Designing a replacement Alps Plotter drive gear - test-drb@ccmp.vtda.org

12 Apr 2013

On Thu, 11 Apr 2013 21:29:30 -0400, Ethan Dicks <ethan.dicks at gmail.com> wrote:
...
  > Mention of
the Alps plotter reminds me that we've had various discussions
> about all the plotter mechs with broken gears but we've never worked out
> the details... 
  I may be able to draw up a gear like this, but
like the "gear
 generator," I'd have to have the correct gear data to know what to
 draw. 
 Understandable.  If I had more numbers up front, I would have given them.

  What specs should I try to draw a gear to?  I
have access to both 2D &
 3D drafting/modeling software (AutoCAD & SolidWorks).
 Do you intend to use a 3D printer to try to manufacture the gear? 
 I do not.  I was contemplating trying to laser-cut some out of Delrin sheet,
 something I did not have access to the last time this conversation made
 its rounds.  As before, FDM printers do not have the resolution for such
 a tiny part, and other technologies (sintered powder or UV-cured resin)
 are likely to be too brittle, especially with such fine features.

I didn't really think so (for the same reasons), but I haven't kept
up-to-date with today's additive manufacturing capabilities.

...
   I suppose
these gears are spur gears with involute gear teeth. 
 As far as I know that is true.

  What is the pressure angle? 
 I do not know.

  What is the total number of teeth?
 What is the pitch diameter? 
 I do not know without digging out a printer to check.  I used to have
 4MP close-up pictures (taken with my gear in 2003 back when
 Electronic Goldmine was surplusing the bare Alps printer mechs
 for a few dollars) but I don't know exactly where those photos
 are at the moment.  My best approach right now would be to
 drop one on a high-res flatbed scanner (1200 dpi) to get a
 reasonable "to scale" picture, but I don't have a gear in front
 of me at the moment (I do have them in town).

If you find the photos or make the scans, please send them.  If
possible, scan the gear next to something with a known scale such as a
6-inch machinist's scale or equal.

...
   You said the
face width is 1/8-inch and the shaft hole fits a 1.27mm
 diameter shaft. 
 That's what I've scraped from previous conversations about this
 gear.  I now own a micrometer so with a gear in hand, I have
 a chance of confirming that to better accuracy.

  Wikipedia (https://en.wikipedia.org/wiki/Gear)
has all this info and
 more -- I'm pretty sure that you are interested in only spur gears
 right now, so you can skim over the other gear-type information on
 that page. 
 *nods*

  Knowing the gear specs may even allow someone to
go to the "small
 gear" catalogs and maybe even be able to select an "off-the-shelf"
 gear for each needed gear.  It's possible. 
 I think we (the cctalk community at the time) looked into that and nobody
 had such tiny gears with so few large teeth in their catalog. 
Sounds like a "non-standard" gear.  I can see why you (cctalk)
couldn't find one in a catalog.

...
  There were
 calls to remove a gear and make castings (most gears are already broken
 making that difficult) or using a lathe to make a fresh brass gear, etc.
 Someone apparently did take a stab at a homemade brass gear but
 declared it too expensive to warrant starting up a small run.  Perhaps
 hobby CNC has evolved to the point now where one could do a gear
 with less labor and make an individually cut gear easily enough to
 justify selling it for around $5.  Or not. 
The hobby CNC world seems to have drastically increased in popularity
in the last few years.  A lot of the (mostly) guys are retired now and
*may* be willing to make a small run for a reasonable cost just for
the challenge.  We'd still have to be able to give them full specs so
they can program their machines.  I recommend we wait on approaching
the hobby CNC community until the gear specs can be determined and you
have a go at the prototype with the laser.

...

 If I have a DXF or EPS of the face of the gear, I can see if the laser
 cutter can make an approximation out of Delrin for nearly free (I just
 spend a few minutes when I'm already working on a run and as long
 as I don't work past the quarter-hour billing tick on the tool, the time
 is already paid for by the other job).  I'm not positive that this will
 work, but the cost is so low that it's worth a stab.  Just need the
 silhouette to scale and I'm good to go.

 -ethan 
Go ahead and measure what you can and send me the dimensions &
photos/scans.  I'll see what I can put into DXF or EPS format for you.

Bob