How to make the hole in an Armstrong tool holder

[ericc]

I am trying to make a straight forged Armstrong tool holder for my South Bend lathe, and I was wondering how to make the square hole for the tool bit. The left and right hand tool holders look straightforward since they are bent, but the straight one is not because it is bent. A hole at a fairly shallow angle has to be drilled in the nose. Then, this hole has to be broached square. This is too challenging for me to do hot. Deep holes more than three times the punch diameter are tricky. I know exactly what is going to happen if I try to drill and file it. I am going to spend a lot of time forging the shank and upsetting and shaping the nose. Then, I'll drill an off angle hole which I'll have to spend several hours looking at as I file this long narrow hole to square with a thin delicate square file. About 2000 trial fits will be necessary to get a sliding fit over the length of the hole. All the while, I'll be looking at the slanted hole, and wishing that it was straighter, knowing full well that if I remade the tool holder, it would turn out the same.

This is really discouraging. There is even more that is discouraging. First, I looked at eBay, and there was a 0-S tool holder with a crooked drilled hole. Even the factory made one could not escape the aforementioned fate. Note that some slanting of the hole does not affect function, because with the short projection from the nose of the tool bit, the functionality is unaffected. Second, I did exactly what I said above, and cannot bear to expend several evenings of effort filing on this obviously crooked hole.

The hole was drilled on a good drill press with a stub length drill. The hole was started with a tiny divot from a center drill, followed by a spotting drill. The tool was clamped to an angle plate, which was clamped to the drill press table. I cannot seem to get the drill exit hole to line up on the long holes.

One curious observation is that in the eBay photo, the exit hole is circular, while on mine, it is oval due to the exit angle. So, the manufacturing process is clearly different, but I cannot figure out how or was done. The lantern post was easy. Just forge and offset the head of a railroad spike and slot punch the shank. The dished ring was also easy with a rounding hammer. I know a quick way to fix this. Slice out the side of the hole with a hacksaw, clean up with a (beefy) file, then tig weld the offcut back in.

Any clues to do it correctly? Is it possible to avoid broaching or even worse, filing? It's obvious that the exit hole is no bigger than 1/4" in the eBay photo, since that would weaken the shank. That would make broaching even more challenging. But, it almost certainly wasn't tig welded, either.

 

[markba633csi]

I always wondered how they made those square holes too- die sinking (edm) perhaps?

 

[T Bredehoft]

Armstrong holders predate EDM.

Could they have been forged around a square arbor?

I'd guess broached.

 

[markba633csi]

You're right, this is turn-of-the-century stuff
Forged around a square arbor sounds more like it

 

[ericc]

Hi Mark. That makes a lot of sense. Kind of like how gun barrels were made. Looking at the close up pictures on Ebay, it looks like this could be the case. Drill large and tap shut around an arbor. This will cause wallowing at the end, and you can almost see it in the shots, although it could be wear. Forging around an arbor would also work with a semi-blind hole, and it could be drilled later at the bottom for a knock-out bar.

Like I said, the right and left hand bars are simply punched/drifted. You can put square tool bits of any length. I am curious how far the square cross section goes on the straight tool. Clearly not to the end, as there would be no need for admitting an extra long bit.

EDM and TIG welding are definitely not period accurate.

 

[RJSakowski]

A square channel could be milled and a flat cap welded or brazed on.

 

[RJSakowski]

Or you could braze a square hole sleeve in a drilled hole. https://www.sdp-si.com/eStore/..\ss\PDF\79004023.PDF

 

[RJSakowski]

If I were forging my own using 19th century technology, I would drill a longer than needed hole. I would make a drift that started as a round but transitioned to my square cross section and drift the hole. If I needed a precise socket, I would make a square broach-like tool with a cutting face to shave the walls to final dimension.

I have used a similar technique to make hex sockets in both aluminum and steel.

 

[benmychree]

The straight tool holders that I have seen have had a slot cut in the bottom to terminate the square hole, this makes the sides of the slot fairly thin and easily broken. In the attached pictures, note that the relief under the holder has been cut with a woodruff type key cutter, on a radius. How the square hole was done, I do not know, but a simple drifted hole would not be very satisfactory so far as accuracy is concerned; I have had tool holders that broke tool bits when tightened. The tool pictured is the only one that I have, and although I have a QCTP, I still do use the Armstrong holders in the old fashioned tool post when clearances will not allow use of the QCTP, for instance, a couple of weeks ago, I had to neck down several #10 screws that hold on the distributor cap for a Caterpillar D-2 starting engine, held in a collet and supported by a tiny center drilled hole in the end; the old fashioned, outmoded tool saved the day!
It appears to my eye that the holes were originally broached, but how it was done is quite a mystery.

 

[MontanaLon]

I've seen some 19th Century tooling that was very clearly hand forged around a piece of tool steel. They included an offset by folding the top over it so the bit was actually running alongside the portion that went into the lantern tool holder. This allowed them to adjust the bit depth and change bits easily. I think the hard part would be getting the holder hot enough to weld while keeping the bit cool enough that it wouldn't weld. It looked much like a boring bar holder for a lantern post but with a square hole.

It was part of a display showing the advancements in tooling from the beginning to now.

I think the most elegant of the holders was formed by drilling a hole and then just squaring the bottom using a file. The top being round really doesn't matter much as the force would be applied against the bottom of the holder at the front and the rear of the holder where you could put another bolt in to hold the rear of the bit in place.

I wish I had a picture of it because I can't describe it very well. Flat bar with a bend which offset the front just the width of the bar so the tool bit hole would be a through hole.