No announcement yet.

changing direction of dc motor (brushed)

  • Filter
  • Time
  • Show
Clear All
new posts

  • #31
    Originally posted by MattiJ View Post

    First time was already enough for me. Some dents on ways and in the chuck remind me of that. Luckily it was at relatively low rpm and chuck missed my toes.
    I had it unscrew when cutting in reverse, as I mentioned. It of course did not fly off and did not even drop (it could not at threading speed, and it only unscrewed because the tool held it from turning) .

    Stories about it spinning off and rolling around are presumably told by folks who have not actually had one unscrew. If you are cutting, it will unscrew and drop down if you do not see it in time, not spin wildly around the shop, because it gets stuck on the tool, and stops rotating.

    Even if it were to come off when "plug reversed", it would have to spin off through several fairly high friction turns before it comes loose, slowing down the whole time. And that is assuming that the drive can produce a shock severe enough to jar it loose, which is extremely unlikely at the higher speeds, where torque is low..

    Most any threaded spindle machine is an old machine and never had the ability to do that. The machines that might, generally are newer and have solid mount, Cam-loc or L series.

    No, Harry, it ain't happening on your old Southbend.

    Keep eye on ball.
    Hashim Khan


    • #32
      Originally posted by MaxHeadRoom View Post

      The one from KB also has a couple of terminals for a braking resistor,
      did i miss it? what "kb" switch is that?


      • #33
        Originally posted by J Tiers View Post

        With PMDC, reversing the armature polarity reverses the motor. With a wound field, field stays same just like the PMDC, so field polarity is not switched, as you note.

        But there is no need to switch OFF the wound field, any more than there is a need to switch off the PM. This is assuming you have a separate on/off switch.

        If you really want a center off type DPDT, (DPDT-center off) then you would be correct. But we seem to be discussing a straight DPDT which is for reversing only.
        That's a good point. I didn't think that a DC motor is likely being controlled from some sort of AC-DC power supply itself would be the main on-off switch for the setup. So then yeah, the switch we're talking about would be just for reversing the polarity to one or the other. And DPDT would do the trick.

        On the topic of throwing the chuck. Again I have to agree that having it run off and chase us around isn't likely. But I do recall when I first got my present lathe and there was no guard pin to stop me switching too far and go into reverse. The lathe has a big box of switch gear and apparently it is OK with instant reversing as I found out..... When I passed through OFF and dropped it into reverse there was an almighty "GROOMP!" sound where it came to a pretty close to instant stop on its way to starting up again in reverse. It didn't because I pulled the switch up pretty darn fast into the "STOP" position. It didn't drop the chuck off the spindle but it did jerk it loose a couple of turns. If it has managed to accelerate up to full reverse I'm pretty sure it would have dropped the chuck onto the ways and carriage though.

        A hole was threaded and a "gate screw" added so I need to withdraw the screw a half dozen turns to let the switch bar select reverse. My heart doesn't need to hear that sound again any time soon.....
        Chilliwack BC, Canada


        • #34
          Yours either IS one that can shock the chuck, or the chuck was not on tightly.

          On mine, it would not unscrew even though I did not even put it on more than just barely snugged. And, when on decently, I made many passes cutting 8 tpi before it came loose.

          Keep eye on ball.
          Hashim Khan


          • #35
            I did some internal threading a while back using the reverse cutting since it was up against a shoulder inside and it was fine. But it was a fine pitch and light cuts.

            To be fair I'm not someone that believes in tightening things until they snap off than back up a little. So I use about 15 to 20 lbs of push on a roughly 18 inch length to tighten the chuck. I'm using the back gears to hold the spindle during this so that's all I feel comfy with pushing out of respect and care for the gear teeth.

            I'm not keen to relive that one time but I've certainly never heard a machine do the equivalent of "OOMPH! ! !" like I'd just kicked it in the nards since... Nor do I want to hear such a thing any time soon in my shop again....

            When I did the back plate I took a fair degree of care to get the cylindrical part of the register size on for size. Of course since then I've learned that the threads will self center to a far better degree. That's fine.

            But it does give me the option of making a pinching collar at some point to see if I can clamp the portion down to bind the back plate down to the register. This idea came up some time back in a thread. I've not forgotten it but it's fairly well down the ToDo list. But if I can squeeze the back plate into grabbing the register bands firmly enough it would give me the option of a lot more reverse turning jobs. And that would be a handy thing.
            Last edited by BCRider; 07-30-2020, 03:22 PM.
            Chilliwack BC, Canada


            • #36
              It would seem relatively easy to add some kind of 'anti-unscrew' device to a chuck, so if this is an issue for somebody, the solution is a simple home shop project away.
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


              • #37
                It could be a simple clip as I show for my HF 9x20 lathe, or maybe just a pin or screw through the chuck into the spindle.
                Paul , P S Technology, Inc. and MrTibbs
                USA Maryland 21030


                • #38
                  The most common way to handle a motor with a field winding is to have one switch for ON and OFF and a second switch for FORWARD and REVERSE. The field winding is simply connected at the switched side of the ON/OFF switch.

                  Actually that is the most popular way for any DC motor. And often the ON/OFF functions are handled with two momentary switches.

                  Someone asked about finding a four terminal reversing switch. There are many sources. Places like Grainger and McMaster will have them but searching may not be easy. And you can forget those under $10 prices; think ten times that much. Also they do not provide complete specs so you may have to do some research for that. Electronic parts suppliers will also have them and their search facilities are usually excellent. I like Digi-Key and Mouser but there are others. The prices will be better than Grainger or McMaster, but not as cheap as on E-Bay or other popular internet sites. The advantage is you can get complete OEM specification sheets for almost every part that they have with a single click from the item description. I searched for "DPDT switch" on both of the sites below. Grainger returned a few dozen hits. Mouser had about 38,000 which I narrowed down to about 8000 toggle switches.

                  Here is just the first switch I found on Grainger's web site. Notice how limited the specs are. And there is no link to the manufacturer's site to get more on it.


                  And here is a switch from the Mouser site. This page gives a clickable link to the manufacturer's spec sheet.


                  These were just the first switches that my search turned up on each site. I did not make any effort to choose one that is suitable for any particular motor. I was able to narrow down the search on the Mouser site to those switches that did have DC ratings. That was something that Grainger did not have any provision for.

                  Originally posted by BCRider View Post
                  I might have missed it but most of the replies are calling for a DPDT switch. And that's fine if the motor uses permanent magnets. But if it has field coils in addition to armature coils I'm thinking that we'd need a double throw with THREE poles switch. Two DT poles to switch on in forward or reverse polarity for either the field or armature and one to simply switch same polarity power to the other winding.
                  Paul A.
                  SE Texas

                  Make it fit.
                  You can't win and there IS a penalty for trying!


                  • #39
                    Originally posted by dian View Post

                    did i miss it? what "kb" switch is that?
                    A bit late, but here is a Pic of the switch and resistor, there is no part No. on it.

                    Attached Files