Sunday, February 12, 2017

Scroll saw throat plate

Part of my scroll saw workflow has been to cut wide of the lines and then spend time sanding or filing, because of tearout.  Also, small pinions and the like tended to fall through the throat plate, necessitating hand tools...

I read that a simple solution to this problem is to make a new throat plate for the saw, so that there is only a little hole for the blade.  Once done, it's almost like a new saw!  I could cut much more precisely -- the blade doesn't tend to drift and the tearout is much reduced.  I cut a whole bunch of parts for the chronometer clock...

Sunday, February 5, 2017

Clock #3, with a chronometer escapement

The next clock design is progressing!

I wanted this clock to have a chronometer escapement, but I also wanted to rework my design workflow a bit.  Previously, I drew plans in Inkscape, which is straightforward, but doesn't easily manage complex designs.  Additionally, the only gears Inkscape comes with are involute, which I didn't want.  So for the timer, I used an online tool to make epicycloid gears, save them as a DXF and import that into Inkscape.  I thought about scripting something in Inkscape to do this better, but found that awkward.

So instead, I turned to OpenSCAD, which claims to be a "programmer's solid modeler".  This is pretty handy as I can develop libraries for later use and re-use, parameterize everything so that changes are easier (and mostly automated), and I can cut/explode/examine every part of my model without too much effort.

So to start, I wrote a library that builds gears according the British Standard 978 Part 2, taken from tables from the back of "Wheel and Pinion Cutting in Horology".  Note: I had to make use of some additional calculations from Swiss standard NIHS 20-10 in order to get some of the curves to match up properly, the description in BS 978 is slightly underspecified!  Now, I can have consistent gear profiles throughout the design.

I then spent the next few months (starting in May 2016) designing a clock movement, working sporadically.  After getting the chronometer detent and escape wheel pitched correctly -- animations were very helpful -- I got the rest of the movement in order.  I'll design the frame later, probably using the new depthing tool I made for the purpose to pitch the wheels.

The train is intended to be mounted on the wall or in standing case, and is weight driven.  It is built as follows:
  • Balance wheel period: 4 s (yes, slow, but that would be kind of mesmerizing...)
  • Escape wheel: 15 teeth and advances one tooth each period
  • Escape pinion: 8 teeth
  • Fourth wheel: 48 teeth
  • Fourth pinion: 7 teeth
  • Center wheel: 70 teeth
  • Center pinion: 8 teeth
  • Drive wheel: 80 teeth
  • Drive pulley: 2 in diameter
Based on this, the clock should run about 4 days with 60 inches of cable.

As you probably can see in the diagram, I decided to try for a non-standard motion work.  It appears in Daniels' "Watchmaking" on page 173 (not an obvious location, at least for me), and also here, where the author notes that Daniels is a but uncharacteristically thin on details.  Indeed, I agree!  I had to do some side calculations to figure out how to pitch each of the components.  Fortunately, those calculations are now enshrined in my OpenSCAD model, so I don't have to worry about them again, even if I change something!

Now, on to building. The first task was to convince OpenSCAD that I wanted flat plans, not a solid model.  I guess a solid model would be better for 3d printing, which seems like a good idea for later, but I want this clock made from wood!  Fortunately, it was an easy matter to explode the parts all onto the xy-plane and cut them all through.  OpenSCAD exports to DXF, which I did, and then imported that into LibreCAD.  From there, I separated out the different parts into named blocks, and printed them all out onto paper.

My previous clocks have been built from 1/2" birch plywood, which is nice and solid.  However, it feels too heavy, especially for something so dignified as a chronometer, so I am trying 1/8" birch plywood.  Surely this cuts much more quickly, but it is also much more delicate.

I'm also trying a different workflow on the saw.  I am rough cutting the wheels with a 34 tpi spiral blade, and then using a scroll sander to do close-in sanding.   It's not fast by any means (the 80 tooth drive wheel above is only partly finished!), but it seems to give accurate work.  At least, it seems to be much more accurate than my previous work, which required many hours afterward with a file.  This clock will probably get its share of filing as well...  Lots of people have mentioned scroll sanders as existing, but since I don't have a fine belt sander it's quite nice.  I also am doing the work under a 2.5x magnifier and a bright light, so that helps too.

For later reference, it looks like I can cut wheels whose teeth have module 2.0 on the scroll saw.  Anything smaller is too fine!  Fortunately my OpenSCAD model is parametric in the module...

New clock depthing tool

Picture says most of it.. I designed a new depthing tool in OpenSCAD that should be a bit more stable than the previous one.  It's also intended that the arbors are fixed and the wheels spin around them.  The design consists of two parallel bars that hold three I-shaped blocks.  The outer two I-shaped blocks are attached to the bars, while the middle one can slide.  The left two blocks have arbors through them.  The sliding block is locked by tightening a knurled nut holding everything together.