn't get punched out, gets pushed to the side. A flat punch just plows through, and it's shape does not naturaly move the material out of the way. A concave punch would actually trap material. A chisel, moves all of the material creating the stretched area and the resulting cold shuts.

Share this post


Link to post
Share on other sites

Ridgeway forge, it is already in "Slitter Geometry", a thread started back in 2009. It covers thin and thick stock and different shaped holes. Try all the different ways out, and report back. I've done that, Dave's done that, and a few people I've shared it with have done the same.

"Knowledge comes from experience." Einstein


"Slitter Geometry"
http://www.iforgeiro...itter +geometry

Share this post


Link to post
Share on other sites

From what I have experanced Tempature plays a critical role in the shearing off of the slug to hot and it will not come off cleanly.


True! Thanks for pointing that out!

Share this post


Link to post
Share on other sites

In my experience, when punching out the slug, the material must be at a red heat. In this operation you are actually shearing the steel slug, and need the material to be cool enough to resist deformation and shear cleanly.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.