Compression latch

Had a little time to do some more tinkering, so I thought I'd work on a latch arrangement for the pressure box lid.

I'd been thinking about various types of latch mechanisms, and recently I decided to try and make a compression latch that would enable me to tighten down the lid as I latched it shut.

In figuring out the various possibilities, I ended up with a latch/knob design that would allow the lid knob to also serve as the latch.

I ran out to home depot and picked up a cheap brass cabinet knob, and scrounged around to see what other stuff I had lying around.

The compression latch consists of a turning handle (the brass knob), a threaded shaft (the #8-32 bolt that came with the knob), a latch finger (I was hoping to grind down a brass bolt I had), and a spring to keep tension on the latch finger.

In theory: when the knob is turned clockwise, the finger move a quarter turn from its recessed cavity on the pressure box lid and engages the catch on the main assembly. Once engaged, if the knob continues turning clockwise, the threaded latch finger moves up the shaft, using a vise-like action to pull up on the  catch and increase the latching pressure.

I was hoping that this would end up giving me the best possible seal for my leather gaskets…

Here are the pieces I gathered up:

As it turns out, the "brass" bolt I had, was not actually brass but just a gold-toned steel bolt (I didn't look at it very closely initially). I went ahead and cut the head off of it, drilled through the side of it, tapped it to #8-32, and ground down the end to make the "finger".

I then realized that I would have to make some modifications to the inside of my pressure box lid. First off, I had to cut out the fancy routered strip (that reinforced the end-to-lid joint) in order to make room for my latch housing.

Since the latch would have to retract into a recessed space, I knew I would have to cut a hole in the end of the pressure box lid. In order to isolate the hole I designed a housing that would be sealed off from the pressurized interior. I also beveled one of its corners so that it wouldn't come anywhere near where the music roll would be winding around the take-up spool.

Before attaching the latch housing, I carved it out so that the mechanism would fit neatly inside, then made a corresponding hole for the latch in the end of the pressure box lid.

Here's how it looks - stained but not yet shellacked. I'll have to wait until I get the lid hinges in place before I figure out exactly where the catch plate should be mounted.

Crank details

Having a heavy load at work this past week, I wasn't able to get much done on the project, but I did play around with the crank parts to get them closer to being finished.


Consulting the plans, I figured out the length of the shaft assembly and cut it to size, then laid out all the parts I thought I'd need:

I first figured out where the mitre gear needed to be, then drilled through it and into the steel shaft to allow the insertion of a roll pin to keep it locked to the shaft:

I had decided to use a knurled steel collar for the drive/friction wheel instead of the wood/emery cloth arrangement specified in the original plans. For the wheel, I went with a knurled microphone base adapter (which measured exactly 18mm in diameter), cut off the threaded end, drilled it out to fit the shaft, and pressed two nylon shaft spacers into the piece.

In order to lock it to the shaft, I drilled a hole partway into the end crank web and inserted a roll pin that would stick out halfway. Then I drilled a corresponding hole into the end of the knurled drive wheel so that the pin would lock the wheel to the end crank (which would lock to the shaft with a set screw).

Here's all the components put together:

It's probably a little more complicated than it needs to be, but once I got started it was almost like playing with legos or tinkertoys...