Fly cutter or Rotabroach for lightening holes?

[Captain Sacto]

Question for the experts...

The rear spar for the vertical stab on the 7A has an eighth inch thick doubler that, according to the drawings, permits optional lightening holes. Two of the holes are 2" dia, and 5 are 1.5" dia.

It appears as though I have two options if I want to cut these lightening holes. I can buy the Rotabroach cutters from the good folks at Avery for $ 175; or, I can get an adjustable circle (fly) cutter from STS for $13.95.

My question is... do the Rotabroach cutters perform in a sufficiently superior manner to justify the >10X cost?

Any and all comments and suggestions welcome.

Thanks in advance! - - Tom from Sacramento

[GAHco]

On all my lightening holes on my six, I used a Malco HC-1.

I bought it from Avery in 98 and it works great.

It is a rotating cutter deal.

There site is http://malco.malcoproducts.com/products/shear/hc1.asp

and it shows a good picture of it.

The trick for nice holes is use the recommended rpm and let the cutter do the work. You must also not quite finish cutting and then stop at about 90% take the tool out an put heavy duty clear packaging tape on both sides of the inner biscuit before it moves, distorts or falls out.

Re install the tool and cut through the remaining aluminum slower than before.

it will come out round and with just the smallest bump where you finish the circle. Pull the tape off and do the next hole.

As with all things of this nature, practice first on something you don't mind throwing away

My experience with fly cutters and aluminum, RUN- AWAY!

I have seen people use them successfully before but one mistake and it's bandages and new parts. Believe me it can happen too fast.

Best wishes.

[dan]

Or you could use hole saws. Just deburr the holes well.

[N674P]

I used Greenlee punches, since I had access to them and not to a flycutter. I have since borrowed a flycutter, then bought one. A flycutter would cut 1/8" aluminum like butter...
The two tricks are to clamp the work down tight (and keep clear if it decides to spin anyway). and to use the slowest speed possible! Don't remember offhand how slow my drill press goes - I think it's 370 RPM. At that speed it works great, and really isn't scary. That's my cheapo Harbor Freight floor model - my bench mount press only goes down to 750 RPM, and it scares me to think about using it in that!

Correction: Just checked my drill presses, and the actual slowest speeds are 170 RPM and 620RPM

[G-force]

Or clamp them in a mill and use an indexable boring head. Of course its handy if you run a machine shop at your work Year of making pieces of metal into smaller pieces of metal and piles of chips have taught me a few things. As mentioned, flycutters, especially the single cutter ones, an notorious for hooking your part, flinging them about, often taking part of your hand or fingers with it. But only when used incorrectaly. It is very important to have your part clamped down well, for the doubler in question two 4" or 6" "C" clamps would be my minumim, there should be no need for your hand to be holding it. Put a piece of wood underneith it and pad the clamp face with a few strips of masking tape. Have the drill press table as high as you can get it and that it is tightened in position. If the cutting blade is adjustable, slide it up so there is very little stickout, perhaps 1/4" or so since your only cutting 1/8". The point is to keep the whole setup as ridgid as possable, you dont want the spindle of the drill press to be sticking down any farther than it has to be when cutting. Your cutting tool should have a slight angle on it so that the outer circumfrence of your cut is deeper than the inside. This way when it breaks through, it will seperate at a nice clean line on the edge instead of the whole swath of the cutter. Turn the cutter slow and use an aluminum cutting fluid like Tap Magic, A9 or even kerosene. Depending on the speed, cutter, and ridgity of the setup you will hopefully get a nice clean cut. Or it will chatter and make all kinds of noise. Just feed it in with steady, constant pressure and keep the lubricant flowing. An "acid brush" and a dixie cup make a good applicator.

[skyyking56]

I agree with Dan.
I did mine with a hole saw (bought the hole saw kit at Home Depot....Dewalt).
I think the kit was $29.95 or so. The holes came out slightly bigger than their nominal diameters (about 1/16"), but were totally acceptable.
There was a lot of noise,deburring and time, but for the cost I justified it.
Just make sure the piece is clamped down and secure and put the hole saw in a drill press.
Next time I will use some kind of lubricant. Duh
Jim

[DeltaRomeo]

Nothing special on my -6. I did dozens of holes with the plain old cheap fly-cutter on the drill press.

If your part is sufficiently clamped down and held in place you can pretty much use any of the methods described above. If it isn't clamped down securely, make sure you have plenty of bandages handy .

b,
dr

[Ironflight]

I also used my fly-cutter building the Val, and still have all of my body parts attached. See all of the notes above for safety tips though! I couldn't find a set of Greenlee punches in the correct size the day I was cutting my panel - they'd be the best for that if you can get ahold of them.

Paul

[Rick6a]

I prefer a good old-fashioned flycutter. Operated at 250 RPM in a drill press, I've never had a lick of problem using em. In fact, I sold my instrument punch early on because I discovered there are minute differences in diameter between various instrument manufacturers and the punch left shear marks that had to be filed away....tedious. I could more precisely cut in each burr free custom hole for all those steam gauges using the flycutters shown. The waste llustrated in the photo represent the total of lightening holes I cut out of my vintage 2000 pre-matched hole RV-6A kit.

Just make sure the work is securely clamped to the press and use a slow RPM with cutting oil and you shouldn't experience any problems. If you hurt yourself or experience other problems, it is a virtual certainty the problem can be traced to poor technique.

[Mel]

250 rpm is still pretty fast for a fly cutter. Back in the "old days" we had to cut ALL holes including lightening holes in the ribs and bulkheads. I never use over 170 rpm with a fly cutter. Like said before, make sure everything is clamped down and gloves are worn.