# Four Different Measurements



## Downwindtracker2 (Apr 18, 2020)

I hope this finds you well. This is as much about humour as machining. I'm making some shock absorber mounts for our travel trailer out of some 3x4 square tubing,2 1/2 "x2 "x 3". Shock absorber mounts, so not too critical.  When milling down the 2 1/2"sides,  I checked the height off the table, with a Fowler depth mic, but I wasn't sure, so used the backside of a Chinese dial caliper to check.  Well, a fair difference.  So I dug out my good tools, I'm a retired millwright so I have some doubles. I didn't trust a depth mic that you popped the rods in with so I had bought a Mitutoyo  depth mic  and I had a Starrett dial caliper as the home one. It could be me, I don't think I have used depth mic much in my work, if ever. But I would have thought at least a couple would have matched. Laughter.  Have fun , stay safe.


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## Brent H (Apr 18, 2020)

How about the "measure twice and cut once" advice - I took out a head for a square and the ruler that goes with it, slid them together and locked it down at 7".  Proceeded to mark the piece, chuck it in the lathe and thread away.......well, that seemed short for some reason.......ahhhh.....I slid the head on and read the 7 inches going from the 12 to 7 instead of 1 to 7.......ugh!    LOL


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## Alexander (Apr 18, 2020)

I often measure hydraulic cylinders I am making at work with a tri mic then a telescopic guage and then a digital vernier. Only because I couldn't explain how I made a whole order of parts .025" to big.


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## Downwindtracker2 (Apr 19, 2020)

It's getting better, still four different measurements, but now they are only a couple of thou different. On this, that's hot worms.


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## Tom Kitta (Apr 20, 2020)

I always measure critical stuff with at least two different measuring tools to make sure I got it right.


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## Downwindtracker2 (Apr 20, 2020)

To be honest, depth mics are a lot harder to use than I originally thought. To get a measurement that was close to agreeing with an outside mic, I had to be softer than even their ratchets.  I still don't have that feel. The Starrett dial caliper when used for depth  matched the outside mic when I pulled it from the vise. Too bad the Starrett is a lot flimsier than the Chinese one.


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## Bofobo (Apr 21, 2020)

I use telescopic but always was taught 3 measurements every 4 inches on cylinders. They take lots of practice but eventually I figured out my mic was squishing the telescopic gauge inconsistently if I waited for the clicker, you have to hover the telescopic guage between the anvil (hover; small sweeping motion like an arc) on the mic until you feel the slightest resistance. And yes depth mic very easy to lift before the clicker “pops”, I never got into it but I wonder if the clicker tension is adjustable


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## Tom Kitta (Apr 21, 2020)

I just put a lot of pressure on the depth mic "feet" so it does not lift easily. 

For telescopic gauge I check both with calipers and micrometer - delicate job both getting the inside measurement and then taking mic of it.


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## PeterT (Apr 21, 2020)

This Old Tony just did a video on rebuilding sketchy  telescopic bore gauges. I had a cheapo set early on I gave up on (maybe prematurely). But its harder to iffy ones to float to max bore when rotating out & repeat consistently when the tube action isn't smooth. Another helpful gizmo is a stand that holds your mic in a horizontal position. That allows you to suspend the bore gage in between the mic anvils a bit more pre-aligned leaving your other hand to screw in the mic until it clicks or whatever your particular clutch setting is. Different mics are more or less sensitive. Just try it on a gage block or something known & see if/how it varies, then use that same recipe when measuring parts.

On depth mics, something that has burned me occasionally is the condition of the blind hole ledge during machining. Good practice just like any other kind of reading - ensure the surface is machined true & clean with no leftover burrs or shrapnel hanging on especially attracted by cutting fluid. That means both the internal ledge being measured & the external datum face. Either will give false readings.


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## Downwindtracker2 (Apr 21, 2020)

I've used telescoping gauges, a fair bit. I much prefer the Mitutoyos over the Starretts.  The first ones I bought for myself were those gritty PA, even with those measurements are possible. But I bought a set of tools off an older machinist/millwright and have a set of Suzuki brand, they look like they were made by Mitutoyo. nice.

My depth mics had only 2" bases making it hard to get enough down pressure . After that, I'ld say the 4" ones are truly worth while. And the Fowler snap in ones weren't even near.  Another thing I found that would throw off the reading is if the rod touched the side. Since I was basically doing heights, that was something to watch out for. 

My 0-6" set is a Polish VIS, so I used it for the 2"-3", the first time after cleaning and calibrating them. Boy those friction thimbles are nice. What I have been missing for 35 years. Lufkin use to make friction thimbles, and Mitutoyo did as well.


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