View Full Version : Forstner drill bits for drilling lensboards? A tour d' forstner or just another boring story?

John Kasaian
18-Nov-2003, 22:59
Hello! My 3 year old son and I went to Costco the other day and came home with a nifty set of umpteen forstner drill bits in sizes that looked useful for drilling lensboards, all in a fitted plastic case that reminded me of Sean Connery's attache case in 'From Russia, With Love.' I chucked one of those bits into the drill press and centered an old Agfa board over some scrap lumber, clamped the whole she-bang down and went to town, drilling from the face of the lensboard. It produced a nice clean hole ever so slightly undersized so a little light sanding should suffice to clear the lip of the flange so I can mount my 14" Artar. After turning the board over though, wooden you know it, the Forstner bit had raised some splinters on the painted side of the lensboard. Harmless, and a little black paint should make it look nicer, but I've seen other lensboards with the same crude splintering. Is this a forstner kind of a thing or is it on account of trying to drill an old piece of joinery? Any recommendations to keep it from happening again when I drill a much thinner Graflex "C" board to mount a 203 Ektar?

Paul Kierstead
18-Nov-2003, 23:11
You could try clamping the board tightly to a backing board. Generally this helps with tear-out on a drill press. Use the slowest speed on the drill press and go easy.

David Richhart
19-Nov-2003, 02:05
Hi John... The backing board can help a lot.

When drilling the hole in a door to install a doorknob, carpenters will stop drilling as soon as the small 'centering point' of the bit comes through the opposite side of the door... then use that small hole as a guide to finish drilling from the opposite side. I hope this simple description helps.

19-Nov-2003, 05:16
I've found that a hole saw bit in a drill press also works well. I drilled an aluminum lens board this way. It still needed a little touch up with a file on the inside to remove the burr left by the bit.

I think that with the bit you used, a good tight fit to a backing board is necessary and I'd try faster rpm on the bit, with a slow feed. Some experimenting will be in order to find the best speed.

David A. Goldfarb
19-Nov-2003, 06:57
As the others have suggested, clamping the board firmly to the backing is the key. I made several wooden lensboards for an old Linhof Tech II this way using a keyhole saw and I used three clamps to hold down the board and didn't have any splintering. A Forstner bit should be even cleaner.

Paul Kierstead
19-Nov-2003, 07:58
Dee suggested a faster RPM. I would note if you to this to be very careful. Large bits such as these have a very high rim speed and overhead extremely easily; consider the rim speed of a 1/4" bit and a 2" bit at the same RPM and you will begin to see where the problem is. If they overheat bad enough to blue the edges, temper can be ruined and the bit will not hold an edge.

Witold Grabiec
19-Nov-2003, 09:22
I agree with Paul, speed must be matched to bit size. It must be fast enough to provide a clean cut, yet slow enough to prevent the burn. Speed at which you feed the bit through place a role in here as well. As others have said, a back board is the only way to get a clean exit cut. In order to get best cut on both sides, you can sandwich your lens board between two other (all you need is a 1/4" plywood) and this will also ensure a perfect entry cut as well (drill a small center hole in the board and another in the top sandwich piece to guide you through).

Todd West
19-Nov-2003, 10:57
Just a small observation that you don't absolutely have to use a backing board. Drill partially in on one side, flip the piece over and drill the rest of the way from the other side (same idea as the precutting often used to prevent splintering veneer laminates on table saws). This requires more alignment and more time than using a backing board, but is sometimes a better approach. When the web in the middle between the two bores gets thin, it will tear out (doesn't matter what kind of bit you use, though I prefer hole saws), but this seldom damages the part unless you're drilling dry, brittle wood. Even then you can usually get a clean hole by feeding very slowly when the web's about to break.

Alan Davenport
19-Nov-2003, 11:33
If the bit is the right size, and the lensboard material soft enough to use it on, go with it. My personal favorite is an adjustable fly cutter; requires a drill press that can go quite slowly, and the usual stuff about anchoring the board to the table. You definitely don't want to be hanging onto a lens board by hand when using a fly cutter to drill...

Alan Davenport
19-Nov-2003, 11:35
Regarding the suggestion for using faster RPM, let your insurance coverage be your guide. Use slow speeds for large holes, esp. in aluminum. If you're using a bit that punches through the center first (includes fly cutters and most Forstner bits) then as soon as the center pokes through, you can reverse the lensboard and finish the hole from the other side, no ragged edges that way.

Witold Grabiec
19-Nov-2003, 17:19
I'm gonna have to disagree with flip-it-over suggestions. While you can get it done this way, you may need a few more boards to get it right. Unless you make a jig that will register your board in exactly same spot on either side. Your small hole is not a good enough guidence to rely on it. I know your cut does not need to be perfect, but since you seem concerned with the quality, use the backing board, which will ALWAYS give you a nice clean cut if used properly.

As for the speed of rotation, you cannot go too slow with a Forstner bit, in fact any bit except a hole saw. Your speed must be matched to material. The softer the material the faster the rotation so as to prevent tear-out on entry.

Bill Laidley
19-Nov-2003, 23:35
Jeez guys... I don't know if we aren't making this too difficult. I can find something to agree with in most of the replies - but I can also find something to disagree with in most of the replies.

Flipping the board: yes this works to reduce chipping. Is there a chance of misalignment? Yes. Is the misalignment significant? That depends on how careful you are.

Using a backing board: this also works to reduce chipping. Is there a chance that it will still chip? Yes. Two things will help - clamping the lens board tightly onto the backing board, but more importantly make sure the drill bit is sharp and properly ground. You decide if a bargain priced forstner bit is apt to be really sharp and properly ground. It also helps for the bit to be turning at the right speed.

Gawd, if we want to get really obsessive we'd could buy $15,000 pin routers, make a set of jigs and have perfect holes in our lens boards.

Aren't we supposed to be taking pictures?

20-Nov-2003, 07:33
Alignment is simple. I clamp two pieces of wood [about 1"x1" long enough to span the drill press table] The lensboard goes between these two. If you're drilling the middle of the board then flipping the board won't be a problem. The board can only move between those two boards. It's fairly simple to align things.

Who makes cheap large forstner bits? All the cheap sets I've seen max out at about 1". Other then the odd 3" clock bits anything over 1" tends to force you to get better bits.

You could try masking tape on the back of the board.

Big bits == slow speeds.

John Kasaian
20-Nov-2003, 08:58

The set my local Costco is selling is called Mastergrip with sizes from 1/4" to 2-1/8" going for about $35 bucks.

I just drilled the "C" board after building a jig to register the piece and flipping the board over halfway through the drilling---no splintering! The registration is just a hair off, but I undersized by 1/16th, so when I sand it down to fit the lens it should be looking good.

Thanks for all the suggestions!

Bill Laidley
20-Nov-2003, 23:29
Okay, to give you a price comparison: Lee Valley (Canadian woodworking tools company) sells a set of 29 bits from 1/4" to 3 1/8" (technically the bits are forstners from 1/4" to 1", and saw tooth above the 1" size) for $249.00 CAD (high carbon steel) or $329.00 CAD (high speed steel). They aren't the cheapest or most expensive place on the planet, but the quality is good.

21-Nov-2003, 11:23
I can vouch for the "cheap" Forstners. They work well but may not stay sharp for very long. I have cut several boards with them on a drill press with no problems. Even quickies in cheap plywood. Simply clamping the board onto another board takes care of most of the splintering as has been said.

But I would like to make a recommendation. I have had to cut a few where the Forstners either weren't the right size or I didn't have a large enough one. For those, I use a circle cutter in the drill press. Adjustabe to any size and easily set accuarately with a caliper. Cuts a hole as well as a bit and will even do aluminum boards with a slow speed and light pressure. Well worth the minimal investment! Just be very careful when using one!

tim o'brien
21-Nov-2003, 12:26
I prefer the hole cutting bits from Milwaukee, slightly undersized, turn over the board before you go all the way through with the outer cutter, finish it off then take the lensboard and chuck it on my lathe. Using a sharp chisel and sand paper, I can get the hole perfectly centered, clean and pefectly sized for the lens in question. It also allows me to put in stepped edges if required for those hard to fit locking rings. Coat of flat black on the back, coat of polyurathane on the front, and voilla!

tim in san jose