Don’t get me wrong, I don’t detest the button-style or bake-lite foot pedals. I just don’t trust them.
Last night, I was sewing my very first applique. The tedium of the cutting and pressing finished, I thought I’d sit down to do some of the stitching. This is slow speed sewing, especially because I’ve never done it before, and satin stitches on my first machine would spark many colorful arguments between myself and the machine.
I made it about half way around the perimeter. Then, speaking of sparks…
I heard them. It was the weirdest thing I’ve ever heard in a sewing room. bzzzzzzt, pop pop…. a weird sound like rushing water, a hum that got louder, and then crackling noises coming from below my sewing table.
This was accompanied by some warm toes
Of course, I immediately unplugged the machine. The sizzling and humming sounds continued for a few seconds more.
I contemplated making a break for the kitchen. I’ve often figured the two best places to put an item if it may be on fire: the sink or in a cold oven. Both should be able to handle high temps without damage, and are reasonably isolated from flammables.
When the noises quieted down completely, I got the screwdriver out. Let’s see what happened.
Disassembled per my post: http://www.archaicarcane.com/singerattheraces/
Nothing appeared immediately wrong… nothing looked melted, nothing was blackened. Hmmm.
I removed the ceramic block (The two screws on the top of it) from the rest of the pedal. I removed the wires that lead to the machine, undid the screw and nut combination and removed that whole assembly from the ceramic rheostat. This will leave you with the screw in one hand, and the actuator portion of the pedal in the other. Put these and the spring that was under the plate aside. I usually put them with my screws in the lid or in a magnetic dish so they can’t get knocked out.
Hmm…. there’s a heck of a lot of black ugly carbon there under the shorting plate (the bent copper piece that’s attached to and lives under the straight plate we used to determine how much to adjust our pedal action). Deciding I needed to know what was under the carbon points (and figuring that the pedal had pretty much had the biscuit anyway) I further disassembled.
To do this, I turned the resistor upside down. This will make it look “U” shaped. The copper leads that both point inward are in our way. I found that loosening the screws 1.5 turns allowed me to rotate the leads out of the way, then tighten the screws up. This lets you get the leads out of the way, but leaves you less pieces to lose.
At the bottom part of the “U” there will be 2 small screws.
Here’s an opportunity to learn from my mistake(s):
Undo one screw, but do it with the screwdriver coming at it from the bottom (upside down “U” now), and gently hold the top of the “point” whose retaining clip you’re unscrewing. After a few turns, the screw will drop out. If you look inside the top, you will see that the retaining clip has been released and the carbon pile can be removed. When I did it, I had been removing the screw with the ceramic turned so that the screw was on the top, and wasn’t holding the point at all. All of the disks spilled through my fingers onto my workbench and I didn’t get to see the order in which things came out.
Note: This is called a carbon pile for a reason. There is the top point that you see, and under that, there are 50 small disks and one thicker disk that lives at the bottom of the pile that work together to provide you resistance capabilities.
You can at this point either take a pair of tweezers and remove the top “point”, then gently tip the disks out, or just dump them out gently onto a soft surface. Do only one pile at a time. Resist the temptation to pour out both piles! 🙂
Please be extremely careful with these disks, they are incredibly fragile. I broke one of them trying to clean it.
Why do we need “resistance” in the foot pedal?
In a sewing machine foot pedal, all that the manufacturer has done is broken the positive (or negative, but usually positive) connection to the motor and inserted this “resistor” into the line. The reason for doing this is to give us the ability to determine the speed that the machine runs at. I don’t know about you, but I’m not a good enough seamstress that I could sew at full speed all the time, as soon as the machine was plugged in.
Enter the carbon pile resistor. These “piles” of graphite disks are loosely in the column that we’ve just released them from above. This is true 1800’s technology. This was a good read about some of the background of carbon pile resistors: http://www.radiomuseum.org/forum/invention_of_resistor.html
As you press harder and harder on the pedal, the shorting bar gets closer, and makes some contact, then full contact with these carbon piles, “tightening” them up. This lowers the resistance, thus delivering more power to the motor, making it run faster.
But resistance can cause fires?
You may have read in your manual, or online, or been told: Never leave a sewing machine plugged in when not in use, it may pose a fire hazard if the foot were to malfunction.
You may have also heard, never leave your foot on the pedal (even a little) when you’re not actually sewing. It can produce heat unnecessarily.
The by-product of carbon pile resistance (or any type of resistance, really) is heat. Power that is unused becomes heat. This is why the pedal will get warm to somewhat hot when using it, especially with low speed sewing. More resistance means more power that’s converted to heat. Too much heat (overheating) will cause parts to fail, or possibly to light on fire.
Interestingly, carbon is naturally created from the graphite disks touching together and rubbing together. As the carbon builds up, it changes the resistance of the carbon pile. Here we should also note that carbon is flammable. You can see sometimes when a carbon brush is in use in an electric motor, that there will be blue “sparks”. It’s called arcing. This is the free carbon dust “burning”. A natural by-product of fire is also carbon. Seems like a self-propagating problem, doesn’t it?
Too much carbon build up and enough heat and you get what I had last night, a small contained fire.
At this point, I had 2 options to me:
- Put all these carbon disks into the garbage with the rest of the pedal and replace the pedal. This will likely be my ultimate choice, at least with my main machine.
- Clean the disks and reassemble to see if I can “fix” it That’s what I did tonight. This way, I can use the machine while I’m waiting for my new pedal to arrive.
Each and every one of those graphite disks needs to be cleaned. Here’s where we can use a little electric motor theory. Armatures tend to get a carbon build up, and the way we clean small amounts of deposits is with a soft pencil eraser.
Please be very careful, the graphite disks are wafer thin, and just as fragile. I broke one in the first pile. I opted to leave it out when reassembling, and it may have something to do with how fast the machine spools up from not running to quite fast.
I touched every single one. Gently hold the disk down, I did this with a full sized eraser, then use the pencil eraser to “erase” the carbon. It will likely not look perfect when you’re done. There may be some streaking and such, but as long as the darkest blacks are removed, this should be fine. When you’re finished with one side, flip the disk over gently and clean the other side too.
Once you’ve evaluated all of the disks, carefully reassemble the pile.
Insert the thicker disk first. I noticed that there were circular markings on one side that seemed to correspond with the bottom of the cylinder, so I put that side down. Then, 2 or 3 at a time, drop the thinner disks in. I did this holding the ceramic sideways (“U” facing away from me) and tilted downward at about 30 degrees. Then I laid the disks against the side of the opening and let go. This way, there would be no serious “drop” to damage them, but they’d be most likely to land flat.
You need them to land flat. Every last one of them. Once you’ve put all of the flat disks back in, put the point in and lay the retaining clip over top of it. Tip the ceramic back up straight (“U” pointing up)
Holding the retaining clip, insert the screw from the bottom and make sure it threads into the retaining clip.
Now do the same for the other “pile”.
I found that when examining both piles, one pile was burnt and the other one wasn’t. This may have been luck, or a sign I caught it soon enough that it didn’t migrate. I believe that left long enough this would have damaged the motor / caused a fire.
Once both piles are reassembled, insert the spring, the plate with the shorting plate and reassemble the linkages to the ceramic. Don’t forget to turn the copper leads back around.
Tighten the metal plate, with the shorting plate under it, until it’s just recessed from the top of the ceramic. I have seen mention of 1/8″ for the amount it should be recessed, but in practice, I think this is a guideline you can use, but if the shorting plate is bent enough, this may be too much, and cause the issue above. I have recently been just adjusting the plate to be slightly recessed. I am more comfortable with the heat this produces versus what I experienced last night. It may be that the two were unrelated, but I don’t really want to take that chance at this point.