It's easier to beg forgiveness than asking permission......

Moderatemixed

Moderatemixed

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Hey there "all knowing ones". I have done something, and it seems to work, so now I am asking if I should have done so and can anyone tell me if I have done anything grossly outside of the safety rules...... So here goes.....

I found a 6 X 18 magnetic chuck at a machine shop that was going out of business. A total basket case, cord cut, rusty.... a total mess; I paid $100 Canadian (So like $75 USD). Turns out it is a Walker multi coil chuck. So I start to read all about these things and learn that they are DC. The cord that was on it had a drum switch and was 2 prong, but if it is supposed to be 115V DC something must be missing; my though process. Turns out I need a $600 control box. "There's got to be a better way".

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I took the whole chuck apart (in true newbie fashion) stripped it, repainted it and put it back together; standard stuff. Here's the cool part. If it is supposed to run on DC then why not just incorporate a bridge rectifier, a $5 item? So I drilled a hole in the box that attaches to the side of the chuck and installed a toggle switch, just an "on/off". Then I ran a new cord into the box (AC in) through the switch and into a 25A bridge rectifier before connecting it to the DC coils. By this point my 11 year old son was involved and we were both of the opinion that we were going to blow ourselves up..... so might as well plug it in and see, lol. We plugged it into a Variac (variable transformer) and hit the power switch. We turned up the Variac and nothing happened. We then realized we hadn't turned the switch on the chuck to the on position, so we did....... Nothing, nada! So I rethought the process...... AC in through the Variac, through the switch into the bridge rectifier, DC out. This chuck should be attracting metal like "flies on stink"; where was my thinking flawed? It was my 11 year old Alex who diagnosed the issue. After a good laugh we plugged the Variac into the wall. The chuck worked amazingly.

So what I learned is that the chuck should have a reversing switch somewhere in the circuit to reverse the current and release the magnetic force quickly. After turning it off, I find the field is weak enough in about 10 seconds to move the item being held so I am not going to modify it.

I do have a few questions though. In my research I determined that most of these chucks need a control box ($600 +). I control my chuck with a $5 part, so what am I missing? Too, when I ran the wattage formula it turns out I am drawing 1A, so my 25A rectifier will be good forever (or at least I think so). Can I use the chuck on my milling machine safely or are these chucks strictly for surface grinding (hopefully not)? And finally, have I done anything dangerous because it seems to work fine and be remarkably effective?

I stand ready now at the firing line awaiting the barrage of criticism which is what I expect given the subject line of this post. I prefer to think of myself as clever. Thank you all in advance for your comments.


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As a long time electrical semi-wizard, I think you did well with the chuck.
As a machinist, I don't know how it would work on a mill, but suspect that if it holds the work it should be ok.
I also assume that the chuck would attract lots of swarf.
 
You made a controller that works. Great! That is how a electromagnetic (EM) chuck works. If you saturate the coils with too much wattage (power) the chuck will get hot and you will know you are driving it too hard. Check often for that until you get the idea of how your controller is working. The $600 control adds the variac or equivalent and an automatic system of demagnetizing the chuck. It switches the current forward and reverse as the voltage is reduced. If you look carefully, during shut down the "rat fur" on the chuck will lean one way, then the other, back and forth, less and less until you can easily lift your part off and wipe the rat fur off the chuck. I would not have the switch on the chuck, redundant, and easily damaged, and might contribute to an electrical hazard. A milling machine pushes against parts harder than a surface grinder that is only doing a thousandth or three or much less cut at a time. The mag chuck will pull more or less on different size, shape and thickness parts. It can work, but you will need to go easy at first and learn what you can get away with. Flying parts are in poor taste in a shop...

Edit: It helps to block the parts in with parallels and other pieces of flat steel, especially important for small, thin, and irregularly shaped parts. Parts that do not span at least two poles will not be held down well, so make sure to block them in well.
 
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Have you looked at the price of a new Variac lately? They are over $400. I was looking to buy replacement brushes for several that I have and was surprised at what they wanted for a simple brush.

If you wanted to demagnetize the surface (and workpiece) quickly, you could switch over to AC and decrease the Variac voltage to zero. Basically the same principle as a demagnetizer.

One word of caution. A Variac is an autotransformer and is not isolated from the mains. Make sure that the chuck is properly grounded and it wouldn't hurt to have a GFIC in the circuit.
 
I bought my Variac on EBay. I paid $45 for it, but it took some time; just had to be patient. Thanks for the guidance and I will run it through a GFI to be safe. Cheers


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