Thursday, January 28, 2016

A Ming-Inspired Cabinet (28)

Post 28 in a continuing series, and 900th post since this blog was started back in 2009.


I got tired of using a screwdriver across the terminals of the otherwise dead start switch on my Zimmermann, and got around to replacing both push buttons and their mounting plate:

The original push buttons were a larger size and in replacing one of them with a 22mm push button operator, I needed to make a new mounting plate, so I ended up getting a new 22mm ‘stop’ switch as well . The new 'stop’ switch is larger and a little easier to hit if you need to shut things down in a hurry. Safety first!

While doing that work, I accidentally grounded a light circuit conductor and blew a 6 amp fuse. The fuses on the Zimmermann are ceramic, a bit like the ones you find in an electric stove, and fortunately I had one spare 6 amp fuse on hand. There are a dozen fuses in the machine and when a fuse blows there is no obvious visual indicator that I can see on the fuses themselves.  You have to test across each one for continuity, which is a little tedious.

Looking in my factory operation manual to dig into the wiring schematics and electrical component specifications, I discovered that the fuses are supposed to be Seimens 'Neozed’ 25amp units, however almost all of the ones in the machine are 6amp. That leaves me slightly puzzled. Either the ones in the machine are wrong, or the operations manual (from 1980) is wrong, or for some reason the specification was changed between 1971 and 1980. I also have a wiring diagram from another year, and it is identical to the first one but there is no electrical parts list included in the other year’s paperwork to confirm which model of fuse was used.

I am tending therefore to think that the correct fuse amperage is 25, not 6. But what if I’m wrong? They have all been working fine so far, but then again, other than the recent accidental connection, there has been no event which has caused them to be tested. They could be fine and they could also be inadequate. I’m not sure what to do. I’m seeing no reason to change anything, but also wondering if the fuses in place are undersized.

I bought a package of 6amp fuses from Germany, and they were pretty cheap actually, and shipping was less than $10.00. Hopefully they will be here soon as I feel nervous having no back up fuses.

This is where I wish I was a bit savvier about motor electrics, and could possibly work out what the correct fuse size should be. I need to study this more…. I confess I am a little fuzzy in my understanding of momentary switch electrics. I do understand that the starter switch is a normally-OFF circuit, meaning the button push connects the circuit, while the 'stop’ switch is normally-ON, and thus pushing the 'stop’ button disconnects the circuit.

Motor controls are not direct in the same way a when you turn on a household light switch. There are magnetic controls between the switch and the motor. When the “start” button is pressed, the motor is not powered directly, rather the electromagnet in the contactor is energized. The magnetic switch in the contactor then engages, simultaneously switching current to the motor and providing self-sustaining current to maintain its own state. Thus when the start button is released, the magnetic switch remains engaged and the motor remains running. I find this fact slightly counter-intuitive, but it is rather similar to the use of relays in an automobile. Instead of routing 460volts through the start/stop switching, the power comes from an internal transformer putting out 30 volts. A low voltage circuit controls a high voltage motor.

I spent the past couple of days doing some CAD work, as my car has been in the shop. The client got back to me and is fine with the design revision to change the steps in the middle portion of the cabinet for shelves. He also wants me to devise a system to attach the cabinet mechanically to the wall in case of earthquake. He lives in LA, so this is a reasonable concern of course. I’ve got some ideas about that and we’ll see where things end up.

Last time in the shop, I was trimming leg tops to a double-bevel or compound angle:

The legs only slope 0.5 in 10, so the slope is hard to see in the above picture.

The legs after the compound bevels have been cut on top - a little more obvious to see the bevel now:

The post tops were the tenoned in one axis:

Another view:

 Another view:

Further reductions lay ahead….

Thanks for visiting the Carpentry Way. My car should be fixed this morning so I will be back in the shop as well.

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