# What's a good technique to use telescoping inside bore gages?



## WobblyHand (Aug 13, 2020)

So, thought I had a handle on this...  Made many successful (consistent) measurements right down to the tenth.  But seem to have lost my mojo. Trying to measure in-situ the bore.  Can't seem to get a consistent measurement any more.  Yes, it has to be my technique.  If I reinsert the gage into the bore, I can tell it's not at the true bore diameter.  Oh yeah, the boring bar is in the way too.  Can't move it far enough away.  As it is, the back side of the boring bar is inside the tail stock.

According to my calculations, I should be about 0.0016" under 1.0000"  I am trying to hit 1.0000.  I now get measurements all over the place.  (+/-0.001).  The cap I have is 0.9991 (tried to hit 0.9995) doesn't fit in the bore, so I'm pretty sure I am under that.  

So I walked away from the lathe.  Don't want to make the hole oversize.  What is the recommended best practice for using these inside bore gage gizmos?


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## benmychree (Aug 13, 2020)

An accurate measurement with a telescoping gage depends on inserting it at a slight angle in the bore, locking it, and rocking it over center only ONE TIME, never reinsert it and wiggle it around, if desired, repeat the measurement from scratch and compare with prior measurements if unsure; this should give consistent results, if the tailstock is in the way of measurement, consider overhanging it past the bed, or remove it.  The telescope gage is not like an inside mike that can reenter the bore and feel around for a final measurement, it is a one shot measurement.


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## Karl_T (Aug 13, 2020)

Even with correct technique, I also don't trust the telescoping guages if real accuracy is needed.

I've popped for inside micrometers, pin gauges and even real bore gauges when its got to be dead nuts on.


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## WobblyHand (Aug 13, 2020)

benmychree said:


> An accurate measurement with a telescoping gage depends on inserting it at a slight angle in the bore, locking it, and rocking it over center only ONE TIME, never reinsert it and wiggle it around, if desired, repeat the measurement from scratch and compare with prior measurements if unsure; this should give consistent results, if the tailstock is in the way of measurement, consider overhanging it past the bed, or remove it.  The telescope gage is not like an inside mike that can reenter the bore and feel around for a final measurement, it is a one shot measurement.


@benmychree Can you explain, "locking it, and rocking it over center one time"?  So you lock it and the rock it, which then forces the pin in?  I have trouble locking it.  The handles aren't very big, so there's not much to grab on to.


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## WobblyHand (Aug 13, 2020)

Karl_T said:


> Even with correct technique, I also don't trust the telescoping guages if real accuracy is needed.
> 
> I've popped for inside micrometers, pin gauges and even real bore gauges when its got to be dead nuts on.


Some day...  Right now, all I have are the telescoping gauges in this size range.


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## mickri (Aug 13, 2020)

I struggled and struggled trying to get consistent internal measurements.  So I feel your frustration.  What benmychree is saying is that once the gauge is in the bore at a slight angle tighten the locking nob on the end of the handle.  Then rotate the handle down.  As the gauge comes to vertical inside the bore pull it straight out.  This is one fluid motion.  I had a lot of trouble getting consistent measurements until I noticed in one video he had his thumb at the bottom of the bore which kept the gauge in place.  Since I started doing that I have been able to get consistent measurements




Are you using a micrometer for your measurements?  I was never able to get consistent measurements with a caliper.  I have a magnetic base with a simple clamp to hold the micrometer that lives on the end of the lathe bed.  This also helped a lot in getting consistent measurements,


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## WobblyHand (Aug 13, 2020)

@mickri Thanks!  Never would have guessed that way of measuring.  I'll try that.  

@mikey convinced me I needed a micrometer stand.  Found one on fleabay and made an offer.  Have to say, even with a stand it's tough to measure these gauges.  The curved surfaces sometimes wants to slide on the faces of the anvil.  I only use the verniers as a quick sanity check, otherwise it's micrometer only.


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## pontiac428 (Aug 14, 2020)

It takes some practice and consistency... I have two sets of General Tso's telescoping gauges that I don't know why I haven't thrown out yet.  Maybe it's because they _look_ like tools.  I bit the bullet and bought some Helios Germany telescoping gauges, and those are smooth and nice to use.  I also use a dial bore gauge, which I absolutely prefer, but it takes time to set up and verify.  It's an import, but matches up with micrometers nicely.

What's the difference between a micrometer stand and a padded-jaw vise?


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## mickri (Aug 14, 2020)

I used a padded jaw vise before I came up with the magnetic base and clamp I posted above.  The only difference was the convenience of not having to walk over to the vise on my work bench and a little easier to read the micrometer.


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## homebrewed (Aug 14, 2020)

The problem with measuring a bore ID with calipers is that the jaws are not knife edges (at least, not on any of the ones I have).  They have small flats.  So they can't _ever_ produce an accurate measurement.  The larger the bore ID the better, but if you need to get below .001 inch you've got to use something else -- plug gauges, inside mikes, etc.  I've been making making my own plug gauges for specific projects, but recently I started pricing out inside mikes.


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## pontiac428 (Aug 14, 2020)

For roughing on the lathe, I like to check this quick way, then take the fine measurements with precision measuring tools on the final finish.


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## francist (Aug 14, 2020)

I have actually switched back to using inside spring calipers and micrometer instead of telescoping gauges. Probably because my set is not a great one (KMS tools, standard blue plastic pouch, enough said) but I get better results with the spring calipers, especially on smaller diameters. I suspect good quality gauges would be a different story but at moment I’m not springing (haha) for them.

-frank


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## mikey (Aug 14, 2020)

Sherline did a video once on how to use ball and telescoping gauges that was really good but I can't find it. Here is one from Don Bailey of Suburban Tools. 

Using ball gauges or small hole gauges is really a feel thing. You are trying to set the tensioning knob so that the contact point of the gauge touches the bore all the way around but not so tight that you cannot easily remove it. The only way to really tell if you have this technique down is to use an accurate ring gauge and practice until you can consistently hit the bore dead on. 

Telescoping gauges are a different animal. It is less about feel and more about learning to set you spring tension on the locking knob correctly. The tension has to be set so that the movable arm can just move when you lever the gauge over as shown in most videos. I find that the tension is the key because you get one chance (per try) to get it right. Too tight and the gauge will read large; too loose and the arm will move as soon as you try to read it. Again, a ring gauge will help you to develop the feel that works for you.

The real advantage of a micrometer stand over a padded vise is that you can position the barrel of the mic so it can be read easily. The tips of the ball gauge or telescoping gauge should just contact the anvil/spindle of the mic; then take your reading. If you use a ring gauge to hone your skills so that you know what the ID should be, it takes only a short time to learn how to handle your mic. I find that if I haven't bored for a while, I have to practice for a few times to regain my feel. Once I do, I can usually nail a reading within half a tenth or better.


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## Bob Korves (Aug 14, 2020)

Don't over tighten the telescoping gauge lock more than necessary.  It should take little effort to rock it over center.  Also, if at all possible, get in a good position for rocking it and try to keep the rocking motion square in line with the hole bore center.  I find that awkward holding positions for using telescoping gauges tend to make them read too small.  The method requires doing it repeatably every time.  Over tightening the gauge also can damage the plunger by galling it.  And, keep the telescoping gage clean and dry, internal grit and rust are NOT your friends with them...


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## WobblyHand (Aug 15, 2020)

Some ring gauges have found there way to me.  Need to practice this art some.  
Guess the art is two fold - correct tension and rocking in the bore, and then correct tension on the micrometer.  The mike ratchet seems too aggressive for this.  But with these rings, hope to find a technique that works.


0.3748, 0.3750, 0.7498, 0.7501, 1.5000


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## mikey (Aug 15, 2020)

When reading in the mic, just touch the anvils to the tips of the telescoping gauge. You do not turn hard enough to click the ratchet. Just touch.


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## WobblyHand (Aug 15, 2020)

Just found out about the mike anvils while practicing.  I think, if the mike is in the stand, it's ok to get the feel for just touching. What's not so easy for me right now is getting the consistent actual transfer of the bore to the bore gauge.  I'll go through the rocking, measure it and say, hmm it's big.  I then bring the gauge back to the bore ring and sure enough, the gauge is bigger than the bore.  If I do it again (rocking through the bore) it seems to be closer to reality.  Guess this is about the correct torquing and rocking the bore gauge.  And, being able to do this in the shop - not sitting at a desk.

I measured the 0.7598 and 0.7501 ring bore gauges 12 times each.  (At my desk.)  You know, to practice.  Mean measurement was 0.749825 and 0.750192.  Standard deviation was 0.0000866 and 0.000108.  I need more practice.  This was with a non-pedigreed "B" telescoping bore gauge.  Don't have the confidence that the measurement I'd take of an unknown would be close enough to reality.  Need to drive that standard deviation down.   


Tried a few measurements with the 0.3750 ring gauges.  Different feeling.  Still hard to get the bore transferred correctly.


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## petertha (Aug 15, 2020)

And then of course there is the physical surface you are measuring. If you are turning with a boring bar, your surface is a series mountain tops & valleys under magnification. And some of the mountain tops have 'snow on top' LOL (micro burrs). So at best your telescoping gauges are running across the tops. That's a 'measurement' all right, but may not be indicative of your final desired surface for precision work. Power feed turning or boring is no different that fine pitch threading although we don't think of it that way. The tool tip for turning has a larger radius so the overlapping scallops help keep the peak to crest undulations minimized, but eliminated.

I have a ritual on my final 2 passes where I dial in the exact same DOC (say 0.005") and I switch to 2 finer feeds. This does a few things. The DOC gives some consistency to the combination of material, tool & diameter. Sneaking up thou by thou can work on certain materials but not others especially if they like to work harden. You are giving the boring bar an opportunity to spring pass (relieve bending in the bar). Different feed also cuts across the hilltops as opposed to just deepening the same grooves like a record player needle if you keep the same feed. Also if you have been doing some roughing just prior the part temperature may well till be elevated. It may measure differently 5 minutes later as it cools.

To get the technique down, just find a bearing race & measure many times. The surface is hard & likely quite precise. So if you are varying +/- 0.001" its probably some combination of technique, gages or micrometer.

When you start getting into half thou or tenths you might be exiting the realm of turning & should be lapping. Even jaw/collet pressure can have a proportionately large impact depending on the material & wall thickness etc.


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## WobblyHand (Aug 15, 2020)

petertha said:


> So if you are varying +/- 0.001" its probably some combination of technique, gages or micrometer.


This is the realm I am working in right now, when I'm careful.


petertha said:


> When you start getting into half thou or tenths you might be exiting the realm of turning & should be lapping.


How is this done?  Soft metal insert and slurry?


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## WobblyHand (Aug 15, 2020)

Also interesting on spring passes at different speeds to smooth out (a little) the bore.  I'm not happy with the bore quality.  There are some micro ridges in places that I can feel with my finger nail.  Other parts of the bore are quite smooth.  I could never measure the ridges with one of these telescoping things.


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## mikey (Aug 15, 2020)

When you bring the telescoping gauge to the mic, be sure the rounded ends of the gauge are in the center of the anvil/spindle contact surfaces and that the handle of the gauge is perpendicular to the axis of the spindle of the mic. 

It is mostly a matter of practice.


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## WobblyHand (Aug 15, 2020)

The mic part is as you say, just practice.  It's a little fiddly, but ok. 

Think the telescoping gauge part requires a lot more finesse on my part.  Not there yet.  Haven't found the just right zone between too darned tight and oops the gauge slipped open.  To make it harder, the feel isn't the same for the different telescope gauges.

Practice, practice, practice.  When you get tired of that, practice some more...  ad infinitum...


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## mikey (Aug 15, 2020)

I've been boring for the better part of 30 years, probably more, and I still practice my technique before doing a precision bore. These gauges rely on feel more than anything else and that is attained through practice. Focus on developing the right tension on the locking knob - just snug, not overly tight. Practice - it will come.


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## petertha (Aug 15, 2020)

WobblyHand said:


> How is this done?  Soft metal insert and slurry?



I'm no expert but yes, lapping tool & lapping compound. I use tools made by Acro. They come in a pretty good range & you can buy replacement barrels which is important if you use progressive grits or wear them out.  https://www.traverscanada.com/barrel-blind-hole-lap-sets/p/112581/

I have seen plans to make lapping tools but there are quite a few kind of pita machining elements to get right - the radial slits, the tapered internal seats on either end, the steel holder, matching tapered adjustment screw... For the price they are good value IMO.

Just know that lapping is excruciatingly slow. Ideally your bore should be within 0.001" with good finish or you will be there a long time. 2-3 though is asking a lot of the lap, coarse compound, potential for deviation etc.

Some pics of my cast iron model engine liners, nominally 24mm bore, within .0001-0.002" of one another. There are some hours there. Actually because I had multiples to do I gave up on turning > lapping. I have a tool post grinder & used that to get them all withing a thou before lapping. That's a different subject (and expense LOL).


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## petertha (Aug 15, 2020)

Just hold your nostrils, look sideways & click BUY haha
These are very useful because you can calibrate them to a bore standard, or for that matter to the jaws of your micrometer if that is how you are measuring a shaft that has to fit. The downside (aside from $$) is they are a bit longish but they also come in different handle lengths. A typical kit comes with a range of anvils to span a bore range. If you already have a decient dial, you may be able to swap it in & save mony by buying the gage only vs inclusive of the dial. Just know the dial holder is kind of specific to the nominal dial OD, plunger length range etc. And the usual caveat, beware of bad clones or wannabees. Some are OK & some are junk just like micrometer or caliper shopping.


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## ericc (Aug 15, 2020)

WobblyHand said:


> ...
> I measured the 0.7598 and 0.7501 ring bore gauges 12 times each.  (At my desk.)  You know, to practice.  Mean measurement was 0.749825 and 0.750192.  Standard deviation was 0.0000866 and 0.000108.  ...



I think that within 0.001" is the best I could do with telescoping gauges.  But, this first measurement is off by nearly 0.01".  Are you sure the gauge isn't sticking?


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## WobblyHand (Aug 15, 2020)

ericc said:


> I think that within 0.001" is the best I could do with telescoping gauges.  But, this first measurement is off by nearly 0.01".  Are you sure the gauge isn't sticking?


Typo.  NOT 0.7598.  @ericc The two ring gauges are 0.7498" and 0.7501".  My measurements (when I knew what the answer would be) were within +/-0.0001 mostly.  But that's because I threw out the bad ones (because I knew what the true value was).  In "real life" I won't know the answer and will have to accept what I measure.

What is the proper etiquette?  Should I edit the original post, with the correct value or just have this?


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## NortonDommi (Aug 15, 2020)

Two cents worth. De-burr the edge of a bore before measuring if using verynears.


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## WobblyHand (Aug 16, 2020)

NortonDommi said:


> Two cents worth. De-burr the edge of a bore before measuring if using verynears.


Good point.  Know I sometimes overlook this.  So I de-burred.  In this case, it didn't matter, but it's good general practice.  Never know when a sharp edge is going to bite!


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## ericc (Aug 17, 2020)

That makes a lot of difference, thanks.  This doesn't look so bad at all.  If you are having a hard time with unintentional bias, you could try measuring one of your own bores, and just looking at standard deviation.  I'd be happy with mostly within 0.0001".  I was just working on boring a piece to fit.  I used a micrometer and a telescoping gauge and hit it right on.  Then, just to see how things were going, I tested with a junk dial caliper, possibly from Harbor Freight.  It was off by 0.015"!  I pulled out an end mill, and it turned out the calipers were right on.  The micrometer was off by 0.015". How could I have nailed the bore?  Of course, ID and OD measurements were with the same instrument.  Somehow, the calibration was off by a huge amount, but I was able to screw the micrometer back to zero.  I don't know how it was off by so much.  The dial caliper can easily be off by that much if a chip gets caught in the rack.


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