custom drill bit diameter

[LCHIEN]

I think I need some holes 23/32" diameter.
I understand that spade bits can be turned down by grinding
the edges a bit, in this case starting with a 3/4" bit which is 24/32", I'll need to grind off 1/32 total or 1/64th from each edge.

So any advice on how to do it, e.g how to mark it, how to keep the removal even and straight and parallel to the shank?

Should I just make a few passes with the grinder, keep close watch with the calipers, and then go to a diamond hone to hone the edges and straighten them by hand?

[jonmulzer]

Why not just a normal drill bit in that diameter? I have that size in several of my indexes and you should be able to pick up one at any of the big stores singly for not too much I would think.

[LCHIEN]

Why not just a normal drill bit in that diameter? I have that size in several of my indexes and you should be able to pick up one at any of the big stores singly for not too much I would think.

Sorry, Jon, my indexed sets of twist bits pretty much all go to 1/2" dia and stop there. And I have a bunch of them.
I have forstner sets and spade sets but they all go by 1/16th inch steps after 1/2"

I'll check the box stores when I go to shop for my sacrifice spade bit but I don't hold out too much hope for one in the 23/32" size at a reasonable price.

[cgallery]

If it were me (I don't have a grinder) I'd use my WorkSharp. I'd be concerned that a grinder would take off material a little on the fast side. I think the WorkSharp 3000 sharpener would allow me to be more precise, and keep the sides more even.

If I didn't have a WorkSharp, I'd use the scary sharp method of sandpaper on a piece of glass or a tile. After all, I've sharpened jointer blades that are 8-5/8" wide using this method, so I imagine a spade bit wouldn't be too bad.

Finally, a quick check of McMaster shows 23/32 bits for as little as $21 (p/n 2933A32). I realize that real men make their own drill bits, but...

Honestly, I think the spade bit does make the most sense, seeing as you'll be able to get the fit to perfect.

Just make sure you check all the conduit before you buy it to make sure it comes from the same batch and there isn't a wide variance in the O.D..

[Carlos]

My first thought would be to use the belt sander with a 60 grit belt. This will keep the sides straight, and a light pass per side back and forth should keep it even.

Actually my first thought would be to buy the one from McMaster...

[Pappy]

I think the disc on a combination sander would yield the best control and flattest backing surface, other than a Worksharp or Tormek. Just make sure it is blown clean of sawdust first and don't hook it to the DC.

[LCHIEN]

on thinking this over some, I was worried about how to take off an even amount off each side as there is no center reference point.

However, since both sides are parallel to each other and parallel to the axis, AND equidistant from the center, and I know the starting dia D1 and the target dia D2., I can remove (D1-D2)/2 from one side, make sure its parallel to the untouched side, then swap over and remove the same amoutn from the second side, and make sure its parallel to the first side.

Since I'm taking off ~.020" off each side, I guess I'll try with the 60 grit and the combo sander and disk and see how fast it comes off.

Thanks for the advice (and yes, I'll check the diameter one more time on the target tubes before going to town on the drill bit).

[cgallery]

Thanks for the advice (and yes, I'll check the diameter one more time on the target tubes before going to town on the drill bit).

It will be interesting to read your report on modding the drill bit. Every once in a while I find a 1/2" O.D. steel dowel won't fit in a 1/2" hole drilled with a 1/2" drill bit. I guess the tolerances just work against me.

Oh, and I have found that polyurethane (Gorilla brand) glue is fantastic for fastening steel to wood. I don't know if you plan on gluing them, but poly is great if you do.

[LarryG]

Loring, are you sure you're not over-thinking this? When I made some signs for a club bike ride last fall, using ripped-down 2x4s for the frames and 1/2" EMT for the posts, I drilled the holes with a 5/8" Forstner bit and found that seating the conduit required only a few taps with a hammer.

If your holes go all the way through, the cut end of the conduit ought to serve as a pretty good "chisel."

[Uncle Cracker]

I'd be willing to bet that the tolerance is not going to be that critical. Remember that you are drilling wood, so getting down to a gnat's ass really won't matter, since the hole is gonna swell a little anyway, just from moisture absorption. Just take a little at a time off each side of a spade bit with whatever grinder or sharpener you have, and drill test holes to check for dry fit. Remember to mark or etch the bit so you won't use it again looking for the stock 3/4" size.

[uglystick]

ShopNotes tip

Loring,

They use a scrap block, nail & file to reduce a spade bit in this ShopNotes tip:

http://www.woodworkingtips.com/etips/etip081800sn.html

I also remember reading a tip about putting a paper or tape shim on one side of the bit shaft in the chuck. This puts the bit slightly off-center, which increases the hole size. Seems like this would only work with twist bits, though.

[LCHIEN]

Loring, are you sure you're not over-thinking this? When I made some signs for a club bike ride last fall, using ripped-down 2x4s for the frames and 1/2" EMT for the posts, I drilled the holes with a 5/8" Forstner bit and found that seating the conduit required only a few taps with a hammer.

If your holes go all the way through, the cut end of the conduit ought to serve as a pretty good "chisel."

Larry, you may be entirely right. I tend to do that sometimes.
Like I said in the other post, I am driving a 0.625" dowel into the .610 ID of the EMT.
.710 OD into 5/8" (0.625) will be wishful thinking but into 11/16ths (0.687") might work... I was thinking about making it reconfigurable, tho, and would want to get them out easily.
I'll play around with it some more before committing.

[LCHIEN]

dang Lowes had 3/4" Irwin spade bit for $3.84.
On the way out I saw the end cap display...
5 piece Irwin spade bit set + bonus 3/4" bit for $7.97.

Duh, I bought the set with the extra bit. So now I have an extra set with
3/8. 1/2, 5/8. 3/4, and 1" bits for another $4.13

I really needed a third spade bit set like I need a hole in the head.

[Uncle Cracker]

I really needed a third spade bit set like I need a hole in the head.

What size hole?

[Tom Slick]

Here is how to lay it out so that you have marks to grind to:
normally you'd use layout dye (dychem) but krylon spray paint or even a sharpie pen is garage substitute
lightly spray one side of the spade bit with krylon so that the surface is covered but not thick.
determine how much you need to take off each side, about .016", and set your calipers or marking guage to that measurement and scribe a line into the marking paint by dragging one jaw of the caliper along the edge of the bit and the other jaw's tip on the paint. it will draw a sharp and even line in the paint
file up to the line and you should be fairly close to concentric/equidistant.