Limit / reverse switch

[Bsmith]

Is it possible for me to get like a limit switch but instead of it cutting the power, changing the polarity?
I have fitted a 12v wiper motor to the x axis of the milling machine with transformer and speed control etc, works well, but now thinking if I could obtain a switch to reverse the motor at the end of cuts it woukld be better still.

 

[jmp49]

You can get limit switches in DPDT (Double pole double throw) in machine grade coolant proof housings made for industrial automation and wire them up to swap the DC polarity, or use the switch to drive a relay to do the same if it draws too much current for the small contacts in a limit switch (likely as they're designed for signalling more than switching heavy loads normally).

Simple diagram to show dc reversing with a dpdt switch. You can use a dpdt relay and wire the double throw limit switch to move its position so the switching current never passes through the limit switch.



How will your wiper motor cope with suddenly being asked to reverse direction with no time to slow down? I guess they do this by design, but probably more inertia in your machine feed than a wiper blade.

 

[Agroshield]

Simple diagram to show dc reversing with a dpdt switch.

The point about instantaneous reverse being harmful is good, but there are some difficulties with the rest of what you say.

How is that diagram relevant to his situation? The logic is incorrect.

When the mill table is in the middle of its travel, nothing is touching the switch. At some point it drives into the switch and triggers it. Then, assuming it does reverse, it drives away from the switch and again nothing touches the switch.

A standard limit switch is NC (so is fault-safe) and goes open when triggered, cutting the power. To drive off the limit switch, there is normally a manual override button that re-enables the motor, but only in the revese direction, so it can back off the switch.

 

[dieselsamurai]

Ive had a wiper motor as a power feed motor for about 4 years now doesnt seem to bothered so far with instantaneous changing direction

One way to use NC switches on limit stops is with a rectifier

 

[Agroshield]

One way to use NC switches on limit stops is with a rectifier

That circuit will not reverse the motor when the limit switch is triggered, only switch off the power to it.

To change its direction, you have to operate the DPDT switch with your finger.

The OP wants something akin to driving a car at a wall and the instant the front bumper touches the wall, the car puts itself into reverse and backs off with no driver intervention.

 

[Pete.]

Is it possible for me to get like a limit switch but instead of it cutting the power, changing the polarity?
I have fitted a 12v wiper motor to the x axis of the milling machine with transformer and speed control etc, works well, but now thinking if I could obtain a switch to reverse the motor at the end of cuts it woukld be better still.

There are enough reasons why you don't want to have a limit switch automatically engage reverse. That's why you never see it on a milling machine. One day you'll hit the limit, invoke reverse and trash the part, the cutter or the machine.

 

[Agroshield]

If a person did want to build a circuit of this type (and I agree it is undesirable), the way to do so is to start with some state (logic) diagrams.

In this case, I think you need five state diagrams:

1. Moving to the right, limit switch not triggered
2. Moving to the right, limit switch triggered
3. Stationary, limit switch triggered
4. Moving to the left, limit switch triggered
5. Moving to the left, limit switch not triggered

Come up with some physical electrical components and wiring that correctly implements the above (and remember you need the same at the opposite end of travel) and that is what is required.

 

[minimutly]

This needs 2 relays - one to power up the motor, another 11 pin to select direction - you can actually buy these flip flop relays, I haven't seen one in a few years now though but am sure they still exist.

Edit to add, as usual @Pete. Is right, I wouldn't recommend it, just wire two stops at the ends and a manual reversing switch.

 

[hotponyshoes]

After reading the comments above, I'm not saying it's a good idea to do it, but it can be done with basic components.
I've done the same thing on horseboxes so a powered locker floor comes out when you press a button then goes back in again when you press the same button.

I'll find the diagram.
Do you want it to go both ways?
Ie, from right to left, back to right, back to left repeatedly..
Or, just go right to left once?

 

[hotponyshoes]

All the wiper motors I have taken apart (mostly older stuff) don't reverse.
The wiper arm is on a cam drive so the motor always spins in the same direction.

Used to remove and relocate the drive pin on the cam so the parking position was vertical in the middle of the screen like a touring car

Basically a regular dc motor behind the gearing though so I can't see why sudden reverse would be any worst for it than any other motor?

I guess the mill table stops moving pretty much instantly when you remove power from your existing motor?
If not I can see you having issues with the motor acting as a generator and reverse feeding the speed control?

 

[minimutly]

There will be a thrust button on the end of the motor in the wiper box, and just a plain washer at the other end - this might get a little hot unless addressed? As said above, wiper motors do not stop and reverse..

 

[dieselsamurai]

That circuit will not reverse the motor when the limit switch is triggered, only switch off the power to it.

To change its direction, you have to operate the DPDT switch with your finger.

The OP wants something akin to driving a car at a wall and the instant the front bumper touches the wall, the car puts itself into reverse and backs off with no driver intervention.

Ah i see sorry should have read it properly I would not want to engage other direction straight away Ive never seen that on a milling machine , usually you take a pass feed in a bit more then repeat in the other direction . I guess it would be handy if you could only mill in one direction say the machine had wear and couldnt cope with climb milling but you would want it to cut hit the stop reverse feed then hit the other stop then let you feed in a bit more before repeating

 

[Beanwood]

All the wiper motors I have taken apart (mostly older stuff) don't reverse.
The wiper arm is on a cam drive so the motor always spins in the same direction.

I agree. An electric window motor though does reverse....

 

[Robotstar5]

There are enough reasons why you don't want to have a limit switch automatically engage reverse. That's why you never see it on a milling machine. One day you'll hit the limit, invoke reverse and trash the part, the cutter or the machine.

How do they do it on surface grinders?, the one we had at work would shuttle side to side on it's own, you just set the limits of the stroke.

 

[not done it yet]

Shirley one should be adjacent to the machine while it is operating? Makes automatic reversing pretty well inappropriate/unnecessary.

 

[Pete.]

Surface grinders are a different breed you run the wheel off the part so it's not cutting then the table reverses. It's normal to have an auto-reverse on those it's often done on a hydraulic trip lever. Once the table starts to be driven back the backlash is all taken up but on a mill if you reverse the table when cutting you can end up with the cutter driving the table in the backlash zone and that usually sounds like bump, bump, bump, bang, f*$$£ !t.

 

[Pete.]

Shirley one should be adjacent to the machine while it is operating? Makes automatic reversing pretty well inappropriate/unnecessary.

A 5 minute cut is 5 minutes wasted if you're standing there watching it. A couple of dozen of those is an afternoon gone. I like to do stuff with my time if the machine is auto-feeding.

 

[Agroshield]

...comes out when you press a button then goes back in again when you press the same button.

That is not what is being asked for.

The table is already moving in one direction. It drives into a switch and while still contacting that switch stops, and then reverses off that switch.

There is no press and then press again. With a knife edge contact, you might activate the switch and the momentum would drive past it to release it. But then how do you achive the second press?

How do they do it on surface grinders?

A lot of machine tool reversing is mechanical rather than electronic. When something touches the trip dog, it disengages the drive and re-engages it such that the motion is reversed. Whatever powers the motion continues to revolve in the same direction. Canonical example flat belt drive planer - the overhead lineshaft always rotates in one direction. The belt shifter transfers drive from an open belt to a figure of eight belt so the table reverses.

What the OP wants is do-able, but it needs a control system with more than simple mechanical switches. Perhaps you might drive onto the limit switch, which would kill power to the motor and start a timer. Once the timer expires, it would fire up the motor in reverse. The challenge is that it might also need a one-way operating limit switch (like a ratchet) so the switch is not triggered again in the reverse direction.

On planers (again), which are also a machine tool that needs to reverse, they had Ward Leonard controls, which would reverse the drive motor direction when the mechanical dogs were tripped by the table at each end of the stroke. But they are not simple on-off switches.

 

[hotponyshoes]

That is not what is being asked for.

The table is already moving in one direction. It drives into a switch and while still contacting that switch stops, and then reverses off that switch.

There is no press and then press again. With a knife edge contact, you might activate the switch and the momentum would drive past it to release it. But then how do you achive the second press?



A lot of machine tool reversing is mechanical rather than electronic. When something touches the trip dog, it disengages the drive and re-engages it such that the motion is reversed. Whatever powers the motion continues to revolve in the same direction. Canonical example flat belt drive planer - the overhead lineshaft always rotates in one direction. The belt shifter transfers drive from an open belt to a figure of eight belt so the table reverses.

What the OP wants is do-able, but it needs a control system with more than simple mechanical switches. Perhaps you might drive onto the limit switch, which would kill power to the motor and start a timer. Once the timer expires, it would fire up the motor in reverse. The challenge is that it might also need a one-way operating limit switch (like a ratchet) so the switch is not triggered again in the reverse direction.

On planers (again), which are also a machine tool that needs to reverse, they had Ward Leonard controls, which would reverse the drive motor direction when the mechanical dogs were tripped by the table at each end of the stroke. But they are not simple on-off switches.

Not what he asked for but the switching will be the same.
If I replaced my momentary switch with a latching one (or just held the button) it would come out, change direction and go back in and so on..

 

[Wedg1e]

On the face of it what is needed is a flip-flop: one press starts movement in one direction, next press reverses, next press returns to forward etc.
Electronically it's dead simple, a couple of transistors. The circuit stays in whichever state until the next press (i.e. while the table is traversing).
However, one point that's been overlooked here as far as I can see is the feed. Table traverses, takes a cut, gets to the end, reverses and goes back to the start... but unless someone's there to shift the Y or Z axis all the cutter will cut on the next pass is fresh air.
In which case what would be more useful would be a high-speed return.
Assuming you did use a flip-flop you'd probably also want a short delay to allow the motor to spool down before it reverses - and that of course might need to vary with the traverse speed.