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Cobalt Drills - right!

I just bought a set of cobalt drills, 29 pieces - fractional set, on ebay. The quality of the drill bits is very "Chinese", - they aren't the quality I expected althought the dealer says they are High Speed Steel Cobalt drills made by Dewalt. (OK - I figure they had China make them!)

Quality aside, - are cobalt drills just a form of HSS bits? - AND - they are brownish in color, - but when sharpened the brown color is only on the outside few microns. Are there cobalt coatings now? ---- If they are cobalt ( and they are marked as HSS CO) then why are thy not brownish throughout? Is there any easy way that I can somehow test them against my other HSS drills to see if they are anythng other than brown coated HSS drill bits?

(p.s. - I have some Cobalt drills made by Precision Twist Drill and they are HUGELY better quality.)

Thanks for any help, Bob Durango

Reply to
Bob
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The brownish color is traditional on cobalt drills to differentiate them from regular HSS drills. The cobalt in this case is an additive to HSS. Cobalt is added to increase the hot hardness of HSS, but it also makes the HSS more brittle. Even without the brownish coating most, if not all, of the cobalt drills I've bought and used in the last 32 years have a thicker web than regular HSS. This allows them to be used at the higher chip loads and higher feed pressure these drills are made for. An example of the increased wear resistance of these drills is a job that I do regularly. The parts get drilled with a .468" diameter drill to a depth of 1.700. This is done with pecking the drill only once and using 8% concentration water soluble coolant. The material is 41L40. I get 1500 or more holes before I need to change the drill. ERS

Reply to
Eric R Snow

----------What constitutes "pecking" the drill ??

Reply to
Jerry Wass

Drill a bit, retract above surface, drill a bit more, retract above surface ...

Wes

Reply to
clutch

Replace retract above surface, to retract to break chip.

Wes

Reply to
clutch

Pecking is when the drill is withdrawn from the hole momentarily. This is done to cool the drill, clear chips, introduce cutting fluid to the hole, etc. ERS

Reply to
Eric R Snow

I think they will turn brownish again when you use them. Either getting them hot, or getting hot in combination with cutting oil will turn them brown. Maybe just exposure to air for an hour will do the same, if they really are cobalt all the way through.

Jon

Reply to
Jon Elson

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