Drill bit life?

G

gshamr

Joined
Jan 18, 2002
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178
How long should a cobalt drill bit last, or stay sharp enough to cut effectively?

I'm using a hand drill with a 7/32" cobalt bit that specified usage for "stainless steel and other tough steels" I drilled 4 holes so far through annealed 3/16" Crucible A2. The first two went pretty fast. By the time I got halfway throught the last one, I was drilling forever to get it finished.

I tried to use a slow speed and some honing oil to keep it cool. Am I getting the metal too hot and hardening it as I drill? Is this an expected life span for a bit? Should I press down really hard as I drill?

I looked at titanium bits but wasn't sure they were any better for thick tool steel.

Thanks,

Gerry Hamrick
 
Thats a good question, they will work better at slower speeds. Sounds to me that the third hole you drilled that the A-2 was not totally annealed or either it got hard durring the drilling process if the steel turned red it got hard. I know the cobalt are suppossed to be better than the ti ones. I believe the ti is just a coating that is applied to the steel drill bit, either O-1 or M-2. As far as how long they will last, I don't know, but do know if they lasted a long time thats not good for business.....Ray
 
I bought three 1/8" cobalt bits from TKS a while ago, I have drilled more holes than I can count with the first one, and it is still drilling just fine. I would guess I am at around 150 holes. With TIn coated bits, I would get about 40 holes before I had to sharpen the bit.
 
I think it may have been hardened by my drilling. I didn't think a cobalt bit should be dull yet.

Can you sharpen it with conventional tools? I saw a drill bit sharpener in Lowes but it was fairly expensive.
 
They drill Great . I Use them all the time never a problem but use cutting fluid /Grease to keep the heat down . they do sharpen pretty well for me
 
Drilling holes should be done at a "slow" speed. I use a drill press and use the lowest speed possible and use oil applied with a small paint brush to extend the drill bit life. I assume you drilled it to fast and pushed to hard making the tip dull out. You should be able to hand sharpen the bits with a grinder if you have one. Personally I buy the cheap HSS bits and just toss them when they get dull.
 
I have seen these dits at shows the demonstrators always have used cordless battery drills with slow speed. Check where you buy the drills they may have a chart showing speed suitable to your size bit.
They may also have the angles to re sharpen at.
One of my first apprentice projects was to make a drill shapening aid
it is basicaly a v cut in at the right angle you just hold it on the end of the bit to check the angle.

Then I spent a lot of time sharpening a box of bits from the tool store. Have a go with of old bit see how you go.
 
Cobalt is good for end mills and bits. It handles heat well but should not be tested or it too will burn. It is best, in my rather limited experience, to run it moderately (not real slow or too fast) and taking shallow depth cuts and pulling soon to cool. This is for steels and other dense material of course. I put cutting oil on the work piece cutting area before each cut, and again pull the bit (mill) often after a very shallow cut that is a plunge cut. When the oil smokes well I pull the bit or mill and allow to cool. It takes patience to allow the bit or mill to remain loyal. A constant coolant spray will decrease the cooling period dramatically upon pulling, however a constant spray does not elliminate the need for periodic pulling.

Cobalt is not the choice for hardened steel. More expensive carbide is my ready choice for that type work and the best (not looked forward too) alternative is grinding wheels. Diamonds I have no experience with.

Roger
 
For high speed steel drills cutting annealed tool steel you need 50 to 60 S.F.M. and medium to heavy feed pressure.

Spindle RPM = (S.F.M. x 12)/(Diameter of drill x 3.1416)

so

Spindle RPM = (50 x 12)/(7/32 x 3.1416) = 600/0.687225 = 873

So run the drill at about 800-900 rpm and use medium to heavy feed pressure (say about 30 to 50 lb.). Lubrication with light oil will extend tool life.

Visually examine the bit every few holes. Rounded off corners indicate overheating. Chipped corners indicate problems breaking through. Dulled edges indicate lack of feed pressure and/or interuped feed. Broken bits indicate lack of rigidity in the drilling setup and/or the use of dulled bits.

You should get a tool life of several hundred holes per sharpening.
 
I tried to sharpen the cobalt bit but I didn't work for me. I then used an old standard bit with a lot of oil and a slow speed. It didn't take long at all.

I dulled that cobalt bit by continued drilling even after it started smoking. I may sound stupid, but I thought it was the oil that was smoking. I guess I dulled the bit.

Lesson learned is to go slow and stop often. Thanks everyone.
 
Gshamr,

If the bit or end mill squeeks it is burning. The smoke was little doubt the oil burning. You probably heard a squeeky sound as the tool was burning (I say burning as a figure of descriptive writing - the tool was over heated). If it squeeks pull the tool.

Roger
 
Learn to sharpen drill bits free hand, not hard to do at all just practice. there is a drill sharping gage smiler to a combination square. I use my Burr King and a 220g belt and I only through away my
drills when thy are too short to use.
I will try to explain how to do it. First grind the cutting edge angle then move the back end down with a slight rotating movement, the back end of the cutting end should be lower than the cutting edge, the cutting edge needs clearance. Finley keep each cutting edge same length. Now I know this is easer said than done but it is not that hard to do, it is a learning process. Gib
 
I use a Drill Doctor which works well, although I've always wanted to learn freehand--thanks for the instructions.
 
gshamr,

I'm curious on the state of the A2? How was it annealed? I've noticed that some steels I get that are annealed are soft in some spots and can be hard in others. I reanneal everything before using it any more because of the hard spots.
 
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