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.
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.
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.
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.