When my drill bits turn "blue", is this an indication that I have burned it up and the bit is now useless? Can they still be sharpened or has the metal been compromised due to over heating?

Thanks,Chet

 

Yeah, blue is bad. Either slow down, or use cutting oil to lengthen the bit's life.

 

A bit that has turned blue can be ground down but the grinding itself will turn it blue or it is a long process. It is not worth the trouble unless it is a valuable bit or the only one you have of that size at 3 AM... Avoid turning bits blue.

BTW- I have never seen a good drill speed chart. If you find any divide the steel speeds by at least half...

 

If I feel like adjusting my drill press' speed, here is what I use:

1/16 - 3/16 - 1500 rpm
1/4 - 3/8 - 1000 rpm
7/16 - 5/8 - 600 rpm
11/16 - 1" - 350 rpm

Since I usually can't be bothered adjusting speed, I drill steel at 500 RPM with 1" and smaller bits.

Always lube drill bits when boring into metal. Cools the bit, prevents bluing, also helps reducing chips from binding in the hole therefore leaving a nicer finish in the hole. I generally use a few drops of gear oil and reapply as necessary.

For aluminum, you can double the above speeds no problem, and use a thinner oil like WD40. WD40 is not a good lubricant for drilling steel, you want something a little thicker.

ATF actually is a great lube for drilling steel, but it smokes a bit. Whenever I change trans fluid in any of the cars since I can't get rid of it, it goes into bottles for drilling, lathing, and milling lubrication.

If you don't have a "drill doctor" you can sharpen your bits on a grinder. I use a diamond wheel running at 1/2 speed to avoid bluing, and getting a razor sharp edge. If the bit is sharp drilling holes is easier, less chance of bluing on the bit even if you run it faster than you should, and the holes come out much cleaner.

You can make a fixture to bolt to the front of the grinder to ensure the angle of the bit is correct at all times, or you machine a groove into the grinder's ledge at the angle and press the bit onto the groove and rotate it.

My fixture is nothing more than a block of steel, with an MT-2 tapered hole, to which I shove in my lathe's MT2 - Jacobs taper adapter, with a chuck on the end. I then spin the chuck by hand and viola, nice sharp bit every time.

I've even learned how to sharpen bits in my lathe, by chucking them in the 4-jaw chuck then sharpening them with a carbide cutter. The reason why this became a useful thing for me is I have several seats of "pilot bits" which bore a smaller hole to get started, then after about 1/8" of depth bore a larger hole that the bit is marked as. So there's a beveled point like a regular drill bit, with a "forrester bit" like body to it of a larger diameter. Sharpening those in a drill doctor or on a grinder removes the pilot hole function so by sharping the tip first, then shaving the wide part, one can restore the sharpness as well as maintain the shape.

Of course this is a two-step sharpening operation and on the smaller bits it isn't worth the hassle. But the larger diameter pilot bits are expensive so I'll put the effort in.

 

Sharpening a drill bit by hand is a skill that can be very hard to learn and takes a lot of practice. When I went through machinist school I had an old school instructor that taught me how to sharpen them by hand. If your bit is smaller than 1/2 inch they are even harder to get right. The only drill sharpening device that I have seen that works is a drill doctor. There is a formula for figuring the correct rpm and feed rate. I dont remember them off the top of my head, If I can find them in my books I will post them. If your but is turning blue you are probably running to fast and trying to feed to fast. You should keep some kind of cutting fluid or coolant on the drill while it is cutting. A feed rate to slow can also cause excessive heat build up.

 

Frederic- How do you get the required rake or lip angle in the cutting edge with your setup?

 

If you are sharpening by hand and your grinder has a rest on it, support the bit with with your index and middle finger. Hold the bit with the cutting edge turned up and paralell to the stone. Start grinding on the bottom of the angle up to the cutting edge until you see the first spark break the cuttting edge . Stop at that point, if you continue until you see sparks all the way across the cutting edge you have rolled the cutting edge over and rounded it off ( most common mistake in hand sharpening). Use a lifting motion not a twisting motion. Remember to constantly dip the end of the bit in some type of coolant every few strokes. With the cutting edge turned up and paralell to the stone you have the correct angle, if your bit was correct to begin with. You can scribe or draw a line on your rest at 64 degrees and eyeball off the line, that will give you 124 degrees total end angle. When done sharpning you should be able to look at the end of the bit, while holding the bit so the cutting surfaces are flat, the web in the center should be at about 45 degrees to the cutting edges. Also the cutting edges should be equal in length. If your bit has more cutting edge on one side you need to take a little more off the short side to even it up. Starrett makes a gauge that is at the correct angle and is marked off in 1/16 inch intervals to measure by. This is how I learned to sharpen bits. You can talk to different machinists and get a different way from each one of them. If you dont have a way to practice this technique or any other hand sharpening technique and get comfortable with it you will probably cause your self more drill bit headaches. I would recomend investing in a drill doctor unless you can practice it and get good at it.

 

Years ago I had a short class in a machine shop. Then worked in a fab shop for several years. If I learned anything, I learned how to sharpen drill bits by hand. I can sharpen bits as small as 5/32". My technique is to start at the cutting edge much like sharpening a knife on a stone. This was just the way I was taught. It's just something that takes practice. It saves on buying new bits.

 

I have seen folks sharpen that way. I tried it but ended up with a roled edge. I prefer to come from bottom up to the edge and stop as soon as I see that first spark break the top edge. What ever works for you. The one thing about which ever way you sharpen a bit is, that , it takes practice to get the technique down.

 

We used that drill doctor and it saved us all kinds of money, We resharpend our half inch masonary bits. I bought those bi level tip ones they really do go through alot faster, But I had to sharpen them by hand with the drill doctor. That was a pain pretty much had to stick it in there and guess at the angle, pull it out and make sure I nailed it then try to reinsert at the same angle. It seemed the resharpened bits lasted longer and cut better or maybe it was all the money we were saving!! It took me two months to talk the owner into breaking down and buying it, once I put it on paper price of 1/2" masonary bits alone, to the price of the doctor I was heading to the store the same day!!

 

I have a Drill Dr., although I have hand-sharpened for years. Love the Dr.!! One thing about hand sharpening - you HAVE to start with a good wheel!! NOT the one that came on your grinder. Diamond is best, carborundom is a good 2nd. Here's a pretty decent diatribe on sharpening:

http://www.essortment.com/home/drillbitsharpe_sbwh.htm

Key on this article - sharpen them BEFORE they get so dull, they turn blue.

And rule of thumb - how can you tell when a drill bit has "gone bad"?? They hang out on the street corner with the cold chisels and ***** punches, smoking cigarettes and telling dirty jokes!!



R