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Tips on match drilling with a hole already behind

Draker

Well Known Member
Continuing the fun started in this past thread, I've got a new pair of longerons bent and fit to the already-drilled skins/bulkheads. Preparing to re-drill them. Some problematic areas will be where I originally drilled through several pieces in a stack. Now I need to drill through just the longeron while somehow ensuring alignment with the other existing holes. For example, skin to longeron to engine mount.

Illustration:

i-zFsjZqC-M.jpg


Any good techniques or tips on how to accomplish this correctly, through 1/8" thick metal? I know from experience that, no matter how well things are clamped and how straight I drill, if I just match drill through one of the existing holes, the bit will usually come out slightly misaligned with the rear hole and/or make it oblong.
 
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How 'assembled' is the stack? Can you peel the skin away from the backing structure? I'd cleco the longeron to the skin using existing holes (you have to start somewhere drilling holes), & drill just the longeron using the skin as a guide. If things are tight, slip some scrap between longeron & back piece to prevent drilling into the back piece. If I didn't trust myself to keep the drill perpendicular to the skin, I'd use a jig of some sort to align the bit. That could be something as a wood scrap, drilled in a drill press. Once the longeron is drilled, I'd then 'match drill' to the underlying structure. The skin/longeron will be 'right', and if there's slight slop in the backing structure, the rivet shop head end will do a pretty good job of filling that. But I'll bet you'll find it pretty close, as long as your original hole was drilled perpendicular to the skin.

Charlie
 
Drill bushing

I found the bit is most likely to wander in the parent hole and widen it. To prevent this, I use a drill bushing to drill the hole as small as possible. If misaligned, it won't egg shape the hole on the other side. You can then use a jig or block to adjust the hole. Difficult with steel but it can be done.
Drill bushings can be sections of brass tubing stacked inside each other or fancy ones. I use a short section of 1/16" brass tibe often. It fits a #40 hole with a #41 bit. Keeps the bit from enlarging the guide hole.
https://drive.google.com/open?id=0B7hyZ-sKQmtrN01tUXdGOWdEUlE
 
I think Larry has a nice idea. That will allow you to drill a perfectly centered pilot hole and you can see how centered it is on the backside and possible correct when you upsize. It is a tricky drill. Chances are just a 90 degree drill jig/block would be off slightly too on the backside as I imagine you just drilled it by hand.

Whatever method you choose practice it on the old longeron before drilling your new one. You can set up the same conditions.
 
If you can use the old longeron as a drill guide, you could follow Larry?s advice and use a drill bushing in it. 1/8? thick material may allow the bushing to set at the original drilled angle. Use the butt end of the drill bit to align the old longeron holes, clamp, drill, then cleco. Check the next hole and keep moving.
However, from the other post, it sounds like the old longeron won?t line up to the newly bent one.

So, I agree with Charley. This is a pretty thin sandwich of material. Any misalignment should be pretty minor. Use a drill bushing as Larry suggests and visually check alignment. It may very well be within tolerance. If not, it shouldn?t be out much as Charley suggests. If it was a thicker assembly, there are ways to fix the hole as long as it was drilled undersized. However, the fix to redrill an existing misaligned hole can be very involved.
 
This is a variation of the problem faced by aircraft builders with wood spars.

The issue is mounting steel straps on each side of a spar for through bolts. A U-shaped jig is usually used - pic stolen from a KR-2 site.

kmlws11.jpg


A smaller version in steel should work in your case. Use a pin on the aft side to locate and then drill through from the front.

Drill the jig in a drill press - the only part that has to be accurate is the straightness of the hole through both 'ears'
 
This is a variation of the problem faced by aircraft builders with wood spars.

The issue is mounting steel straps on each side of a spar for through bolts. A U-shaped jig is usually used - pic stolen from a KR-2 site.

kmlws11.jpg


A smaller version in steel should work in your case. Use a pin on the aft side to locate and then drill through from the front.

Drill the jig in a drill press - the only part that has to be accurate is the straightness of the hole through both 'ears'

I have done this on straight spars. However, the longeron is an angle, and not straight. This complicates the drill guide. Still can be done, but keep in mind, you would need a jig for each hole, or long jig to capture all of the holes as each could have been drilled at a different angle, and the spar is also bent. That is a lot of work, and unless you have very good machining skills, the chance of making this work is slim.
I don?t think this solution would be practical. Still good info for those that have not been down that road. I am currently doing this to refit wing tip castings that where sandwiched at the end of the wing spar (not an RV).
 
I have done this on straight spars. However, the longeron is an angle, and not straight. This complicates the drill guide. Still can be done, but keep in mind, you would need a jig for each hole, or long jig to capture all of the holes as each could have been drilled at a different angle, and the spar is also bent. That is a lot of work, and unless you have very good machining skills, the chance of making this work is slim.
I don?t think this solution would be practical. Still good info for those that have not been down that road. I am currently doing this to refit wing tip castings that where sandwiched at the end of the wing spar (not an RV).

I disagree... you are assuming a close-fitting jig.

The jig need be only a loose fit around the longeron, and after each hole is drilled a cleco can be used. No need for multiple jigs. Different angles make no difference. The only important thing is that the drill itself is a good fit in the hole - which is why I suggest a steel jig, but drill bushings would work.
 
If this is the top longeron of a -7, there should be fewer than a dozen holes in each longeron that have material behind it. The weldment at the firewall has about 7 holes; the only others I see with a quick look are the vertical angles over the spar.
 
I disagree... you are assuming a close-fitting jig.

The jig need be only a loose fit around the longeron, and after each hole is drilled a cleco can be used. No need for multiple jigs. Different angles make no difference. The only important thing is that the drill itself is a good fit in the hole - which is why I suggest a steel jig, but drill bushings would work.

Yes, I have only used this technique with a close fitting jig and much thicker material that can easily be indexed.
I guess as long as you can securely clamp this in place so it doesn?t move after you index it....
 
Yes, I have only used this technique with a close fitting jig and much thicker material that can easily be indexed.
I guess as long as you can securely clamp this in place so it doesn?t move after you index it....

I'm not sure you even need to clamp it. :)

The pin in the back will locate it - it just needs to be held reasonably still for the short drill through the longeron.
 
There are many ways to to do what you need to do. As one person mentioned, a "strap duplicator" works well. Also, you can use a "bushing". Basically a hollow tube equal to the diameter of the preexisting hole. Or you can simply drill through it as best you can, then if the holes are slightly oblong, you can oversize them. If you went with a HiLok you can use a 1/2 over size.

If it were me I'd go with bushing method. (https://www.grainger.com/category/m...+Dia.|0.014~~Outside+Dia.|5/32"&filters=attrs)
 
My take

For all but the firewall holes the back piece should be a tab that will have some movement. I think a tender touch will go a long way to making this a non issue
 
Thanks everybody. After thinking about the problem a little, I came up with something combining Gil's jig idea with Larry's bushings, that I think will fit the bill:

Let's see what I've got here: A bunch of brass tubes, 1/8", 5/32", and 3/16" O.D., a 3/8" hardwood dowel, and a squeezer yoke that will fit around the longeron:

IMG_1477-M.jpg


Cut off a length of dowel, and chuck it in my drill press, sanding it down:

IMG_1481-M.jpg


Until it fits snugly in the yoke:

IMG_1478-M.jpg


Now cut about 3" of the 3/16" rod, chuck it in the drill press, and file off just enough so it fits snugly in the other side of the yoke. Slide the 5/32" rod inside of the 3/16" rod:

IMG_1479-M.jpg


Secure the yoke against my drill press table and support the dowel from underneath:

IMG_1482-M.jpg


With a long 1/8" bit, use the tubes as a drill guide to drill dead-center through the dowel:

IMG_1480-M.jpg


Now, cut two lengths of 1/8" rod, slide one through the dowel, and the other through the other rod stack-up. Now I've got an adjustable tool that I can drill a #41 or 3/32" hole directly through without any chance of missing:

IMG_1485-M.jpg


It fits right in the existing holes!

IMG_1486-M.jpg


Not quite ready to drill yet, but I'll report back later on the results. I'll be trying it out with some scrap first, of course, but I think this is going to work.
 
Brilliant. Send that to Kitplanes. More will benefit from your idea.

I concur! Kudos to Larry and Gil and even more for your ingenuity in taking their ideas and making it work for you. I have no doubt it will work well.
 
Credit

I concur! Kudos to Larry and Gil and even more for your ingenuity in taking their ideas and making it work for you. I have no doubt it will work well.

To be fair and honest, my Mentor, Dave Paule gave me the tubing tip. He taught me so much.
 
Verdict: Mixed results

I could reach 4/7 holes with the tool and they turned out perfect. The engine mount flange gets wider towards the firewall and for the forward-most three holes, the jaws of the yoke would not fit, so I had to use another method to get them. Used a drill block and the 1/8" rod as a bushing, and drilled from the skin side inward with a very small bit, then a larger one, etc. until final drilling #30. Turned out fine.

Thanks (yet again) for VAF coming through with critical inspiration!
 
You have attained Puzzlemaster status.

Verdict: Mixed results

I could reach 4/7 holes with the tool and they turned out perfect. The engine mount flange gets wider towards the firewall and for the forward-most three holes, the jaws of the yoke would not fit, so I had to use another method to get them. Used a drill block and the 1/8" rod as a bushing, and drilled from the skin side inward with a very small bit, then a larger one, etc. until final drilling #30. Turned out fine.

Thanks (yet again) for VAF coming through with critical inspiration!

I was just telling a non-builder that it was like a giant puzzle to build an airplane. You are a puzzle master, sir!! Well done.
 
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