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219PB

Well Known Member
I just saw another post about firewall penetration but I did not want to hijack.

I am making my initial firewall penetrations and used a step drill bit on the first one. I drilled it to 3/4" and it made a perfect hole. I needed the hole to be 1" so I put another bit in that would go larger and the hole got sloppy. It bent the metal and made "waves" all around the perimeter. I was able to dress, press, and clean it up but I am leery about the next hole.

It is possible that maybe I needed to apply more axial force while I was enlarging the hole. Has anyone seen this issue or have a tip for using these unibits for thin stainless steel?

I looked into purchasing knock-out punches but the ones that I could find reasonably priced were for electrical conduit and the actual hole sizes are 7/8", 1 1/8", 1 3/8".

What are the most typical sizes that need to be made? What have others used successfully to make these holes?
 
219PB said:
I just saw another post about firewall penetration but I did not want to hijack.

I am making my initial firewall penetrations and used a step drill bit on the first one. I drilled it to 3/4" and it made a perfect hole. I needed the hole to be 1" so I put another bit in that would go larger and the hole got sloppy. It bent the metal and made "waves" all around the perimeter. I was able to dress, press, and clean it up but I am leery about the next hole.

It is possible that maybe I needed to apply more axial force while I was enlarging the hole. Has anyone seen this issue or have a tip for using these unibits for thin stainless steel?

I looked into purchasing knock-out punches but the ones that I could find reasonably priced were for electrical conduit and the actual hole sizes are 7/8", 1 1/8", 1 3/8".

What are the most typical sizes that need to be made? What have others used successfully to make these holes?
Click to expand...

I had the same experience with ragged perimeter. I use Greenlee punches, but still sometimes need to enlarge and treat the edges. The step drill has worked well for me up to 3/4. Larger either I used the punch or a carbide burr and enlarge it to prevent the damage you noted. In drilling, it seems that high speed and medium pressure makes the best holes. This is in contrast to the heavy, slow for stainless recommendation. A perfectly sharp bit is a requirement. I might think your 1" step was too slow, too much pressure, and/or dulled (or some combination). Even the punches can distort the edges sometimes, grease helps.

Good luck, happy building.
 
Another thing is to back up the work. A block of soft pine works well. Drilling through the wood is not doing your step bit any good; stop often and let the bit cool. But the holes you make will be superior. You could also back with relatively thin soft steel or aluminum, which will let the heat dissipate better and soak up less of the cutting oil. You ARE using cutting oil, right?
 
I overkilled it by getting the hydraulic hole puncher from Harbour Freigh.
Some of the larger holes (1.93 for heat valve) needed a 3/4 hole to get the tool positioned. The 3/4 hole I made with step bit looked horrible, but the 1- 15/16 looked great.
 
I am just glad our planes are not made out of this stuff, It was terrible to work with. Barrowed a commercial hyd punch for the big holes and Unibit for the small ones.
 
large hole for heat muff

I cannot think why your drilling a 1 inch hole but for the large hole for the heat muff. I got one of the round hole drills that have a drill bit in the center and saw teeth around the edges that the hardware store sells I then drill a 3/4 plywood sheet with the saw. pry out the round piece of wood that is in the saw drill a hole in the firewall the size of the pilot drill and bolt the wood to the firewall. I also had a piece of wood on the inside of the firewall as well. take the pilot drill out of the saw and using the wood to hold the saw in place drill the hole with cutting oil. got this off vansairforce several years ago worked good
 
Thanks for all of the replies. I made the 1" hole for the nosegear bolt access. The plans call for a 3/4" but that size would not allow a socket to access the bolt head. I enlarged it to a 1" and the socket just goes through the hole.

I did not even think about cutting oil for this thin material. I wish I had some scrap material to practice on.
 
You'd do yourself a real favor by delaying things until you can practice on scrap! The ol' "ounce of prevention" thing!

mjb
 
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