Cobalt drill bits

admin

I was wondering if anyone can shed some light on using cobalt drill bits for me,  The salesman who sold me a set said I should never drill pilot holes because it would dull the bits.  But I cant imagine drilling a 1 inch hole through plate without a pilot hole.  also what kind of speeds and lubricant should I use. On the weekend I drilled 4 holes through half inch with a new 3/8 cobalt bit on slow speed and it took forever Everyone told me cobalt bits cut fast and clean so what do I need to know?
Reply:Cobalt bits don't cut any faster than steel bits if turned at the same speed.  Pilot holes are bad for bits because they increase pressure on the outside edges of the cutting edge and cause chipped bits.  Pilot holes are sometimes necessary, though, because you can't drill straight without them.  If possible, use only a center punch.  Drilling slow will prolong the life of your bits, the rule of thumb for cutting tool speed over mild steel is 100ft/min when using HSS bits.  Cobalt bits have a higher cutting speed but will last longer at slower speeds.  Not all cobalt bits are created equal.  Try some bits from Triumph Twist Drill Company and they will blow anything else you've ever drilled with out of the water.  Greenfield, Precision Twist, Cleveland Twist, etc... all make excellent bits as well.
Reply:what kind of lubricant should i use with these bits
Reply:I regularly drill without a pilot on sizes up to 2 inch.  I do however use a Center drill to spot the location and I have 15 horsepower on the press.  I find cobalt bits fragile.  They certainly hold their edge but if they catch breaking though or are misaligned they snap more readily that standard HSS.  On large bits you need a quality coolant, water soluble oil of some type. ( flood the bit)
Reply:Lubricant should'nt be needed if you just keep an eye on the shavings.If they start turning a blueish-black color,you are putting too much pressure on the bit,causing the bit to overheat.This may cause bit damage.
Reply:thanks for your input i just spent 700 on bits so I dont want to ruin them
Reply:I would have saved my money and bought quality high speed steel bits.  Only buy cobalt when you are intending to drill hundreds of holes at a time.  To use cobalt alloy bits you need lots of power and a rigid drill press.  It is the same thing for carbide tipped tooling.  You need greater power on your machine in order to drive the cutters efficiently.  It is like driving a formula one race car down a one lane dirt road.
Reply:Not everybody is an expert drill bit sharpener and cobalt bits are necessary when time between sharpenings is long.  I can grind a nice standard point on a bench grinder.  I have tried split point with no success.  I would like to buy a tool grider and grind my own bits, but I haven't seen any for less than $400 and those don't include any tooling.  A carbide wheel for one is $100+
Reply:I use 1/16" cobalt bits for drilling Grade 8+ bolts and allen heads for safety wire. I wouldn't do it with HS bits that's for sure. I usually just use WD40 as a cutting fluid. Works fine.