Help drilling stainless

[KiloWhiskey1]

I?m trying to enlarge a 3/16? hole in a solid stainless tail spring and I?ve broken two drill bits. So far I?ve tried to use a slightly larger diameter bit to step the hole up gradually, but I broke a titanium and a cobalt bit. The tail spring and it?s bulkhead bracket are already riveted on the air frame, so I can?t use a drill press or mill. I need to get the hole enlarged to .2450? (ish) so that I can use a reamer to finish the 1/4? hole.

Any tricks for using a hand drill to enlarge a hole in thick and round stainless? Is step drilling with gradually larger bits a bad idea? Should I just go directly to the final size?

I have some M35 Cobalt bits on order that are supposed to be good for stainless. Any other tool suggestions would be appreciated as well.


Thanks,

Keith

[MED]

Some stainless steel will work harden when drilled, transitioning into a very hard phase which is almost impossible to drill. Without reheat treating the metal, it may be necessary to grind the hole to a larger size rather than trying to enlarge it with drills.

[Kyle Boatright]

I'd use cutting fluid or boelube. That stuff works.

On stainless, you use low cutting speeds and medium/high pressure. Do not let the workpiece get hot. That means lubricating the drill and going slow/stopping as necessary.

[Phil]

Once you know the trick, you never worry about fighting stainless again.

https://youtu.be/FMzIHl1HMXc

[ColoCardinal]

What Kyle said. The lube is VERY important. Old timers used to use lard on stainless steel. You may also want to back relieve the drill bit very slightly. Use a small stone if you have one. Stainless will often "grab" the bit and snap it. If you can lessen the angle of the cutting lip slightly, it can help. Sort of the same technique as grinding a bit for plastics except you don't want to be as aggressive (don't create a negative rake). Just take the steep edge off of the outer corner of the lip.
What MED says is also true. Let's keep our fingers crossed that this hasn't happened and try it again.

[Norman CYYJ]

If you have work hardened the metal take a small cutting stone bit and run it in the hole to grind away the hard metal surface then follow the directions above to drill the hole to proper size.

[ty1295]

As mentioned anytime you drill/machine stainless slow rpm, high pressure. If you have spun the bit at high speed you have work hardened it and it will require even more pressure and sharp bit to get thru at a low speed.

[KiloWhiskey1]

Thanks for all the suggestions. Carl described the exact problem that I?m having. The bit grabs and snaps before I can make any progress. The sudden torque from the bit grabbing moves the drill in my hand and the bits snap. The good news is that I doubt it has gotten hot because the bit hasn?t had time to spin at any significant RPM.

I?m having to lay on my back, so keeping lubricant flowing is a problem. I was using motor oil on the bit and in the hole, but I may try a constant flow of water and just accept the water torcher dripping in my face. I?m also going to try tapering the edges of the bit in hopes that it will stop the bit from grabbing. Hopefully I can do that without dulling the bit in the process.

Any thoughts regarding using increasingly larger bits to step up to the final size versus going directly to the final size bit?

Thanks again for all the tips!

[F1R]

Now that you have the location quite well marked, consider taking the tail spring to a machine shop where it can be held securely in a milling machine.
Buy your local machinist a 6 pack of his preferred beverage for an in position inspection of your situation.

OR

If you are resigned to in position
Machine a long guide bushing (4-5 X the diameter of the hole you are drilling) with a radiused end that fits the curvature of the housing. Then either epoxy and or clamp it securely to the OD and drill at a low RPM. Continuous feed and lots of cutting fluid( like rapid tap) are the key to preventing work hardening.

The guide bushing will keep you aligned and prevent the snagging issue.

[snopercod]

I have successfully used Guhring screw machine drill bits (available from www.mscdirect.com) to drill hundreds of holes in 304 stainless. I used soluble oil as a coolant. If you're drilling a spring, though, that's gotta' be exceptionally hard stuff. Good luck.