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# putting longer tips on test indicators

#### Blix666

##### Plastic
was wondering how to figure out how you calculate the readout difference when using longer tips with test indicators. we usually use the standard 11/16" length tips that come with the indicators, but for some new parts we're about to start we'll need to use longer tips.

#### Spencer in NH

##### Stainless
Calculate the ratio of the calibrated tip to the longer tip: Divide the length of the long tip by the length of the calibrated tip. These lengths are from the pivot on the instrument to the contact point on the tip. The reading on the instrument is then multiplied by this ratio.

For example, if the new tip makes the pivot-to-contact-point distance twice as long, the instrument is going to be half as sensitive.

Hope this helps.

#### Gary E

##### Diamond
was wondering how to figure out how you calculate the readout difference when using longer tips with test indicators. we usually use the standard 11/16" length tips that come with the indicators, but for some new parts we're about to start we'll need to use longer tips.

Spencer is right ...but look at it this way...
You replace the tip with one 2 times the original tips length..
When the longer tip moves 1 unit... does not mater if it's 0.001 or any distance, the new longer arm love 1/2 that arc... and the dial LIES to YOU...

GET A NEW LONGER INDICATOR....
1... it will be ACCURATE
2... you wont have to futz with correction factors
3... the old indicator WILL STILL BE CALIBRATED... you do calibrate them, right??
4... the new longer tip will be calibrated... and accurate...

Dont be a cheeeeepskate... indicators are not expensive..
SCRAPPED PARTS ARE

#### The real Leigh

##### Diamond
Dial test indicators (with the wiggly arms) aren't particularly accurate in the first place due to trigonometric errors when the point is moved.

You can buy various measurement stems. Just install whatever you want.

If you want some degree of accuracy, calibrate the device by mounting it on a mill table with a tenth-reading DRO, or use gage blocks.

- Leigh

#### cecilstrange

##### Aluminum
Here is an illustration of the significance of cosine error when using a dial test indicator:

For non-Interapid test indicators measurements are accurate when the contact point is at zero degrees to the surface to be measured, i.e. parallel (for Interapid use 12 degrees). If you measure with the contact point at 20 degrees instead of zero, cosign error will cause the reading to be too large by 40 millionths of an inch.
021 : Test Indicator Contact Points

#### Peddler

##### Hot Rolled
Leigh, aren't test indicators are generally used for comparative measurement? Once you establish a good base (0) reading, usually using gage blocks, they seem to be quite accurate in reading deviation from the master.
Certain brands of test indicators repeat and read accurately far better than others.

#### Davis In SC

##### Diamond
32 years in the shop, I just use a DTI as a comparative gage.. 99% of the time, it is dialing in a part in the Harig spinner... All I look for is almost no needle movement. No way I trust the numbers..

#### The real Leigh

##### Diamond
Leigh, aren't test indicators are generally used for comparative measurement? Once you establish a good base (0) reading, usually using gage blocks, they seem to be quite accurate in reading deviation from the master.
Yes, absolutely. That's what they're designed for. The scale reading can be reasonably accurate for very short movements. Errors increase with arm angle.

Certain brands of test indicators repeat and read accurately far better than others.
Very true. As with any other product, you get what you pay for.

- Leigh

#### winger

##### Stainless
I saw an ad for an elliptical tip that was supposed to reduce the error compared to a round tip. but even that was with a limited range. The way test indicators are made they are really measuring change in angle.

Dave

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