Twisted!

This isn't a project log like most of my posts, because the project at hand is so geeky, and so well documented elsewhere, that I wasn't even going to post it. But, something interesting happened so I wanted to log that, at least. The project (now completed) is to convert my mini mill from a spring-type lift to an "air spring" - sort of like an automotive shock absorber to lift the mill head.  The bit in the red circle below is what went away - the spring is under the round cover:

Part of the process is to precisely locate, drill and tap some holes in the column and in the mill head. That's hard going, especially with the cheap drill bits I buy. A real machinist might spend $20 or more on a single drill bit, where I recently bought a set of 115 for less than 50 bucks, shipped. The downside is that they are cheap junk!

Why would anyone need 115 different sizes? The average home center set might have 13, with 1/16" between each size. The larger standard sets have more, with 1/64" between each size, but 1/64" is actually a huge amount of "slop" if you have the hole wrong. So there are metric sizes, of course, and then something called "Letters and Numbers" sizes. They go from A-Z and 1-60, and the sizes are interspersed among the standard "inch" bits in 64ths.

I was to drill and tap holes for an M6 X 1.00 metric bolt, which calls for a Number 9 drill bit. I was real clever, and pre-drilled the hole with a standard inch size smaller than Number 9, since I have a bunch of those and they are easy to replace at Home Depot. But drilling in cast iron is tough, and one of the holes "ran out" - that is, the bit bent in the hole, came out the side of the work and then shattered. That was a treat, getting the remains of the bit out, but I was able to do it.

So, then I put my only Number 9 bit in, and gingerly tried to enlarge the hole. At first all was well, but then it snagged and ... check out the photo below. The top bit is normal, with a spiral that is uniform along the length. The lower bit (the Number 9) sort of "unwound" - another split second and it would have snapped:

That called a halt to the proceedings until I could get another Number 9 bit. I couldn't find anywhere in Lancaster to buy a single numbered bit on a Saturday afternoon, so I bit the bullet and went to Harbor Freight and bought another cheap numbered set, 1-60. It was on sale for $19.99 and I had a 20% off coupon, so the damage wasn't too bad. Now I have spares for everything except Number 9. I'll pick up a single bit the next time I order from Amazon.

The insult added to that injury is that I later realized the hole that caused the problem was not really necessary - it was for a limit bolt that doesn't exist on my mill. Dang!