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Tip: Greenlee punches in thin stainless

flion

Well Known Member
This is bound to have come up before but I never saw it. Working with stainless steel is the bane of my building experience. I ruin tools, get weird holes, sharp edges, etc. And, of course, I have needed to put in large holes - in the RV-6A, it was the holes to pass the DB connectors for the Lightspeed ignition. In the -10, it is the holes for the firewall CPCs that I am using. Well, the Greenlee punches seemed obvious and at first looked to be doing the job. But then the punch quit cutting and the stainless formed into the punch instead. This forming work-hardened that area, so trying to finish up with a grinder was painful and ugly.

I wanted to do a better job on the -10, so I contacted a machinist friend who is building an RV-7 to see about possibly milling. He laughed because he had been working on his firewall and knew that this would not be a good idea. "Do you know how to tell when the firewall is finished?", he quipped. "The bleeding stops." But discussing how to approach this seriously, I proposed sandwiching the steel in aluminum to see if it would stop it from feeding into the punch. He didn't think it would work, but proposed another idea, and off I went to try them out on some scrap from an old firewall before tackling the -10.

FP08082013A0002P.jpg


The top hole was produced with the aluminum sandwich; it's the same result as I got before. Half of the punch works but the other side pulls into the punch and leaves a tag where the bend is hardened and difficult to remove smoothly. The second hole (and the subsequent ones on the RV-10 firewall, were done with his technique and came out much better though not perfectly smooth. The punched out scrap to the right gives the clue to how it was done. First you drill the hole for the draw bolt and set up the punch. Tighten the bolt and begin the cut until it has gone around 1/3 on both sides (1/6 of the circle). Undo the punch and rotate it so it starts the cut just past the end of the previous cut and repeat. On the third time, continue the punch until the part separates - the little tags between the cuts will show on the edges of the cut but won't be work hardened and can be filed away. For really large holes, I'd make more cuts, kind of like using an old-fashioned lever can-opener. The idea is that the little (1/32" approx.) tags between the cuts prevents the stainless from drawing into the punch until the last cut when they are easily sheared by the punch.

Of course, there's still plenty of stainless work to cut yourself on, but at least you can get decently round holes without too much effort. I hope this helps.
 
"Do you know how to tell when the firewall is finished?", he quipped. "The bleeding stops."

Your friend's joke made me laugh and cry, remembering some thin stainless firewall material I was working on for a project last month. Some of my wounds from that are still healing!

I left that project frustrated and rueing the day I get to my own firewall. Thanks for your great tip, Patrick. I'm feeling more confident about taking this task on.

--
Stephen
 
Interesting.
I did not have any trouble with my Greenlee punches on the firewall.
I punched the 2" heater hole and a few other smaller holes. Yes it did leave a turned up "burr" that took some work with a roloc pad and right angle air tool but I did not have the trouble you did with the large uncut flap.

My Greenlees are not new. Inherited and undoubtedly purchased used at a hamfest.
 
Interesting.
I did not have any trouble with my Greenlee punches on the firewall.
I punched the 2" heater hole and a few other smaller holes. Yes it did leave a turned up "burr" that took some work with a roloc pad and right angle air tool but I did not have the trouble you did with the large uncut flap.

My Greenlees are not new. Inherited and undoubtedly purchased used at a hamfest.

+1 I had no real trouble either, at least not yet. My Greenlee's are 25 yrs old from my Dad, and were relatively unused. I did find that greasing generously helped the process. It did leave a burr, but less with the greasy process. Are the newer dies still made in USA? Obviously YMMV.
 
You guys raise a good point. Mine were purchased new about 8 years ago. They work great for aluminum and mild steel but thin stainless forms into the dies on all of the sets. It looks like the die pairs are close enough tolerance if they didn't shift relative to each other during the operation, which lets the tolerance go too high on one side. A tighter fit of the draw bolt through the female die, perhaps accomplished with a shouldered bearing, would probably fix it. Be that as it may, this is what I had to work with and how I made it work. I guess the other lesson is to practice on scrap before using a tool.
 
“Interesting.
I did not have any trouble with my Greenlee punches on the firewall.”

+1 purchased new and used the kit for all the larger holes on the whole project.
 
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