Question for the Pros

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November X-ray

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I need some step calibration blocks for an ultrasonic thickness gauge, easy enough to make but my question is how many thousands oversize should I leave the machined 4140 metal for final filing, sanding and polishing to the final dimension. I am thinking .003 above my final dimension and I know I could simply experiment and determine this but I am short on time and long on work currently.

Thanks,

NXr
 
I thought that might be the consensus, looks like $500 bucks worth of online ordering will occur.
 
UT mics I have come with a .2500 cal block in the case. Yours must be different. You also need to know the transmission coefficient for the specific material. It's density related, but you can dial it in on all UT mics I have seen. You know, Glass is different than lead, for example. The are most accurate calibrated with the same material, or as closely as you can get to it as you will be measuring. I've never found them thrillingly accurate, but they certainly can be useful.

What kind do you have?
 
author=Tony Wells link=topic=3667.msg26791#msg26791 date=1316741405
UT mics I have come with a .2500 cal block in the case. Yours must be different. You also need to know the transmission coefficient for the specific material. It's density related, but you can dial it in on all UT mics I have seen. You know, Glass is different than lead, for example. The are most accurate calibrated with the same material, or as closely as you can get to it as you will be measuring. I've never found them thrillingly accurate, but they certainly can be useful.

What kind do you have?
Click to expand...

Tony - you are correct about the 1/4" cal block with the unit, however this client has specified each crew (unit) will have a calibration block in 0.125, 0.250, 0.375, 0.500, 0.625, and 0.750 respectively.

We have Stresstel T-mikes but these are no longer supported and they are probably 15 - 20 years old, good units and served their purpose well. Anyway we are being assigned new units that someone in procurement paid nearly four grand each for and did NOT get any calibration blocks with. I am all too familiar with the material densitys and planned to make the calibration blocks out of the same material for the current job and as the need arises do the same for any other specific materials (steel, plastic, etc, etc.) I was going to have the "step" water jet cut oversized essentially making 2 steps out of one chunk of material and then machining and polishing to the final dimensions.

Ultimately someone thought they were saving money when in reality we could have purchased units for less than half the money and gotten everything included to meet the client's specifications.

Oh well I get older and dumber everyday, just ask my kids!

Thanks for you help Guys!

NXr
 
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That's the same mics I have (T-mike). Seems I have a box of 4 or 5 of them. Not much you can do if the customer demands it. I haven't noticed a great effect with varying surface finishes, at least on the workpiece. No one expects to polish a spot to an 8 Ra finish just to get a thickness measurement. In my experience, they aren't all that accurate anyway. Besides, that's what the couplant is for. Have you thought about just making them close and marking the actual? It can then be used for a verification test. If you made the block 0.1275, and that's what it checked....all is well.

This reminds me to drag out that box and see if any of them are still good. We had lots of trouble with coolant making the cable to the transducer awfully stiff. Some were breaking at the transducer, and having to be replaced. If I run into that on mine, I'll have to try a repair I guess, since you say they no longer support them.

Sorry if I was preaching to the choir there, X-R.....I didn't know how up on them you were. Hope your project smooths out for you. Pleasing an unreasonable customer can be trial. They do know that these things aren't particularly linear, right? What range are they expecting?
 
Tony, the job is to ensure, while field bending 20" X-70 pipe, the wall does not thin below their acceptance criteria which I cannot remember at this moment and do not have the specs with me currently. You have an excellent idea regarding cutting the blocks and mic-ing the actual dimension and stamping it on the side, I never thought of that as most cal blocks are in the series listed above and I know they will not check the block itself, only the calibration certs. I also have a 10 pound 4 ounce chunk of 1-1/2" steel I use as a cal block for my mag particle to ensure the unit is capable of pulling 10 pounds of force the code requires. The chunk has "certified papers" for it's weight and it looks like someone chopped it off with an axe however it really was just a piece of scrap but was the perfect weight.
 
Don't know if this would be expedient enough, or within budget, but you could buy individual gage blocks. High chrome tool steel used for gage blocks has a similar coefficient to most of the 41xx series. Don't quote me on that, but that's what my memory says.
 
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