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Query drill bits (again)

glenhead

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Nope.

Brass, copper, and bronze are "grabby". When they're cut, the resulting chips are different from those in aluminum or steel or plastic or just about anything else. The conventional cutting edge on a drill bit digs in to the surface deeply enough that chip formation fails. That causes the bit to stall suddenly and all-too-frequently snap off inside the hole. A tool steel drill bit stub locked in a hole is considered a Bad Thing.

"Normal" drill bits have sort of a scooping leading edge on them, making steel or aluminum or whatever peel up and make a curl. That's what causes the problem in copper alloys. Dubbing the bits removes that scoop and turns it into more of a scraper. The cutting action of a dubbed bit makes it shave the brass away cleanly without grabbing. (It all comes down to shearing planes within the crystal matrix of the metal.)

I bought a (somewhat) cheaper set of drill bits for use in brass and wrote BRASS on the front of the index. It took very little time to dub them all. Yeah, the idea of intentionally modifying a brand-new bit so it ain't worth a dang for steel is certainly cringe-worthy, but I don't know of a factory-made set of bits for brass. I guess I probably never looked, frankly. A specialized set of tools like that is going to jack the price up a ton; it's a lot cheaper to just buy another set and dub them.

Comments on the listing:
  • M35 is a flavor of high-speed steel (HSS, also known as tool steel) that has 5% cobalt in it. (Saying "HSS M35 Cobalt 5%" uses a lot more words to bang the drum.)
  • Cobalt is added to HSS to make it harder so the edge lasts longer when used to cut harder steels. The edge will still last longer if you use them only in brass, but it's not really worth the extra expense to go for the M35 vs regular HSS.
  • It used to be that 135° tips were more of a specialty item, but now everybody has shifted to them. If you can find a set of 118° bits you'll probably save a bit of money up front and they'll work just as well.
  • The split point makes the 135° tips "walk" a bit less (the 118° bits aren't as squirrelly), to the point that some people don't see the need for a pilot hole anymore. Other than the split tip, this set is still ground with conventional tips, so it'll still be safest to dub them if you're going to use them in brass.
Hope this helps.

Glen
 
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NEW65

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Thanks Glen so much for the information you've provided on drill bits. In fact I've printed it off! You certainly know your stuff.
Can I ask you if you prefer to use a stone to dub the bits? Is a stone the best choice to use ?
Cheers Glen
 

glenhead

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Up at the top-right corner is a little gizmo that says "Search". (At least it's there on my screen.) If you use that to search for things you can find an amazing amount of information on these forums. Using it to search for the word "dub" didn't return much, but searching for "dubbed" returned a bunch, including this one:

Thread on dubbing bits

There's also a ton of information available with a Google search.

Glen
 

Mike Phelan

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I have a set of drills for steel and another set for brass where I've ground for zero rake (didn't know the term "dub"!).

Most of the smaller holes I drill with spade drills which I usually just make from tool steel.
 
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NEW65

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Thanks again Glen for the great info - i did a search on ebay for drill bits and found these... here is the link:
105pc Cobalt Drill Bits Set for Stainless Steel Metal HSS-Co Cobalt Bit Titanium | eBay
The old saying is: If it sounds too good to be true, it probably is
I just find it hard to believe that the quality of these drill bits can be good at this price. However, I dare that if used in brass they may be okay.
Be good to hear your view Glen
Thanks
 

bruce linde

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uhm... i just paid 10x that for a seriously nice quality (and way smaller) set of bits, just so i can turn the old set into 'brass only'.

as you say, might be ok for brass....
 
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NEW65

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Cheers Bruce. I have seen the type of set you have bought, as you say much more expensive and probably excellent quality. I just don't see a point in buying cheap inferior tools. I guess I have finally learned my lesson!
I think I'll buy a set like yours and adapt them to use in brass. They will probably last for years :)
 

gvasale

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To "dub" a drill bit, take a sharpening stone and rub it across the cutting edge until the edge of the helix has a flat which is on axis with the length of the drill. This takes away the ability of the drill to grab and get pulled into the work when working with soft metals.
 
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