Other People Projects ==> Bridgeport 8F Power Feed

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BptFeed01.JPG
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DESCRIPTION:

These are pictures of a retrofit project to fix the speed control
electronics of a Bridgeport power feed unit.

Posted by Jon Spear .  

The following description was provided:

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BptPowerfeed.txt, created September 9, 2002, by Jon Spear, of 
San Francisco, CA

The 8 photo files, BptPowerfeed0*.jpg, where * goes from 1 to 8, 
document the procedure I followed for swapping out the old control 
board for my Bridgeport mill's electronic power feed unit.  The 
original board, made in 1979,  had gone bad, and I wanted to replace 
it with a more modern commercially available unit.  

The unit I chose was made by Minarik, which I got from eBay for 
about $45.  The model number is MM23001C.  I don't think that this 
board is especially unique, but it does have the following nice 
features, which is why I selected it: 90VDC (5amps) output to armature, 
100VDC (1 amp) output to field windings. Feedback control;, with 
adjustments for maximum torque (i.e., armature current limit), and 
IR compensation (a feedback parameter which is adjusted for a balance 
of keeping the motor speed insensitive to load perturbations vs. keeping 
the speed stable).  Also, the board was small enough that it fit into the 
original box, from which I removed the old board.  This allowed me to 
upgrade the machine while not altering its original apearance.

Figure captions:

1.This is the old board, made by SECO, and the box in which it was mounted, 
which attaches to the side of the mill.

2.  Same stuff, flipped to see reverse sides.

3. Here are the wires, switches, speed control potentiostat, that mount 
underneath the feed unit and motor assembly.

4.  This is what the replacement electronic control unit looks like.  I 
mounted it onto a scrap piece of 1/8" aluminum plate, with holes cut so 
that it would mount where the original board had gone.  I was able to use 
almost all the original wires, and added a couple of 18 gage wire leads 
for the modifications I made to the armature outputs.  The new unit used 
4 fewer leads than the original, so now there are some unused wires 
between the box and the feed unit assembly.  The original 500 ohm pot 
was removed, and replaced with the 10k ohm pot that came with the Minarik 
drive. Also, the single pole "on-off" switch was replaced with a new 
Common-NormallyOpen-NormallyClosed"  switch.  this switch allows for rapid 
traverse.

5.  Here's what the back of the box looks like, with the protective back 
cover plate that seals out contaminants like chips.

6.  Here is the original electrical diagram for the original board.  Note 
that the "reversing switches" control the polarity of the motor's field 
windings, which carry about 1 ampere of current.  There are 11 leads plus 
ground, 4 more than the replacement unit.

7.  Here is the way I decided to wire up the replacement board.  The notable 
difference is that now, the reversing switches control motor direction by 
changing the polarity applied to the armature, which can be up to 5 amperes.  
When the unit is powered up, and the power feed lever is in its "neutral" 
center position, the armature is disconnected.  However, the filed windings 
are always energized, using up about 100 watts of electricity, so the motor 
will get warm if the power is left on constantly.  I think this was the best 
possible solution with the hardware I had available.  I haven't had any 
problems with it in the past few weeks, since I installed it.

This is the diagram that I thought would be worthwhile, to share with other 
Bridgeport owners who might have an old broken power feed board.

8.  Here is a photo of the assembled unit now in use.  Looks pretty close 
to original... The feed rate can go from about 0.4 inches per minute, up to 
40 inches per minute.

If you have questions or comments, my email address is 

   jdspear@alumni.princeton.edu

p.s.  Currently, I see that the same model of dc motor drive, Minarik Model 
MM23001C, is available from a place called, "Surplus Center," of Lincoln, NE.  
Their US phone number is 1-800-488-3407, and the catalog item number is 11-2269, 
for a price of $56.95.  

Good wishes, 
Jon