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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I'm just drawing up the layout of the main switchbox, and I've decided, now that the circuit has 24 volts as well, it makes more sense to put the emergency stop button in the 24V circuit before the forward/reverse switch.
This would mean the 'stop' button would cut the 24V supply to the relays, which would release, tripping the NVR switch as well, due to the 'A1 (fifth) terminal of the NVR switch being connected to the live 'motor run' feed after the relays.
I should have the relays on Monday, if all goes to plan.
EDIT: Here's the revised circuit diagram, with 24V 'stop' switch, and a first drawing of the layout of the main switchbox.
I'll probably add some fuses to the 24V circuit, and I want to improve the layout a bit before it's finalised.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I've now divided the circuit into two. The right hand side below shows the 'front panel' of the main control box, with some terminal blocks mounted on the reverse, and the left hand side below shows the layout inside the box.
It's still 'work in progress', but I know I now need a box around 200mm by 200mm by 75-100mm.
I'll tidy the drawings up a bit more, and I now have most of the components.
I was looking at some plastic enclosures, but I don't think a modern plastic control box is really suitable for this lathe. I'll see what else I can find.
EDIT: just noticed I've not put wires 'L' and 'M' into the laft hand side circuit. It was a late one last night. I'll sort it later.
If anyone can see any other issues, or has any suggestions, I'd appreciate it.
I've still not put any fuses in. I also left out the live run wire from the left hand relay to the motor, I'll add that too.
This is still 'work in progress', the final drawing shouldn't take much longer, though.
Registered Member #2390
Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381
I might just be splitting hairs here but here goes. To me it looks like your e-stop circuit would work just fine. That said, typically when i work on control system designs we would use a safety relay or safety contactor. Your e-stop will drop out the 24v to the coils on your reversing circuit (which should be electrically or mechanically interlocked). Since your motor is not a 3 phase unit, it might be better to add a 4 pole contactor with a 24v coil as a "safety relay". 4 normally open contacts, 1 for your 24vdc feed from the supply (before any devices, excluding your led e-stop indicator and the switch itself) and 2 to break the ac that feeds the reversing setup. You can add redundancy to the contact blocks on the e-stop button if you chose to but they rarely fail. The reason i suggest the additional relay is just incase you get a stuck relay in your reversing setup. If you had a brake wire to the motor you could also use the relay to control that. You would just need the correct contact arrangement on the relay. That said, what you have, again, should work fine. Where i work overkill is pretty much standard when it comes to safety.
Edit: Give ebay a look for hoffmann enclosures, you can usually find a box and panel pretty cheap!
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
doctor electrons wrote ...
I might just be splitting hairs here but here goes. To me it looks like your e-stop circuit would work just fine. That said, typically when i work on control system designs we would use a safety relay or safety contactor. Your e-stop will drop out the 24v to the coils on your reversing circuit (which should be electrically or mechanically interlocked). Since your motor is not a 3 phase unit, it might be better to add a 4 pole contactor with a 24v coil as a "safety relay". 4 normally open contacts, 1 for your 24vdc feed from the supply (before any devices, excluding your led e-stop indicator and the switch itself) and 2 to break the ac that feeds the reversing setup. You can add redundancy to the contact blocks on the e-stop button if you chose to but they rarely fail. The reason i suggest the additional relay is just incase you get a stuck relay in your reversing setup. If you had a brake wire to the motor you could also use the relay to control that. You would just need the correct contact arrangement on the relay. That said, what you have, again, should work fine. Where i work overkill is pretty much standard when it comes to safety.
Edit: Give ebay a look for hoffmann enclosures, you can usually find a box and panel pretty cheap!
What you are basically saying is I should have another relay 'in series' with the feed to the coils on the other relays, in case a relay sticks, as it's very unlikely that both will fail to open simultaneously?
Seems to make a lot of sense. Another three pole relay, but what happens if the first relay sticks?......nothing happens......I don't quite follow.
What am I missing?......do you mean that just one contact in a three pole relay could stick, and if it was the 'live feed' the motor would stay on, but an additional three pole relay would still kill the motor as long as one set of contacts opened?
EDIT: Wiring up 'idiot lights' to tell you when one contact has stuck, but it's still 'apparently' working ok sounds like a real headache.
EDIT: I did wonder if I needed a relay in the E-stop circuit
Registered Member #2390
Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381
Sometimes i do have a little trouble explaining ;) The additional relay would break the dc to the two coils as well as the motor power to them.
E-stop pressed would kill 24v system wide (except the e-stop circuit) and break the mains feed to the motor. (the feed to both motor relays)
The stuck relay: Where i work one of our standards is to have backup safety for relays or contactors that see a lot of cycles. The logic is that the more they operate the better the chances of a stuck contact, we have seen it happen quite a few times. More so with dc circuits as they tend to arc more, but it does happen with ac motors that aren't driven by a vfd.
The number of operations your safety relay would see is going to be incredibly less than the number of operations the motor relays will see over the life of the machine.
A note on safety relays: Normally a safety relay will be a red color. The contacts inside are different than normal relays, they are whats called "positive displacement" with most newer high end components like Allen Bradley for example, all of their contactors now come with positive displacement contacts regardless if they are labeled safety or not.
Normally in our e-stop circuits the e-stop button will drop out the safety relay and require pressing a "control power on" button after the e-stop button is reset. The control power on button pulls the safety relay back in (or turns it back on). One coil is all that you need to control. The safety relays will kill power to areas of the machine that would be considered a hazard, such as motors, cylinders, etc. Motion or any operation that needs to be disabled to render the machine safe. At the same time, using the safety relay in that manner allows you to keep power on in areas that are not a hazard and would be required for fault handling, like a plc or hmi operator interface.
Hope that makes more sense!
Edit: One relay to rule them all!
Edit: Quote:What am I missing?......do you mean that just one contact in a three pole relay could stick, and if it was the 'live feed' the motor would stay on, but an additional three pole relay would still kill the motor as long as one set of contacts opened?
Yes! As long as the additional relay operates the motor relays could totally fail closed and you would still have a safe condition. Coils can also short, and cause a failure to open as well as contacts sticking or welding themselves.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I think it makes sense to add a safety relay on the E-stop circuit.
I'm just puzzling over how to add indicator lights to tell me the actual positions of the relay contacts, as there is no power to the contacts ubtil the NVR switch is activated to start the motor.
Maybe I need to use four pole relays instead of the three pole relays depicted, and wire 24V 'on' and 'off' indicator bulbs to the NO and NC contacts?
The only other way I can think of to ascertain whether one relay has 'stuck' would be to switch the direction and power up, and see if the fuse blows.
EDIT: I've decided to include another relay in series with the fwd/rev relays. I'm thinking I can use a three pole relay and use two poles for live and neutral (I only really need to interrupt the live feed, but interrupting the neutral as well gives additional protection, I think.
Releasing either the safety relay or the fwd/rev relays will release the NVR switch. I can't see a reason to have an additional reset button, unless the NVR switch has also stuck closed.
I do have a spare four terminal NVR switch wgich I could place between the safety relay and the main NVR switch. It would then have to be operated before the main NVR switch, thus acting as a 'reset' button, I think. If it was also placed before the 24V power supply feed it would also reset the safety relay, I think.
I'll have to do a drawing, no point trying to explain it
EDIT: Ok, I need the safety relay AFTER the 24V relay, otherwise I can't reset it (pretty obvious really). I can't use one of my spare NVR switches as a 'reset' switch because I need to momentarily apply 24V to the safety relay coil to 'reset' it. The three poles on the safety relay are live, neutral, and the 24V supply to the safety relay coil, so that whae the E-stop button is operated, it cuts the 24V supply to the safety relay, thus releasing it, and once the E-stop button has been reset, you need to momentarily apply 24V to the relay 24V output/coil input to reset the relay. (If it was a 240V relay, you have an NVR switch made from a relay and a 'momentary on' switch).
Now I need to order a suitable switch.
I acquired six relays and three bases earlier today from a fellow 4HV'er, and I think I've found a suitable enclosure on Ebay..
Next job is to redo the circuit diagrams to include the safety relay and reset button.
EDIT: I think I could use a five terminal NVR switch as a 'reset' button if I place the NVR switch first, then the 24V supply, then the safety relay, and then connect the 'A1' terminal to the live output from the safety relay, so the NVR switch can't be switched back on until the E-ston button has been reset. The NVR switch will then act as a reset button for the safety relay, I think. I'll have to draw it.
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
Hey, I just got a lathe too! Now I'm in the club... hahah.
It's similar to yours, but its a post-war model, '57 I believe. It has a similar drive system, and back-gear for higher torque for harder cuts. You have to make sure to disengage the pin from the bull gear to the main drive pulley when you have the back-gear "in" otherwise the drive train is locked-up, also locking up the drive train is one way to remove the chuck. This one also uses the rocker tool post, which is hard to setup. You have to be aware of the height and angle at which the tool faces the work. The rocker key under the tool holder gives a fairly wide adjustment range for the tool height, so paying attention to "tool center" is very important. If you can't face-off a round completely, you will need to re-adjust the cutting tool height a bit higher.
Most of the tooling is HSS and has to be hand-ground, this is because of the tool holder itself. The tool holders have a 16 deg. back-angle, and today's tooling has a 0 degree back-angle, so that has to be factored in, unless you purchase a tool holder with 0 degree back angle, then you need to shim-up your tool holder so it is back on center-line.
I am using a right hand threading/facing ground tool with a straight tool holder and it is working fairly well. Getting setup to cut and cutting properly are a bit daunting for me, this is my second time with a lathe and I'm fairly inexperienced, but youtube helps.
This one is an Atlas TH-54, with the screw cutting quick change gear box, which is pretty neat. I also have a forward/reverse switch, which is probably going to come in handy, and of course it has a lamp cord that needs to be changed out.
Another thousand dollars or so, and it will be fully tooled with sharp bits, quick change tool post, etc. (don't toss the rocker post, but we do both need quick changes so we can use modern tools and inserts)
I also found out that now I need a bench grinder to grind tools and sharpen! That's another setback, but if you're going to use a lathe be prepared to grind tool bits.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Very nice, HazMatt. I've not made any progress with mine since early July, as the shed ended up getting filled with junk again over the summer.
I think I now have a suitable enclosure for the switchgear, etc. I just need to clear sufficient space in the shed to get back to work on it, although with it now being a day away from December, and not having any heating in the shed, I'm not expecting to have a great deal to report until the spring.
I used the back gear on mine pretty much all the time, for slow cutting stainless steel.
Incidentally, if yours is belt drive (it's difficult to see the drive train in the photo) keep it that way, it's much better to have the belt slip than to have any other mishaps, in my opinion. If the belt slips while cutting it usually means you either need to sharpen the tool, or you're trying to take off too much material in one pass. Give the lathe an easy time
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
Oh yea I definitely agree. Most people I know see a belt as detrimental, but I see it as a benefit. Limited slip is a good thing, it prevents over-loading of the machine and a possible explosion when something goes a-miss.
Mine is not flat belt, its more modern V-belt, but like any belt drive it will eventually slip if the load is great enough.
For my Atlas milling machine it has more slip than usual because I had to replace the drive belt with a link belt and they tend to slip quite a bit, which is not really helpful, but it is hard to replace belts on Atlas machines (have to take the spindle apart to do so)
In the meantime, you can read up on sharpening tools and practice for spring. I have to get a grinder now and a 80 grit and 120 grit wheels, so one new toy is forcing me to buy ANOTHER toy.... the saga continues.
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Progress with my lathe.
