Is it possible to punch non-round holes from only one side "blind" in sheet metal?

Is it possible to punch non-round holes from only one side "blind" in sheet metal?

I'm trying to find a tool or way to punch D shaped holes in sheet metal equipment cases. These cases are for older equipment and the metal has already been bent and formed. There are round punches that you can drill a hole, install the punch and make a hole, but I need D shaped holes and not round. There are hand tools that will do this, but they typically will only reach a few inches in from the edge of the sheet. The sheet metal cases are typically 16ga or near that and the holes are less than 1/2" diameter. I have a punch and die that is the correct shape and size, but I can't figure out how to make it work due to the size and geometry of the cases. So, can you punch holes blind from one side into sheet metal or is there some other way to do this? A thought I had was to put a block of wood or HDPE on the opposite side of the sheet and just hit the die with a hammer and hope that might work but I thought I would ask before I ruin the punch.

An industry standard hole?

Perhaps the Greenlee catalog is your source?

That is a pretty small hole to make, I don't think you will be able to drive the punch through with a hammer.

Off the top of my head, I'd suggest you drill a smaller round hole, maybe 3/8" and then begin to file a flat on one side. Then go and buy yourself a 1/2" round file, probably a double bastard cut roughing file (with tapering end) and grind a flat down one side of the file. Let this flat rest on the flat you filed in the hole. Alternate files until you get it large enough to serve your purpose

I would think the hole you 'guide' hole you start with wouldn't have to be the full diameter of the radiused part of the D. So, 1/2" nominal D hole with a 1/4" guide hole. I'm pretty sure Greenlee chassis punches will do that. Both side access required, but I think you have that...

Here's a non-Greenlee source for manual or hydraulic operation, in a little video.
Panelpunches.Com

One could also epoxy a d-shaped 'washer' in the correct orientation, and then install your lock (or whatever) in that new orientation. (I'm assuming spot-welding is out, for finish reasons.)

If this application is in a row of, say, connector jacks or rotary level controls, you can also prevent rotation by mounting provisions behind the panel. Either a locking bar across several components, or a sub-plate that contains you D hole, but also extends to either side to be sandwiched between the front panel and adjacent components. So it's like the above 'washer' solution, but held in by neighboring components rather than epoxy.

Chip

Greenlee offers a 0.5" x 0.469 D punch / die, looks to be the smallest they offer as a standard.

The 'customary and ordinary' way for Donkey's Years on low volume prototype has been round hole PLUS 'Dee' holed washer ...with a TAB.

A much smaller hole - often 'blind' - is placed where one wants to anchor the tab for any chosen angle of the Dee washer.

Much less nuisance and waaay faster to drill or punch round hole, then place that indexing hole than it is to muck about with files or punches.

Only once into significant volume of a nailed-up design, do dee punches become the better answer.

Bill

I have dozens of Greenlee punches, including "D" shape ones and have used them for decades. I have never seen a Greenlee punch that was smaller than 1/2".

I have done some research work on these punches and can tell you that the draw screw becomes the most critical element as the size of the punch decreases. They cut at four places (old style) or two places (newer, slug buster style) and a 1/2" punch requires just about as much force as a 3" one. But the draw screw is required to be smaller. A 1/4", grade 8 bolt is barely marginal for a 1/2" punch. Smaller bolts as required for smaller sizes are going to be problematic. Perhaps it could be done for punching soft AND thin aluminum.

SteveBausch said:

An industry standard hole?

Perhaps the Greenlee catalog is your source?

Click to expand...

Thanks all for the suggestions. The greenlee punches are all larger than I can use and as EPAIII says, the issue is the draw blot has to be a very small diameter for small holes and there is a lower limit on what you can do with that, hence a lower limit on the punch diameter (too large for me). I think I found the easiest solution and that is to drill a minimum size hole that just barely fits the threads and flat of the connectors and then use a sheet metal nibbler to cut out a "negative keyway". Looks like a double D hole with one of the D's flat or a D the "wrong way" with the curved portion on one of the short sides rather then a long side. Hope that makes sense, as my username says, I am not a machinist and I don't know the correct terminology to describe things. Thanks again for the help, I am sure I can put all the suggestions to use in my projects.