What did I do wrong drilling these holes?

   / What did I do wrong drilling these holes? #1  

SmallChange

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I have 1/2" and 3/8" mild (low carbon) steel plates I'm joining with a pattern of 5/16-18 bolts, and am drilling 11/32" through holes for these bolts. The first three holes went OK, but halfway through the fourth one, the drill started overheating and smoking, and stopped progressing. It was now blunted. So I stepped down to 21/64" and tried again in the same hole, but that one overheated and blunted, too. It looks molten around the leading edges.

Since I had already drilled 11/64" pilot holes and had the assembly clamped together to preserve hole alignment, I could flip it over and drill from the far side, following the same pilot hole. I only had these 11/32" and 21/64" bits, now ruined, so I used 23/64". That worked fine, and my holes are done.

But I want to learn from the experience. So, what's the lesson? What'd I do wrong?

More info: I'm drilling at about 1000 rpm per a chart from a drill bit maker. I'm using a fairly big nice drill press. These bits all happened to look pretty new, especially the /64" ones. The work is in a drill press vise clamped to the table. The holes are all perpendicular to the plates. I'm using a light oil as cutting fluid.

Maybe the first bit was just worn out (even though it looked good) and that made it overheat. But then why did the second one overheat in the same hole? Can an overheating drill bit harden the work beneath it? If this is what happened, I guess drilling to that point from the other side, the cutting edges now at a different angle, got through it just fine. Nothing felt funny about completing that hole.

Thanks!
 
   / What did I do wrong drilling these holes? #2  
I've found there to be a big difference in quality of tools, including bits. Home owner - tradesman - manufacturing. Cutting oil has chlorinated oils and sulfated oils as part of the mix. I can't tell you what either of them do, but as cheap as cutting oil is, and as expensive as are drill bits, I use the cutting oil.
 
   / What did I do wrong drilling these holes? #3  
I use one of these rigid oilers on mine. Makes bits last long time. But i also use a drill doctor to clean up my bits after use.only thing i dont like with these oilers…kind of messy.

also, what angle are the bits. 118° work better on wood, can cut steel but dull quicker. 135° better on steel.
 

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   / What did I do wrong drilling these holes? #4  
First drill most likely went dull then work hardened the hole. All the other drills didn't have a chance after that. You may still save it if you drop the RPMs a lot and go slowly with cutting oil.

Go slower on the RPMs and use cutting oil, whatever type you have or if you don't have cutting oil, just use WD40, ATF, etc. Heck, I just use straight up hydraulic oil that I save when I have to pull cylinders apart.

Yes, understand the chart says 1000 RPMs but a chart doesn't always know every single situation in hand. Sometimes it's better to go by feel. I would go like 500 to 700 RPMs for those sizes of drills. Drill will last a lot longer and there isn't as much risk of having the issues you're having.
 
   / What did I do wrong drilling these holes? #5  
I've found there to be a big difference in quality of tools, including bits. Home owner - tradesman - manufacturing. Cutting oil has chlorinated oils and sulfated oils as part of the mix. I can't tell you what either of them do, but as cheap as cutting oil is, and as expensive as are drill bits, I use the cutting oil.
^^Agree....That RPM would drill fine all day if it was a high quality HSS bit, you were using a quality cutting oil or coolant, and feed rate maintained a good chip load. Using a lubricating oil instead of a cutting oil and running maybe a bit high RPM with that oil and questionable bit quality, and if the bit rubbed too long instead of having a chip load, could contribute to the drill problems. If the cutting edge gets chipped and starts rubbing, it will overheat and will deposit high carbon to the hole as it tries to friction weld itself. That will in turn chip the next drill and will start the same process again.
 
   / What did I do wrong drilling these holes? #6  
I've always thought recommended drill bit speeds were designed for production work when you needed many holes as fast as possible,

I've had a single speed drill (500 rpm) and a drill press (150 rpm). No problems running any size bit at those slow speeds.

Bruce
 
   / What did I do wrong drilling these holes? #7  
As said, Too high drill RPM was your first problem, then dulling the drills, then overheating them, then work hardening the holes, then only a fresh start from the other side, as you did, or a carbide drill are going to complete the hole. Drill at a silly slow RPM, with cutting oil, and it will work fine.

Some new drills come very poorly sharpened, or at least sharpening not well suited for some applications. Drilling cast iron, mild steel, hardened steel, aluminum, brass, and plexi glass all are optimized by a sharpening not as well suited to the other materials. They'll do it, but getting the sharpening of the drill, and the RPM right make all the difference. If the drill is getting hot, smoking, or not drilling at the rate it was in the last hole, it's time to stop and investigate....
 
   / What did I do wrong drilling these holes? #8  
I have had similar problems. The only solution I found to work consistently is to use good quality cobalt bits……
 
   / What did I do wrong drilling these holes? #9  
I have had similar problems. The only solution I found to work consistently is to use good quality cobalt bits……
I haven't bought anything but cobalt bits for the last 20 years.
I have a half drawer full of old HSS bits from 30-40 years ago that I never use.
I take that back, I did buy a 7/8" Silver and Demming HSS bit because I didn't want to spend $85 on a cobalt bit.
 
   / What did I do wrong drilling these holes? #10  
Too fast on the rpm
NEVER use petroleum lubricating oil. It acts as a coolant at best on steel. Canola oil, or any vegetable oil is 100 times better, but Messy. WD-40 is Worthless for steel, but great for ALU.
Use water to keep thing cool. (100 times better than lubricating oils.) (soda water keeps things from rusting up)

Also, did you mention the size of the pilot hole?, It should be about the size of the web of the final drill.

When drilling steel, MAINTAIN THE CHIP! that is, keep the cutting lips cutting, don't RUB.

As others have mentioned, once the hole gets hot and blues, it's going to take a good High Speed Steel (HSS) or better to penetrate the hard parts. Or a few trips to the grinder to re-point the drill. If the drill bit gets burned, it's a thankless job to re-point. You are best off just to cut off the business end and grind a new point. An acquired art. DON'T Burn the lips when sharpening!

It's all so hard, but as your post testifies, we git it done! I've got hundreds of twist drills with spoiled cutting lips or broken corners*, I'll get to them "someday" (*from my aviation days)
 
 
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