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View Full Version : Forstner Saw Tooth Bits Are Causing Burning



CameronPotter
26th April 2006, 04:35 PM
Hi,

I got some Forstner Saw Tooth bits from Carbatec (well the local Tassie supplier anyway) on recommendation by forums users and I was using them to bore out a blackwood pepper mill. The damned things kept on burning my wood, so I slowed my lathe right down - still burnt. I thought that maybe the wood was just REALLY dry, so I tried it on a wet apricot bowl blank that I was roughing down (thanks Sheddy) and it STILL burnt.

What am I doing wrong? Was a silly to assume that the bits come sharpened?

Any help gratefully appreciated.

Cheers

Cam

Ashore
26th April 2006, 04:52 PM
What Diam bit and what speed are you using, ?

My Carb- Forstners came sharp and ready to use.

They do dull faster than expensive ones , Have a vermont I put all my dog holes in with through masonite and mdf and still good as new , the Carbi- same size needed re sharpening after two holes then after each hole thus I bought a much better one $ 54 I think for 1 3/4 forstner , so if you are going to do a lot of the same size and using hardwoods you might considder a better one for that size

Rgds

CameronPotter
26th April 2006, 04:56 PM
Thanks for the fast reply.

I am using 44mm, 38mm, 25mm and 22mm bits.

I am using them at 600rpm.

They seemed pretty blunt from the word go... As a matter of fact I then tried a $2 shop spade bit that I bought for other reasons than drilling (I don't normally buy tools from $2 shops... honest! :rolleyes: ) Anyway, it clearly outperformed the much better quality (and design you would think) forstners...

:(

Bodgy
26th April 2006, 05:19 PM
Thanks for the fast reply.

I am using 44mm, 38mm, 25mm and 22mm bits.

I am using them at 600rpm.

They seemed pretty blunt from the word go... As a matter of fact I then tried a $2 shop spade bit that I bought for other reasons than drilling (I don't normally buy tools from $2 shops... honest! :rolleyes: ) Anyway, it clearly outperformed the much better quality (and design you would think) forstners...

:(

There may be a lesson here!

CameronPotter
26th April 2006, 05:24 PM
There may be a lesson here!

I hope you aren't recommending what I think that you are recommending! :D

However, I don't have any $2 bits that are big enough for some parts of the pepper mill...

echnidna
26th April 2006, 05:46 PM
spade bits always drill faster than forstners

Groggy
26th April 2006, 06:01 PM
I am guessing that you are drilling into end-grain. This is a common problem with forstners, especially the cheapo's.

What I have tried in the past is re-filing the sawteeth and also putting a few notches (offset!) into the clearance cutter with a fine file.

CameronPotter
26th April 2006, 06:05 PM
I am guessing that you are drilling into end-grain. This is a common problem with forstners, especially the cheapo's.

What I have tried in the past is re-filing the sawteeth and also putting a few notches (offset!) into the clearance cutter with a fine file.

Yep. Drilling into endgrain and it is being a PITA. You say that you have tried putting notches in, but you don't say if this improved performance... I just want to be clear before I go and start hacking up my bits.

As for spade bits drilling faster, why then do people use forstners? :confused:

Cheers

Cam

rsser
26th April 2006, 06:20 PM
Not had problems with new Forstner bits, even Carbatecs.

So thinks....

- drop to your slowest speed; 600rpm is definately too fast

- just drill in a cm at a time, withdraw the bit and blow out the shavings [edit: don't withdraw it completely; leave the tips in; and as you bore get a wooden chopstick in and pull out the debris]

- make sure your piece is fixed solidly in the chuck and everything is nicely lined up

- the carbon steel bits wear quickly in hard timber so touch up the cutting edges with a triangular file often

Hope this helps.

CameronPotter
26th April 2006, 06:27 PM
Thanks Ern, but my lathe is a Woodfast MC900... I can't go any slower than 600rpm.

Damn!

As for the other comments, I was already drilling and stopping and drilling a stopping while clearing the waste (with compressed air - gotta love it).

The piece was VERY securely attached and well lined up.

I think that a quick sharpen might fix my problems somewhat (and a better lathe). :rolleyes:

rsser
26th April 2006, 06:37 PM
K, well I used to do them on my MC900, so yep, sounds like sharpening is the go.

Groggy
26th April 2006, 06:38 PM
Yep. Drilling into endgrain and it is being a PITA. You say that you have tried putting notches in, but you don't say if this improved performance... I just want to be clear before I go and start hacking up my bits.Cam, I am not in the habit of passing on bad experiences as suggestions, but if you are willing to listen and have a week or two to read them.....

Seriously, about a month ago I was in a store and got involved in a discussion with the proprietor and a cranky customer who had the same bit and same problem. Cutting into end grain and the bit would not cut and also heated up too much.

He was offered a replacement bit, but was more interested in a working solution so we did some experiments. We found the end fibres would not clear and needed to be scored, cut and cleared.

After a few failed attempts we hit the idea of the 'v' notches in the clearance cutters, which, coupled with the sharpening, improved performance by probably 20%. I'd take a pluck and say the notches helped about 10%


As for spade bits drilling faster, why then do people use forstners? :confused:
A forstner will drill a more precise hole as it should follow a true path through the wood. A spade bit can wander with the grain sometimes. Edges are generally cleaner and the bottom smoother (again, generally, there are exceptions).

If you are worried about notching your bits then don't. I just thought your initial post was seeking ideas.

CameronPotter
26th April 2006, 06:49 PM
No, I was certainly asking for ideas.

I will certainly do it as you say that you noticed improvement. I just wanted to clarify it wasn't one of those - "Well this might work" comments.

Also, your second post is very helpful as I wasn't thinking about a V-notch but was going to file a square notch... :o

Thanks for that. I will post how it goes (providing I remember).

(oh, and I will resharpen them I think).

Cheers

Cam

Ashore
26th April 2006, 07:03 PM
I think the major problem is the speed Last week was drilling 60mm ,into some end grain, for thermometers, at 180 RPM and that was just a touch too fast , was only red cedar though not a real hard timber. That works out at .05m/s cutting speed at the circumfrence 'I think ?' and that seems to be about right for softer wood , slower for hardwoods.

Nuggett
26th April 2006, 07:43 PM
Just a thought are you using the saw tooth HSS ? these are much sharper and you can hone with a diamond stone.
This is what I use and it works fine.:)

Jim Carroll
26th April 2006, 08:27 PM
Cam for these bits you need the speed down to 350 - 400 rpm. Especially endgrain on blackwood.
Once you have burnt the edge they dull of very quickly and need to be resharpened, it doesnt matter what timber you use afterwards they will still burn the hole till you sharpen them.
Use a triangle file on the sawtooth parts and a flat file on the main cutting flat edges

Groggy
26th April 2006, 08:41 PM
Cam, I should have added what the others have so clearly mentioned as the first priority - slow the speed down if at all possible. Can you see blue marks on the bits? If so they may well be heat damaged and any sharpening effort will have a short life.

soundman
26th April 2006, 08:59 PM
I'll reinforce jims comment about speed, forstener bits are only a couple of generations removed from hand driven brace & bit, I find that they like being used good & slow. How slow I dont realy know but I like to be able to see the blade cutting & ejecting chips.

The other thing to understand is that this type of bit is slicing off large widths of materal on each pass and needs to be able to clear its waste.

I have a carbatec set & oooeeee yeh I supose they were sharp when I got them but the edge shape & clearance on some of the smaller ones wasn't ideal. I.ve been hand sharpening them one at a time as I need them with saw files or mill files as needed.

This generic sort of bit is reasonably soft material I is fine in softwoods commonly used overseas but on our harder woods will need frequent sharpening.

I'm happy with the set I have and what I expect from them & I would buy the same again but they have limitations in harder & more abrasive material.
Consider the whole set cost me less than two decent carbide tipped bits.

They certainly work better once they have been tuned and they do need regular sharpening.

pay attention to the cutting angles of the flat slicing blades, file the relief on the underside carefully, the upper side benefits from flatening and smoothing to help both the mating edge point & to smooth chip ejection.

The smaller ones particulary, pay attention to the chip channel, you may have to file the upper side of the chip channel to smooth it and to make sure it has a taper opening wider away from the edge.
A couple of my smaller ones cloged badly before I tuned them.

The saw teeth should be sharp and have relief on both inside edges of the tooth.

to sharpem you will need the smalest mill file you can buy, and a couple of small triangle files. The realy small ones you will need a small needle file.

If you have expectations similar to modern HSS twist bits you will be disapointed and will not get the best from these.
But if you think of them as a tool that has bearly jumped from the dark side into a power tool they will do many usefull things for you.

They drill a nice clean flat bottomed hole particularly in softer woods.

cheers

rsser
26th April 2006, 09:47 PM
Last I looked Carbatec had HSS bits .. might keep their edge a bit longer.

And of course we're talking about saw-tooth bits, not Forstners, if I've got the difference right.

Skew ChiDAMN!!
26th April 2006, 10:11 PM
And of course we're talking about saw-tooth bits, not Forstners, if I've got the difference right.

I've noticed there's a distinct trend, even by the manufacturer's, to lump saw-toothed bits under the "forstner" category.

They've even started to add a central drive spur (or point, or whatever you want to call it) which, traditionally, a forstner doesn't have as 'tis supposed to cut a square bottomed hole. As distinct from a square bottomed hole with a dimple in the middle. :rolleyes:

I wonder how long it'll be before you can't get a proper forstner for love or money?

macca2
26th April 2006, 11:13 PM
I use the carbatec forstner without the saw tooth. When drilling the large hole for the pepper mill I only drill in about 5mm to start, then change to a small maybe 22mm. Then follow with a 38mm and finally drill to depth with the large 45mm.
It is time consuming but gets the job done without burning the drills.

Is it possible to sharpen the "none sawtooth forstener"
cheers
macca

Munga
27th April 2006, 06:58 AM
Slow as possible speed and as fast as possible feed, a forstner needs to cut not shave the timber (I hope that makes sense)

Arch

Lance Stunning
27th April 2006, 07:56 AM
I drill 3" and 3-3/8" dia. tubes for bird feeders. 22770I started with HC steel chiwanese bits which promptly dulled due to heat generated at 250 rpm. So, I resharpened and sprayed moulder lubricant which helped. Dropped the speed to 125, and loosened the drive belt on the lathe. If the friction gets too high the belt slips a bit. Yeah, I know, bad form. So, before you get your panties in a bunch, I only did it to complete the project. Speed's not high enough to hurt, much. I switched to HSS (3-3/8) saw tooth which solved the dulling problem. Heat is still a problem on longer tubes. As written before, back out, blow out the chips, start again. 22769 This tube is 18" in length, ID is 3-3/8", took about 45 minutes to bore. FWIW
Sharpening saw tooth or forstners is not difficult. I do mine on a 1 x 42 belt sander.

CameronPotter
27th April 2006, 09:51 AM
Thanks guys.

It sounds like I might be in a little trouble as my lathe is too fast to run these bits! Damn...

I haven't burnt the bits or anything, but I will sharpen them and use them VERY slowly if I need to use them.

As for forstner being different from a saw tooth, I know, but these days the term is so interchangeable, I thought I would be safe and mention both terms.

Special thanks to Soundman for his info on tuning them... Maybe I can figure out a way to do my drilling on the drill press.

:confused:

Thanks

Cam

Sprog
27th April 2006, 04:49 PM
Download the Wood Screw Chart and Drill Press Speed Chart here, just click on the appropriate links.

The charts are in PDF format.

Wood Screw and Drill Speed Charts (http://www.woodmagazine.com/wood/story.jhtml?storyid=/templatedata/wood/story/data/203.xml)

CameronPotter
27th April 2006, 05:36 PM
Thanks Sprog!

Auzzie turner
7th May 2006, 07:42 PM
What speed were you boring at? The larger diameter forstner bit, the slower your lathe speed should be. Also, bear in mind that boring into end grain, does not make life any easier.

Regards.....Auzziewoodturner

Pharaoh_25
13th May 2006, 08:37 PM
Cam,
They do come sharp but it wouldn't hurt to give them a going over with a 240 and then a 360 diamond pad. The forstner bit will go dull very quick if not used in the cutting motion. If it is not cutting, just rubbing will cause friction and the bit will get hot and loose the edge. The larger bits cut well with lathe running slow and constant pressure, take small cuts. The smaller bits clog up. Blunt router bits burn timber, saying that blunt forstner bits may as well. Sorry for rambling

Pharaoh_25
13th May 2006, 08:42 PM
Couldn't of said it any better

CameronPotter
14th May 2006, 08:42 PM
Thanks.

I think I have it sorted now.

Cheers

Cam