Cutting the Spar Lighting Holes

[planecrazy]

Ok so I am in the process of cutting the lighting holes in the main spar for my Bearhawk 4 Place and have run into a big problem when trying to use the fly cutter to cut these holes. Simply put it is a hess of a mess. Any suggestions to make this job go a little easier?

[Ray Strickland]

Because a fly cutter is inherently unbalanced, you must have the drill press firmly bolted/clamped to a heavy, solid table. The spar blanks also needed to be clamped to the drill press as well as being supported by tables adjacent to the press. Rig the press for its slowest speed. Keep the pressure light as the cutter engages to aluminum and go slow. Use lubricant throughout the cut. Clean-up of the edges of each hole will be required. Use a drum sander in a hand drill followed by hand-sanding with progressively finer aluminum oxide paper.

The above process worked well for me. I believe that others have had good results using a router. Perhaps some of them will chime in.

[Chewie]

Here's what I did.

[fairchild]

ROUTER ---- ALL THE WAY !!!!!! :-)
Make a MDF frame that will surround the part on 3 sides- just enough slack to slide it down the line. Make a hollow doughnut that will guide the outer edge of the router plate. Put
the guide in compression with some clamps front to back...….

The scary part (to me) of using a fly cutter on that would be the instant the cutter breaks through the surface--- if there is any significant flex or give in how the work has been
immobilized---- the cutter can "bite in" and just wad the whole part up into scrap metal in about a millisecond. Another way might be to use a Dremel rotating around a central
pivot point--- but that pivot point would have to be supported from the back so as not to block the rotating arm that the Dremel is on. Might be slow-- but could work OK.
Could sub. a small laminate trimmer for the Dremel for a bit more guts.

Make the sliding block touching on back and edge flanges. Has a 1/4 inch pin which sticks up through the center hole for the lightening hole. Electric cutter (Dremel?) with arm
which end of arm threads onto pin. Have a depth adjustment so you can cut about .010 each pass. 3 or 4 passes (or more) and you are done. slide wood down to next hole
position and clamp down. repeat. That should work -- right ? you could set the arm to cut about .010 small. then make a final finish pass after the center disk had dropped out.

Might make the wood horse so it fits inside the sheet metal---- that way you could use as big a router as you wanted and you would not be running into the bent
edges of the spar.

Tim

PS-- I will be having to do this soon enough----

[S Lathrop]

I double the router advise. I did not use a fly cutter on any wing part. I made templates for all the lightening hols (ribs and spars) and used the router. I use a hand held router and clamp the work and templates to the bench.

[BTAZ]

Not sure about the size needed for the 4 place but for my LSA I was able to purchase appropriate hole saws on E-bay. Each was around $15 and IIRC I needed about three sizes..

[geraldmorrissey]

I used a fly cutter with great results. Read Stricklinds post, it's all right there. No tooling necessary, just big stupid clamps, lots of them. I've never had a problem when the cutter breaks through. The cut piece may spin a little, but no "ball up and ruin everything". If it does you didn't clamp properly.
Cheers
Gerry
Patrol #30

[fairchild]

Yes--Im sure it CAN be done--- but you dont have ANY wiggle room when it comes to clamping the work down--- I dont think I could EVER
get it clamped tight enough that I would have the expectation that it would be foolproof 100% of the time. I wouldnt want to risk throwing
away a spar blank as much trouble as I expect it to be to get measured and bent PERFECTLY. To me--- router seems to have MUCH less
stringent set-up requirements and fewer "suprise" failure modes......

T

[Chris In Milwaukee]

I understand your hesitance with the fly cutter. It can be a dangerous thing. But the reality of it is this thing is .032” thick 2024 and over six feet long with two huge bends in it. It’s crazy rigid, massive, and you’re not going to ball it up.

https://bearhawkforums.com/forum/bearhawk-patrol-plans-build/23645-spar-party?p=26793#post26793

When I cut my spar lightening holes, I used two handy clamps firmly squeezed to the table top and it was immovable. I think you’ll be pleased with the results regardless of the approach you take.

[Chewie]

Exactly what Chris said. Two clamps, decent fly cutter, radius set, locate the center, hands clear, go slow and you're done. Keep that radius set and do all the other holes that same size. Then move to the next size.

My advice for telling if a method will work, in your shop with your tools with you doing the work, is not to speculate but to test it on scrap and see what you get.

[fairchild]

My own personal habit--- when Im doing an operation on a high value part--- for my own emotional security--- I revert to whatever
method that I think will have the least failure modes -- even if that method takes 5 times longer to setup. Maybe a possible advantage
to router may be that the cut edge needs little clean up. But -- if the fly cutter is as sharp as it should be--- it could make a nice clean
hole too. Just wouldnt want to let it get dull---- :-)
T

[planecrazy]

Thank you everyone for all your advise. I ended up using a wood cutting whole saw and it really worked very well. All the holes were cut in about 45 min. Clamping the spar to a 2x4 underneath stabilized the pilot bit.

[fairchild]

the sharing is what makes it fun !

[way_up_north]

ROUTER ---- ALL THE WAY !!!!!! :-)
Make a MDF frame that will surround the part on 3 sides- just enough slack to slide it down the line. Make a hollow doughnut that will guide the outer edge of the router plate. Put
the guide in compression with some clamps front to back...….

The scary part (to me) of using a fly cutter on that would be the instant the cutter breaks through the surface--- if there is any significant flex or give in how the work has been
immobilized---- the cutter can "bite in" and just wad the whole part up into scrap metal in about a millisecond. Another way might be to use a Dremel rotating around a central
pivot point--- but that pivot point would have to be supported from the back so as not to block the rotating arm that the Dremel is on. Might be slow-- but could work OK.
Could sub. a small laminate trimmer for the Dremel for a bit more guts.

Make the sliding block touching on back and edge flanges. Has a 1/4 inch pin which sticks up through the center hole for the lightening hole. Electric cutter (Dremel?) with arm
which end of arm threads onto pin. Have a depth adjustment so you can cut about .010 each pass. 3 or 4 passes (or more) and you are done. slide wood down to next hole
position and clamp down. repeat. That should work -- right ? you could set the arm to cut about .010 small. then make a final finish pass after the center disk had dropped out.

Might make the wood horse so it fits inside the sheet metal---- that way you could use as big a router as you wanted and you would not be running into the bent
edges of the spar.

Tim

PS-- I will be having to do this soon enough----

This sounds like a very interesting idea, can you post some pictures of your jig....on how to use the router on the spar

[fairchild]

I am still doing ribs----- but I can maybe sketch out sort of the kernal of the idea - as I havnt made the jig yet.....
I am thinking of laying the spar on its back on a table with the flanges down. (so the flanges spread as it goes down towards the table.)
The construct a 3 sided "box" from heavy wood so that the box fits snugly on top and sides. Maybe make it about 2 feet long. Since the spar SHOULD be the same
all the way down -- it should slide. ( I could be lined with thin felt on the inside maybe--) In the top of the box--- cut a circular hole the same size as the lightening hole.
Carefully finish the holes edge to be smooth and even. To secure the wood saddle--- make some wood clamp plates that compress the flanges -- and tighten from below-
to prevent any sliding when you dont want to slide--

Then you can drill a -- say 3/4 inch hole n the spar with a uni-bit near but just inside the hole. You router will have a duplicating bit with the bearing on the router side.
Start the bit in the drilled hole--- then go SLOWLY around allowing the bearing to ride the edge of the saddle. It will cut a round hole as perfect as the wood edge it follows.

I would probably get a piece of 1/4 inch plexiglas and make a bigger router base---- one that is big enough that the router can NEVER tip any ---- so that the plexi is overhanging the hole at all positions. ( most standard router bases are 4 or 5 inches--- you wouldnt be supported on the inside- only on the outside.)

Once that hole is done -- loosen the 2 grabbers below--- slide down to the next hole and start again. Probably-- from what I have done-- all that will be needed is to
work the cut edge a little with some 22 or finer aluminum oxide paper and a de-burr tool to get the cut edge to a polished condition. The router bit leaves a fine texture
which the paper takes away.

You Could make some kind of removable centering plug out of plexi that you could drop in the hole to center the saddle over the next hole's center.

Well -- thats as far as I have thought out the idea------

Tim

PS--- dremel makes a circle cutting device--- but I could never find on in stock locally. It might work fine too-