Hand machined mechanical pencils

Test indicator contact tip bar (20/01/13)

I needed to turn something eccentrically in the lathe and had to ensure that the offset was a specified distance. The test indicator was useless because it only had a point contact tip - if the axis of travel of the indicator is offset from the rotation axis, then it will indicate a greater travel than is actually the case. This problem is solved by using a bar contact tip - see the animation below. The bar contact (on the left) shows the correct travel of 4mm, but the point contact (on the right) shows a travel of 4.31mm.

I used a piece of 1/8" carbide (broken PCB drill shank) as the contact. It is a light press fit in a small aluminium holder which is screwed to the end of the test indicator plunger in place of the original tip. The last two photos show it in action.

Here's a PDF drawing of the holder. Mill out the slot a little bit at a time until the bar is a suitable press fit.

Original point

Protective pieces to clamp the plunger with

Hold plunger with pliers and unscrew the tip

New tip pieces