Drill bit material and type

I'm looking to invest in a good set of drill bits but I am a little confused as to which type is best. I will be drill mainly mild steel and occationally either aluminium or stainless.

My question is two fold, first: what is the difference betweem black oxide, TIN, bright, cobalt, etc. the second part, for what I'll be drilling (predominently less than 1/2" mostly in the 1/8" to 1/4" range what are the best bit type to use.

Thanks in advance

first,

"black oxide, TIN, bright," these are surface treatments. If cash was no object i would go with the TIN, if money is a issue and you were ding mostly ferrous then i would go with black oxide. I do mostly AL so I use bright, as Al has a tendency to stick to the oxide.

cobalt, etc. At this point i think you are asking bout cobalt vs high speed steel (HSS). Cobalt is longer running then hss but more expensive. I then to buy drills in cobalt under 1/2" hss 1/2 and over (just a generalization, job specific tooling may be exceptions)

other thing is split vs chisel point. For the drills that you start a hole with I like the split point for me that is under 3/8" I think most of the cobalt drills come with split point but just something to look for. Also i don't know how well a split point will hold up under steel. maybe some one else can comment.

split points work well in steel, stainless, aluminum. i would also opt for 135° point drills. 118 point are ok but the 135 tend to break the chip a little easier. me personally i would invest in a good set of cobalt bits that are uncoated. i assume this is for home use, as opposed to shop use, as shop use i tend to buy drills as need for the job usually ten at a time to keep some spares. that way you don't end up with a bunch of drills you'll never use. plus buying 10 of every fraction, letter and numbered drills will get expensive.

Given no specific requirement for what they'll be used to poke holes in:

118 piece set (1/16-1/2 fractional/A-Z/#1-60)

135 degree split point

Uncoated cobalt

USA manufacture


That'll cover the odd bit of titanium or stainless, and will still cover the AL and mild steel, and plywood, and plastic you'll encounter.

A nice set costs upwards of 3 bills, and is money well spent.

I keep a post-it on the lid of the index w/ a list of the missing/burnt/too small to warrant sharpening drills, and reorder from McMaster/MSC/etc. when placing regular orders.

If you do a fair amount of aluminum, get the bright bits. As mentioned, you'll get more welding to the oxide and TiN.

I think I am the only one in our shop that sharpens drill bits. I split the points on everything I sharpen, even up to 1"+. I have been running these 118 degree split points in our G&L Bickford radial in sizes from under 3/16 to 1 1/2" with no trouble at all. I don't go crazy on the feed, but they work fine.

TiN coated drill bits are great for a production environment, or when you're after that extra edge for speed/feed. The titanium nitride coating also helps the drill's lubricity, or chip flow thru the flutes- works great in all materials, but a good set costs the cabbage. Once you dull the tip and regrind it, you lose the effectiveness on the cutting edge unless you have them recoated and most of the time it's uneconomical. Gummy materials like aluminum like to adhere to the drill flutes, especially if you're not treating them right. Aluminum likes lots of SFM and LOTS of cutting fluid. 135degree split point bits are great, penetrate easier at start, and they center a bit better, but are trickier to regrind by hand. 118degree in most practical cases will work fine. "Cobalt" bits are still High Speed Steel, but with cobalt added for added toughness, hot hardness and wear resistance. They cost more, but are worth it if you know how to treat your drills. In my opinion, go for a nice set of uncoated ("bright") cobalt drill bits, 118 or 135. Buy a quality set- don't buy JUNK. You'll notice the difference not only in appearance, feel, and quality of grind, but also in performance and how nice they regrind as well.

A word about cobalt. 35 years ago, while in the Air Force, I had this terrible task to accomplish for my electronics hobby. I had to drill out FOUR 6-32 x 2 1/2" steel screws that were frozen in the varnished steel laminations of a transformer, and replace them with longer screws.. Why? I needed to put mounting lugs under the screw heads and nuts at the other ends. I bought a bunch of hardware store grade HSS drillbits, got out my hand drill, and started drilling. I ruined all my drillbits before I had drilled 3/8" deep. I gave up, and walked over to the base laundry to do my clothes. When I looked in one of the washers I found two drillbits just the right size for my project. I figure they had been in the pocket of one of the aircraft mechanic's uniforms, and fell out into the machine. The shank was marked CO. They were split point, and had a slower than normal twist.

I took the bits back to my barracks room. I proceeded to drill out ALL FOUR 2 1/2" screws, without cutting oil, with ONE BIT. Took me about 1/2 hour.

So whenever I look to drill steel, I buy a good quality cobalt bit.

All excellent advice.

If you are a home shop machinist, there is a way to save a lot of money, though:

Buy low cost (Chinese) version 118 pc drill set as mentioned above. Whenever you dull or break a drill bit, replace that size with 1 dozen cobalt bits. While far, far from ideal, the Chinese drill bits are still quite tough for the home shop environment. With this method, all your most commonly used sizes will be cobalt without the cost of buying a high quality cobalt set outright.

LM

Probably the best thing you can do is get stub length.

HSS, uncoated is perfectly fine for hobby use.

LiquidMetal said:

All excellent advice.

If you are a home shop machinist, there is a way to save a lot of money, though:

Buy low cost (Chinese) version 118 pc drill set as mentioned above. Whenever you dull or break a drill bit, replace that size with 1 dozen cobalt bits. While far, far from ideal, the Chinese drill bits are still quite tough for the home shop environment. With this method, all your most commonly used sizes will be cobalt without the cost of buying a high quality cobalt set outright.

LM

Click to expand...

That's a badass plan. By the time he's ruined his 1/4", 5/16", and 3/8" drills he'll have (36) new drills worth a shit, (115) that aren't, and be out $388 for his troubles.

John Welden said:

Probably the best thing you can do is get stub length.

HSS, uncoated is perfectly fine for hobby use.

Click to expand...

Yes, but you really, really need COBALT to drill thru a #6-32 machine screw
covered in varnish.....

i believe the answer is a question ... " do you sharpen your own drills?" the answer should make
a difference.

TiN coated drills get worn, the same as not-TiN coated drills. when i grind a fresh end on one, it
becomes a "non-coated tool" . unless you have an autoclave handy to recoat the business end... it is
now a non-coated drill.

same goes for cobalt drills . the web is enourmous, and must be ground to a split point. that means
it takes twice the time at the pedestal grinder or T+C , if you have one .

i'll buy hi-pro drills for volume jobs in stainless and toolsteel ... everything else gets budget
black oxide . it works just fine.