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my mill just let out the magic smoke :(

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  • my mill just let out the magic smoke :(

    finally made keys for the vise, trammed it in (thanks Stepside!), got the ER25 collet chuck and drawbar sorted out so it would fit, slotted 4 holes in some alu right angle brackets (for the roller stand) then "kapow!" and a flash of light came from the back of the machine. After flipping the garage breaker, then the house breaker when that tripped, then unplugging the mill and then flipping the garage breaker I had a look around

    terminal block screwed onto the back

    it is clearly toast. Any ideas why? I was running at ~1700rpm with what looks to be a blunt 3/8" rougher (lots of material push out instead of chips) and a 1/4" DOC, so current draw was probably high but not enough to trip the garage breaker (20A). Screws mounting the box to the mill look rusty, so perhaps some kind of dry/ rusty joint = heat and then light/ smoke?

    Looking for a replacement now - any tips, especially regarding current rating? The motor is a 1 1/2hp, 13A, 110VAC job, but I've read that motor start up = higher current, so is a 25A or 30A 300VAC/DC strip going to be enough or should I go up to a 65A current rating?

    I had a look around inside the mill at the terminal blocks/ transformer/ breakers (not flipped) in there and nothing looks untoward. I can post up pics if it helps.


  • #2
    Loose/corroded connection on the terminal strip.
    Judging by the amount of rust it has been under salt spray test and they forgot to flush out the salt water after shipping it on open deck from china to us..
    Location: Helsinki, Finland, Europe


    • #3
      Yep, resistance in the connection.


      • #4
        Have you looked at the motor??? Something caused that much heat. Any chance metal chips got into the motor. I would say you are showing the result of something else going on.


        • #5
          Looks like maybe L1 and/or N terminal was loose and vibration finally caused enough movement to short them out. Or if this is a 220 machine L1 and/or L2 were loose. Did you wire those up, or were they already wired up? I don't see any reason why that would happen unless the terminals were just loose and shorted out. The good news is you can probably just replace that terminal block with another one and you should be set to go -- I doubt anything else was affected. Make sure your using the correct size breaker too in your panel.


          • #6
            Yep high resistance connection due to corrosion or loose connection will get hot and eventually burn. I would see what wire goes where on the terminal strip, remove the wires strip and clean. Twist together wires like they were connected on the terminal strip and solder them together. Then cover with a wire nut and tape.


            • #7
              My thought is that the block was already melted before they shorted out. Resistance in a connection causes plenty of heat.


              • #8
                Now is the time to replace that motor with a 3 phase one and add a VFD for speed control, FWD/REV, soft start and much more. I did my Grizzly mill and what a difference. Just dial in whatever speed you want. The VFD also provides protection.

                The Auttomation Direct VFD. The large box is optional

                The VFD controls.
                Grantham, New Hampshire


                • #9
                  And how much did that cost?

                  Originally posted by CPeter View Post
                  Now is the time to replace that motor with a 3 phase one and add a VFD for speed control, FWD/REV, soft start and much more. I did my Grizzly mill and what a difference. Just dial in whatever speed you want. The VFD also provides protection.

                  The Auttomation Direct VFD. The large box is optional


                  • #10
                    Hey Matt,

                    The first comments are correct in my estimation. The rust on the motor lead connection on the left is the cause. If you look at the way the plastic melted to the right it was just the one wire terminal that was the start of the fault.

                    To fix it, the easiest is to use regular wire nuts and do away with the terminal strip all together. Since the motor leads coming in the back of the box have marginal insulation, I would remove the box and check to see if there is any slack in the wires to get to solid insulation, if short get butt splices that crimp on and add a length of 12awg to the affected lead. Put it all back together and wire nut the cord wires that you recut to the motor leads and you should be ready to go.

                    I would be pretty confident that the motor itself is OK. You could use a VOM on resistance and check for a short to the metal casing between each lead should read OL and then take a reading between the 2 leads and you should get a low ohm's reading in the single or low double digits.

                    I hope this helps. I'm sure more will be posted that can be of help as well.

                    Mr fixit for the family


                    • #11
                      thanks all! Yes, rusty or loose connection was my thought too, with some vibration (I hate cutting slots) and extra current (blunt endmill) causing the melting then shorting. Motor is wired for 110V, although this is just the line in box - all the forward/reverse, 110/220 etc stuff is inside the column. It also came this way and I didn't think to check it, lesson learned.

                      I don't think that metal chips in the motor are a problem - looks clean and the motor is hung upside down under the other end of the head (it's a Clausing 8530 clone). I didn't test for excessive heat, although chinese motors seem to run hot if my bandsaw is any indication.

                      Didn't think to solder and nut them, that I can do easily. Any cons vs. using a terminal block? Just saw Chris' post - yep, plenty of wire on both sides to cut it back to fresh, strip and start again.

                      CPeter - lovely looking mill! Treadmill motor and controller is on the cards for the mill. I've put them on my drill press (formerly my mill too!) and lathe and they're a wonder. Can't wait to put one on the mill, but I have a long list of other projects I have to get through before that happens. I'm sure everytime I use it I'll be thinking "wish I could change speed without fiddling with the belts". As some point that'll get annoying enough that I'll start on the conversion, at which point all the current circuitry will be moot.
                      Last edited by mattthemuppet; 11-20-2017, 12:15 PM.


                      • #12
                        +/- $200 for the VFD, $20 for the remote control and 3 phase motors are cheap. You can buy a new 2 hp three phase motor just like that one for about $200. I did the same thing to my lathe ( and 10 other machines). I bought that Marathon motor on Craigslist for about $100. The remote switches and potentiometer came from EBAY and actually total cost was less than $10. For the cable to the remote I use computer ethernet cable. You need six conductors. It is so simple that the on first one, I call tech support (they are in the USA and speak English) to verify that it was as easy as it looks. The programming is easy, too.

                        The switch and pot

                        The Wiring

                        The remote in a handy box
                        Grantham, New Hampshire


                        • #13
                          I have used both AC & DC motors and controls and 3 phase and VFD is much superior IMHO. Instant reverse is nice for tapping.
                          Grantham, New Hampshire


                          • #14
                            CPeter, thanks for sharing that; it's inspiring and I didn't realize it's that easy. Probably a stupid question, but - you're using a single phase 220V input to the VFD, correct?

                            Is there a good resource for those like myself who are not really electrical guys to learn about this kind of setup? I'd like to set my Bridgeport clone up like yours at some point, but just never really knew exactly how to do it.

                            Instant reversing is one of the big draws for me, since I tap in the mill often.


                            • #15
                              For the moment, I would not fiddle with the VFD.

                              I concur with the high resistance connection diagnosis. Fix that issue and you can be back in biz quickly. There is nothing wrong with the box, as far as I see, so yes, you could use wire-nuts, or another terminal strip, just as you choose.

                              That failure is a really good reason NEVER to use a modern style thermoplastic terminal strip. Use the Bakelite type, they do not melt (although they do char). Your failure seems to have been as follows: Bad connection heated up the metal, the thermoplastic softened, and allowed the wire tension to push the metal connections in contact with each other, which shorted the input and opened the breaker.
                              CNC machines only go through the motions