How to Use a Telescopic Bore Gauge
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- เผยแพร่เมื่อ 18 ก.พ. 2018
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In this video, I will be showing you how to use telescopic bore gages. Telescopic bore gages are used to measure the inside diameter of a bore or hole. These do not take a measurement themselves but is used to transfer a measurement to another tool. Typically a micrometer is used to take the measurement from the bore gage.
As you can see, I do have a full kit here so not one gage fits all sizes of bores. The smallest I have is 5/16” to 1/2” or 8mm to 13mm and the largest I have is 3.5” to 6” or 89mm to 152mm. This kit is made by Starrett, usually, it’s best to invest in a higher quality brand kit to achieve accuracy and best operation.
There are also two types of gages available which I have here. Both will have a knurled thumbs screw on the end to lock or unlock the measuring plungers, can also be referred to as legs, anvils, pistons, or spindles. This particular model has two plungers. They both move independently and are under spring tension. Once that thumb screw is locked, it holds the plungers into place. Once released, the plungers extend outwards.
Now for the other type, which has a single plunger and the opposite side is stationary. Again it also has the same style of thumb screw to lock and unlock that plunger.
To demonstrate the accuracy, I will be using my Fowler 1 to 2 inch micrometer that is able to measure to 4 decimal places or one-tenth of a thou. Always make sure your measuring tools are properly calibrated and in good working condition so the accuracy of readings will not be jeopardized.
Ensure the lock is loose, push in the plungers, and tighten the thumb screw. I’ll show a couple different angles so you can see exactly what I am doing.
Ensure both the part you are measuring, the bore gage plunger faces, and micrometer measuring surfaces are clean. Insert the gage and loosen the thumb screw to expand the plungers. Wiggle the gage to ensure it’s centered, tilt the top of the handle of the gage about 15 degrees off center and tighten the thumb screw. Just tight enough to keep the plungers in place, but not over tight where they can’t move.
Tilt the top of the handle back, just past the center about 5 degrees and remove. Do this only once, otherwise, it can affect the accuracy of the measurement.
At another angle. Ensure plungers are compressed and the thumb screw is tightened. Insert, loosen thumb screws to expand plunger, wiggle to ensure it’s center, tilt handle off center about 15 degrees, tighten thumb screw, tilt handle past center about 5 degrees and remove.
And again at a slightly different angle using the single plunger bore gage this time around. Ensure the plunger is depressed, insert, loosen thumb screw to expand piston, tilt handle at about 15 degrees, and tighten. Tilt handle past the center about 5 degrees and remove.
Typically I hold the micrometer in my right hand, so this will be a little more challenging. With this I am trying to demonstrate the accuracy of feel, you will want the same amount of resistance when the bore gauge was inside of the bore between the measuring faces. Slightly twist and rocker the bore gauge and the measuring faces on the pistons are slightly domed, so we’re trying to find the high spots. I cannot see the reading of the display on the mic, therefore I can’t make up my own measurement. The first measurement is 1.250 and 7 tenths.
Now the second bore gage. This does take some practice and feel. There should be a light drag when sliding the gage in the micrometer. Do no use the ratchet as this may cause too much pressure and push the pistons on the gage inward giving you an inaccurate reading. Now to check, we have a measurement of 1.250 and 6 tenths.
Finally the same measurement of the same bore using the third gage. If you are somewhat new to this, it’s probably best to take a few readings to be safe until you build up that comfort of feel. And our last reading is 1.250 and 6 tenths.
As you can see we have an accuracy of one-tenth of a thou. Results will depend on user feel, experience, quality of bore gages, and surface finish with that bore.
Moving into the last example for a different measurement and another view. Depress the plungers and lock them into place. Insert the gage into the bore, loosen the thumb screw so the plungers expand. Wiggle around the gage to ensure it’s on center, tilt the bore gage about 15 degrees and lock pistons. Tilt the handle past center about 5 degrees and remove.
Wiggle the gage between the two measuring surfaces of the micrometer, finely adjust the thimble and then take the reading. We have 1.072 and 5 tenths.
Excellent! You really explain that very well, Thank you!
You're very welcome!
Very nicely explained! I had to figure out the rocking part on my own, but I found your video to show to a friend. by matching the feel of the t-gage in the bore being measured and the way it feels in the micrometer, you can get very precise measurements.
Thank you so much, greatly appreciated!
I can't believe you typed the dialogue. great demonstration
Thank you!
Best video for explaining how to use this tool. Everyone always skips over some steps.
Also when removing the gauge. Can you just tighten it more so the micrometer can be used to ratchet without having to worry about the plunger depressing.
Thank you so much for the kind words. And yes you can, the ratchet should be a very gentle tighten anyway.
Super helpful! Thank you!
You're welcome!
Nice demo. The only thing I might change is to not lock the gage before putting it in as when it springs out when letting it go it could it the inside bore harder than it needs to. Just depress the plunger(s) when putting the tool inside the bore.
Thank you and great tip as well.
Matt, thanks for the video. I've never understood why you rock the tool after tightening it, it seems like if it's at an angle, by rocking it, it would read wider than it really is. What am I missing?
No problem, always happy to help. Think of it as a right angle triangle. When you have the bore gauge on an angle, it's wider than the bore, so that would be the longest line of the triangle. Once you angle it back in the opposite direction, and it'll go slightly past being a perpendicular line. The lock holds that inner dimension, so the plungers are forced into place to some extent.
That helps, thanks Matt
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