Wednesday, March 30, 2011

Notes on drilling stainless steel with DIY tools

I had to drill some mounting holes in a stainless steel tray I bought.  It was harder than I thought.

Here's what worked, in the end:

  • Use cobalt bits.  Home Depot sold some cobalt bits my Milwaukee called "Thunderbolt" that were expressly stated to work well on stainless and iron.  They did the trick for me where gold titanium bits just dulled and broke.
  • Use a slow drill speed.  The second-slowest speed on my 18V cordless did well.  I tried high speeds, and they just made the drill bit tip cherry-red, melted and useless.
  • Use moderate pressure.
  • Drill straight down.  Drilling on an angle makes it more likely you'll bust the bit off.
  • The above notwithstanding, I found a little bit of wiggle on the drill helped the bit gain some bite.  YMMV.  I wouldn't do this on thicker metals.
  • Use a nail or something to dent the metal where you want to start.  The "Thunderbolt" bits did a good job of staying put to start, as long as you went really slowly, but having an initial detent helps a lot.  It's a must with titanium bits, mine skated everywhere.
  • As always, start with a small hole and work your way up gradually.  Note, however, that thinner bits are weaker.  I had good success with cobalt 3/32" bits, but titanium 3/32" broke before they really bit in.
  • Oil it.  No idea why it works, but it does.  To some extent, the more oil I put on, the better it went (i.e. re-oil frequently).  Don't use WD-40 or something that evaporates.  They say to use "cutting oil", but Home Despot did not have that.  I used an old can of "3-In-1 oil" and that worked fine - I imagine any light oil would also work.
  • When using oil, you WILL see metal coming off - assuming you're making any progress.  If not, stop and fix what is wrong - dull bit, no oil, or wrong speed.  Keeping at it will do you no good.
  • Change bits frequently.  Even in thin stainless, titanium bits were only good for enlarging 1 or 2 holes before dulling in to uselessness.  That chromium makes it damn hard.
  • Once the hole has been made, gold titanium bits may be enough to enlarge it.  But the cobalt ones did a much better job for getting the pilot hole done, and would likely be better all around.  I only bought one size of cobalt bits, so I had to use titanium to finish up.
  • Don't try and get more mileage out of a broken drill bit.  Re-using the stub won't get you anything.  I tried.
  • Scrap all your dull bits right away.  You may cringe, but it's already worn out.  Don't keep it hoping for a few more pennies worth of mileage.  Bits are disposable, it's the cost of doing the work.

All of the above also applies to aluminum, with the exception that higher speeds work OK.  Not high speed, mind you, but 1 or maybe 2 notches over minimum on a cordless hand drill.

I've read that carbide bits, although hard, are not recommended for metals.  Ask around.  If in doubt, it's prudent to not use them.

If you're doing a lot, look at 'real' tool stores for 10-packs of bits.  HD does not have these.  I imagine 10 cobalts would be pricey, but the usefulness is 10x that of titaniums for this kind of work.

I will admit my first six broken bits came out of a 101-pc Ryobi master kit sold at Home Depot for a 'ridiculous' price.  So they were cheap - sue me.  They're drill bits, not gold jewelry.

The kit has been good enough for 99% of what I need, and has virtually every size and type of bit, which is far easier to manage than a job-by-job run to the store.  In the few cases it's not adequate, I go and pick up the specific expensive stuff for what I need.  Like today.

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