Can I pick your brains? How would you solve this puzzle....

[HankMcSpank]

Me again, had the chance to mull this a little.

Here's the immediate problem - just about every implementation of gun drilling has the drill static & the workpiece turing - that's fine when your workpiece is circular (gun barrel, kids musical recorder etc)...just lob the workpiece in a collet - it's perfect centred...job done.

But if the workpiece is a guitar neck (ie odd shaped!), then that's not gonna work very well (to much potential for it to be misaligned ...especially at the far end)

Therefore I reckon I need to have the gun drill bit turning & mount the guitar neck on a static table/bed - the problem here is, gun drills always have compressed air squirted down their centre (to expel the chips from the cutting end) ....so how do you supply/maintain compressed air down the centre of a drill bit that is turning? (ie what's the name of the part that allows a static pneumatic hose to connect to something that is spinning)

[Robin Hewitt]

Therefore I reckon I need to have the gun drill bit turning & mount the guitar neck on a static table/bed

Not sure that works. You'd think all rotations were relative, but are they? :naughty:

If you spin the workpiece you define a line through it, the axis of rotation.

If you spin the drill you define squat.

If a turning drill goes off line, it's centre is still the slowest moving part of the cut.

If a stationary drill goes off line, it isn't.

Question is, is that how a long gun drill bit keeps itself on target?

They cut and burnish.

[BillTodd]

Robin has it bang on (saved me a lot of typing :)) gun drills only work because the job is spinning.

That said: I think, with the right technique, you could drill 500mm into a soft-ish material without too much drift.

First, grab yourself some wood of the same type as the neck to practice on.

Use a sequence of drills each a 1/2 mm or so smaller than the last. (you might be able to do it with one drill, but that doesn't give you the option of 'correcting' the path slightly)

Fix (braze or silver solder) the smaller drills into the end of steel rods the diameter of the previous drill. (concentricity is obviously important, as is a correctly sharpened drill).

I've drilled an 8" deep blind hole in steel using this technique with a 4mm jobber bit that was just a press-fit into the end of a silver steel rod ( [edit]I pinned it to the rod to stop the drill turning)

The first drill hole is the most important. Make a sequence of sleeves to align the rod in the first hole, so the drill is guided for its whole length.

Drill using lots of pecking (3-6mm in, then pull out and clear the chips) .

If you don't have a long bed lathe handy, then some kind of long sliding rig is essential.

It's a difficult, not an impossible job.

Bill

[Robin Hewitt]

Robin has it bang on

Blimey, that's a rarity, thanks

I just took pics of a gun drill. They are a bit odd.

[HankMcSpank]

Blimey, that's a rarity, thanks

I just took pics of a gun drill. They are a bit odd.

As you say...odd (thanks for the pic)

Re not being able to keep the workpiece static (& rotate the bit), this page suggests it's one of the methods....

http://www.gundrillingsolutions.com/Pages/basicpr.html

Bill, the method you outlined I'd considered...I have no sliding rail though, so would have to contempate making one...but then I saw the cost of a pair of support rails @1.2mm long - gulp.

[BillTodd]

Bill, the method you outlined I'd considered...I have no sliding rail though, so would have to contempate making one...but then I saw the cost of a pair of support rails @1.2mm long - gulp.

They don't have to be that strong, they just have to be straight.

[HankMcSpank]

They don't have to be that strong, they just have to be straight.

Yes I realised that (as there'd not be much deflection force operating downwards), but I'd imagine getting 'straight' at a length of about 1.2m might prove a little difficult (I'm also thinking bass guitars might be an option so now allowing up to 600mm - so 600mm drilling 'travel' + 600mm to retract the neck before/after drilling it = 1.2mm rails needed)...which is why I looked at supported rails...they be straight!

i'm open to siggestions though (I can feel a custom rig coming on!)