Member postings for martyn nutland

That's all very well...but could someone please help me with using a dial test indicator!

I have a Mercer (0001" reading) I bought on Ebay. When it's at rest in it's case it reads 13 increments anti-clockwise of zero. I assume this creates an initial pre-load back to zero when you start to work? I'm trying to use it to centre the spindle of my milling machine over a 23mm hole in the centre of an adaptor plate I'm trying to make for a lathe chuck.

I insert the Mercer in a Jacob's chuck in the mill quill. I know I should have it in a collet but at the moment I don't have a suitable collet only Posilock ones. I then eye things up to roughly centred by manipulating the X and Y axis of the mill and then push the stylus of the Mercer out with the tip of a scriber until it makes contact with the wall of the hole. Now when I rotate the quill/chuck/spindle by hand the stylus loses contact with the hole 'by a mile' (or at least by so much it's clearly visible to the eye).

Even if I manage to keep the stylus in contact I have wildly extreme readings of eccentricity (e.g. 170/180 increments on the Mercer dial. I try to eliminate this by 'halving the value' by manipulating the X or Y handles according to where I think the high spots are, but never get very far. Should this be done only every 90°. I.e. get the stylus lying in the same plane as the X axis and half the discrepancy, then move on 90° until it's in the plane of the Y and repeating.

And even if I can get a zero reading through most of a sweep it's usually because the stylus has lost contact with the hole altogether.

Clearly this is an incompetent mess on my part and I would be very grateful if someone could tell me how you use a dti - Mercer or otherwise! Much on the internet but, of course 'theirs' always sweeps to within a couple of 'thou' straight off.

Martyn N

PS In the case there's a flat rectangular bar about 2.5 inches long. It has an attachment at one end to house the stalk of the Mercer and I assume it considerably extends the diameter the stylus can sweep. But in what would you hold, horizontally (or vertically) such a flat bar to sweep a diameter?