Lathe - foot pedal

[TimeWerks]

Just a quick post on another lathe maintenance item.  I have a Boley 8mm watch makers lathe which has a foot pedal control to adjust the speed of the lathe.  As I pressed the pedal down the speed was not consistently increasing.  I had just serviced the lathe motor and lathe body, so the only thing left to look at was the pedal.

Cleaning the pedal switch improved the quality of the speed control, but it was surprising to see the switch construction.

Outside is pretty boring, but functional.

Back plate took a bit of convincing to slide it off.  There are grooves either side of it and you have to sort of pry the end of the plate over the edge of the pedal body to get past the two indented stops.

The switch is much larger than expected, held in place by one screw in the middle.  When you press down on the body, it causes the lever to draw in the pair of thin metal pieces which gradually make firmer contact with the contact at the bottom.  It gradually squeezes the end and in so doing somehow decreases resistance causing more more current to flow.  Needed to unhook the thin metal blade from the lever arm.

 

And the surprise for me was when I took the top end of the off, all these very thin carbon wafers  spilled out, along with the end piece at the bottom of the picture.  The resistance went down because more pressure caused better surface contact across all of these wafers!  

You can see they are varying shades going from gray to black.  The blacker they are the more carbon build up from current.  The carbon build up reduced the flow of current. So the solution was to clean both sides of all these wafers with Electrical Contact cleaner very carefully.  Too much pressure on a slightly uneven surface would cause the wafers to snap into pieces.

I broke two wafers in the cleaning process so did pretty good.   But in order to keep the same overall thickness of the switch I had to replace the broken wafers with a couple of washers that were the same diameter.

I sanded the end contacts and the thin metal strip that touches the contact at the bottom.  I dropped the bottom contact down the hole first, then carefully coaxed the wafers in after it until I filled up the gap to the other contact assembly and attached the two screws.

After assembly the speed control was smooth again.