Large Forstner Bits... the Myth Exposed

[SARGE..g-47]

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. Most agree.... And I was concerned getting a DP that starts at 500 rpm as I mainly run Forstner bits.

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.

[B]At 600 RPM I got a super clean cut.. no grab.. no squeal and positively to me the fastest without much plungiing and releasing. Just enough to clear the chips... The cleanest cut by far.... PERFECT!

So... I found 600 rpm to be perfect. I was surprised as you might be alsol. But.. don't take my word for it and go try it yourself. Since I mainly run Forstners on my press.. it is now set to 600 rpm and will stay there. And... Hello Steel City Variable Speed... when can I expect you?

[ragswl4]

Great information. I'm gonna try it on my DP. What do you think, smaller bit higher or slower speed or leave at 600? Maybe there is a new chart in our future!

[SARGE..g-47]

Great information. I'm gonna try it on my DP. What do you think, smaller bit higher or slower speed or leave at 600? Maybe there is a new chart in our future!

I have a feeling that somewhere in that area for both but I did not try the smaller bit. What I found was once you get in the range it didn't matter if it was hard-wood or soft-wood. I also found that there is a too slow and a too fast. Keep in mind I don't have a RPM run-out meter in my shop so I am relying on Rigid's chart to set belt speed.

So.. the range is from basically 390 rpm to 650 rpm with testing at 240 through 990. Those are the speeds I have on the Rigid DP. There might be an optimum between 510 rpm which is when it really starting performing well and 650 rpm when it started dropping off in performance so just try it on your DP as I think no two presses will be running exact rpm.

That can be effected by worn and stretched belts.. loose belt adjustment.. etc. I kept the lid propped up in this case to be sure I was not getting belt slip or the lock adjustment working lose to cause a loose belt. So.. the speeds were checked under those conditions allowing about 5 minutes between each hole for the carbide bit to cool down.

I doubt I will have time to do a full chart but I will try some 1/2".. 3/4" as those I use most tomorrow to see. But.. I have a strong feeling that the 600 rpm on my machine will hold in all sizes on my machine.

Now... if someone were paying for a full chart.....

Regards...

[Uncle Cracker]

I've had good luck drilling with big forstners by using the same "two steps forward, one step back" touch that I use when drilling pen blanks with smaller bits. I don't force the bit into the wood, and I meticulously clean the outer rim of the bit, and then put a thin coat of paste wax or beeswax on it just before I make the cut. This also helps with those deep cuts with a hole saw. Speed is 700 rpm, or thereabouts...

[LCHIEN]

the Wood magazine drill speed chart recommends 250-500 RPM for forstners up to 2" and softwoods and hardwoods.
So your results are not totally inconsistent.

I think I would still like slower speeds down to around 300 RPM on a DP for forstners up to 3" (altho the biggest I own now is 2.125") and maybe those beam type cutters.
And There are different types of forstners - some have those saw-tooth edges and some do not. I would expect that to make some difference.

I do think its some good hands on research, though, that you did - especially relating to your bits for your technique. I will definately be more experimental next time I have to drill a bunch of forstner-holes rather than just accept the old adages.

[SARGE..g-47]

I've had good luck drilling with big forstners by using the same "two steps forward, one step back" touch that I use when drilling pen blanks with smaller bits. I don't force the bit into the wood, and I meticulously clean the outer rim of the bit, and then put a thin coat of paste wax or beeswax on it just before I make the cut. This also helps with those deep cuts with a hole saw. Speed is 700 rpm, or thereabouts...

Basically the same approach as I take Uncle Crack but.. I call it plunge and release. Keep in mind that no two presses or attack pressure will be the same. I simply found out something and challenge each to find their range on their DP which may vary with bit size and attack pressure.

But.. what I have found is the old myth of very low speeds on large bits is a bunch of bull as I see it after late this afternoon.

Regards

[SARGE..g-47]

the Wood magazine drill speed chart recommends 250-500 RPM for forstners up to 2" and softwoods and hardwoods.
So your results are not totally inconsistent.

I think I would still like slower speeds down to around 300 RPM on a DP for forstners up to 3" (altho the biggest I own now is 2.125") and maybe those beam type cutters.
And There are different types of forstners - some have those saw-tooth edges and some do not. I would expect that to make some difference.

I do think its some good hands on research, though, that you did - especially relating to your bits for your technique. I will definately be more experimental next time I have to drill a bunch of forstner-holes rather than just accept the old adages.

I have a feeling it might vary from machine to machine and with different folks applying lever pressure. But.. I can tell you that below 300 rpm on my machine will squeal as a wounded pig... grab and then simply stop once it grabs as it doesn't have enough momentum at that speed to continue.

A simple challenge for everyone to find their range on their machine as speed does matter when all else is basically equal. I don't have faith in the charts as you cannot re-create the conditions they were derived at and that leaves too much unknown ground.

Regards...