Thread: Shipping damage and its repair on a Mitutoyo Optical Comparator's Protractor Screen.

Or how the bacon was saved with an ER collet.

A few weeks back I brought home a Mitutoyo optical comparator. It is a model PJ 250 B from the late 1960s I reckon based on my catalogue collection. Imagine my surprise a couple of days later when I found a similar vintage 1 minute reading protractor screen, new in its wooden box, on US eBay for 56 bucks buy it now. And buy it now I did. There was an element of uncertainty with the purchase. The protractor screens for the early model comparators were satin chromed, this one was black but it had the old Mitutoyo script. It was a post within the States only deal so I had it posted to my brother in New Jersey. He scanned the front and rear of the screen and also scanned a scale rule so that I could check the emailed images. Exactly the same size as the plain screen on the comparator I had.

The screen arrived on Friday along with two other examples of near unobtainium, replacement belts for my little mill. The wooden box containing the screen had not fared well. The base had come adrift and two of the four felt lined mounting blocks had split in two. The screen appeared OK. So I thought. My brother had removed the thumbscrews and hold down fingers and placed them in an envelope.

It was upon opening the envelope and looking at two of the screws that I realised that their threads had been sheared off inside the brass rim of the screen. This would have occurred while the screen was shipped from the seller to NJ.

I thought I could probably turn up a couple of replacement sections of thread and graft them to the satin chromed, nicely knurled thumbscrews. Mitutoyo for some weird reason, used a non standard 3mm x 0.6mm pitch thread on the one intact screw used for locking the screen. Knocked the lazy idea on the head of using a modified section of an existing standard pitch screw. All this was based on the successful removal of the broken off threads. And that wasn't successful. I purchased a new No.1 sized Easy Out that failed to live up to its name. The brass crumbled.

I resorted to drilling and tapping the hole to suit an M4 fixing. I drilled the thumbscrews to a depth of 10mm with a 3mm bit and turned down a section of an M4 screw to suit. They were an interference fit. I filed a flat along the 3mm section of the screw to allow the expulsion of air when I glued the bits together with Loctite RC 635 retaining compound.

I was concerned about damaging the knurling on the thumbscrews while driiling. I used an ER collet chuck with the nut finger tightened and my fingers crossed and all went well. Fortunately.

BT

ps I've added a few photos of the Mitutoyo in action. What a fantastic thing.