Combined FWD/REV switch plus potentiometer for mill power feed.

Hi everyone,

I have a small motor on it's way to me via Ebay (Scania 24Volt DC wiper motor) which will be part of a power feed for my WM-16 milling machine. I will control the speed with a PWM circuit which I already have, but I'd like to source a combined Forward/Reverse lever type switch and potentiometer all-in-one unit to mount on the front of an enclosure. As in, the directional lever is mounted radially around the potentiometer shaft?

Does anyone know what such a switch might be called?

Many thanks,
Ed.

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Ed ... Not sure if this helps, but; if it were two pots, the unit would be called 'dual concentric'

MichaelG.

The power feed manufacturer Align use a combined control you describe....

You could make your own, a metal sleeve through which the post spindle passes, the sleeve in turn operated by a lever and operates two micro switches to control direction ( by switching the motor polarity).....basically.....needs a bit of refinement to be effective....food for thought?

Mark looks to have beaten me to it but this one has a pot and switch

Have seen what John suggests work fine. Use a long shaft potentiometer, preferably one of the good quality metal bodied cermet or cermet over wire types as these can be got with solidly mounted shafts up to around 2" long. Spendy, probably around £15, but the lower priced mostly plastic types are a bit weak in the body for machine tools.

Forward - Off - Reverse selection needs an effective detent. I have used a simple drum with notches and plain V3 micro switches. Switch button falls into the notch to hold position. OK on the lab gear I use it on but not, I think robust enough for a machine tool. Bridgeport use a tension spring and over centre lever which is very effective and not impossible to re-work into home shop production form. Tried the ball and spring in a threaded sleeve thingies and didn't get on that well. Seem to be quite sensitive to detent hole size and shape. After a couple of tries got to "Works OK, I suppose" but never got the nice warm fuzzy well engineered feeling.

Ordinary button micro switches should be fine but the roller lever type seems more appropriate for an engineering job. Never used roller and lever with notches as a detent system but it might work well if you use the long lever type with enough over travel to put the lever in bending to give a leaf spring effect increasing detent holding force. If you try this note that the switches have to work "backwards" with the normally closed connection being the on state. Arrange the lever of the on switch to be clear of drum or cam whilst the other one is hard off to give the detent. Use both for the off position detent so its more positive.

Clive.

In the original post Ed CLEARLY says " a combined Forward/Reverse lever type switch and potentiometer all-in-one unit to mount on the front of an enclosure"

Nothing about two pots.

John Rudd has come up with the best explanation but on the Align unit this switch assembly isn't an actual assembly but built up from parts that fit into the main die casting, to whit a lever operated cam that selects 3 micro switches, one for forward, one for reverse and a small one to energise the control board.

The pot is on a long shaft running concentric thru the lever shaft and is completely separate.

Just for Michael's sake, here are the manuals.

More Bumph to store on the hard drive.

However these power feeds have one serious problem that is not easy to resolve.

Because the pot is concentric with the lever shaft after a while as the pot wears and free's off, continually selecting forward and reverse alters the post setting so it has a tendency to alter the speed when you are least expecting it.

Delving inside, because of the complexity of cramming everything into a small space it's impossible to fit a friction brake to the pot shaft.

You can't fit one on the outside easily short of a brake bracket off the pot shaft but clear of the FWD / REV lever.

The X axis power feed on My Warco WM40 has now got to this point and has cost me a few small cutters and is one of the jobs I need to look at shortly.

So if you do design and build your own make sure you leave a gap between the FWD / REV lever inside and the pot to fit a friction brake of some design.

.

But he needs a two position switch if using a switch for forward and reverse

.

John,

For pity's sake please give it a rest

I only mentioned the two pots to introduce the term 'dual concentric' ... and thereby the general idea of 'concentric' being the word to search for.

... If you read Ed's opening post it is fairly obvious that he was struggling to find the right word.

MichaelG.

Thank you guys, I must say I do like the sound of John Rudd's solution. I'll need to machine a dog clutch anyway to engage/disengage for manual positioning so a bit more work on it to make a switch shaft and cam should be do-able. I've been watching the video on YT by Stefan Gotteswinter for his power feed. The guitar switch might work if it has multi-position rotary detents, some of them are push/pull switches.

I'd like somehow to link the power to the power-feed motor via a relay which is energised by a connecton from the spindle control panel, so if the fuse on that blows (as it has done a number of times with some heavier hobby cutting), it will also take out the power-feed. But I need to think about this because if the fuse does blow I might forget to put the power feed in neutral and it could move the table when powering up.

The Align type switch looks exactly what I'm after operation-wise, I didn't realise it might be just a basic mechanical spindle with limit switches.

JS: you lost me a bit there John sorry, I'm not sure what a friction brake is, still learning . Is this something mechanical to stop the pot shaft moving to an unexpected position because of parts moving around? or power resistors that soak up current when the feed-motor stops?

Certainly lots of things to consider.

Thanks again,
Ed.

Ed,

Sorry, friction brake is something mechanical bearing on the shaft to stop it moving under vibration or being carried along by another moving part.

Could be as simple as an elastic band round the shaft and fixed onto a screw.

I just wanted to make you aware that the dual concentric shaft idea of a lever feed and pot does have disadvantages if you don't factor in a brake.

Ah I see now, thanks for the clarification.

Ed.

Hi Ed

It would be simpler and long lasting to use a DPDT with centre OFF for driving and reversing the motor combined with a seperate pot for the speed control function.

Emgee

Thanks Emgee, I've seen DPDT's used for power feeds on youtube but I would prefer a lever. To me, a lever just seems to require a more definite action and is a better visual reference for direction of travel.

Regards,
Ed.

Ed Duffner; If you are interested there is a fairly simple electronic solution using a single pot controlling the inverting input of an op-amp. The non-inverting input being biased to mid-rail voltage and an appropriate amount of negative feedback. When the pot is at its mid position there will be no output. Moving away from the centre point will cause the op-amp output voltage to move either towards the negative or positive rail depending upon the direction the pot is moved.

Edited By Brian Oldford on 07/03/2017 22:56:10

Probably the nicest way to induce a bit of friction under the knob to keep things in place is to trap a greased O ring between the knob skirt and the panel or support bush. Need to use a collet fastening knob so you can adjust the position to get the right degree of nip. Only works well with a good quality metal bodied potentiometer as there is inevitably a bit of tension on the shaft. Probably best to have a relatively large O ring, 3/4 - 1" diameter maybe supported centrally by a grooved washer or bush.

The official nip system is an O ring or nylon washer trapped under the head of a special nut screwed onto the potentiometer mounting thread. Quite expensive, not universal so usually you need to get the one made for your potentiometer and needs an extra lock nut to keep a preset drag. Generally a right faff as a drag device. Really made as a locking device to use with a short shaft screwdriver set pot.

A nylon sleeve nut with an extension drilled to lightly grip the potentiometer shaft screwed onto the mounting thread might work. Getting the grip just so could be difficult. Perhaps cutting saw slots across the end of a slightly too tight bush making a set of flexy fingers rather than a full bore might work.

Making a nice job of this won't be a trivial exercise. Back of the envelope drawing won't cut it. You will need to design it properly if its going to be nice to use with just the right clearances not to flip and flop everywhere. i started one before deciding life was too short and opting for separate lever and pot. My basic plan was to run the direction selector lever on a deep bush, about 1 1/2" diameter, with a flange to trap it against the mounting panel. Countersunk screws at the back of the bush to hold it on and a thread in the middle to screw the pot into. Microswitches on the back of the panel. Projections on the lever going right through the panel to operate the switches. Which was about where I gave up and went for something simpler.

Clive.

Before anyone jumps down my throat I know this isn't what the OP asked for, but when did that stop us. I find three of momentary push buttons ( left/right/stop) with a pair of latching relays better than toggle type switch, as you can then fit microswitches to the table to stop the drive. You can tie the stop button in to the machine stop button as well so that when you stop the spindle the table stops as well, which is handy. If you want the circuit send me a pm. Having said that on the bucket list is a lever operated set of switches, left/right/down(stop) which would be neater, but a lot of effort. The joysticks for early computer games could have been adapted, but they have all gone to landfill by now, and weren't all that robust.

Brian, Clive and Duncan, Thank you chaps. The electronics approach is a bit beyond my skill level at the moment, I do have some very basic electronics knowledge (used to be a sparky). I will probably be using Fusion 360 and Coreldraw X3 for working out designs etc. Thinking about it there's probably an Arduino solution to this but that's not something I into, at least for the moment.

Duncan, thanks for the kind offer of a circuit diagram, I will probably be ok though, I think.

Cheers,
Ed.