Large Forstner Bits.. Exposing the MYTH

[John Thompson]

I have always heard larger Forstner bits require very low speed. I am getting the new Steel City 17" Variable Drill Press and someone noted it was a "loser" when they found out the variable speed was from 500 rpm to 2500 rpm as they said they run large Forstner bits and they should be run at 300 rpm or under.

In 37 years of WW.. I have heard this often and many will argue the point adamantly and most agree.... And I was concerned getting a DP that starts at 500 rpm as I mainly run Forstner bits and run them around 400 rpm with OK results.

But.. is what is thought to be the "Holy Grail" on rpms for those larger bits "really" the Holy Grail? I set out to find out today while waiting for a glue-up to dry. I mounted a 2 1/8" Forstner and used 4/4 or 1" stock. A piece of pine and a piece of oak. A plunge and lift to keep the bit cool and and not over-power it to avoid grab and too big a bite. I started with 240 rpm and ended at 990 rpm. The results might surprise you.. really surprise you.

At 240 rpm.. the bit grabs and sqeals.. I made it to 1/16" in oak and 1/4" in soft-wood after I allowed 6 squeals. Unsatisfactory with the bit completely stopping...

At 350rpm.. proved the same squeal and grab making it to 1/4" in oak and 1/2" in pine. Unsatisfactory as the bit completely stopped several times...

At 390 rpm... got completely through without much squeal or grab so it was OK...

At 510 rpm... basically breezed through with no squeal and little grab and left a clean cut.. OK with one star..

At 650 rpm.. a repeat of 510 as got through with no squeal and little grab and left a clean cut.

At 990 rpm... basically poor performance and got through the oak only 1/8" before the squeal.. excessive grab to the point of the belts trying to jump pulley track. Stopped before they did. Completely unsatisfory at this speed.

At 600 RPM I got a super clean cut with no grab.. no squeal and no real slow down other than an occasional release to clear chips. The cut and speed at which it cut was as close to perfect with that monster bit as it gets.

So... I found 600 rpm to be as close to perfect as I am going to get. Between 390 rpm and 650 rpm was in range but the 600 was almost perfect. I was surprised as you might be also. But.. don't take my word for it or pass it off as he's full of it. Go try it yourself.. forget what you have heard repeatedly as I have... then past judgement.

Since I mainly run Forstners on my press.. it is now set to 600 rpm and will stay there for that task . And... Hello Steel City Variable Speed... when can I expect you?

Sarge..

[Mitchell Andrus]

The better the bit, the faster I run it. Good bits tend not to rub and burn, so speed makes a better cut. YMMV

[Bob Mircetic]

I applaud you for taking the time to actually test what is generally considered established fact.

However, I don't know if you can jump to the conclusion the 600rpm is perfect for *all* forstners, as I imagine different bit sizes would generate different results.

[Clifford Mescher]

Did you mention the type and manufacturer of the bit? Clifford.

[Roy Hess]

Interesting post. My old Sears drill press also starts at 500 rpm. I have a full set of Forstner bits which I use frequently. They are not even a "quality" set. I have always run them at 500 rpm in Oak or Ash, and they work fine.

Never thought of it any other way

[Pete Bradley]

Thanks for that. I recently had some smart guy acting as if I was some sort of hack for using a DP that "only" goes down to 470 RPM. After years of excellent results, it came as a surprise to me that I was actually doing it wrong.

Like you, my experience is that slower isn't necessarily better especially if speed is necessary to eject chips.

Pete

[Mike Henderson]

At the woodworking school I attend, they teach that if your forstner bit is burning the wood, you need to increase the rotational speed.

So they agree with what you're saying.

Mike

[John Thompson]

Mitchell.. the 2 1/8" bit I used was a carbide tipped Ole Hickory which were made by Oldham. The bit was brand new as I have never had the call to use a 2 1/8" in my WW projects and I mainly build large carcass items even though I have had the set for years.

I can use a more expensive bit tomorrow as I have CMT forstners and hollow mortise chisels. But.. what I found is definitely contrary to the better the bit.. the faster you can run them. What I found is that there is definitely a too fast and a too slow. So.. at this point I can't agree with you but I wll re-test... 240 - 510 - 600 - 990 tomorrow and if I find different... I will get back and say so.

Sarge..

[John Thompson]

I applaud you for taking the time to actually test what is generally considered established fact.

However, I don't know if you can jump to the conclusion the 600rpm is perfect for *all* forstners, as I imagine different bit sizes would generate different results.

As I stated Bob.. I don't want you to jump to "any" conclussion.. go test the smaller bits tomorrow and see what you find. I only used the large bit to see if the 500 rpm limitation on the Steel City Variable would be reason to re-nig on it. It was not and it really surprised me.

But.. it won't take you that long to draw a conclusion using various size bits. It is a sad day if we have to depend on someone else's word on a WW forum as frankly.. experience walks and BS talks. But you can't always distinquish between the two sitting behind a computer desk.

So... by all means run you own test and report back what you found as I did. The whole thing took about an hour waiting for glue to dry.

Fair enough... ?

Sarge..

[John Thompson]

Did you mention the type and manufacturer of the bit? Clifford.

The brand is Ole Hickory I have had for years. The 2 1/8" was new as I have never had the call to use one that large on furniture. I definitely have on construction but that is played on a whole different playing field.

I can run some CMT's tommorrow as I purposely used the cheaper carbide set I have. Personally.. I really don't think it will matter greatly but just a guess at this point.

Sarge..

[John Thompson]

Thanks for that. I recently had some smart guy acting as if I was some sort of hack for using a DP that "only" goes down to 470 RPM. After years of excellent results, it came as a surprise to me that I was actually doing it wrong.

Like you, my experience is that slower isn't necessarily better especially if speed is necessary to eject chips.

Pete

I have always admired your knowledge with machinery, Pete. So.. I am glad to be of help and I should have done this sooner as it took about an hour. I have a feeling "they" were more assuming than speaking from experience as I also for years believed what I read but settled on 390 rpm on my machine. I shoud have tried it at 510 rpm which is a big improvement and the most improvement on my DP came at 600 as mentioned. 650 was OK but it started to do some grabbing. 990 simply was pitiful.

Again I am glad I did it and will pursue some smaller bits tomorrow as I use the 1/4" - 3/4" the most but I have a hunch I will find that somewhere around that 600 rpm range on my machine will be the mark. Keep in mind I do not have a rpm run-out but the belts were good and I left the lid propped open to make sure there was no slippage to throw off results.

Regards...

Sarge..

[Sonny Edmonds]

I've watched tons of trainees misuse a drill bit in presses and by hand, trying to learn.
One thing you have omitted, or overlooked, is the amount of pressure you are exerting on the bit at any given RPM.
That will make one hell of a difference to anybody trying to follow your results.
I've watched, stopped, and taught apprentices proper bit pressure before, during, and after they burnt bits.
I always find that one true "Achilles Heal" to most summations about wood related work lack consistency in the material itself.
Charlie Browns Oak isn't going to be anywhere near the same as Fred Flintstones Oak.
Unless you can make allowances for Material, Tool material, Pressures used, and accurate RPM applied, it really becomes a moot point.

Kinda like me with my results of measuring my air flow at my machines. Not really worth a poot to anybody but me. Because it is doubtful the results could be duplicated in another's shop.

[John Thompson]

At the woodworking school I attend, they teach that if your forstner bit is burning the wood, you need to increase the rotational speed.

So they agree with what you're saying.

Mike

The only time I have ever had burn was at high speed Mike. One day I didn't feel like changing speed and dropped a 3/8" in at over 1700 rpm. That 3/8' dis-tempered and was junk. I replaced it and latter bought a premium set of carbide bits but frankly.. as long as this cheap set is sharp they cut just as well as the CMT's.

But.. tell your school there is a too slow. Try that bit in hardwood at 240 rpm and the bits squeals and then stops as there is not enough rpm to fight it's way forward.

Sarge..

[Jason Whitaker]

John,

excellent post and very interesting to read your results. I have been making some decorative candle holders and using a 2 1/8 bit probably about 30 times in the past two days in woods like walnut, padauk and purple heart... and I have to say you are absolutely correct. My DP only goes down to 600RPM but I find that my cuts are clean, w/o burning and are relatively easy to make....And the bit I am using is not great quality, though it was brand new and relatively sharp. And I thought I was doing it all wrong too.....

[John Thompson]

I've watched tons of trainees misuse a drill bit in presses and by hand, trying to learn.
One thing you have omitted, or overlooked, is the amount of pressure you are exerting on the bit at any given RPM.
That will make one hell of a difference to anybody trying to follow your results.
I've watched, stopped, and taught apprentices proper bit pressure before, during, and after they burnt bits.
I always find that one true "Achilles Heal" to most summations about wood related work lack consistency in the material itself.
Charlie Browns Oak isn't going to be anywhere near the same as Fred Flintstones Oak.
Unless you can make allowances for Material, Tool material, Pressures used, and accurate RPM applied, it really becomes a moot point.

Kinda like me with my results of measuring my air flow at my machines. Not really worth a poot to anybody but me. Because it is doubtful the results could be duplicated in another's shop.

Sonny.. I am not a beginner or apprentice and do have a clue after WW for 37 years on what type pressure to apply to a drill bit and how it responds to a given piece of wood. I work in my shop 4-8 hours a day.

But I do agree that pressure will play a role. This was done with a with moderate pressure and release to allow the chips to omit at lower speeds to attempt to avoid the grabbing. At the mid range the same pressure but found that the bit was not grabbing so I continued until I felt the slightest grab or squeal and then release.

At the 990 rpm the bit began grabbing after about 1/32" inch and grabbing so hard the belt felt and looked like it would jump track as I propped the lid open to watch for slip.

So.. if you read carefully this is not a cut and dried "this is the way it is" or "follow my results" in all conditions thread. Merely what I found and what I found was that the myth about using slow speed on large mortise bits is a fallacy as far as I am concerned.

But... I also challenged and encouraged you.. him.. them to go find out for yourself in lieu of taking my word that I have discovered the "Holy Grail". I haven't nor did I say I did.. simply that you don't believe everything you hear and accept it if any doubt. Just try it for yourself and the truth will be revealed about how your machine acts.

And frankly what I have found is most definitely worth something to me. If others have a look they may find out that it could also mean something to them. If it doesn't so be it...

Sarge..