If you're squeamish when it comes to using precision tools as anything more than toolbox ornaments, steer clear of this post. I'm about to do some pretty sacrilegious things to an unsuspecting depth micrometer.
It started when I received an eBay purchase a few weeks back. I had acquired a nice NSK depth micrometer with interchangeable rods. The first problem was my own fault. I didn't realize that the sizes listed were 1-2, 2-3, 3-4, 4-5, and 5-6. For some reason when I read the description I thought it covered the 0-1 range, which is of course the one most people would use pretty often.
The second problem was a bit worse. When I received the mic, the 1-2 rod was installed in the body. It was pretty crunchy when I turned the thimble, but I've brought some pretty abused tools into excellent working condition so I got to work with a disassembly and cleaning. Although there was a lot of crud and a bit of oxidation it cleaned up really great,
--BUT-- here's the kicker: the 1-2 rod was bent about 1.8" from the end, causing about 0.020" TIR when rotating the thimble. Inaccurate measuring aside, the thing was rubbing inside the barrel pretty bad too.
I ended up getting half my money back from the seller, on my suggestion. Most of the set was good but it was going to take me some time and effort to make useable. After a quick search for some cheap surplus rods that would fit, I decided it was not going to be an easy find. I decided to shorten some of the longer rods to replace the bent 1-2 and the 0-1 that never existed.
Ok, if you got through that - or if you skipped it, here's the fun part! The pictures! I didn't snap any of the initial cleanup because it happened in the kitchen while making dinner right after opening the box. ;) Gotta love tools that are mailed to you.
After a couple of tries at setups, I opted for bolting the depth mic to a 1-2-3 block. The washers didn't work great, but did hold it tight against the block. This enabled me to accurately check the rod without removing anything from the setup, and gave me a surface to run an indicator over and make sure it was dead flat.
I marked the rod with my favourite tool, a sharpie marker, and went at 'er with a grinder. I had a cutoff wheel mounted and was holding it in a bench vice. I left about 0.030 material to remove in the next step.
This is where I got out my light saber ..... no, not really. The way this picture turned out though I couldn't help but mention it.
Checking how much material to remove...
...and here's how I'm removing it. I've mounted the 1-2-3 block in my milling vice, on the milling machine. Attached with hose clamps (don't snicker) to a plate coming off the side of the spindle is a Dremel rotary tool and a small mounted stone. And I just have to say here, that is a
precision wood spacer block above the Dremel guys!
How did I proceed? I set the anvil at the same height as the 1-2-3 block (actual 1", but not reading correctly yet). I touched off the grinding wheel very lightly, then lifted it off by 0.025 so that I'd clear the block. Then there was the painstaking process of running the cranks on my poor man's surface grinder.
End result? I had actually started on the bent rod, figuring I couldn't F* it up more than it already was. I got it within about 2 tenths (no vernier scale on the barrel) of the zero reading when I was done. It's actually pretty easy because I was using the micrometer itself to set the depth of each pass.
Here's the second one that I was working on. I went about this a different way after realizing that the rods aren't hardened in the middle. I marked as before and then took it to the band saw. That got me within 0.020", and then I carefully took more and more off on my bench grinder (checking regularly) until I had only 0.004" left to grind in the precision setup.
I decided to harden the ends of my modified rods as well. I'm not quite done the whole process yet, but I anticipate complete success, despite my limited equipment.
