Thread: DTI Repeatability- A question to other users.

Normally I put that sort of thing down to being caccy handed or things not being mounted rigidly enough. I think I have a tenth indicator somewhere - would you like me to send it over next week so you can compare?
I've just finished up a week here at "head office" and talked to the guys that do the calibration here. To do DTI & DIs they mount it in a really rigid stand and using a large calibrated micrometer head wind the plungers up and down. The interesting thing was that I was told the error is plotted for the various readings. At the end of it they can tell by the period of any errors on the plot which of the 3 gears inside the indicator is worn. I'd never thought of interpreting errors that way.

Michael

Try the same again with your gauge blocks against the height micrometer to minimize the drop between the two. The indicators have less of a tendency to jump if you drag the tip onto the blocks etc., from under the indicator rather than push towards it or from the side.

One other thing to mention. If you want to maintain the faces of your gauge blocks when working on a granite surface plate you should always use a "wear block" on the granite. It is one block, .0500" or .1000", 1mm or 2mm for eg., in the set that is always on the bottom of the stack with the same face against the granite and the good side up. That way the rest of the blocks don't get scratched and will wring together easily. If you choose not to use a wear block it doesn't take long before the rest of the set get scratched and they in turn scratch others when wrung together.

Pete

Your indicator point "appears" to be pointing down in relation to the granite plate. The ideal position is to be parallel or pointing slightly up. The velocity at which you move the indicator also has some effect on the reading.

Gene

My experience with 0.01mm DTI's when used like that is when I move the dti forward onto the workpiece, slide it over, then bring it back I get a 0.01mm different reading..

I think consistency is the key to repeatability...

What happens repeatability wise if you leave the stylus on the blocks, zero it, and then carefully lift and return the stylus with a pin or scribe?

Rob

Michael, Pete, Gene and Richard,

Thank you for your replies. Yesterday was the first time I've used the gauge blocks. I will keep in mind the "wear block" and its importance. I'll head back up to the shed and try a couple of different mounting options being mindful of the angle of approach. The DTI could be 40 years old and though it looks to be in good nick, wear may play its hand in the outcome of the readings. It's interesting to find that RC's indicator performs in the same fashion.

Michael, if it's not too much of a pain in the neck to send me your tenth DTI (if you can find it ) I'd greatly appreciate it. I have a couple of 0.005" dial test indicators but half a thou is a bit rubbery for this application.

Bob.

Originally Posted by nearnexus

What happens repeatability wise if you leave the stylus on the blocks, zero it, and then carefully lift and return the stylus with a pin or scribe?

Rob

I'll give it a go Rob. I've noticed that when traversing something while attempting to set it parallel, the DTI will read differently when the traverse is reversed. Only a whisker, maybe a tenth. Could be wear or it could be just the nature of the mechanism. Even happens with my box fresh Compac dial indicator and the 1um Mahr Millimess. As RC suggests, if it happens consistently its probably not a problem.

Bob.

Hi BT,
I always find that it is better to move the object being measured rather then the indicator. This also means that you are using the same spot on the rock to compare off, which with the little carbatec ones is a good idea, as i don't think they are as flat as the report says. (mine certainly is not as flat as the "tombstone"-that what SWMBO calls it)
Stu and i bought some comparator stands earlier this year with the plan of making some super rigid holders from them. Maybe something else to keep an eye out for? https://www.woodworkforums.com/f65/to...ml#post1627877

Cheers,
Ew

Originally Posted by Anorak Bob

I'll give it a go Rob.

Bob.

I did the same with my cheap CTC DTI this morning. I set it up and then lifted the ball tip with a feeler gauge within the readable range and then returned it.

The repeatability was EXCELLENT. Pretty amazing for such a cheap DTI.

However, I found that there was a very slight difference between lowering the tip back slowly and letting it slam back - slight, and not as bad as you're getting, but there.

So I recon you might have a case of stiction, where the inertia of the tip is having an effect on the reading point. Maybe the oil is a bit sticky in the unit.

Cheers

Rob

Hi BT,

What you are seeing when the movement reverses is just the hysteresis in the DTI, check the DTI specs and see if it's within the tolerances specified. With a little care in how you move the tip you can minimize the error and provide more consistency in the results.

When you are at the 1 um level, you need to have super rigid mounting, and hold you tongue just right for every measurement. don't forget for a 50 mm high (steel) stack will change 0.65 um per degree C..

Don't forget to dust carefully....particles of dust that are visible in a shaft of light are approximately 50 microns. The eye can see dust particles down to 10um, I found it interesting that you can "feel" dust on a surface plate at much smaller sizes than you can see..

Regards
Ray

Originally Posted by QC Inspector

Try the same again with your gauge blocks against the height micrometer to minimize the drop between the two. The indicators have less of a tendency to jump if you drag the tip onto the blocks etc., from under the indicator rather than push towards it or from the side.

One other thing to mention. If you want to maintain the faces of your gauge blocks when working on a granite surface plate you should always use a "wear block" on the granite. It is one block, .0500" or .1000", 1mm or 2mm for eg., in the set that is always on the bottom of the stack with the same face against the granite and the good side up. That way the rest of the blocks don't get scratched and will wring together easily. If you choose not to use a wear block it doesn't take long before the rest of the set get scratched and they in turn scratch others when wrung together.

Pete

Pete
Thanks for this helpful information.
I am somewhat of the "old school" & although I have a Granite Surface Plate have not used it for setting up using gauge blocks.
Did not realise the granite might cause scratching of the blocks.
I have hand scraped a couple of Cast Iron surface plates using the granite as the master.
So will continue to use the cast iron plates for any precise use, with gauge blocks, to avoid the scratching problem you mentioned, or if need be, use the .050" gauge block as you have outlined.
I found this hint very helpful, thanks again.
regards
Bruce