Sharpening Twist Bits, Neander Style?

Hi guys,

I picked up some vintage Union Twist bits with tapered, square shanks. They fit quite nicely in a small 6" brace that has a square chuck with a retention screw -- I've become quite fond of this lightweight little brace for drilling pilot holes, counter sinking, and driving screws. Light weight, handy, and quick to change bits.

Anyway, the twist bits are sharpened at a very obtuse angle meant for metal. As such, they require a *lot* of downwards force to drill into wood, especially the larger diameter ones. This has always been the case with twist bits sharpened at obtuse angles, even relatively new and sharp ones. These might also benefit from being sharpened, though they are reasonably sharp still.

I've never sharpened twist bits, and the info I come up with online is unsatisfactory at best.

My questions are basically:

1. Is there supposed to be a relief angle behind the cutting edge, IE, meaning, that the tip should not be perfectly canonical in shape, but rather a little more steel should be removed behind each cutting face to make sure that the cutting edge, and not the bevel, is in contact with the wood?

2. At what angle would you sharpen these for wood? 30 degrees? I understand that obtuse angles are used for metal, I guess for durability, but if I also wanted to, just occasionally, drill metal, but mostly wood, would some compromise angle, like say 40-45 degrees, be advisable?

I'm guessing I should just chuck them in an egg beater drill somehow so I can spin them (maybe make a wooden adaptor)... or maybe some kind of jig that holds them steady while I spin them with a brace, like a hole drilled in a piece of wood at the desired angle and stuck in a vice, while I spin them on a stone or abrasive paper to get the desired angle, and then I can come back with a small diamond stone and relieve the metal behind the cutting edge (the would be "bevel") a degree or so if necessary. Am I thinking right here?

Hi Luke

I suggest you search "twist drill bit geometry" that will give you great info about twist bits. The required relief angle, IMHO, will not be achieved with your method.

Have a look at a drill sharpening guide that has been around for a long time, the General Tools bit sharpener. It takes care of the compound angle needed to produce the relief angle. It is used on the side of a grinder wheel. It sweeps the bit onto the wheel at a continuously varying angle.

Hand grinding was the way to go, and still is for some people, before such jigs were available, but probably always with a rotating grinder of some kind. The hand motion is something to be mastered...

Regards, Andre

Luke, my bits with metal drilling geometry generally do not have any problems drilling with a brace. It just goes a little slow.

Heck, even with a tap holder the standard bits seem to work fine for me:

Attachment 471517

Are you sure your bits are sharp?

Often my bits are touched up by hand. Other times my power sharpening system is used. There is also a hand Drill Doctor® in my shop for restoring bits in really bad shape.

Bits for drilling through wood do have a much more acute angle. Some of mine can be checked tomorrow if my angle gauge will work on drill bits.

As Andre mentioned, search drill bit geometry to find out about relief angles.

My dad told me a long time ago point on the is supposed to have a small line, not a perfect point. This is what breaks the surface to allow the cutting edges do their thing.

jtk

Quote:

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Originally Posted by Andre Packwood

Hi Luke

I suggest you search "twist drill bit geometry" that will give you great info about twist bits. The required relief angle, IMHO, will not be achieved with your method.

Have a look at a drill sharpening guide that has been around for a long time, the General Tools bit sharpener. It takes care of the compound angle needed to produce the relief angle. It is used on the side of a grinder wheel. It sweeps the bit onto the wheel at a continuously varying angle.

Hand grinding was the way to go, and still is for some people, before such jigs were available, but probably always with a rotating grinder of some kind. The hand motion is something to be mastered...

Regards, Andre

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Quote:

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Originally Posted by Jim Koepke

Luke, my bits with metal drilling geometry generally do not have any problems drilling with a brace. It just goes a little slow.

Heck, even with a tap holder the standard bits seem to work fine for me:

Attachment 471517

Are you sure your bits are sharp?

Often my bits are touched up by hand. Other times my power sharpening system is used. There is also a hand Drill Doctor® in my shop for restoring bits in really bad shape.

Bits for drilling through wood do have a much more acute angle. Some of mine can be checked tomorrow if my angle gauge will work on drill bits.

As Andre mentioned, search drill bit geometry to find out about relief angles.

My dad told me a long time ago point on the is supposed to have a small line, not a perfect point. This is what breaks the surface to allow the cutting edges do their thing.

jtk

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Thanks!

I think you're right, Jim -- they seem not to have been all that sharp.

I googled Twist Drill Geometry as Andre suggested and got the gist of it, along with studying my larger bits. The fact that there is in fact a relief angle, and that there should be a chisel point were a big help.

I then proceeded to sharpen up one of my two 3/16 bits, just following the geometry that was already there with a credit card diamond stone. The bit that I sharpened works much better than the unsharpened bit now, so it appears I've succeeded!

I just stuck with the original geometry and didn't change the angle. The compound+canonical angle of the bevel is a bit intimidating but I think I can manage grinding it by hand -- I'll try with some cheap modern twist bits a few times before risking my vintage ones.

Anyway, this is a game changer. I've thrown away so many twist bits in the past because I didn't know how to sharpen them. Never again!

Sharp fixes most things like this. Get a Drill Doctor and don't look back.

Too much angle on the drill bit's tip and they'll self-feed... aka screw themselves into wood. There's sort of a balance with brace bits, you want them to self feed a little, so you don't have to push down hard.

Another thing that will help is drilling pilot holes.

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Anyway, this is a game changer. I've thrown away so many twist bits in the past because I didn't know how to sharpen them. Never again!

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Even my broken bits get saved. They can be used to make marking gauge pins and other useful tools.

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Another thing that will help is drilling pilot holes.

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This is especially true with larger auger bits. They can split a piece of wood with their large lead screw, especially when drilling near the end of a piece.

When using various bits for screw pilot holes the bit for the countersink can be used first. This leaves a dimple to drill for the thread pilot that can then be followed by the shaft bore.

As an example with a #6 screw use an 9/32" bit just deep enough to reach the full diameter of the bit for a flush fit of a flat head screw. Then drill the center with a 5/64" or 3/32" (smaller bit for softwood larger for hardwoods) for the threaded area and then a 9/64" for the shank if any.

Attachment 471528

Note the shank size is often half the head size. This is the case with many wood screws.

jtk

Peter Galbert's Chairmaker's Notebook has an interesting appendix on regrinding regular twist bits into brad points (with extra long tips for help in drilling at angles) and into a special shape he came up with for drilling into end grain. I haven't tried them yet, but it is interesting and looks relatively easy to do.

Many decades ago a gentleman in a factory where I worked taught me to sharpen twist bits by hand on a bench grinder. It wasn't hard once I knew how! I've sharpened lot of fairly large bits this way, most recently for drilling holes on the metal lathe. Basically you hold the bit so the cutting edge will contact with the grinding wheel with appropriate two angles. Then grind while rotating the bit while at the same time dropping the handle a small amount. This gives a useful clearance angle so the bit will cut. The motion must be coordinated. Sharpening this way is is easier to demonstrate than describe. It's also easier the larger the bit. For small bits I use a cheap drill doctor or another jig I have, can't remember the name. You can also grind a flat on the cutting edge at the right angles but for most bits I prefer the curved bevel.

Quote:

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Originally Posted by Jim Koepke

Even my broken bits get saved. They can be used to make marking gauge pins and other useful tools.



This is especially true with larger auger bits. They can split a piece of wood with their large lead screw, especially when drilling near the end of a piece.

When using various bits for screw pilot holes the bit for the countersink can be used first. This leaves a dimple to drill for the thread pilot that can then be followed by the shaft bore.

As an example with a #6 screw use an 9/32" bit just deep enough to reach the full diameter of the bit for a flush fit of a flat head screw. Then drill the center with a 5/64" or 3/32" (smaller bit for softwood larger for hardwoods) for the threaded area and then a 9/64" for the shank if any.

Attachment 471528

Note the shank size is often half the head size. This is the case with many wood screws.

jtk

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Nice chart! I usually figure this out on the fly by just drilling various holes for the screw and shank and trying out each screw individually. I work with metric size screws and imperial bits, so I don't really know the perfect sizes. Maybe the screw sizes are the same though? I don't know.
I notice that even here, the sizes of small twist bits, while labelled as fractions of a mm, seem to line up with imperial sized bits (1/16th, 3/32, etc.), until you get to larger sizes (maybe 1/4 and up?) at which point they are graduated by 1mm, no funny fractions to match imperial fractions. So, maybe it doesn't really matter that my bits are metric or imperial all that much.

Great point about using broken bits for marking gauges and the like. I actually plan on making a bunch of marking and mortise gauges. I found a coat hanger with wire that has a surprisingly high carbon content (it's springy but not super hard) which I can heat up red hot on the gas stove in the kitchen and quench in brine, and get something just hard enough to make a pen knife. I "forged" such a tiny pen knife between two hammers, using the stove to heat it up in this manner, and it holds an edge way better than mild steel and gets much sharper, but is nowhere near as long lasting as proper knife steel. I'm guessing the hardness is somewhere around 45-50 HRC or something. Anyway, I thought I would use this for my marking gauges. I'll start saving my broken bits though :)

Quote:

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Originally Posted by John C Cox

Too much angle on the drill bit's tip and they'll self-feed... aka screw themselves into wood. There's sort of a balance with brace bits, you want them to self feed a little, so you don't have to push down hard.

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I think I discovered that angle accidentally. I was careful to maintain clearance and actually ground away slightly more than what was originally there on the back side of the bevel. Some of my bits just sorta self feed a little bit now, but not too excessively. I can see how this could be problematic if I increased the angle any more than what I have now though.

Quote:

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Nice chart! I usually figure this out on the fly by just drilling various holes for the screw and shank and trying out each screw individually. I work with metric size screws and imperial bits, so I don't really know the perfect sizes. Maybe the screw sizes are the same though? I don't know.

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At times, especially if a chart wasn't handy, a drill was chosen by just holding it next to or over the screw to pick the size.

Not sure if it works for metric screws but with imperial screws the head is often twice the size of the shaft. (The shaft being the unthreaded part of the screw.)

In my drill indexes the bit holders have a chart stamped into them for machine screw tap drill sizes. On the inside cover of some of my indexes the sizes of drills for wood screws are written with a Sharpie.

American bit sizes are in three measurement groups; fractional by 1/64 of an inch, numbered from 80 - 1 and letter designation. The numbered set is usually sold in a set of 60 - 1, 0.04" - 0.228" (1.016mm - 5.791mm).

Attachment 471633

Usually where there is a larger jump between the numbered sizes a fractional bit fills the void. As an example between the #48 (0.076") & a #47 (0.0785) is a 5/64" at 0.0781".

jtk