Latch Repair

       An researcher at work recently brought in a latch they needed repairing. Apparently the latch was no longer available from the supplier or too expensive for their budget. The latch itself is a standard die cast zinc part you could find from any number of suppliers like Item or Parker. The problem with it was the steel pivot pins had worn out the holes in the Zinc casting. The holes were now oval and the latch wasn't securing properly.  

The disassembled latch

    The moving elements of the latch were held in the body with two swaged pins. Removing the pins were a task because of a countersink in the body. I eventually ground the heads off the pins and drove them out with a punch, but not without slight scratching to the body casting. Projects like these are filler jobs for us and moderately bothersome. As such we don't get too worried about doing a perfect job when it comes to aesthetics.

    The original request was to make replacements for both the latching link (left most linkage) and the connecting link (center, light colored linkage). I thought that measuring and duplicating these parts - while possible - would spend more time than I wanted to. I decided instead to overbore and bush the holes.

    The parts had enough flat sides that I could clamp them directly in the vice. The largest gauge pin that would fit was selected and used to center the existing holes. Because the holes were worn, they weren't round or parallel. When indicating, I did both concentrically and both X and Y rotations. I didn't worry about getting it perfect and was happy with a few thou run out.

Checking vertical run out

   
    Boring the holes was straight forward. A solid carbide boring bar was used due to the small size. The latch used 3/16 inch (4.7 mm) pins and I saw no reason to not continue to use these. The holes were bored to a diameter of .239 inch (6.3 mm) giving a press fit of .001 inch (.02 mm) on the bushings.

    The bushings were simple and made out of stainless; outer diameter .240 inch and a reamed 3/16 inch bore. I had to come back afterwards and drill the bore larger. The pins I'm using for the pivots ended up being more than just a few tenths over sized.

Making the bushings

      

        We recently bought new machines in the shop and the new lathes and tooling are really cheap. They don't have any power feed so the outside finish was lacking. The longer of the bushings ended up having a slight taper and scored its bore a bit when pressing it in. Pressing was done with a mill vice. 

All holes bushed, ready for assembly

     

All the bushings aligned neatly and the pins rotate freely. This project ended up taking longer than expected and was more annoyance than anything. I decided to write an article about this because it highlights design oversights that are often made for the sake of cost. This latch was designed to wear out, and it was designed to be replaced when it wore out. You don't always have to completely replace something when it breaks, or even replace only one part of it. In this case, repairing the parts rather than replacing was a faster, easier and cleaner option. It also didn't require throwing out any parts. 

Stay curious.

.David.