How do I accurately enlarge existing holes in wood?

[JohnLindsay]

I need to increase the diameter of some aluminium bushes in a wooden bulkhead and also in the wing spars, from typically 0.75" to 1.0". The bulkhead is 0.75" thick with 0.25" ply on both sides (1.25" total). The spar is 1.5" thick.

It is relatively easy to cut a very smooth 1" hole in wood from scratch but are there any clever techniques/tools/ideas for cleanly enlarging an existing 0.75" hole, whilst accurately retaining its position? In the case of the wing spars, I need not only to enlarge the hole but also to do it through as small an opening in the fabric as possible?

Suggestions so far include dreaming up an appropriate cutter/hole saw with a spigot and inserting a guide bush with a hole for the spigot into the back side of the bulkhead/spar, perhaps drilling half way through from each side so as to meet in the middle. Alternatively, turn up a finely tapered plug with sandpaper glued to the outside and a central spigot; hold the spigot in a drill chuck and gradually wear away the unwanted wood.

All ideas very gratefully received. Better still, if you have been faced with the idential problem and have solved it by creating a special tool or jig .................!

John Lindsay

[macconnacher]

The only technique I can think of is to make up some plugs of doweling with pilot holes in the centre done on a lathe. Glue in the dowling in the exisitng hole and then drill out to the new dimension removing the dowling in the process.

[bertdeleporte]

Hi,

other current repair technique:
-cut a metallic plate 10''x10''
-drill a 1'' hole
-properly position the plate centered to the existing 0.75'' hole; you may use, if possible, a 0.75"/1" plug machined on a lathe
-clamp it FIRMLY (use a counter plate on opposite side if needed, to avoid dent on wood)
-remove the plug
-drill the wood through the hole in the metallic plate, making sure you keep the drilling machine orthogonal to the surface

The best is to work on a drill press if parts are split and handy.

If you need a sketch, tell me.

Beware your fingers while drilling,

Bertrand

[JohnLindsay]

Gentlemen,

Thank you so much for your most helpful replies. What you suggest makes perfect sense; my only concern is why I was unable to come up with such ideas myself!

Because the aircraft is assembled and there is not much space, I shall pursue your suggestion Stuart but, if possible, incorporate something of your idea as well Bertrand to reduce the possibility of my drill wandering off-square.

Fingers crossed (while I still have some!).

John.

[Rob Swain]

John,

I'm guessing you are trying to up the MTOW of your VP1 from Series 1 to Series 2 spec.

My partner in aviation crime has done this on a fully covered and otherwise ready to fly VP1 (G-BAPP) and I think we still have most of the tools he used to do it with, which we may be able to pass on to you.

It involves a holesaw and a custom made mandrel. For more detail, like how to do it in the wings, you'll have to talk to him.

Call me at the weekend and I'll put you in contact with him.

[JohnLindsay]

Thanks, Welshman. I'm very new to wood and have never owned or seen a router in action but what you suggest sounds as though it should be perfectly straightforward.

Thanks also to you, Rob. I am indeed turning a Series 1 into a Series 2. I'll certainly give you a call tomorrow (Saturday) because nothing would give me greater reassurance than to speak to someone who has actually tackled the problem. If there are some tailor made tools that I could perhaps borrow, my cup would runneth over.

Where would I be without this Forum!

John.

[Ian Melville]

Where would I be without this Forum!

John.

On a diet to keep below the MTOW

On the subject of routers you can get small ones called palm routers (Bosch GKF600) which would fit in a confined space better. Check it has enough power first though.
Routers can be used with templates, so make a MDF/steel/alu plate with required hole size in middle and clamp to spar in exact position. Then rout out the excess wood. Make sure you use a bit with the bearing. eg laminate trim bit.

[Nick Allen]

I asked my housemate, a German master carpenter, how she would do this, and she recommended along Stuart's lines, i.e. cut a 0.75in. plug; glue this in place. If possible clamp some wood flush to the back of the assemblage -- protects the rear surface as you cut through. Then use a very sharp 1in. hole cutter...

[Rob Swain]

I asked my housemate, a German master carpenter, how she would do this...
_________________
If you want to build a ship,
just get your housemate to do it!

[Nick Allen]

(It would be an excellent ship...but I'd be waiting a long time to set sail: such is the downside of the obsessive German quest for perfection!)

[JohnLindsay]

Just to say that, thanks to all the above very sound advice and the generous loan of some tools from Rob Swain's partner in aviation crime, I have now done the job successfully without tearing even one strand of the fabric on the wings. The holes are drilled and the new bushes are currently being machined to size.

In short, the approach was very much along the lines outlined by Stuart Macconnacher and others above which was to leave the existing metal bushes in place, use a very sharp extra-deep (new) hole saw with a bespoke mandrel to suit the hole in the bush, thread a flexible drive through one of the the rear spar inspection holes in the fabric and drill gently and with patience. It needed a second person to work the drill and occasionally stop the flexible drive from trying to tie itself in knots.

A problem that I thought was going to set me back months is solved and the work almost complete.

Once again, thank you all.

John Lindsay.