Drill bit problem

J

joeyp1971

Joined
Apr 14, 2009
Messages
36
I'm having a problem in that i keep breaking drill bits drilling through steel. I have Delta 12" press with 1/3 hp and about 1750 rpms. I'm using short leangth carbide bits from a place called Drill Bit City and I'm drilling through soft 440-C, D2 and ATS-34. I can't understand why I keep burning through bits. Do I need to start with a smaller diameter bit and work my up to the larger sizes (as you would with other drilling applications)? Is my press simply underpowered? Any help would be appreciated.
 
are you using any lubrication and how fast is your press turning? Try some cutting fluid and the slowest speed
 
From what I understand Carbide bits like speed and the smaller the bit the faster it should go. But one of the machinist should probably answer here.
 
Get yourself a set of the Dewalt cobalt bits, or just buy singles in the sizes you use. I bent one of them hammered it back straight and it still cuts like a champ. No need to use carbide bits for soft steel and as you've found out they are very brittle and do not tolerate any wobbliness; rigidity is a must.
 
I've been having a similar problem with bits, but not carbide ones. I cant find any that won't burn up an lose their temper drilling annealed 1080. I've got a similar delta press running at it's lowest speed, and I use cutting oil. Any suggestions of good brands or whatever would sure help.
 
I just use cheapo HSS/Ti-nitride bits or the MSC single bits. The MSC ones are the gray/black oxide bits. They are very inexpensive and work fine. You'd have to be getting them pretty hot to affect their temper. HSS drills are usually M2 and are tempered high. The only way to get it really hot like that would be to hold it down and have it spinning instead of cutting.
 
I find the faster I drill I actually harden the steel a bit and thats it. Bit becomes dull and steel becomes hard.
 
Frank Barron said:
I find the faster I drill I actually harden the steel a bit and thats it. Bit becomes dull and steel becomes hard.
Click to expand...

Good old work hardening.

One thing I do but don't know if it helps or not is when I'm going to let up to apply more water or oil I pay conscious attention to the pressure I am putting on the bit and make sure I am either applying enough pressure to cut or quickly lifting the bit. I don't allow the bit to sit and turn without enough pressure to cut and when I restart the cut I make sure I don't hesitate and get it cutting right away.

The faster the bit is turning the more likely it will turn on the material with just enough pressure to work harden it.
 
You need to be much slower on the rpm's. 200-300 if you can get that low, if not just make sure you keep the bit cool.
 
If you are drilling 1/4" or larger holes, the pilot hole will certainly help. The pilot should be or there abouts the size of the web on the large drill you will use. Frank
 
I use cheap HF drills, very slow never over 800, make a little dent, apply cutting oil just a little and watch for curls, the curls tell you, speed is good, feed matches speed, and bit is sharp. If it stalls or makes a noise into the trash, these cost about $0.10 -$0.15 each in packs of 10 on sale.

I think your problem is speed.
 
My .02:


tomh said:
Try some cutting fluid and the slowest speed
Click to expand...

:thumbdn: Not bad advice, but not likely to help much.






AcridSaint said:
For annealed steel, you shouldn't need carbide. I would go to regular HSS drills.
Click to expand...

:thumbup:





Rusty McDonald said:
From what I understand Carbide bits like speed and the smaller the bit the faster it should go.
Click to expand...

:thumbup:




69_knives said:
Get yourself a set of the Dewalt cobalt bits, or just buy singles in the sizes you use. I bent one of them hammered it back straight and it still cuts like a champ. No need to use carbide bits for soft steel and as you've found out they are very brittle and do not tolerate any wobbliness; rigidity is a must.
Click to expand...

:thumbup:




69_knives said:
Good old work hardening.

One thing I do but don't know if it helps or not is when I'm going to let up to apply more water or oil I pay conscious attention to the pressure I am putting on the bit and make sure I am either applying enough pressure to cut or quickly lifting the bit. I don't allow the bit to sit and turn without enough pressure to cut and when I restart the cut I make sure I don't hesitate and get it cutting right away.
The faster the bit is turning the more likely it will turn on the material with just enough pressure to work harden it.
Click to expand...

:thumbup: This is the key.




blindviper1 said:
You need to be much slower on the rpm's. 200-300 if you can get that low, if not just make sure you keep the bit cool.
Click to expand...

:thumbdn:


This is a common misconception, but it is not accurate. Feed is more likely the problem.


RPM should be appropriate for the drill diameter, drill material, and material being drilled. I drill 1/4" holes in D2 over 1000 RPM with HSS and get hundreds of holes. Drills dull and steel work hardens because of underfeeding. Drills break because of poor feed control, especially during exit. Carbide needs high speed to work, and thus needs a rigid setup to avoid chatter.
 
In fact, depending on the thickness of the material - I wouldn't even bother with pecking or coolant or oil. Just drill the hole. Lean into it enough to maintain a curly chip, and let up some on exit because once the web exits the force required to cut goes way down and it is easy to over feed and break something.

The only reason to go under the recommended spindle speed for the recommended SFM is a setup where chatter is being an issue (under feeding causes chatter, BTW) or an underpowered machine that can not generate enough torque at the "correct" RPM.

Poorly annealed (somewhat hard) steel will require more force to feed properly. Look at the chip coming off. If you're just rubbing, you're dulling the bit. You can drill unsphearoidized steel, but you gotta lean into it (within reason).

I normally only use carbide drill bits for production drilling of aluminum or plastic, for the wear resistance. It generally isn't tough enough for drilling steel.
 
Nathan nailed it, although I would add to make sure your workpiece is secure and not prone to spinning or shifting once you break through the other side of the material. While carbide is harder it is necessarily therefore more brittle and it won't take much lateral movement to shatter the bit, especially if they're small diameter.
 
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