Cobalt Drill Bit Question: Twist

[ddc9999]

So I fully understand why they make left and right hand twist on drill bits. And I get why most Mechanic’s Length Cobalt Bits are left hand twist since normally if you’re drilling and need that short bit it’s for car work and you’re dealing with a stuck bolt. So if you gotta drill it out anyway you might as well maybe get lucky early in the process.

I already have a set of left hand cobalt drill bits from the Irwin Extractor Kit (3101010). I’m not sure if they are considered jobber or mechanic’s length.

Question is there any reason to get a right hand twist cobalt drill bit set? I honestly can’t think of a reason since it’s not like the metal cares and a hole is a hole at the end of the day. All I could think is if the drill ***** in reverse for some reason relative to forward, but drilling into metal is a slow low power process so who cares. Less is more and all. Not ripping giant holes here.

Thanks!


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[pstemari]

Typical drill presses don't have a reverse gear. If you're using cobalt drills, I assume you're machining stainless or other troublesome alloy. The cobalt drills have a thicker web and are more brittle than normal HSS.

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[ddc9999]

Typical drill presses don't have a reverse gear. If you're using cobalt drills, I assume you're machining stainless or other troublesome alloy. The cobalt drills have a thicker web and are more brittle than normal HSS.

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Thanks! So for me it seems like I’m all set. Looks like not having a drill press is a good kind of problem. It means less stuff I can tell myself I need to buy.

I’ll stick with the left hand bits for now and buy something only if the situation arrives. Thanks! I almost wasted some money on those Milwaukee cobalts that I wouldn’t need.

Lastly, I did notice the cobalt drills in the Irwin set are from Brazil (rest is USA). Anyone know how Brazil drill’s quality is?


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[ez-duzit]

p--good post.

I have RH twist cobalt bits, which are used mostly in the mill and drill press.

 

[driz]

No point I can see. They come in handy though for helping out broken off tubs and bolts now and again but you really only need a couple. They come in handy like easy outs up here in The Rust Belt. The trouble I have with em is if they get mixed in with the rest some way well they sorta don’t [emoji85] drill so hot if you know what I mean. Spray paint em red so they stand out or otherwise segregate them.


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[6PTsocket]

So I fully understand why they make left and right hand twist on drill bits. And I get why most Mechanic’s Length Cobalt Bits are left hand twist since normally if you’re drilling and need that short bit it’s for car work and you’re dealing with a stuck bolt. So if you gotta drill it out anyway you might as well maybe get lucky early in the process.

I already have a set of left hand cobalt drill bits from the Irwin Extractor Kit (3101010). I’m not sure if they are considered jobber or mechanic’s length.

Question is there any reason to get a right hand twist cobalt drill bit set? I honestly can’t think of a reason since it’s not like the metal cares and a hole is a hole at the end of the day. All I could think is if the drill ***** in reverse for some reason relative to forward, but drilling into metal is a slow low power process so who cares. Less is more and all. Not ripping giant holes here.

Thanks!


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The primary function of any bit is to drill holes not extract broken screws. Left hand bits are useful as they will sometimes spin out a loose screw stub. Cobalt bits are made for drilling very hard alloys but the trade off is that they are very brittle and best used in a drill press. Drill bit design goes far beyond this forum Bits are designed with different alloys, different coatings, spiral rates, web thickness and tip angle and even how they mount. I guess if you have to use a cobalt it is best to keep it short to reduce flex and the inevitabe snap.

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[matt_i]

Many times you will get a 135 degree split point with a cobalt RH twist drill.

Which is great for virgin materials, they do start fast, but I think the point is a lot less robust and damn near impossible to hand sharpen on a grinding wheel as opposed to a 118 degree drill bit.

Also a 135 deg point does not like to follow a pilot hole. It will typically chatter at the start, a 118 deg point has no problems with following the leader.

For me I would prioritize the point angle over the drill bit material.

 

[MushCreek]

As an aside, short LH bits are (or were) known as screw machine drills. In the glory days of American factories, screw machines pumped out billions of machined parts. But- screw machines had two spindle speeds, and one was the reverse of the other. Any job that involved tapping required low speed, RH for the tap. This meant that all of the other, high speed tooling had to be LH. I ran Brown & Sharpe screw machines, as well as others. We would machine brass parts at 6000 RPM, and tap at 1200! The tap goes in at 1200, but comes out at 6000 RPM. It all happens in the blink of an eye- this on machines designed before WWI.

A favorite prank when someone came to the screw machine department to borrow a drill bit was to give them a LH bit (without telling them).

 

[ez-duzit]

Cobalt drills are most excellent on drilling stainless steel. That's where mine get used.

 

[CobraRed]

Lastly, I did notice the cobalt drills in the Irwin set are from Brazil (rest is USA). Anyone know how Brazil drill’s quality is?

Brazil drill bit quality is usually decent, although as a cobalt drill bit almost certainly M35 5% cobalt instead of 7% M42 like the USA made ones. Which shouldn't really matter unless you're pushing its cutting rate/heat limits, which you can't with a cordless drill anyways.

 

[ddc9999]

Brazil drill bit quality is usually decent, although as a cobalt drill bit almost certainly M35 5% cobalt instead of 7% M42 like the USA made ones. Which shouldn't really matter unless you're pushing its cutting rate/heat limits, which you can't with a cordless drill anyways.

Thanks! That’s what I was thinking. M35 is perfect for my needs and even that is probably overkill.


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[danielbuck]

what are you using for drilling? If it's a hand drill, I'd probably just stick to HSS bits. In my experience you're more likely to snap a drill bit (smaller ones particularly) with a hand drill through metal, since you can't keep the drill perfectly inline the entire time, and your downward force tends to "punch through" the end of the hole, which often can cause a snag. HSS is less brittle than cobalt. And cheaper

If it's in a machine, then I'd say get whatever fits your budget, you'll get more life out of cobalt and other more expensive metals/coatings. Or just use HSS and sharpen them more often.

why would you not want right handed drill bits? Usually most folks keep left handed bits around just for extracting broken bolts. Unless there's a situation I'm not aware of?

 

[visionguru]

I recently drilled through a broken grade 8 bolt stud which is 20mm long with Irwin Cobalt bits. Started with 1/16"... all the way to 3/8". I'm not so experienced with drilling, but the cobalt bits don't seem to be easy to snap.

 

[pstemari]

As an aside, short LH bits are (or were) known as screw machine drills. ...

"Screw machine drills" from all industrial supplies I've dealt with are RH by default. You can get LH ones, but you'll pay out the nose for them.

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