POTD was making a punch for an upcoming reproduction Erector set part. Short version is the part has a 0.650” hole through the center of a sheet metal blank (0.036” stock). I have 5/8” and ¾” punches, but not the proper size. I thought about using the 5/8” punch and opening the hole with a reamer or boring on the lathe after the fact, but opted for a new punch and die.
I use O-1 for punches and dies; my Roper Whitney #218 press uses punches in either ½” or 1” diameter, and dies at 1 ¼” or 2 1/8” (don’t quote me on the 2 1/8”, not in my shop to measure). The hole size matches the punch diameter, so the target there was 0.650”. The die hole size has clearance to the punch which is determined by the material being punched and the thickness of the stock. The formula for plain carbon steel is:
Total clearance = 20% x Material thickness
The softer the material, the smaller the clearance. In my case, it’s 0.036” steel so the total die/punch clearance should be ~0.007” (0.2 x 0.036”). That means the die hole target is 0.657”.
I started with a chunk of 1” O-1 for the punch. Faced leaving a nib which will locate the punch to a center-punched dimple in the blank. Then turned the diameter to 0.650” and chamfered the transition from 0.650” to 1”. Also cut a relief area on the body of the punch for number punching the size. Flipped the O-1, faced the back side to overall length and cut a back-angle near the top. It’s not a big deal with round punches, the RW #218 has a pocketed receiver for the punch that’s 1” diameter. There is an anti-rotation screw about 1/2” from the end which clamps the punch in place so it doesn’t rotate. Naturally, that’s more important for non-round punches, but the round ones also have this feature.
Chucked up a length of 1.27” (oversized stock) O-1 and faced. Plunged a parting tool a touch over 0.625” down the length to mark the height of the die. Then cut a relief area in the middle for number punching the die size. Center drilled, and drilled a clearance hole in prep for boring to 0.657”.
After boring to 0.657”, flipped the die and faced the back side to an overall length of 0.625” (normal die height). Then bored out the back side of the through hole to around 0.725” to give some clearance for the punched slugs to drop. Roper Whitney dies typically have about 0.2” of material at the proper diameter and clearance below.
Punch and die before heat treating; they're setting on the Erector set part I'll be making
I didn’t show it, but number punched “650” and “657” on the punch and die before heat treating. That was done with a Thermolyne muffle furnace for hardening and a Thermolyne hot plate for tempering.
Proof of the pudding was knocking in some holes in 0.036” stock. No problems!
Thanks for looking, Bruce
