Trimos Height Gauge - Calibration?
I have just bought a second hand Trimos height gauge/micrometer. It is one of the wholly mechanical models, 1 division = 2 microns. It has been out of calibration for a few years, but it appears to have had careful use. Having checked it as well as I can using slip gauges (gauge blocks) I suspect it needs adjustment. I don't know how to do this and don't have good enough equipment to try.
1) How costly is recalibration likely to be? I am in UK, but local US dollar prices would be useful for comparison.
2) I assume that it is possible to bring these height micrometers back to original calibration - how is this done and what should I look for in a supplier to do this?
Trimos height gage calibration
A few years ago I also bought a second hand Trimos height gauge. Though mine has a digital scale, I believe they are quite similar. I checked/calibrated the height gauge myself - I don't need an official calibration report.
I used a calibrated step gage block in vertical position with 10 mm intervals. It allows both up and down measurements with the height gage. And the base of the step gage block is larger then the gage blocks you used.
I assume there are some calibration laboratories in the UK who can perform a decent calibration and provide a quotation for the job. If you don't know where to go, I would check the UK Accreditation Service. They'll probably be able to tell which companies have an accreditation for calibrating 1D (or 3D...) "CMM's". Or you can check their scope at The United Kingdom Accreditation Service
I run a tool workshop, also providing CMM contract measurement. The height gauge I only use in the workshop - for pre-setting Z-offset of sinker-EDM elektrodes. After comparing the gauge with my step gage block, I know it's within a couple of microns, which is sufficient for me. For really accurate measurements I use the CMM - which is calibrated.
So, I can't give you a price, but hopefully some UKAS-accredited companies can.
Just for curiosity: do you have a picture of your height gauge? I've never seen a fully mechanical one. Sounds rather interesting!
Thanks for comments. I realise my description was not really precise so you may have been thinking about another type of measuring equipment. This is a Trimos height micrometer. It may of course be a bit worn, but I would hope for better than 20 micron accuracy. If I can get around 10 micron or better I think I would be very happy.
There are plenty of UK calibration places, some quite local so no problem with finding one. I just wanted to get a bit of warning about likely costs.
Nobby - this type of mechanical height micrometer (I said gauge - without thinking!!) was pretty much a standard type of kit years ago, perhaps they still are in many places. Some people used to refer to them as lighthouses for obvious reasons. Starrett and Mitutoyo Height masters are similar. The mechanism is interesting, the bottom row of numbers rotates to show further numbers as you adjust the height.
Cromwell charge £53 for calibration.
Quote: “Having checked it as well as I can using slip gauges (gauge blocks) I suspect it needs adjustment.”
Are you asking how to zero the scale? I see on your first picture a thumb screw on the righthand side, does this allow you to move the datum point? I had an older version of the Trimos than yours which had a small grub screw/socket screw which served the same purpose.
Obviously this has nothing to do with calibration over the range but judging by the condition of yours & the wear that can take place I would think the only worry over out of calibration was if the instrument had been dropped or similarly damaged.
Great pictures. I hope you're right about the accuracy and put my doubt to shame. If Mark is right about the price for calibration (£53) then I'd go for it. Of course I'm guessing that the price doesn't include postage and "handling". I usually have an optimistic nature but I'm guessing the calibration price (everything included) will probably be closer to £100.
Can't you beg, borrow or scrounge a couple og block gauges (100 and 200mm?) and do a quick test? You'll probably need a dial test indicator (lever type) with a 0.001mm or 0.002mm reading.
Good luck and let us know what happens.
daredo222 managed to post while I was writing mine
Gordon -I have already checked with a selection of slip gauges and with a very sensitive dial test indicator. I get a consistent high reading on the height micrometer, which to me suggests that there is no problem between steps on the micrometer, but that it needs to be re-zeroed as a whole. The trouble is I have no idea how to do this and am reluctant to dismantle it to find out. Even if I did know how, it is probably better that this is done in a temperature controlled room with the highest grade standards. The local calibration service is 20 minutes driving distance so no problem with that. If it is GBP 100 or less then probably worth doing.
Ray - no sign of it being dropped or any major damage. The old calibration stickers indicate that it was well looked after for a fair while. The thumb screw is to lock the thimble (at least that is what it does on this model).
This looks like the same model. The page shows the specification.
Thanks for the help.
It certainly was a nice bit of kit and probably can be made so again. I entirely agree about the need to have this recalibrated properly and I will start the ball rolling on this today.
As far as the dial test indicator is concerned I have to disagree a little. The use in this case is purely as a null indicator. I can get extremely consistent reading repeatedly checking the same slip gauge. I get repeatedly consistent but different readings when moving from slip gauge to height micrometer. Not only that but the difference seems to be about the same for any slip gauge that I use, off any of the steps of the height micrometer. I am hoping that there is a simple adjustment that allows the whole head assembly to be twisted a bit, just like an ordinary micrometer and that this can be done at reasonable cost.
The slip gauge accuracy point I have to agree with. 2 microns ? I doubt that my workshop set are that good. These obviously have to better than that to achieve the level of calibration accuracy that I would be happy with. Hopefully a calibration lab can sort that out.
Anyway thanks for help.
Originally Posted by billmac
All DTIs are null indicators. That's their only purpose in life.
The calibrated scale exists only to give you a general idea of the magnitude and direction of the error, not its actual value.
Nice tool. Good luck with it.
I thought I would just close the loop on this thread. I took the height micrometer to a local UKAS calibration laboratory. They agreed to adjust and recalibrate it to the best it was capable of. I just collected it today and to say that I am pleased with the result is a bit of an understatement. The worst error on any step is now 3 microns. Many of the downward facing steps are 0. The price was very reasonable.
GBP 40 + VAT. (about 46 euro). That was everything - adjustment, calibration, certificate etc. I delivered and picked up myself - same city.
According to the laboratory owner, these height micrometers can have problems with wear on their feet and the step surfaces can get scratched and worn. Fortunately my example was OK.
Sorry Gordon - I should have known that with all the noise in the news at the moment.
Trimos height micrometer
This is a very high quality height micrometer. I worked in the technical dept. for Fowler for many years and Fowler the the US agent for Trimos. To the best of my knowledge, these height micrometers have been discontinued for many years now(I'm guessing about 15). From what I heard, Trimos had one gentleman who used to assemble these height micrometers. When he passed away, they were not able to replicate the same quality as the guy who originally assembled them for Trimos so the product was discontinued. There are still many of them out there and the quality of them speaks for itself.
Its nice to hear from someone who knows these micrometers well. I am very pleased with my purchase and the recalibration. I use the micrometer fairly often and enjoy working with a quality instrument. It is a shame about the loss of the one guy who knew how to assemble them - it sounds like a major loss to the industry. I can understand that there is some 'magic' to the process, since it looks like the whole series of levels stacks up from bottom to top and you would need some clever selective assembly to keep errors under control.