View Full Version : correct use of telescopic gauges

02-13-2004, 04:27 PM
what is the correct procedure for using telescopic gauges. I know that you set it on an angle and lightly tighten the locking screw and then arc it thru the bore. But my question is this,are telescopics self aligning or self centering to the line of the bore or do you need to rock they side to side to find the correct position(like when you use calipers).I do rock they from side to side to find the largest diameter but I am never too sure,so I end up measuring the bore several times before I'm sure.Some-one told me they are self aligning , self centering? Any help on the correct use, would be greatly apprieciated.

02-13-2004, 05:15 PM
You are right on the first part. Put the telescoping gauge into the bore at an angle, tighten the scew lightly and then pull the gauge up past the vertical centerline to compress the gauge to the size of the bore. Once you have done this pull it out,snug the screw and take a reading with your micrometer. Never!! put it back in the bore and take another reading. If you don't think you got a good pull when taking the reading, start over. The gauges are self centering because they have convex surfaces and are in a round hole but depending on the type of material you are measuring the gauge might drag a little. By this I mean, if you are measuring iron it will drag a little, if you are measuring steel or brass the gauge will tend to slide better so that you get a more accurate reading. I always take two readings, if they are the same then you can assume that you were in the center of the bore. If one is different from the other then take a third reading. You know the old adage, measure twice, cut once. I hope this helps

02-13-2004, 05:20 PM
As I interpret your comments, I think you're doing it about the way I was taught. Tighten the knob down close to (but not quite) the point of locking it, then wiggle around so it can expand to max dimension, then simultaneously tighten and lock while rotating (or arcing) out of the hole.

02-13-2004, 05:29 PM
Here is how I have done it. Compress the ends of the tele gauge, put in the hole at an angle, loosen the screw at the end so the ends expand out. Tighten screw, pull past vertical, do not rock it, one pull, slow and sure.

measure, but not tight. I use fingertip pressure, even forgoing the clutch as some clutches on mics are too tight, and the pressure on this will compress the ends in. Measure very light, This is my addition. Older tele gauges will compress back easier than new ones.

Do this twice to check, and if not sure, as many times as you need. As noted above.
When you become competent and sure of your use of the gauge, you can check for egg shaping as well by trying different spots (axis) in the bore.

Forrest Addy
02-13-2004, 06:04 PM
Adding one thing, be sure to move the telecsope gage stem in a radial plane. If you angle it away from the centerline the gage can be biased off the diameter and measure a chord instead.

02-13-2004, 09:12 PM
Thanks from me too. I've been using it wrongly.

02-14-2004, 12:14 AM
A tiny drop of light oil on each face helps the self centering process on most things.

gary hart
02-14-2004, 02:09 AM
No expert, just a novice without much training. Had developed my technique and was getting consistent results. Then things changed and couldn't get accurate or repeating reading. Explained my procedure to a machinist friend and he told me to quit tightening the second time after swiping through. This solved the problem for me with my set of telescoping gauges.

02-14-2004, 07:04 AM
Thats great,Thanks very much for the help guys.

02-14-2004, 09:10 PM
Pretty much as the others have said. It's one of those things that takes a certain amount of "feel" and practice. One can't be ham-fisted about it.

T Wise
02-16-2004, 07:37 AM
According to Starrett, the second tightening is not done, I had this discussion with my instructor once, I lost, only because he was the one giving out the marks. If you think that you tightened the micrometers too much and changed the reading then do it again.


02-16-2004, 10:31 PM
Yup, no second tightening.

I will try the light oil coat tomorrow....good idea. I found that on brass and aluminum, the gauges leave a slight mark on the surface, of which I wondered did I add a tenth (.0001)? Probably not, but alas I wonder....Will try the oil if for nothing else but peace of mind, and BECAUSE IT SOUNDS LIKE A SOUND IDEA!!!!