Drilling holes in stainless

I tried to drill a couple of holes in 316 10mm thick stainless. I want to tap the holes to accept a M4 grub screw. I ended up with a broken drill snapped off in one hole. The other hole I gave up after blunting two drills. I'm holding the work in a milling vice and using a mill to try and drill the holes. Would I be better of using a end mill or a slot drill.

Hi Dave,

I had the same problem a while back. Use a decent TiN coated drill bit with plenty of oil, the slower you go the less heat will be produced. I would probably do it around 600rpm at the most. You will also need a spiral point tap in the black oxide coating otherwise you'll probably find your taps won't last long either.

If you are trying to drill holes in 316 plate, the likelihood is that you will have problems. I believe it's due to plate being made to be weldable, whereas 316 bar is made to be machineable by the addition of (I think) a dollop of sulphur.

A million (well 20) years ago this exact problem was encountered where I worked, trying to make some aero dynamic fins out of 316 plate, virtually impossible to machine. The material supplier was contacted and he said (quote)..."....you're trying to machine 316 plate???, can I be the supplier of your cutters??". It was then explained about the use of sulphur. We got some bar and as the problems went away, we did have a lot more swarf to dispose of though!!

Hope this helps.

cheers

Bill

Dave

I agree with Joe. I use a lot of 316 SS and it can be a challenge by times. I use a low cutting speed and a good cutting oil but I use a drill press to drill the hole. Using the drill press allows me to use a very gentle feed as the drill begins to break through. The breakthrough is when drills usually snap. Ditto on the spiral tap. I use only spiral taps on all through tapping. They do a nice clean job providing you clean the chips away before extracting the tap.

Cheers and Good Luck

Bill

I have found those carbide tipped masonry drills very good for getting through hardened stainless

Sharpen them up, use the backgear(slow speed high torque) and lots of steady pressure on the drill

Better than wrecking a nice hss drill, (don't leave too much meat for tapping the thread either or the squeak of death will wreck your tap (as I call it.))

UK Drills (and I am sure others) sell TCT drills specifically for SS. I have used and they are very good on any hard metal.

http://www.ukdrills.com/Product/index?catid=262&productid=150

A good deep center punch also helps!

Clive

Try using Ambersil Tufcut fluid when screw cutting, tapping or drilling stainless steel. It is also OK for turning if you take things easy. It is designed use when cutting tough metals. It is widely available from e.g. Axminster, Cromwell, RS etc.

Where I work we drill 316L stainless pipe everyday up to 100mm diameter with hole saws, a lot of sizes with broach cutters (trepanning cutters) and smaller ones using coated stub drills Ø3 for pilots and then step up to the finished size. It is always done on a large Richmond or Asquith radial drill (the morse taper is M5 so that gives an idea of size). It is always drilled with power feed and about 0.1 to 0.2mm of feed per rev. Cutting fluid is sometimes but not always used. The speed is not important as long as it is not too high. Slow speed usually gives higher torque which is good but the power feed is essential. If you work out the feed rate required for manual feed with high rpm it will probably be far higher than you would expect. Constant pressure as earlier posts have stated must be used, do not dwell or work hardening will occur. This is where power feed helps. If you back off for peck drilling you must back off fast and far enough to avoid work hardening and then go back with the feed rate needed to make sure you cut on contact without rubbing. CNC makes this sort of task easy but with practice it works with manual feed. Its a bit like the problem of parting off, you need to have confidence and go at it without trepidation.

When screw cutting use a larger drill than the standard tapping drill. The material is tough so a 70% thread will usually be as strong as needed and the tapping process will be a lot easier on your nerves and the taps.

Martin

Do you get the same problem with silver steel or is it just stainless. The bits I'm making don't need to be stainless.

If it doesn't need to be stainless then use carbon steel. Silver steel is for making cutting tools. If using plate then S275 (common material for steel plate) has Proof stress 275 MPa. 316 is 290 MPa, EN3 (normalised bar) is 215, EN8 (normalised bar) is 280

in my daily job I work with industrial washing machines made from 316, to drill it we use a sharp tungsten coated drill with plenty of cutting compound, slow speed and lots of pressure, for cutting threads plenty of cutting compound and steady as you go with frequently backing the tap of.

Martin P

Using a centre punch on 316 can leave a work hardened spot that makes starting a drilled hole difficult.

Ian S C

Knowing a center punch mark can make a hardened spot-priceless! How to over come that problem?

Looks like I'm stalled any way. When I went back to the job to day it looks like I have stripped the teeth of the gear train on my x2 mill.

Looks like a strip down.

Mark the spot and start with a spotting drill. Think the hardest stainless drilling job I'v had was drilling the holes in the stanchions for the handrails on my Brother in Law's 16M catamaran, and the base plates for them. The 5/8" holes got a bit of tube MIGed in and a steel cable was pasted through this.

Ian S C

Yep I have stripped the plastic drive gear wheel