What Cause Drill Bits to Break?

[xroad]

What is the cause for a drill bit to break? Cheap steel? Poor technique? No cutting oil? Bit too long and it flex too much? Global warming?

I saw somewhere ... selling "stubby" drill bits. I suppose that will made it more stiff and less flex. Can I just shove the bit deeper into the chuck?

 

[Mickey O]

Break or dull? Breaks are usually caused by flexing the bit, not clearing out the chips (especially with aluminum), to hard of drilling pressure (when you first break through the drill follows the twists and then snaps) and cheap or crappy bits. You can put the bit further in the chuck but check to make sure it's spinning without wobbling (will happen if you go up to far).

 

[DHCrocks]

breaking is caused by either too much side loading on the bit (not drilling perpendicular) and snapping or if the bit gets stuck and it twists to snap (usually only very small bits).

 

[xroad]

Break, not dull.

I just picked up a 13 pcs set of Harbor Freight REVERSE CUT drill bits. It is to be use for extraction of bolts/screws with broken heads. In a stomach dropping case like this, I really do not need to have the bit break on me and get it stuck inside the hole. Then I am really screwed! SO, are cheap *** drill bits really that bad? I think I don't want to find out that way. I should get some better quality bits. Even it it breaks, at least I can say I did my best and tried my best.

 

[xroad]

breaking is caused by either too much side loading on the bit (not drilling perpendicular) and snapping or if the bit gets stuck and it twists to snap (usually only very small bits).

Good quality or poor quality, can I minimize the likelihood of twist breakage by not going too deep into the hole, and incrementally increase the size?

For example, start with the small bit, go in 1/4 inch, then next size larger bit, go in that same 1/4 inch. Then go back to the small bit for the NEXT 1/4 inch deeper. That way, the bit, any bit, will never see mare than 1/4 inch buried into the metal. (sorry for the convoluted description, newbie with limited experience here).

Regarding shorter bits, I would think a longer bit allows more flex tolerance due to me holding the drill "not quite perpendicular". Having one whole inch of bit length to flex that few minute of angle error in m y holding the drill is better trying to flex that same few minutes of angle over a 1/4 inch length of drill bit. Make sense?

 

[Elroy]

What is the cause for a drill bit to break?

Simply put: Too much stress.

Look at it this way. There are combined stresses. First is the torsional stress required to drive the cutting tool. That combined with the build up of chips dragging the inside of the hole.

If your in a hand drill then there is a good chance there is some bending stress from not pushing on the drill motor squarely.

Then there is the compressive stresses from the quill or your hand depending on the set up.

All of these stress combine to general a peak principle stress. Combine that peak stress with the fact the drill is rotating and like magic you now have a stress reversal condition where the principle stress goes from a maximum compressive load to a maximum tension. Stress reversals damage materials much quicker than a simple load. Next thing you know the material is fatigued and fails

Can I just shove the bit deeper into the chuck?

On the surface that might appear to be a good choice but there are some problems. First off you'll be chucking on the flutes of the drill. There is a stress concentration that occurs where the flutes transition onto the smooth round shank. Chucking deep on the drill up on the flutes aggravates the concentration factor. So you're reducing the bending stress by having a shorter beam but throwing it all out the window by chucking on the weak part of the drill bit

 

[HandyManny]

Lot's of various things cause them to break. How are you using them.

A lot of times it's steel that's both too weak and brittle to handle the amount of torsion that's being placed on the bit. Could just be cheap bits.

 

[DHCrocks]

Go slow with a steady even pressure and that will lessen the likelyhood of breaking. Just because a bit is expensive doesn't mean it will not break. Take Cobalt bits for example, I have snapped way more cobalt bits then HSS bits. Cobalt while more hard and durable is more brittle and will snap if you side load it.

 

[Zrexxer]

SO, are cheap *** drill bits really that bad?

Cheap *** drill bits are REALLY that bad.

 

[scooby074]

Cheap *** drill bits are REALLY that bad.

+1

Bits usually break from too much side loading, or fetching up in the hole. Ive had cheap bits just break from looking at them funny nothing worse then breaking a bit off in a stuck bolt. Some lower grade bolts (read Chinese) have really crappy grain structure in
them that just love to catch and snap bits.

Another great wrecker of bits is BFM (bed frame metal). I go to great lengths to avoid drilling in that ****.

Go to a machine shop or supply house, and get a couple singles in the most common sizes you need. (i like Dormer bits). Wont be more than a couple bucks each.

While were on the subject, i havent had much luck with lefty drill bits on anything but the loosest broken bolts, so it may be a better investment to just buy good regular bits and good easyouts.