Monday, April 25, 2011

Taig lathe cabinet -control panel and switches

To conclude the discussion about the construction of the lathe cabinet, I'll now cover the switches, and finer points of the control panel

In my day job (which seems to cover days, nights, weekends, and other times as well... but that's another story) I am quite familiar with industrial emergency switches - aka E-Stops, or "Lock off stops" (LOS)
Industrial E-stops tend to have replaceable contact blocks which bolt to the back of the switch mechanism, and these contact blocks can be double sided, stack-able, and able to be used in a variety of configurations to suit the control need.
I have one or two of these switches which I've salvaged from discarded equipment, but the contact blocks add at least 40mm (1 1/2") of depth behind the faceplate - unsuitable given the space constraints at the tail stock.

Figure 1 - Two N/C switches with perspex circles glued on

Since I only had 18mm (3/4") behind the switch at the tail-stock, I decided to build my own e-stop switch until a commercial alternative presents itself. I purchased a number of N/C (Normally Closed) momentary push-button switches from one of the e-bay stores (Virtual village from memory) to use as e-stops.
I also purchased some N/O (Normally Open) momentary push-button switches as well... for some reason the N/C switches were only available in Yellow, and the N/C in Red - no matter.

I then cut out some suitable sized circles of perspex (Hole-saw with the pilot drill removed - whole job done by clamping in a drill press) and glued the circles on the button face using a cyano-acrylate based glue (Loctite Prism, or some other form of "super glue" - aka "crazy glue")

Figure 2 - N/O switch with smaller perspex circle glued on, next to bored out PET bottle cap

The two e-stops were then painted red using a sheet of paper glued over the perspex, and paint sprayed onto that. The paper makes the perspex opaque, and helps it take the paint better.

A similar method was used to make the button on the N/O switch slightly larger, and painted green.

The N/O button was to become the "Start" button, and as such I felt should be shrouded to prevent accidental activation. I could have made a nice professional shroud using pipe with an end cap soldered in and bored out, or I could simply grab a lid from a PET soft-drink bottle and bore it out to match the switch body.

Figure 3 - Test fit of "Start" button in shroud - using tail-stock mini-panel for support.

The panel was marked out and drilled for all switches prior to painting, and the back was marked up to make wiring easier.

The PWM circuit (commercial kit from Oatley electronics - Kit K252 ) was grafted on to a surplus Pentium heat-sink, which was then screwed to the top inner surface of the control panel so the fan blew directly on it. The on-board potentiometer was replaced with a wired external unit which is accessible as the speed control knob on the control panel. I had a few hiccups with the kit, but not as a result of any problem of Oatley's... All resolved now, but issues included one terminal block cracking when tightened up, and a dodgy soldering job on one oscillator pin.

Figure 4 - Back of control panel with PWM circuit assembly resting on it

I used some of my salvaged spiral wrap for the cabling - turned out to be a disaster since the wrap was so old the plastic was brittle. I had a chat with Wayne at Rexel and bought some more.. cheap chinese stuff instead of the the Cabac brand we use at work, but certainly much cheaper... and it seems OK for my use.

Figure 5 - Control panel front view prior to painting or labelling.

The cabling of the control panel was covered in my previous post and will not be repeated.
The control panel was simply painted "Bender grey" along with all the panel work and a label was made up.
The label was made from creating the text and dial markings, warnings etc  in Paint, and then assembling them on a page and printing it out. I then cut the paper up, and rearranged them to match a tracing already done from the finished panel. Securing screws, holes for breakers, relays, controls etc were marked in and the text placed around them. Once completed, the finished sheet was then placed through a colour photocopier and trimmed for effect. A pass through a laminator and re-trimmed and it was ready to be glued to the control panel. Peter Homman described a similar method for the prototyping of control panels for products, including membrane switches - it's a good idea I was grateful to be able to learn from.

Figure 6 - completed control panel with paint and labelling

A quick few notes:
  • I won't be adding anything more to this Taig lathe cabinet article series unless someone needs clarification on something... use the "contact page" to send through any questions.
  • I don't plan on offering any drawings or plans.. I can take a few more photos, but that'll be only if requested.
  • If you're in Australia... look at Oatley for cheap kits and other interesting bits and pieces. The motor currently fitted to this lathe is one of their 300W scooter motors. Since there isn't a cheap source of treadmill motors in oz (similar to the big surplus stores in USA) this is a good alternative. I've bought from Oatley over the years and found their prices and range reasonably good for a number of products.

Lastly... what would I do differently if I repeated this project?
  • Use a finger brake for the panel work for a neater finish
  • Make the swarf gate bigger, and the swarf container smaller (so an industrial E-stop with contact block can fit)
  • Use more flexible cable for the 20A DC wiring
  • Add a separate circuit breaker to  allow the motor drive (PSU, and PWM, etc) and the fan to be operated separate from the supply to the GPOs
In the words of Porky Pig, "That's all folks"... next post will be back on one of the many other projects I'm trying to get off my "to do list"


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