Making Big Holes

 

  This was supposed to have been posted ages ago.  I thought I had posted it.  But I've searched the blog for various key phrases, and it seems I never posted it.  If I did, and you're reading it twice, my apologies.

 ~≈{👁}≈~ 

When one wishes to poke a hole in something, of arbitrary size, and especially when it's bigger than any drill bit you have... there are ~~several~~ well, two, machining options and one sloppy option.

The tool shown offers one thou adjustment per screw graduation. 
Some will even do a tenth. (Bring money)

Ze Boring Head

• Chinese units are expensive.  Good ones are breathtakingly expensive.

• Large; the smallest I've seen (some rare beast Mr. Pete found), is about 1.5" in diameter (minimum hole size)

• Capable of very precise cut adjustment

• Convenient to use- once the tool is set up, increasing the depth of cut by a thou is a matter of turning a screw

• Lots of off-center mass, tries to shake/sway/bend your machine's column, so best used in a full sized knee mill or a VERY stiff, square-column bench mill.  You will typically have to run at lower speeds than the SFM would normally allow

• Uses boring bars which usually have carbide inserts, some are just brazed - maybe you can sharpen it, but probably not.

• Best for deep holes like engine cylinders

Ze Fly Cutter

• So cheap they're nearly free.  Seriously, HF practically gives them away as door prizes (nearly - a set of five might be on sale for $10), and because it's basically a hunk of funny-shaped steel which has no built-in need of precision, there's no need to shop for anything better.  This is a tool fetishist saying this.  Cheap ones work fine, it's the cutter you'll want to spend your time and money on.  (cutters are almost never included)

• cutters can be made from inexpensive HSS square tool stock, which you grind to a cutting tip yourself - this is less convenient than carbide, but ever so much cheaper cost of ownership, and a very useful (some would say mandatory) metal shop skill to acquire. Use both ends of the tool stock.
You can also use cheap Chinese insert holders in these.

• "tool setting" - that is, setting a fly cutter's tool to a specific, precise diameter - is fiddly but you get used to it.

• readjusting depth of cut once you've set it is a simple matter of doing the tool-setting job all over again - wait, what?  Yeah, that's a fiddly disadvantage.  Did I mention they are cheap?

 AFAIK, that's it for precise and semi-precise holes.  The other large hole cutting options I can think of are not adjustable, and they make relatively imprecise holes.


 If your hole is a nice common dimension and not say, "pi inches", you might find a shell mill cutter that will do the trick, provided the material is less than an inch or so thick.  These are expensive.  The cost of re-sharpening shell mills is nearly as much as buying a new one.  Used ones on eBay tend to be dull.  These are non-trivial to sharpen, it requires a "universal tool grinder" and knowledge/skill.


 A third option, for making approximate holes - not cut to any tolerance at all, is ordinary hole saws. 
They are cheap.
  Good (AKA “bi-metal”) hole saws are quite capable of cutting steel, aluminum, brass, whatever, provided one:


• uses copious amounts of a good-quality cutting fluid*

• backs off the cutter frequently; lets fluid back into the hole, allowing teeth tips to cool a bit

• uses low cutting force - if you tend to burn up drill bits, this method is not for you


 
 I have a few garden-variety, hardware-store hole saws that have seen _extensive_ use, including cutting lots of circles out of .25 and .5in steel plate,
 and they are still sharp.  Done right, this method is _messy_.  It can be less messy if you’re willing to burn up your hole saw halfway through cutting a hole, but I recommend letting it be messy.

 On numerous occasions I have rough cut a hole with a hole saw, then cleaned it up and dimensioned it with a carefully-set fly cutter.  i should probably do an entry about how to do that, although there is probably one on YouTube already.  I'll look and maybe link it.

 ~≈{👁}≈~ 

_____


* ie; preferably _not_ the stuff sold in the hardware dept for threading pipe - that’s the Lowest Common Denominator of cutting fluids; it works for carbon steel only, its high sulfur content stains copper and brass, it does you no good with copper, soft aluminum or stainless… 
personally I'm not a fan.


 I prefer Tap Magic™(standard grade).  It is expensive but I brush it on by hand so I only use a quart every year or two.  There are plenty of other good productss.
  Just use _something_.  The thing about the high performance fluids like Tap Magic is that they make difficult-to-machine materials like copper, 300 series stainless, and (gods-help-you) titanium, much easier to machine, with a cooler running
 tool, better chip breaking, and better surface finishes because little or no Built-Up Edge.  High-sulfur cutting oil from the
 plumbing department does nothing for these materials. And plumbers don’t much care about the surface finish of 
their pipe threads either - you might. ;)