How many holes can I get from a 1/4" drill bit before resharpening?

[thackman]

I have a large benchtop CNC mill and I need to profile long narrow part out of a 3" x 10" bar of 3/4" 1018 steel. I'm planning on predrilling 40 to 60 holes around the perimeter and milling out the webs but I don't know how many holes I can get before the drill dulls and I don't want to snap off a drill in the part after 10 holes. The CNC mill will give a much more consistant feed than a drill press and I can rig up flood coolant which should result in a longer bit life but you don't have the feedback that you get from a drill press so I don't think I'll have much warning when it starts to dull.

I know that a bit from Harbor Freight might get me 1/2 of a hole before it twists off and real machine shops can get 1000 to 10,000+ holes out of a $50+ Ghuring bit. A typical $2-$3 dollar bit is somewhere in the middle. I have several screw machine length HSS bits on hand from Enco that are between "1/4" and "3/8". Some are from Triumph Twist Drill and some are "made in USA" house brand. Does anyone have a ballpark estimate of how many holes 3/4" deep I can get between tool changes? Is it 10, 100 or 200? I can't imagine it 200 or more.

Thanks

[DouglasR]

The drill make and quality has alot to do with it. So does machining parameters. If you are driving the drill at hyper-sonic speeds and feeds it's going to have a short life.

My recommendation is to calculate about a .003 to .005 inch per tooth feed rate and about 100 SFM speed.

Formulas are in the machinists handbook.

You may also want to peck drill to break the chip and keep it from clogging the flutes. Coolant is critical as well.
-Doug

[hpowell]

The starting speed and feed 1/4" drill into 1018 steel should be 1528 RPM @ 10 ipm.
The rigidity of your setup and of the machine can affect these figures, so I'm assuming that rigidity is good.
For a 3/4" deep hole I suggest the chip break cycle (Usually G73) in 0.1" steps. Use the best quality HSS drills you can afford with plenty of coolant and you should be able to drill all the holes without resharpening.

To make these calculations easy you can use the Machinist's Calculator from http://www.machinist-calculator.com/ Full disclosure: Machinist's Calculator is a product from my company.

[RCaffin]

I doubt you will achieve anything much by drilling all those holes with cheap drill bits except wasting a lot of time. The amount of wear on the milling cutter will be about the same with and without the pre-drilled holes.

I've been down that path. It didn't work. Good tools are a delight.

A better solution would be to bite the bullet and buy a decent TiN-coated HSS-Co milling cutter. It will last you a long time. Or go the whole hog and buy a carbide cutter.

Cheers