Findlay Punch

Home shop machinist.

1
The author’s incomplete set of Findlay punches. Missing are the 7/8- and 1-inch diameter punches. Second from the left are the raw material blanks to shop-make the AWOL punches.

Hole-making punches, also known as knockout punches, are a quick and easy way to make accurate holes in aluminum sheet. Punching works by shearing the material between a male and female die. Provided the dies are sharp, the material is not too thick, and the force is adequate, punched-out holes are nearly perfect and require only minimal deburring.

Click to view a large format version of this drawing and a chart of basic dimensions for all the punches from ¾ inch to 2 inches.

All of the usual aircraft supply houses sell commercial knock-out punches; the only downside is they can be a bit pricey. Unless you have a lot of holes to make, it might be hard to justify the cost, which brings us to this month’s project: making a “Findlay” punch. Patented in the late 1930s by one E. H. Findlay of Pomona, California (ref: U.S. Patent number US2069231A), the Findlay design has a number of attributes that make it unique (uniqueness being a critical necessity for the granting of a utility patent).

The following description is an excerpt from Findlay’s patent:

  • “My invention has for an object the provision of an improved tool by means of which holes may be punched accurately and expeditiously in sheet materials without appreciable deformation of the material surrounding the punched hole.
  • “Another object of my invention is to provide a tool for punching holes in sheet material whereby the shearing edges of the tool are accurately aligned during the punching operation.
  • “Another object of my invention is to provide a punch from which the blank can readily be extracted.
  • “Another object of my invention is to provide a punch and die from which the material in which the hole has been punched can be readily removed after the punching operation is completed.”

I was recently given a set of Findlay punches from my brother, Jim. The punches were marked as having been made by our dad’s company, B.H. Hadley Mfg., which would date them to the late 1940s. If you’ve never heard of Findlay punches, don’t feel bad. Virtually no one knows about them. There are no records of how many sets were made or sold. For all we know, only two sets may exist. (If you have heard of Findlay punches, I’d like to hear about it! Post your comments below the article ↓.)

It takes a really sharp blow with a heavy sledgehammer (left), but the resulting hole made by the Findlay punch is clean and distortion free. The inner rib on the female die and outer rib on the male die combine to dish the slug, which draws it away from the sides of the die for easy extraction (right).

What is amazing about these punches is how good they work. They make perfectly clean holes, the dies don’t bind, and the punched-out slug doesn’t get stuck in the female die.

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Findlay punches are designed to be used with a heavy engineer’s hammer or sledgehammer. When punching holes in thin aluminum (0.035 inch and thinner), the pointed guide pin has no trouble piercing through, but on thicker stock I found that drilling a pilot hole to match the guide pin diameter made it easier to line up and get a clean hole. Either way, when punching material thicker than 0.050 inch, it takes a good whack with a big hammer… a really big hammer.

Step one was facing the female die blank to length (left). Any hole drilled in a lathe-made project should begin with a spotting drill (right). This assures the twist bit will be lined up on the true center of the workpiece.

If you have an arbor press, it makes the process a bit easier. Holes larger than 1 inch and/or in sheet 0.050-inch or thicker will still require a solid blow with a sledgehammer to the top of the ram.

Through drilling for the guide pin. Drilling in 4140 alloy steel requires patience and cutting oil.

The set I got from Jim was missing the 7/8- and 1-inch punches, so replicating the AWOL sizes seemed like a great project for the home shop. All of the work was done on my 8×14-inch bench lathe. While most of the project was straightforward lathe work (O.D. turning, boring, and drilling), special operations included chamfering (see “Chamfers and Tapers,” January 2020), making an interference fit (“Interference,” August 2018), and grinding a custom cutter (“Daily Grind,” September 2017).

The secret to making straight holes is “peck” drilling: retracting the bit after every 0.050- to 0.100-inch “peck” in order to clear the chips from the flutes.
The next step was to make the counterbore. A ½-inch end mill was used to establish the depth of the counterbore and make room for the next operation, which was to enlarge the opening with a boring bar.
A boring bar (left) was used to enlarge the counterbore to the desired ID. A shop-made gauge (right) was used to confirm the inside diameter.
To make the chamfered relief cut in the female die, a bench grinder was used to make a “form” tool from a ¼-inch high-speed steel tool blank. This relief is what dishes the punched-out “slug” so it won’t get stuck in the die (left). Counterboring the base per the drawing and deburring edges not involved in the punching process (right).

The material used was 4140 alloy steel. The punch can be put to work immediately, but if you plan to knock out a lot of holes, plan on getting both male and female dies professionally heat-treated to Rockwell C 46–50 (the same as the original Findlay punches).

The male die starts with facing to length and then OD turning the narrow part of the stem (left). Using the cross slide set to 45° and a right-hand diamond tool, a chamfer was made on the back of the male die (right).
The male die is finished after drilling for the guide pin and making the counterbore using the same “form” tool used to chamfer the female die (left). Fitting the guide pin required close-tolerance precision (right). The goal was to turn the guide pin to slip-fit the first ¾ inch into the male die, with the last ¼ inch a press, or shrink, fit.
The shop-made 1-inch punch. The discoloration of the male die was from heating it to 450° F in a toaster oven to “freeze fit” the guide pin. The discoloration was later removed using a Scotch-Brite pad.

It occurred to me after making the 1-inch Findlay punch that the design might be adapted to work like modern knockout punches by using a threaded drawbar to pull the two dies together. That’s a project for another day. But if it works, you’ll be reading about it here. In the meantime, get out in the shop and make some chips!

1 COMMENT

  1. I very much enjoyed Bob Hadley’s piece on the Findlay punch. It solves the problem of clearing slugs from dies that I have fought for years.

    However, there is one dimension not given, the diameter of the punch.

    This is another problem altogether.

    Machinery’s Handbook says the best clearance between the punch and die depends on the thickness of the material being punched. The clearance based on radii of the punch and die is usually about .05 to .06 the thickness of the metal being punched. Thus the a punch for .060″ sheet would have a clearance of about .003″ per side, or the punch would be .006″smaller in diameter than the die. Depending on the class of work the clearance may be as much as half or twice this figure.

    You put out a great magazine. Thank you.

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