Quick Switch Fix

[scmhogg]

I am rather proud of myself when I fix something that I think is unrepairable. Suffice it to say that I have limited mechanical/electrical experience.

My HF 16 speed floor model drill press has been a surprising success. It has virtually no run-out and great power.

The other day, I was drilling something and when I finished I turned the drill press off and it kept running. I had to unplug it.

I was instantly depressed. I have had some experience getting parts from HF. Three or four months last time. And, I knew that the switch would not be standard.

I took the switch apart and little parts started to fall out. The metal, business end, of the switch fell out. Then I noticed that this piece was bent and both contacts looked like they had welded together. They fell apart when I opened the switch. I bent the part straight, sanded off the rough spots on the contacts and, viola, I am back in business.

Any thoughts on why the contacts welded in the first place?

Steve

[LCHIEN]

a spark jumps an air gap when the voltage is there and the gap is small enough.
This happens when a switch is closing and happens before the contacts actually close. In the spark (we call them Arcs) electrons jump and some positve material (e.g metal) is also transfered.

It also happens when the switch contacts are opened, the inductance in the motor windings makes the current want to continue to flow and it will build up a back EMF voltage to keeps the current jumping the opening gap even farther than when the switch was closed. This is usually worse than the first situation where the switch is turned on, with motor loads.

How severe the arc is depends on the speed with which the contacts separate, the size of the contact points and the material and the voltage and the current load at opening.

If the switch is cheap a number of factors can make the arcing so bad that the contacts weld as metal is transfered. small point contact area instead of wider, flatter contact area. Poor plating, some platings reduce arcing. mechanical advantage (e.g. springs and leverage) that pulls the contacts apart faster. The BT3100 switches that failed regularly with the same issue you had (staying on) were allegedly replaced with switches that had stronger springs.

If you just filed the contacts you will probably have the same problem again after a while. what little plating there was to prevent arcing was probably removed by the arcs and the filing.

[cabinetman]

Kudos to you for fixing the switch. I know the elation you felt. I get the same feeling when I have the shot-in-the-dark fixes that turn out positive. I must say, that my tools that get used a lot, the switch is the first thing to go. It may be more prevalent with click type trigger switches versus slide switches.
.

[tfischer]

Wow. A good reminder to pull the plug (and not rely on the switch) when adjusting the belt & pulleys.

Congrats on your fix.

-Tim

[BobSch]

Wow. A good reminder to pull the plug (and not rely on the switch) when adjusting the belt & pulleys.

Congrats on your fix.

-Tim

Always a good idea, no matter what the tool. Well, except maybe a hammer.

[ironhat]

Leaning on Loring's rational about the likelihood of a future failure for this switch I know how let down I get when a repair goes awry. I'm just saying, have yourself steeled for that eventuality no matter the probability.