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RE: gEDA-user: Fitting a hobbyist design into someone else's form factor

Most bent sheet metal enclosures are toleranced to +/-0.015 inches. So anything on a face of the enclosure at worst case will be +/-0.015 inches off from the expected dimension. If there are bends between the mounting holes then the tolerances will be greater(it's difficult to hold the tolerances around a bend). However since you have to measure the part to get the dimensions I would guess you will need at least twice the slop to get everything to fit.

Generally molded pieces will have higher tolerances, perhaps to 0.005 inches.

As a for instance for mounting holes for a #6 screw the mounting hole on the PCB will be 0.156 inches in diameter. I believe a #4 is 0.120 inches. Again, since you are measuring this by hand you will need much larger clearance holes.

Since you don't have a drawing on the part I suggest using a calipers(they're available for <$30 from places like Sears) to measure inside edge to inside edge of holes, then measure the diameter of the holes and subtract the difference to get the correct dimension. Of course this will be difficult if the holes are lined up.

Don't forget the keepout areas around the mounting holes, otherwise you run the risk of shorting traces to the enclosure.

Good luck!


From: msokolov@xxxxxxxxxxxxxxx (Michael Sokolov)
Reply-To: gEDA user mailing list <geda-user@xxxxxxxxxxxxxx>
To: geda-user@xxxxxxxxxxxxxx
Subject: gEDA-user: Fitting a hobbyist design into someone else's form factor
Date: Mon, 29 Jan 2007 04:46:32 GMT

Hello fellow gEDA/PCB users,

I would like to fit my design into the form factor from an existing
gadget so that I don't have to invent my own mechanical design and can
utilise a ready-made enclosure.  This means that I need to carefully
measure out the existing board in order to reproduce its mechanical form
factor.  Here is my question to the list: what level of measurement
precision is required to make a generally acceptable fit of the form
factor?  Will a metric ruler with mm ticks be sufficient, or would I
need something finer?  I don't know if the form factor I'm trying to
copy was originally designed as metric or inch-based.

The mechanical aspects that I need to measure and reproduce in my design
are:

* Board dimensions and shape.  It is shaped like this (read in a fixed
  width font):

  +----------------------------------------------------------+
  | RJ45                      RJ45        AC mains           |
  |                                       connector          |
  |                                                          |
  |                                                          |
  |                                   +----------------------+
  |                                   |
  |                                   |
  |                                   |
  |                                   |
  |                                   |
  |                                   |
  |                                   |
  |                                   |
  |                                   |
  |                                   |
  | LEDs                              |
  +-----------------------------------+

  I guess such a board is fabbed as rectangular, then a corner is cut
  away, right?

* Location and drill diameter of mounting holes.

* Location of some connectors along the back side.  The existing board
  has two RJ45-style jacks and a "kettle plug" AC mains connector, and
  mine will have identical connectors in the same locations.  The
  existing board also has a block of 4 RJ45s stuck together (Ethernet
  hub), but mine will have a DB25 in its place.  The DB25 is smaller
  than the 4 RJ45s, so I'll have a little bit of wiggle room for it.

* My board will have some surface mount LEDs on it, and I want to put
  them exactly where the existing board has its LEDs so that they are
  visible when my board is put into the existing enclosure.

I would appreciate any advice on how much precision is required to
achieve what I've just described.

TIA,
MS


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