Cutting 2" cabin heat hole in firewall

Well, I dulled my fly hole cutter blade and only scratched the firewall with my 2" hole saw (made for wood really) so I am scratching my head yet again. How have others cut the 2" hole in the firewall for the cabin heat? Guess I'll have to get medieval with the thing and use a reamer or something. This stainless steel is tough. I guess I was hoping to cut a nice quick hole with even edges.
Thanks!

Mike, I used a hole saw w/o any troubles.. it went right through -- I did lube it pretty good though.

PS. It was one of those "aircraft grade" hole saws from Lowes

Hole saw

The hole saw works well for this but I would recommend someone hold a block of wood on the backside of the firewall. The SS is tough but if the hole saw catches a tooth it will actually make a small tear in the metal. Use lots of Boelube.

Andy

Electricians punch set

I found that a punch set used by electricians works great. You still have to drill a smaller pilot hole with a standard drill bit but the finished diameter is cut by the punch. It leaves a nice sharp edge with no chance of scratching anything up. There are manual and hydraulic hand pump assisted models but if you know any electrician I'll sure he will have one that he may let you borrow. Most of them are made by the Greenlee company and can be found at any electrical supply store or even on Ebay.
NYTOM

Red or Green

Draw a 2" circle then just make a 3/4 hole with a unibit and cut the hole with the green or red snips
just take smaller bites and sand with a 1.5 inch
sanding disk.. cover it with the heat box and move
on.. you won't ever see the hole again..

godspeed said:

Draw a 2" circle then just make a 3/4 hole with a unibit and cut the hole with the green or red snips
just take smaller bites and sand with a 1.5 inch
sanding disk.. cover it with the heat box and move
on.. you won't ever see the hole again..

Click to expand...

This is very true. The hole is covered by the heat box on the engine side and the flow diverter/cover on the inside. Don't sweat it too much.

Stop by the "KCUB RV Factory" ...

Mike,

Sorry I missed this as you were JUST there hours ago!!!

I think we have all the tools you need. I will show you the results and loan you the tools you need for it.


James

mikehoover said:

Well, I dulled my fly hole cutter blade and only scratched the firewall with my 2" hole saw (made for wood really) so I am scratching my head yet again. How have others cut the 2" hole in the firewall for the cabin heat? Guess I'll have to get medieval with the thing and use a reamer or something. This stainless steel is tough. I guess I was hoping to cut a nice quick hole with even edges.
Thanks!

Click to expand...

Use the electrician's hole punch. Keep in mind the punch set for 2" actual size holes will be marked bigger or smaller (CRS). Measure the female die to be sure. Deburr the edge.
Steve

A 2.25" chassis punch used for instrument panel works great. It will give you a 1/8" clearance around the heater valve.
Mel...DAR

Hole saw, block of wood

I bought a hole saw from the Sears Craftsman Aviation Department. I backed it up with a block of wood. The paint on the holesaw smoked a lot, but it made a decent hole. It was a bit ragged, so I cleaned it up with a Scotchbrite wheel on the die grinder.

Like you, I first dulled the blade on my fly cutter before making the trip to Sears.

Regards,
Martin

Cutting 2' cabin heat hole after action report

Cut the 2" hole with the Greenlee hole punch set. It cut fairly nicely, but I had to dress up the hole afterwards to make the edges even. I would recommend backing the firewall up with a sheet of .063 alclad to give the hole punch something thick to punch thru as the firewall is thin and isn't really thick enough for the punch set. Thanks for all the comments!

Holesaw work/ stainless steel

Excess speed when using any kind of cutting tools with stainless can cause glazing and work hardening of the surface. A good cutting oil will certainly help and I suggest you use one of the slowest hand drills you have. I generally use one of my electric screw drivers when cutting sheetmetal with a hole saw. A speed of approx. 450/550 RPM is enough. Doug Sr.

Perhaps too slow

Perhaps I went too slow. I guess I was cutting at about 6 rpm with the manual ratchet set. Regardless, being a novice at a lot of what is required for building this airplane, I'd back it up with the .063 if I did it again to avoid any problems. I used some lube on the blade and die to help. All in all, it turned out much better than if I'd gone crazy on it with a fluted reamer or something.

The Greenlee punch is "the" tool for the job. They are expensive!!!

If you choose a hole saw, get a Bi-Metal one. It has hardened teeth and will last. ALSO... lubricate the pilot bit & saw with, get this... Parmolive Gold Dishwashing liquid. It is a trade secret and works like a charm on stainless steel!

I learned it from a plumber friend after watching him cut in a kitchen sink. I wouldn't have believed it if I hadn't seen it with thine own eyes!

I have subsequently proven it own my own projects. It works!!!

Hope this helps!

CJ

Dremel reinforced cutting disc

Just as another data point, I tried using my hole saw, which up until this point has performed flawlessly for cutting out aluminium lightening holes, however even with copious amounts of lubricant, the hole saw had its teeth shaved in short order. I then resorted to a reinforced cutting disc on my dremel tool, making small cuts just inside the 2" perimeter. Once I had removed the center, I then used the dremel tool cutting disc like an angle grinder and cleaned up the hole, before finally finishing it with some fiber backed strips of sand paper. I was very satisfied with the result, it only used about half of one cutoff disc, and it really didn't take that long. In hindsight I wish I used the dremel method from the start. If you are going to use a dremel to cut the hole, don't forget to wear a respirator when doing the grinding, as grinding and welding stainless releases chromate nasties.

Tom.

Another REALLY easy way to cut large holes in the firewall is with a Rotabroach cutter or equivalent. They only come in a set (very pricey), so the best thing to do is ask around and see if some tool-maven you know might own a set. The Rotabroach cutters cut that SS like butter; takes maybe 10 seconds at normal RPMs.

Use Bo Lube on stainless

I didn't have the right punch or hole saw, so I used the Medieval method of drilling a bunch of small holes around the inside of the perimeter, connected the holes with a Dremel tool and removed the stainless "throwing star", then used a drum sander (about 1.5" diameter) to smooth out all the teeth. Crude but effective!

I ruined my 2" hole saw in about 90 sec, then used a unibit to open up a 3/4" hole. Then I removed the majority of the material with an HF air nibbler, then I smoothed the edges and brought it to size with a carbide burr on an air die grinder (the $15 Horror Fright type). I have 5 of the HF die grinders so that I don't have to change attachments . The air nibbler is a really handy tool for cutting large internal holes in anything. I used cutting oil with the nibbler and unibit. I am sure the other methods mentioned work well too - it comes down to what you have available.

One thing about that thin stainless is that if you have something dig in and stretch an edge it can get messy and look bad. So you have to be careful to cut, not press and stretch. Also, the burrs created when cutting stainless will rip open your flesh worse than freshly cut glass. YOu really want to deburr thoroughly and be careful working around the hole while deburring. it is very easy to "open up the plumbing" and nobody wants blood all over their nice clean firewall! Have fun and protect yourself. I know people who have sliced tendons and lost mobility in their fingers while being careless around sharp metal. Please don't let it happen to you.