The Shop > Tools
Depth Mic Modding
sparky961:
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.
cwelkie:
Good on ya for making a silk purse out of a sow's ear using what you have to hand.
You mentioned hardening the rod tips - good idea but can't help but wonder if the "precision grinding" could be left until after that just to be sure that process doesn't effect the length for some reason.
Cheers
Charlie
sparky961:
--- Quote from: cwelkie on February 05, 2015, 09:37:50 AM ---Good on ya for making a silk purse out of a sow's ear using what you have to hand.
You mentioned hardening the rod tips - good idea but can't help but wonder if the "precision grinding" could be left until after that just to be sure that process doesn't effect the length for some reason.
Cheers
Charlie
--- End quote ---
It's good you pointed this out.
I omitted this information because my post was getting a bit long and boring. I realized after finishing the first one that the rod middles weren't hardened so I hardened that one plus the second one that I cut just shy of the final dimension. After re-checking the first one, it seems to have grown by 0.001! I double and triple checked this, even removing and re-inserting the rod a few times. Same thing every time, so I'll be putting this back on the "surface grinder" and taking a very light pass.
Long story short, yes - harden then grind.
philf:
--- Quote from: sparky961 on February 05, 2015, 05:21:05 PM ---After re-checking the first one, it seems to have grown by 0.001! I double and triple checked this, even removing and re-inserting the rod a few times. Same thing every time, so I'll be putting this back on the "surface grinder" and taking a very light pass.
--- End quote ---
Are the heads of the rods solid or are they adjustable? On my M&W depth mike they are adjustable so the odd 0.001" is easily dealt with without machining.
Phil.
sparky961:
--- Quote from: philf on February 05, 2015, 06:04:32 PM ---
--- Quote from: sparky961 on February 05, 2015, 05:21:05 PM ---After re-checking the first one, it seems to have grown by 0.001! I double and triple checked this, even removing and re-inserting the rod a few times. Same thing every time, so I'll be putting this back on the "surface grinder" and taking a very light pass.
--- End quote ---
Are the heads of the rods solid or are they adjustable? On my M&W depth mike they are adjustable so the odd 0.001" is easily dealt with without machining.
Phil.
--- End quote ---
You, Sir, are the man of the hour! I went down to the basement tonight for a short shop session before bed, thinking that I'd snap a few close-up pictures showing how they aren't adjustable.
Well, as it turns out it took me staring at the close-up through the camera lens for it to click that those are wrench flats on the top you dummy! (me) The many times I had previously looked at it, I just thought the top cap was pressed on because I didn't see any threads. So, I tried carefully turning it and lo and behold, it turns and there must be threads under it because I was able to adjust the rod by quite a few thousandths either way.
That means that when I'm grinding the next one (and I may replace the bent one yet, since it still does wobble a bit), all I need to do is get within a few thou of the right size. Preferably on the long side because that will allow for future resurfacing of the anvil if needed.
I could have posed this next shot, but this is as it appears. It's a right-proper measurement, clean surfaces and holding everything together tight, and using the ratchet on the mic. Quite pleased, I am.
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