forstner bit measurement

[durango dude]

Just noticed the mortises that I was drilling (1/2") were a little smaller than my 1/2" Wood River chisel. The difference is a tiny amount - but enough to notice. (the chisel will go in if you force it)

Being a little obsessive - I got out the vernier caliper, and discovered that my new freud/diablo forstner bit is 1/128" shy of 1/2". I also checked out my older PC bit - and found the same thing.

Oak's pretty hard - and 1/128" difference is enough to cause some headaches (I could likely persuade pine to deal with tolerances like that).

Seems kind of odd - is this a standard thing? After all - both my bits measured the same. I suppose it's conceivable that my caliper AND chisels are off ---

[cabinetman]

My half inch chisels measure the same as my half inch Forstners.

.

[eezlock]

forstner bit measurement

It could be and probably is where the bits were made is the real problem.
Look closely and see if they were made in China or Taiwan if so, they are
actually mm not standard inch sizes like good old American made tools are!
That is kinda' like buying good quality plywood these days at Home Depot or Lowe's .....3/4" plywood is actually 23/32" not a full 3/4" as it used to be.There always to be some sort of trade -off when we need/ want something these days.

[JR]

The runout on my drill press would take care of the difference.

[dbhost]

Hmmm.

Freud forstner bits I thought were made in Italy.

For what it's worth, it sounds to me like a 13mm relabelled to 1/2". You can, and probably should adjust your tenon to fit your mortise. 1/128" is pretty miniscule, but hard woods are often very unforgiving when it comes to that sort of play... Oak is bad, pecan is worse... I have had it split on me when test fitting. Yuck!

[durango dude]

well - the good news is that I follow Charles Neil's motto ----- "sneak up on it....."

[LCHIEN]

FWIW, are you measureing the hole you cut, or the diameter of the bit, when you say its undersized by 1/128? and if we're talking calipering, you should probably be talking about errors or out-of-spec sizes in thousandths, not 1/128s.

[LCHIEN]

FWIW here's some measurements from my tool chest:
Marples Chisel (marked 1/2" and 13mm- which is like .511) .497"
Craftsman Chisel 1/2" .492
Carbide brad point drill 1/2" .500
Woodline Carbide forstner 1/2" .490
Drillmaster (HF) HSS Forstner 1/2" .502

A little runout on the DP might enlarge the hole a bit or the brad point of the forstner/brad point might center the bit enough to take out the runout. The telling thing might be to caliper actual holes drilled...

But anyway there's some tolerances on actual cutting tools.
Anyone else have any to report?

[durango dude]

loring -

All started when I noticed the mortise that I drilled out was slightly smaller than my chisel. LIke I said --- feeling a little obsessive, I took my vernier caliper to the bit, just to check. I measured 0.492" (approx - my General Tools caliper)
My caliper is analog - and goes up to 1/128"

I don't have a digital caliper ---- just happened to notice the mortise didn't match the chisel - and wondered why.

I can deal with this just fine ---- in fact - have been cutting downright tight mortise/tenon joints - and really enjoying it.

[Joe DeFazio]

loring -

All started when I noticed the mortise that I drilled out was slightly smaller than my chisel. LIke I said --- feeling a little obsessive, I took my vernier caliper to the bit, just to check. I measured 0.492" (approx - my General Tools caliper)
My caliper is analog - and goes up to 1/128"

Hi Durango Dude,

Just out of curiosity, did you measure your bit across several diameters (like 9 o'clock and 3 o'clock, 11 o'clock and 5 o'clock, etc.), and at several distances along the length of the bit (like right along the scoring/cutting edge, along the walls of the body of the half-inch-or-so head of the bit) and the like? An interesting diameter to check is the one exactly in line with the planing surfaces (the flat cutting edge that hogs out the flat bottom of the hole).

I have found it to be common to find the larger bits to be out of round by at least a few thousandths, or to vary at different distances along the bit length. And runout often takes care of bringing the hole up to spec.

A good test of your hole size is to use a measured set of transfer punches or drill rods; that will tell you how big your holes really are. After all, at least for my work, the holes are what's important, not the bit.

And, some other things to consider: if your holes are for dowels, the dowels will most likely be far more than a few thousandths out of round (I come across dowels more than twenty thousandths out of round, and dowel roundness can vary greatly with changes in humidity). You can always sand dowels a little (or hammer them through a die if you have one). If your holes are for half inch bolts to pass through, the threaded portion will be considerably undersized as well, so a slightly undersized hole can often be acceptable. The •unthreaded• portion of a bolt (near the head) can be over-sized, though; that's something to watch out (and drill out) for.

Joe

[durango dude]

am guessing there's not a lot of run-out - since the 1/2" mortise is tight compared to the 1/2" chisel.

Good news is - I match more than I measure when I build stuff - so I can live with this imperfection.

In a nutshell - I walk over to the toolbox, dig out the shoulder plane (Stanley #92) and sneak up on it....... (thanks Charles Neil).

It ain't worth walking over to the band saw and resetting everything. I'd get more error than 1/128" by moving the fence.

Only takes a few strokes with a hand plane to coax 1/128" off a tenon.

Key is - be sure to take equal strokes on each side of the tenon.