Destroying Drill Bits

[Calandrod]

I seem to have an issue with burning up my drill bits. Today was particularly frustrating and I could use some advise.
On the lathe, I was using a 9/16” drill bit to bore a hole in mild steel (.75” diameter). I had previously used a 3/8” bit as a pilot hole. The 3/8” hole was 2.25” deep and though I also had problems with that one, I am focusing on the 9/16” problem. I used the formula (100 x 3.82 / .5625) which gave me an RPM of 679. I chose the 560 RPM on my lathe. Initially it appeared as though all was well. I had nice curly chips about an inch long rolling out of both sides of the flutes. I had medium pressure and it was a steady chip load coming out. When I got about .75” into it, the chips rapidly became dust and everything overheated leaving me with no chips coming out, a scorched blue part, and yet another trashed drill bit. I might add all my drill bits are used but it appeared to be cutting very well at first and sharp. I’d like to fix my drilling problem before investing in nice new bits so I don’t destroy them.
This is almost the same problem I had with the 3/8” bits except they both lasted 1” each. I tried using cutting fluid on the second one but that made no difference.
I appreciate any help.

 

[400bird]

I'm too green to have a good answer and mostly responding to see what others say.

How hot is the work piece? I wonder if you need coolant? Or pauses to let the drill bit/steel cool down.

 

[toprecyler]

I would venture you still a bit fast on RPMs. My speed calculator shows about 449 RPM
for soft low carbon steel, 9/16” size HSS drill. Now if you have a lower grade drill bits, I would cut that speed in 1/2. Use coolant, or oil, to help keep heat down. If you start getting any color in the chips, you are still turning or feeding too fast.


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[Ken226]

It's like a multivariate equation. Lots of variables that depend on one another for the final output.

Are your drills good quality Carbide, HSS, or cheap carbon steel from the likes of Harbor freight?

Assuming the steel you are calling mild is something like 1020, and the drills are quality HSS, the 500 rpm should be fine. Try peck drilling in 1/4" ish increments per peck, use high sulfur cutting oil and you wont have any problems.

If the drills are carbon steel, low quality, or if the material you are cutting is tougher, then you need to slow down the lathe rpm, use shorter pecks, clear chips more often and use more cutting oil.

So, as an experiment, try slowing it down to about 350ish rpm, use plenty of cutting oil and peck drill. If it works, then you'll have a point of reference.

 

[Calandrod]

I repeatedly backed the bit out to free it from chips too.

I'm too green to have a good answer and mostly responding to see what others say.

How hot is the work piece? I wonder if you need coolant? Or pauses to let the drill bit/steel cool down.

 

[Calandrod]

I would venture you still a bit fast on RPMs. My speed calculator shows about 449 RPM
for soft low carbon steel, 9/16” size HSS drill. Now if you have a lower grade drill bits, I would cut that speed in 1/2. Use coolant, or oil, to help keep heat down. If you start getting any color in the chips, you are still turning or feeding too fast.


Sent from my iPhone using Tapatalk

Very helpful. Thanks. Should I always be able to touch the drill bit without getting burned? I read that somewhere but don’t know if it’s true.

 

[Calandrod]

It's like a multivariate equation. Lots of variables that depend on one another for the final output.

Are your drills good quality Carbide, HSS, or cheap carbon steel from the likes of Harbor freight?

Assuming the steel you are calling mild is something like 1020, and the drills are quality HSS, the 500 rpm should be fine. Try peck drilling in 1/4" ish increments per peck, use high sulfur cutting oil and you wont have any problems.

If the drills are carbon steel, low quality, or if the material you are cutting is tougher, then you need to slow down the lathe rpm, use shorter pecks, clear chips more often and use more cutting oil.

So, as an experiment, try slowing it down to about 350ish rpm, use plenty of cutting oil and peck drill. If it works, then you'll have a point of reference.

It’s kind of a collection of random bits when I bought all the tooling at once. So I have no idea on their quality.
I’ll give that a try and see what I can make happen. Thank you, I really appreciate it.

 

[Ulma Doctor]

Slow it down and use cutting fluid or anchor lube

 

[Ken226]

It’s kind of a collection of random bits when I bought all the tooling at once. So I have no idea on their quality.
I’ll give that a try and see what I can make happen. Thank you, I really appreciate it.

For drilling I use high sulfur thread cutting fluid. Whatever I can get cheap on Amazon Prime or the local Ace Hardware. This is what I have at the moment but jeeze, the price seems to have doubled:

Amazon.com

For drills, I like 135 degree Cobalt split point drills. I've been using a Drill Doctor DX750 for about a decade to resharpen them, kinda like a pencil. All the way down to the stump, then I toss the stump and replace the drill. So far i've had the set for about 10-12ish years, and have sharpened away and replace about a dozen of the commonly used sizes. The 1/4" and 3/8" i've sharpened to stumps, tossed and replaced several times.

Amazon.com

I also have a cheap set of these Harbor Freight cobalt drills. Also 135 degree split point, kind as a spare set. They work surprisingly very well for the price:

https://www.harborfreight.com/cobalt-drill-bit-set-115-piece-61886.html

The Drill Doctor 750 (at least mine did) sucks when doing the split point because the rear edge of the grinding wheel isn't quite centered. After putting the wheel hub on the lathe and bumping the shoulder back about ten thousandths to get the back edge on center, it now does a pretty decent split point.

 

[m1kemex]

Could be many things.

First thing that comes to mind, and the most obvious, is that the drill is not the expected grade of HSS, so you're running it too fast, and ruining it.

But I doubt it.

The way you describe it, I think it's the bar itself. It may have some nasty incrustations. For example, high silicon impurities often make stuff nearly unmachinable. A friend of mine always told me, that stock stuff you find in junkyards is not to be trusted for that reason, as there is usually a reason why it ended there.