Moderators: Random Orbital Bob, nev, Charley, CHJ, Noel

By gcusick
I wonder if others have observed that (some) Forstner bits cut measurably undersize. I have 2 definite instances - a 14mm Axcaliber bit that cuts 13.6mm diam, and a newly-purchased Bosch 24mm bit (which I think is probably made by Fisch) that measures 23.6mm.

Neither bit has been re-sharpened.

I’m off to spend a happy afternoon measuring Forstner bits - you have to find your fun where you can :), but I’d be interested in others’ experience.

By sunnybob
The very first question to ask, is do you know if your vernier is accurate to start with?
When youre arguing over 0.4 of a mm thats an important question because its very odd that two separate measurements are off by the same 0.4. :shock:
By gcusick
Good point, Bob. I have used 2 different calipers, but I’ve also just ordered a 20mm gauge block because I can see the need to calibrate the vernier. However, the reason I started measuring the bits was that I used the 14mm one to fit some Hafele Zysa hinges, specified as needing a 14mm hole. I drilled a test piece, and found that the only way to insert the hinges was with a hammer! I picked up a 14mm reamer to open the holes out, which gave a perfect fit. Hence measuring the bit and finding it under size.
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By Eric The Viking
This thread I shall be floolwing with interest!

Did you power the bits with a drill press or a hand drill, or do something else?

I'm on my fourth generation of battery-powered hand drill now. The better quality ones, by far, were the first two. The latest one, bought circa 2010, has more runout than a break-in at a white mice breeding lab. It might be that the manufacturers of the bits are attempting to allow for this (fool's errand if that's it), so 13.6 would be supposed to give 14mm in a "normal" sloppy tool.

The other thought is that most of the decent Fortners I have are significantly tapered. If they are ground that way, any error in the factory sharpening might upset the radius they cut.

Watching with interest!

By hodsdonr
Forstner bits usually get 'red' hot when drilling so maybe the manufacturers are making allowance for the expansion and only the 2nd 3rd holes etc are true sizes. ( that we could be so lucky and the manufacturers think that far ahead)
By owen
The size probably changes slightly depending on what material you're drilling into aswell I guess. There's no need to be measuring anything with vernier calipers in woodworking, that kind of accuracy is just not needed.
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By Eric The Viking
gcusick wrote:Except, of course, when you're fitting a metal or plastic part that needs an exact fit.

Yup, absolutely.

We live in a world of factory-made components. These often require tight tolerances - as you pointed out at the top, hinges are one such. Of course you can put them in/on the door with oversize holes, but many designs require "exactness".

I have seen older ones that need a 35mm hole for the door side of the hinge, but are held in position by small countersunk chipboard screws driven down the side of said hole between the hinge and the chipboard - nasty but effective.

And our existing kitchen, old and pretty nasty too, has the cabinet side of the hinges secured merely by plastic bungs pressed into 3/8"* holes. It was bespoke, so it's hard to imagine the maker didn't have options. To my surprise, where other parts have failed down the years, these have held up so far (23 years and counting for our ownership, but probably at least 10 years old before that). Some have had to be "settled" by percussion maintenance, but are still going. They wouldn't be, however, if the fitting holes weren't pretty tight tolerances.

Then there are things like dominoes, biscuits, various Lamello joiners, etc. all of which really need the tight tolerances they have.

If I make something with trad joints, I can adjust if necessary to get a nice fit and finish, but I live in a world where I mostly work on factory made stuff that needs repair or modification.

Don't get me started on Ik<cough>...


*might be metric but I doubt it.
By gcusick
Yes, the Zysa hinges I mentioned are a case in point. They are cylindrical concealed hinges that fit into a hole bored in each element - door and frame, for instance. The body of the hinge has a knurled finish, and is retained in the hole by expanding the body by means of a pointed grub screw. The expansion can’t be more than 0.2-0.3mm, so the hole has to be exact. Much more ‘engineering’ than traditional woodworking.
By owen
Do they really only expand 0.2mm? That seems like nothing. I have never used those kind of hinges so have no idea
By gcusick
I measured one earlier. A 14mm hinge expanded by 0.4mm by really tightening the clamping screw, but realistically by 0.3mm. The other point is that the clamping screw expands the bottom of the hinge barrel, and is intended simply to retain the hinge in the hole - for stability, the top needs to be properly supported at the surface. And that requires an accurately-sized hole.
By Boringgeoff
I've had a set of drill bits (not Forstners) which are marked in increments of 1/2 mm up to 12mm, which I suspected were really imperial bits rebranded. Could this be the case with yours? Using an online calculator 13.6mm converted to 17/32" and 23.6mm equalled 59/64".
By gcusick
I doubt it, particularly in the case of the 24mm bit, which is Bosch branded and marked “Made in Austria”. I don’t know where Axminster source their Axcaliber bits, but their range is pretty much all metric, apart from some key points, eg 1/2”.

At the risk of provoking a reaction, for me, the sooner we get rid of this inch malarkey the better! (And I’m closer to 70 than 60)
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By Phil Pascoe
It'll exist til the USA goes metric. I read long ago that our governments were persuaded to agree to going metric because they understood the USA was going to go metric as well. I prefer to work on larger scale work in imperial as I cannot estimate or visualise anything in metric - I don't have a clue whether a 1.8m chap is short or tall, how far 17k is without converting and I know not whether 9c is quite warm or quite cold. 67 this year, at school we had the option of imperial or metric drawing papers.