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[How do I?] Ambrose Shardlow & Co depth mic calibration

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pdentrem

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#1
I was given a tool cabinet and assorted stuff for helping an elderly gentleman move and downsize. In the mixed stuff is a depth mic that either his dad or himself had. It had been set aside and the rod was lightly seized. I applied light penetrating oil and finally got it to move. There was lots of light rust inside the spindle and in the hole that the rod passes through as well as the rod. I ended taking it apart completely. In resetting the rod my question how to calibrate? As per the photo it comes with 6 rods in total and I hope that the 5 long ones are correct. The shortest one was in the mic and I removed the pinch nut to clean and lube before attempting the reinstall. I will take it in to work and use some gauge blocks, but here at home I am getting some differences in readings. Mind I am using a dial vernier caliper. When I set the mic to 1" and the vernier agrees, I run the mic back to zero I see that the rod is actually slightly inside the anvil by about 0.003". Am I missing something?
Pierre

IMG_1301.JPG
 

Bob Korves

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#2
The biggest trouble with depth mics is getting a reliable, consistent reading. The issue is that the base can lift up slightly before the ratchet clicks. The base needs to be held down quite firmly to avoid a bad reading, or must be used without the ratchet with the spindle only touched down lightly on the work. First get reliably consistent readings, then calibrate to match them.
 

hermetic

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#3
Shardlows was VERY good equipment, they specialised in drop forgings, and made the crankshafts for the Merlin engines. based in Sheffield, Yorkshire UK.
 

pdentrem

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#4
Shardlows was VERY good equipment, they specialised in drop forgings, and made the crankshafts for the Merlin engines. based in Sheffield, Yorkshire UK.

That is very nice to know. I am an aircraft buff and I love this kind of stuff!
Pierre
 

woodchucker

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#5
Shardlows was VERY good equipment, they specialised in drop forgings, and made the crankshafts for the Merlin engines. based in Sheffield, Yorkshire UK.
Love the sound of the Merlin's . Watched a Utube video of a guy rebuilding one and the first firing after restortion.
 

Dinosaur Engineer

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#6
Any gauge that relies on slender threaded rod adjustment is prone to tampering and misuse. I've seen many bent rods. I've found that trying to adjust the rods to correct errors is very difficult and time consuming.
 

pdentrem

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#8
Love the sound of the Merlin's . Watched a Utube video of a guy rebuilding one and the first firing after restortion.
Also I get goose bumps when the scene near the beginning of 12 O'clock High, where Dean Jagger's character is standing on one of the hard stands and you hear a radial engine starting to crank over and start up.
Pierre
 

Bob Korves

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#9
Any gauge that relies on slender threaded rod adjustment is prone to tampering and misuse. I've seen many bent rods. I've found that trying to adjust the rods to correct errors is very difficult and time consuming.
Amen to that!
 

Tony Wells

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#11

pdentrem

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#13
True I have the short 1" rod in at the moment, but later it could be any up to 6". So this is good to know. I have been wanting some gauge blocks anyways.
Pierre
 

4gsr

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#14
A quick easy way to calibrate a depth gage is to use a 1-2-3 block that you know is accurate within a tenth or less. Just put the depth rod thru one of the holes against your granite plate or equivalent and calibrate. Good for 1", 2", 3" rod or stack two 1-2-3 blocks together and go up to 6".
 

Dinosaur Engineer

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#15
Adjusting a depth mike barrel to calibrate one rod isn't going to calibrate the mike for other rods. The only way for multiple rod depth mikes ( or internal mikes) is to calibrate each rod in turn by adjusting the rod nuts at the end of each rod. Trying this is needs the patience of a saint !
 

Tony Wells

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#16
Realistically, most depth measurements are not all that critical. Not that there aren't exceptions to that, to be sure, but there is no practical need, particularly in a hobby shop, to spend all the time to get every rod to zero out within a tenth. If you have the time, and as above, the patience, by all means, go for it. I have a digital Mit that reads in 50 millionths, and it came from the factory very, very close. I have never had to adjust it. I have had it in a clean room environment and it passed. If it ever does get off, I am willing to say that it is the user, or possibly debris in the wrong place. It's really hard to put enough wear on a depth mic to really require much resetting. They need to be checked, but if they are found suddenly to be off, look for why before starting to change things.

But on the other hand, of your set is short a rod or something requires a replacement, you have little choice but to spend some time setting it. But be realistic about it. Think about what you will be using it for and proceed accordingly. For example, I have some spare rods for one of my sets that are ground to a near point, leaving a very small flat on the end, which is original. But since they came from a salvaged mic set, they had to be set for one of my mic bodies. I was content to get them within 0.0005 and be done with it. Now of course, they have to stay with that particular mic, which is a little frustrating at times. After doing a few over the years, it isn't a bad job, unless you want to really set them to zero.
 

Bob Korves

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#17
Realistically, most depth measurements are not all that critical. Not that there aren't exceptions to that, to be sure, but there is no practical need, particularly in a hobby shop, to spend all the time to get every rod to zero out within a tenth. If you have the time, and as above, the patience, by all means, go for it.
If you just wait until you need the depth mic for a job, you can calibrate it then for the accuracy you need for the job you are doing. That saves a LOT of time calibrating rods you might never actually use, or only need it to be a skinnier pocket ruler for the job at hand...
 
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