Punching thick bars?

[danielsmith]

Hey everyone, I wanted to slot punch a hole in a bar that is approximately 2" thick (2"x2"x 6"). this is in preparation for a mortise and tenon joint. Can anyone describe the tool need for this? do i use a flat bottom punch or one with a slight point on the end?
any advice would be helpful.
dan

[ThomasPowers]

Do you wish to have the bar bow out around the mortise hole or be flat sided?

Flat bottomed punch for flat sided and slitter/drift for a bowed out is how I would do it.

Also high alloy tools (so they stay hard even when hot---H13, S7, etc) and a striker with a sledge as that's large stock to work! PRACTICE before trying it on your "good stock"

[CurlyGeorge]

Hey Thomas. Is the flat sided what you would use on a hammer to keep the sides flat?Like a standard type cross pein. The sides are flat. I was wondering about how to do that. I guess that does make sense, though. Thanks for that info.

[thecelticforge]

I started cheating years ago with this. I started by drilling as many guide holes as I could and then use small punches to open them up. Then to a chisel, then to a very tapered drift for the finish. The drifts I use are long so I can use the narrow end as a handle. Hope this helps!

[Frank Turley]

The London book, "The Blacksmith's Craft," talks about slot punches. The business end is proportionately narrow, parallel sided, and with rounded ends. You will remove a burr of steel when using it, forepunching and backpunching. Many smiths prefer this method to using a "slit chisel," as described and shown in Ernst Schwarzkopf's book, "Plain and Ornamental Forging." The thinner and sharpened slit chisel may leave small shuts either end of the cut which can allow for cracks to develop. If cracks do not develop, you nevertheless may see results of the slit ends even after drifting to size. I recently used a slot punch on a hatchet eye before drifting and going through 1 3/4" stock. I made my slot punch from S7, although one could make it from an old round punch or a ball peen hammer. It is a top tool with an eye and wooden handle.

You may know that some of the old hammer eye punches had the haft (handle) put on at a 45

[gaylan veater]

Very good Frank,
Mark Aspery teaches the use of a slot punch. That is what I have used every since his classes and that is what I prefer to use on hammer head, flat bar etc. That is my 2 cents it works great for me.
Gaylan

[Jack Evers]

Frank,

Back in the 70's (about when I first saw you work) I watched Bruce Daniels punch a hammer head. He talked about sticking the punch and was sprinkling some coal fines into the hole ahead of the punch to prevent sticking. Have you seen or done anything like this?

[HWooldridge]

Coal fines work quite well to help release a punch and I use that technique every time I hot-punch a hole, whether in thick or thin stock. The coal provides a level of lubrication in the hole plus I've been told the low level of gas pressure generated when the coal burns can help expel the punch (although I think the lubrication factor is more important). Of course, a lot of folks only have gas forges so there is no coal around to utilize. In those cases, a commercial forge release agent or even a few grains of crushed charcoal can be used.

Another point not yet covered is the importance of draft in a punch. It is quite common in all types of industries to add taper to any tool which forms a deep hole - in a variety of processes such as forging, molding, casting, cold heading, etc. This detail becomes even more important in those cases where a punch tip is upset slightly by heat, which effectively locks it into place and hamstrings the smith until he can remove it. Subsequently, I'd agree with using a slot punch as described above but with a minimum of about 5 degrees of draft added per side to assist in removal. A proper hot-work steel such as H13, proper punch design and a release agent like coal or some other commercial product, will all combine to maximize your efficiency and output.

Edited September 26, 2009 by HWooldridge

[Hofi]

What is the hole demention (inner diameter) of the hole you want to get ???
Hofi

[Mark Aspery]

I like to use a handled slot (flat ended) punch with the handle set off at about a 30 degree angle.

Rightly or wrongly, I typically punch from one side only on bars 1 inch thick and under - there are pros and cons to this. Anything over 1 inch and I come in from both sides. This means that your layout has to be pretty close prior to starting.

One of the disadvantages in using a handled punch is that you cannot turn the punch around after each cooling. If you are off with the grind of your tool, turning it after each cooling helps you to 'zig-zag' along the centerline of the stock as opposed to drift off to one side. Make sure the tool is ground square.

Punching such a large bar will tend to cook your tool pretty quickly so I would use an air hardening alloy or something like Atlantic 33 (which is a water quench and no temper tool steel - very handy for this type of work)

Steel (generic) tends to shrink about 1/4 inch over 12 inches when cast from liquid to solid. I think that works out to be somewhere near 2% shrinkage.

If you intend to pass another bar through your hole when finished then your drifts need to be a little over-size. I would try for something larger than 2% to allow for a little slop during assembly.

Drifting is the key to this little problem. From my experience, drifts push rather than pull. That is, if your slot is too long, you will see evidence of the slot post drifting. If you want thick sides to your hole you will have to slot long and then upset the material around the slot prior to drifting (in the same way as a square blocking)

I would have multiple drifts on hand, each bigger than the last, to open your hole out by degrees.

You will have to find a way to support the bar as you drift. You may have to drift over the jaws of the vice if you do not have a swage block (or something similar) with a large enough hole in it. I would make a bolster plate (or two) to drift over - to prevent the material being pushed down any hole that you use.

You will be using very large bars - consider the radiant heat and dress accordingly.

If you do not intend to upset the slot punched hole prior to drifting, then the hole should be about as long as your final drift is in diameter. I would use a smaller sized slot punch and use it like a hot cut - for example use a 1 inch long slot punch and overlap the cut by 1/2 inch to create a 1 1/2 inch long slot. This can help with the sticking punch problem. Make sure that the edges of your punch are half rounds and not corners.

I understand that a large part of this text is teaching you how to suck eggs, but other, perhaps less experienced, smiths will be reading this thread.

Edited September 26, 2009 by Mark Aspery

[Frank Turley]

There are proprietary punch lubes, but for the small shop, coal dust works fairly well. It's best put on with a spoon or alternatively, if the end of the punch is cool enough to wet and hold some surface water, you can dip the wet punch in the dust and it will stick.

[kevan]

I started cheating years ago with this. I started by drilling as many guide holes as I could and then use small punches to open them up. Then to a chisel, then to a very tapered drift for the finish. The drifts I use are long so I can use the narrow end as a handle. Hope this helps!

There is no cheating at all. In fact it makes more sense.
I use a milling machine.

[nuge]

Whether you slot punch or slit chisel, you will obtain a frog eye (side swelling) when drifting which is part of the design and provides chiaroscuro

sweet word, that "chiaroscuro". something to aspire to with forgework...

[danielsmith]

thanks to all who have replied, i will let you know how it goes!