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I'm trying to install an endpin jack in a finished guitar, but my end block is a little too thick for the hardware to screw together. I figure if I can figure out a way to countersink about 1/4" on the inside I'll be in good shape. I know I've seen a tip here on how to countersink a drilled hole inside a closed body while working from the outside (maybe on an archtop?), but all my searches have come up empty. Anyone have a link to the thread I'm talking about, or have any clever ideas?

Thanks,

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George :-)

Reverse grind an old spade bit..........

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Goggle "reverse spotfacer". You may need a bushing for the shaft to rotate without wobbling in the endpin hole if you have already reamed it out for the jack.

With a properly supported shaft they give a very clean cut.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

That's the ticket! I've been banging my head over this all night.

Thanks!

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George :-)

Just had to do this today on a 70's Martin pickup install. I have a few reverse ground spade and forstner bits I keep around for this job.

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Eschew obfuscation, espouse elucidation.

We use 3/4" Fuller counterbores with ring-style collar stops for depth-of-cut control for this job, as well as for counterboring neck bolts on conversions and bolt-ons where the builder forgot to mill the counterbore before closing the box. As the boss says, 'DAMHIKT' . The Type B countersinks are several times more expensive than a good quality 3/4" spade bit, but save the time involved in the conversion and take a stop collar without modification. Mild steel from the big box store works for the counterbore's shaft, but a 3 foot long length of 5/16" drill rod is not expensive and is stiffer.

If the tail pin hole has already been opened, it is easy enough to find nylon or steel bushings with the correct inside and outside diameters at Home Depot - this works for both spade bits and forstner counterbores. A shop-made wooden bushing can be made with a plug cutter and a brad point drill on a drill press.

Type B Counterbores:

http://www.jamestowndistributors.com/userportal/show_product.do?pid=498&familyName=Type+B+Counterbores

Collars:

http://www.jamestowndistributors.com/userportal/show_product.do?pid=495&familyName=Stop+Collars

Worth mentioning that by removing the screw-on shell, you might gain enough length on an acoustic jack that a counterbore might be avoided. Be sure to add some LocTite to the remaining nut, and use vinyl electrical tape over the hot terminals and copper shielding tape over that (and in contact with the outer ground lug) to provide the shielding effect lost with shield removal. This will usually pick up from 1/16" to 1/8" of extra length on even a short (1-1/8") jack, which might be enough to avoid more woodworking. Removing the shell is also insurance against it loosening and rattling, which is not uncommon to see in the shop (we use Blue 242 LocTite, which is removable, on all acoustic jack interior locking nuts to avoid the issue).

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For the times they are a changin'

- Bob Dylan

Reverse ground spade bit.............................OH my gosh, David is human after all. I thought it would be laser guided automatically measured for dia. and depth.........!! Just kidding of course, have always admired your engineering and approach to accurate work..............also your teaching style is top drawer. That's enough of that for now. Cheers.
Tom

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A person who has never made a mistake has never made anything!!!

Pictures worth a 1000 words, this is for a neck bolt pocket, you get the idea -- you'll need a sleeve (nylon from a hardware store) to keep centered in the current hole

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Ken Cierp

http://www.kennethmichaelguitars.com/

Excellent help, fellas! Greatly appreciated.

Thanks again,

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George :-)

If it helps restore my image at all, I actually use a reverse ground Forstner for this job, with a sleeve turned down to snuggly center in the hole, so I did make it a little bit more complicated.



That said, even if the shaft doesn't make for a tight guide, and even if you're using a spade bit that's not that precisely ground, it's funny how little difference it seems to make - if you pull straight and start it clean, they can still cut a remarkably clean counterbore.

Of course having the shaft ground to keep it as well centered as possible helps you keep the start of the cut clean (avoiding a rough looking edge at the surface). Then a full round shaft like on this Forstner or a proper reverse counterbore can really help if you're cutting in to an uneven surface, like those Guilds that have the bizarre strip inside the end block which serves no reasonable purpose I know of.

Once you get the hole started though, any decently filed spade bit can cut a very clean counterbore. For depth, it's easy to just mark the shaft to the target end thickness, and stop when you see the mark come out of the hole.

There are good reverse counterbore systems available, and they're good to have around the shop. When I need one though, I usually need it now, and it's quicker and easier to grind one out of an old bit (lots of those around the shop) than to search, order, and wait.

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Eschew obfuscation, espouse elucidation.

Just wanted to say thanks for this thread and for the OLF. I needed to open up the counterbore in the block on a uke I'm working on. This post and the reverse ground spade bit did the trick! The pic shows a test, but the counterbore in the block turned out pretty good.



Brad

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I've used Fuller B4 style counterbores to recess neck bolts that were drilled after closing the body - the same approach could work for end blocks.

I had to address this issue when I replaced a neck that had a dovetail joint with a bolt-on. After filling the dovetail "mortise" in the heel block, I drilled the two 1/4" holes in the block, then reversed a 3/4" Fuller counterbore on its drill bit. After feeding the drill bit through one of the holes in the heel block (from the inside, of course) I tightened down the chuck of the drill motor onto the bit, then ran the drill in reverse and pulled the motor toward myself, until I had a nice, flat-bottomed hole in the block. Then repeated the process for the other hole. Hope this makes sense. If I can find some photos, I'll post them.

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michael propsom
www.propsomguitars.com

You could also use one of these -



With a removable countersink section like this you could just flip it around on the bit... or onto a longer bit of bar stock if needed.

FTL

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www.leddingtonguitars.com

Some sort of counter bore is better, as you end up with a flat bottomed hole.

I've found that even wrapping the shaft of the spade bit with tape can be enough to get a pretty good start. It's surprising how well they tend to center.

How does finished bore get the required flat bottom? Sorry not seeing how those bits could work

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Ken Cierp

http://www.kennethmichaelguitars.com/

Keep in mind most forstner bits have a little taper in the rim for clearance. If you re-grind one, you may need to remove material from the outside. This can make it slightly undersized. If you don't, you may be limited in depth by the increasing friction as it wedges in the hole.

This is the Fuller counterbore that works for me. The tape marks the depth I wanted to stop. The majority of the cut is dead flat.

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michael propsom
www.propsomguitars.com

The original post just mentions needing to countersink... not mention of a flat bottom. It was just a suggestion dude!

FTL

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www.leddingtonguitars.com

I used one of these - a little pricey but it's a one-time buy and it works great. I used a long piece of 1/4" steel rod through the 1/4" bolt holes in the head block, instead of the drill bit shown in the picture.

https://www.amazon.com/gp/aw/d/B01MF8TZ ... UTF8&psc=1


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