Destroying Drill Bits

[Parlo]

Check the land on the flutes. If there is wear, you must grind it back to the correct diameter.

 

[Jake M]

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.

Observation- That drill bit got "cooked". It's more than likely lost it's "hard" to a depth that's far deeper than a quick resharpening is going to fix.

Personal experience- I don't use "feeds and speeds" for drills. When I pull the drill out, I want to be able to touch it without pain. "Hot" is fine, but if I can't leave my finger there... No good.

Recommended speeds for drill bits are for industrial processes where there are a LOT of controlled variables. Drill bits of a higher quality than most of us keep (although I'm quite sure there's exceptions). How deep is the hole (how long is the drill exposed to the heat). Coolant? Power feed for a controlled chip formation. All kinds of things that are out of our hands on manual machines.

I don't have a "formula", but my "rule of thumbs" to get started and experience says that if I were doing a 9/16 hole in mild steel, I'd probably do it at 200 RPM on a drill press where I'm yarning on a lever, and perhaps (slightly) higher on a lathe where I'm cranking a screw which can apply more force, in a more consistent way.

In the end, hobby shop or big industry, recommended feeds and speeds are a starting point, the end process is always driven by results. At the hobby shop level, there's typically a LOT more variation from the tables, and it's almost always on the slow side, and sometimes a lot. except the drill press. There's a LOT of those that don't go slow enough to make much beyond a 5/16 hole. lots of us get by with just "drill a little", wait a few seconds, drill a little, wait a few seconds.... There's a lot of workarounds to make do, but with speed, feed, lubricant, (maybe) sharpening back to where the drill hasn't lost it's heat treat, whatever you do, managing the heat is what's going to solve the problem in those pictures.

 

[yota]

in the 2nd pic, the tip of the bit looks very dull. doubt it got that bad in one hole.

 

[Chipper5783]

As Parlo said, make sure the lands are full size right up to the cutting edges. If the bit was previously pushed hard and the edges rounded back, then you have to take off a good amount, perhaps half an inch - so that you are back to full size at the lands. My guess is that is your main problem. All the suggestions above are good, but I’ve drilled a lot of holes in steel with no lube - it still works fine. Of course the drill bit edge doesn’t last as long, the surface finish won’t be as good - but it doesn’t stop cutting after a half inch deep hole!

Pilot holes are sort of a mixed blessing, sort of good, sort of not. The typical rule is to only make the pilot hole the size of the center web - that is a good place to start. The issue with too large a pilot hole is that there is a tendency to feed too fast, so that small outer edge that is cutting metal has way too high a chip load (drills are somewhat designed so the outer lip is pulling the drill in to push that center web through solid material - except with a pilot hole the center is just air). When hand feeding the larger bit into a pilot hole it is almost hard not to over feed - especially when you slow down the spindle speed, then it feels like the feed is ridiculously slow.

 

[pavyan]

If you allow me, I will throw in my two cents

First and foremost, the rotational speed of the lathe spindle for such a drilling diameter is too high, I would set a safer ~300-400/min (for regular HSS; HSS-Co5 or better HSS-Co8 is a different matter).

In my old bench drill, I have the rotational speed set to approx. 400/min and in practice I only use them (this model does not have lower ones) - and I use them even for a 1mm diameter drill bit because I do not feel like changing the transmission belts But even in steel, this 1mm "goes" relatively well at a low 400/min.

The second thing, equally important - is lubrication/cooling. From my experience, it looks like this: high cutting speed - COOLING+lubrication; low cutting speed - LUBRICATION+cooling.
Of course, capital letters mean what is more important in the given conditions.

As for lubricants... mineral waste oils/lubricants are the last resort, in truth, they are not suitable. But cheap vegetable oils are good enough, and the best - in my opinion - is linseed oil. Not so much linseed varnish, because it contains siccatives and dries quickly (a few days), but pure linseed oil (for frying); it also dries, but it takes months.

I mention this because you have to clean the tools - otherwise, they will soon "cover" with a skin of dried linseed oil.

However, the absolute best are advanced modified oils, specially designed for such purposes (threading, drilling, etc.)
For example - like ROCOL RTD oil, mentioned here by Mauzinette; I don't know it, but I'm sure that in Poland I use a very similar product: it's TEREBOR (by French Molydal or Polish FANAR), a nice blue "oil for difficult drilling, tapping and machining" (from manufacturer's description).

I've been using it for years - and I don't know a better product for that: threading with a fine-pitch M16x1 tap in stainless steel or acid-resistant steel using linseed oil - that's ~3 holes and the tap is already protesting.
Using TEREBOR I will thread 20 such holes... and then thread another next 20 - and tap, although slightly blunt - can still be used. For me - pure revelation.

Of course, I also use TEREBOR for drilling and sometimes for turning; if TEREBOR is not available in the US, look for and buy e.g. ROCOL RTD, it is probably an equivalent - or another similar special cutting oil, they will advise you in the specialist shop.

The third thing, which probably does not need to be mentioned: is a well-sharpened drill!
I sharpen my blunt drills myself, "by hand" - they drill very well; I have several decades of practice - but I will add here one more important thing: if you drill WITHOUT A PILOT HOLE, it is absolutely worth using drills with the so-called split point correction (in the photo: "ścin", in Polish). This reduces the necessary feed force by up even to 60% and also causes significantly less force causing deflection of the drill from the hole axis. Drill bit blade split point correction is especially important for large-diameter drills, but small ones are very "like it" too. The hobby driller likes it even more, l assure you
I perform the mentioned blade correction of drill bits on a bench grinder (large drill bits) or (small drill bits) by the irreplaceable DREMEL, i.e. my favorite jack-of-all-trades tool

I'm pasting images, because a picture is worth a thousand words:

 

[ChazzC]

I have a 7x16 Mini-Lathe and Little Machine Shop (LMS) 3990 Mini-Mill, both small. Hobbyist machines. I routinely use the LMS Speeds & Feeds online calculator, which was developed for Hobbyists. I also usually start at lower speeds than recommended, increasing if it seems to be OK to do so.

Speeds & Feeds

The premier source of tooling, parts, and accessories for bench top machinists.

littlemachineshop.com

 

[ericc]

When this happens to me, I always spark test the drill bit. Usually, it will give a simple carbon steel spark, not a high speed steel spark. That means two things: first, run them half speed, second, if you see that much blue on a carbon steel drill bit, it will require a lot of grinding to get back to good hard material. Annealed carbon steel will dull rapidly and heat up.

 

[RJSakowski]

M2 HSS doesn't lose its hardness below 1000ºF, at which point it is beginning to glow a dull red in a dark room. Carbon steel, OTOH, start to soften around 400ºF. You can easily distinguish HSS from carbon steel tooling with a spark test. Sparks from HSS are a single spear without branching and a red color while carbon steel sparks are highly branched and yellow to orange in color.

 

[pacifica]

If you allow me, I will throw in my two cents

First and foremost, the rotational speed of the lathe spindle for such a drilling diameter is too high, I would set a safer ~300-400/min (for regular HSS; HSS-Co5 or better HSS-Co8 is a different matter).

In my old bench drill, I have the rotational speed set to approx. 400/min and in practice I only use them (this model does not have lower ones) - and I use them even for a 1mm diameter drill bit because I do not feel like changing the transmission belts But even in steel, this 1mm "goes" relatively well at a low 400/min.

The second thing, equally important - is lubrication/cooling. From my experience, it looks like this: high cutting speed - COOLING+lubrication; low cutting speed - LUBRICATION+cooling.
Of course, capital letters mean what is more important in the given conditions.

As for lubricants... mineral waste oils/lubricants are the last resort, in truth, they are not suitable. But cheap vegetable oils are good enough, and the best - in my opinion - is linseed oil. Not so much linseed varnish, because it contains siccatives and dries quickly (a few days), but pure linseed oil (for frying); it also dries, but it takes months.

I mention this because you have to clean the tools - otherwise, they will soon "cover" with a skin of dried linseed oil.

However, the absolute best are advanced modified oils, specially designed for such purposes (threading, drilling, etc.)
For example - like ROCOL RTD oil, mentioned here by Mauzinette; I don't know it, but I'm sure that in Poland I use a very similar product: it's TEREBOR (by French Molydal or Polish FANAR), a nice blue "oil for difficult drilling, tapping and machining" (from manufacturer's description).

I've been using it for years - and I don't know a better product for that: threading with a fine-pitch M16x1 tap in stainless steel or acid-resistant steel using linseed oil - that's ~3 holes and the tap is already protesting.
Using TEREBOR I will thread 20 such holes... and then thread another next 20 - and tap, although slightly blunt - can still be used. For me - pure revelation.

Of course, I also use TEREBOR for drilling and sometimes for turning; if TEREBOR is not available in the US, look for and buy e.g. ROCOL RTD, it is probably an equivalent - or another similar special cutting oil, they will advise you in the specialist shop.

The third thing, which probably does not need to be mentioned: is a well-sharpened drill!
I sharpen my blunt drills myself, "by hand" - they drill very well; I have several decades of practice - but I will add here one more important thing: if you drill WITHOUT A PILOT HOLE, it is absolutely worth using drills with the so-called split point correction (in the photo: "ścin", in Polish). This reduces the necessary feed force by up even to 60% and also causes significantly less force causing deflection of the drill from the hole axis. Drill bit blade split point correction is especially important for large-diameter drills, but small ones are very "like it" too. The hobby driller likes it even more, l assure you
I perform the mentioned blade correction of drill bits on a bench grinder (large drill bits) or (small drill bits) by the irreplaceable DREMEL, i.e. my favorite jack-of-all-trades tool

I'm pasting images, because a picture is worth a thousand words:

View attachment 508500View attachment 508501

If you look at the outer lip on these photos you will see they are sharp. This is where a lot of the cutting is happening. Also why if you have a 135 degree drill the chamfer of the hole should be 140 degrees so you can enter the cut with less stress on the lip edge.That edge can get destroyedx or degraded in just one action, then won't drill well or not at all.

Check your drill bits with a magnifying glass to see how they are.

Same thing happens with end mills and why we use radiused end mills.

 

[Red96]

Probably more than one issue; can't disagree with any of the other suggestions.

IMO you have too large of a pilot hole, so the drill is only cutting on the outer 1/8" lip of the bit, quickly overheating and dulling that area of the grind. And you are turning too fast, which only amplifies that problem. Pilot hole should only large enough to accommodate the webbing.