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.  

One Day Builds: Center Punch

    This post is the first of what I hope is many to come. Because I am a full time student, and work 20 hours a week, my shop time has been greatly limited. This is why I have been posting articles about design work rather than shop work. I don't like being away from the shop for fun this long. While I do work there, helping students and doing job work isn't the same as fun projects.

    Most days I have an hour or two between classes that I use for studying, napping, or otherwise killing time. I hadn't previously considered using this time for shop work because it was only an hour or two; not long enough for the setups most of my main projects use. I decided to try the idea of a One Day Build, similar to what Adam Savage does for Tested. The premise is you try and build a smaller and simpler item in one shot, one day, finished product. I expanded on this idea a bit to include smaller projects with simple setups. With simple setups, I can quickly set up and clean up in the small periods of time I have.

    The idea for the first One Day Build came from a broken tool. I was at work rifling though drill bits, sorting strays and looking for dull ones to sharpen. I found a bit on the larger side with the tip absolutely destroyed. Oddly, the bit didn't have any heat damage so I can only assume someone was drilling composites with it. It turned out it wasn't just the tip that was destroyed but both margins too. Dulled, chipped or melted tips we can repair, worn margins we can't.

    I always hate throwing out tools, even when they are broken. So much time and resources went into making that special piece of metal only for it to be thrown away one day. Always seemed like such a waste. I decided to save it and find something useful to make with it. That useful thing ended up being a center punch because I didn't want to store the drill for a long time.

    I began by chopping off most of the flutes. This was a task in it's self. The abrasive chop saw did not like the HSS. It finally cut through and I was disappointed to see the web wasn't as big as I would have liked it to be up near the shank. Moving forward anyway, I roughed out the taper on the bench grinder and then moved to the surface grinder. I used a spin index tilted on a magnetic sine plate. I wasn't super concerned about the dimensions of the punch, so I eyeballed everything.

Finished center punch

     I set the sine plate with a few pieces of scrap to get it to match the rough grind I had done. The surface grinding went quick. When roughing, I use a .01 inch (0.25 mm) cut with .01 inch (0.25 mm) step overs. This is my preference, I understand there are many schools of thought with grinding, this is what I use. This is a low precision tool so I used the same 46K wheel for my finish pass at .0002 inch (0.005 mm) as I did for roughing. I didn't even bother redressing.

    For the struck end. I did some quick chamfers and a flat end. Again, no measurements were taken. The shank is relatively soft, so I don't know how it will hold up over time. I'm happy with how this project turned out. I really like the look of the remaining flutes on the taper and the drills size still visible. HSS isn't the advised material for centerpieces, but I couldn't find anything saying don't do it. I guess because there is no heat build up in center punches, no one ever tried to make out with HSS. Even if this tool only lasts for a year or two, its still worth it as I didn't have to throw something in the scrap bin.

A ghost of a previous life

    Sorry there aren't any photos of the process. Because I'm short on time when doing these things, I don't have time to take lots of photos. In the future I'll try to do better.

.David.