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Thread: Lathe Mechanical E-Stop
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9th Jul 2019, 07:11 PM #1Golden Member
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Lathe Mechanical E-Stop
My M250 lathe was originally fitted with a mechanical E-Stop assembly on the rear of the motor.
It is a shaft mounted plastic disc between two plates, one fixed, one floating axially against axial springs.
The stationary electrical control winding pulls the floating plate, so the assembly is electric release, spring apply.
The brake was rated 415 Vac and connected directly to two motor phases.
The brake was tested at 240 Vac as Ok, but never used because it just sounded "wrong" to apply such forces to the headstock gears.
The forthcoming delivery of a new Powtran VFD prompts me to complete the VFD installation, including getting an E-Stop functional.
The VFD is complete with dynamic brake.
What other style of mechanical brake is used on lathes ?
What experiences can anyone provide to help me sort out an acceptable arrangement ?
I made a dividing plate for the lathe that clamps directly to the spindle outboard end.
It looks like a ventilated disc off a pedal/motor cycle.
Is this approach viable ?
Look forward to any suggestions made,
John.
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9th Jul 2019, 08:37 PM #2
The up models of my lathe have a drum brake inside the double 2 step pulley feeding the gearbox. I have the shoes (100mm diamiter) but have never installed them or the mechanism to make them function.
I've seen others go down the track of the small motor bike disc brake ( thumpster motor bikes) they come in cable or hydraulic calipers and are cheap on eBay.
My lathe stops pretty quickly, which is why I haven't given major thought to fitting the brake. If I was to fit a VFD I would just rely on the braking resistor to slow it quickly.
I'd like to see some pictures of you set up, have never seen that style myself.Using Tapatalk
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9th Jul 2019, 10:56 PM #3Member: Blue and white apron brigade
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Can't see why not.
I use a home made mechanical disc brake for my WW bandsaw.
The disc turned up out of a Bisalloy plate and it's so massive it barely gets warm even after multiple stops - this is the way I normally stop this saw.
The pads are from a Mitsubishi Colt Galant and the cam operated mechanical mechanism from a hospital gurney.
With 2 x 450mm cast iron wheels on the BS there's a lot of rotating mass.
Using the pedal turns off the VFD and can stop the band in about 3 seconds, faster if I wanted to but I worry about stresses and strains on multiple uses.
Pedal.jpg
On my lathe my e-stop just cuts the power to the Powtran VFD (this model does not have the brake built intuit) but it still stops the lathe in under a second.
The main reason for this e-stop is more to isolate the VFD and beyond from electrical power which is something a mechanical brake is unlikely to do
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9th Jul 2019, 11:37 PM #4Mechanical Butcher
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M250 - Is that a Harrison?
My M300 has a foot-actuated brake that uses a belt V-groove as the braking surface.
As per many veteran motorcycles with V belt drive, a shaped wedge is attached with linkages, to provide braking when pushed into the vee.
The Harrison M300 is still being made.
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10th Jul 2019, 06:09 PM #5Golden Member
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Harrison M250.
For Dave J (and other members) who would say, "without the photos it didn't happen" :
M250 Brake Mount End Compr.JPG, M250 Brake End Compr.JPG, M250 Brake Bits Compr.JPG, M250 Brake Assembled Compr.JPG,
The motor mounted disc looks and feels like bakelite.
The assembly is what it looks like on the motor.
The 3 capscrews locate the mount end plate to the motor, the 3 draw bolts pull the balance of the assembly up to the end plate, compressing the 3 springs on the the 3 locating spigots.
Note that this would be a pain to mount, as the springs need to be compressed to get the draw bolts to engage their threads.
There are 2 untapped holes in the end casting and floating plate, but cannot see how these would be used to pre-compress the sub-assembly to make for easy mount to the motor.
Perhaps external clamps.
As I will not be using it, I guess I will never know.
BobL,
Your words convince me to relax and test the stopping power of the braking resistor.
John
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10th Jul 2019, 07:08 PM #6Most Valued Member
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John I had a colchester with a brake very similar to the one in your pictures, it was mounted on the end of the input shaft situated just to the rear/behind the chuck.
I hated that brake.
With a foot brake you can pretty much stop where you want, with the electric brake it was much more hit and miss, and often more miss than hit.
Stopping in the right spot can be important in some situations such as cutting an imperial thread to a shoulder with a metric leadscrew as you can't disengage your half nuts.
I have not used a lathe with VFD and brake resistor but I have heard that you can adjust them to stop almost instantly if wanted.
cheers, shed
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12th Jul 2019, 01:26 PM #7Most Valued Member
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I fitted a cable operated caliper and disc to the pulley on the motor of my lathe.
I have a thing about(where possible) using your "E-stop" most of the time, its the first thing you are going to go for when it hits the fan and with any luck at all, you know its working. Also a foot brake gives you the option to stop fast or slow or even change your mind halfway through. While VSD's can be set up to stop lathe very quickly I'd rather not be locked into using that every time. On top of that I think a foot brake is just more convenient to use (even when it was just a switch for the motor it was better than using the lever on the carriage).
E-isolator Not saying an isolator on a VSD is a bad idea, but I have a hypothesis* that my brake would happily stop my lathe even if the motor wasn't switched off, even when the starting winding switched back on. Maybe one day I will test it as it shouldn't harm anything other than possibly blowing a fuse or tripping a breaker. Do you think a VSD would(should) trip in this case?
*based on car/bike brake performance and your motor power test rig.
I installed a switch that clamps to the lathe bed and is run into by the carriage to switch the lathe off, it works ok but I think it would be far better with a VSD setup.
You know you can open the half nuts? You just have to use the thread chaser to keep track of where you opened them and close them again at the same point going back.
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12th Jul 2019, 03:26 PM #8Member: Blue and white apron brigade
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Most likely it would trip. The way around this would be to set the VFD to a "coast to stop"mode and when you hit the brake it triggers a micro switch that turns off the VFD. This is how I set up my WW BS with the footbrake. Then you can hit the footbrake as hard as you like at any time - maybe that's how yours is setup?
I don't think an E-stop should be the usual way to stop a machine and that it should be reserved for real emergencies as repeated super fast stopping is not going to be good for most machines. For usual stopping, as you say something like a mechanical brake has advantages mainly to be able to vary the stop time to suit what you are doing. This probably could be done via a VFD using some sort of variable resistance braking resistor but I have yet to see one.
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14th Jul 2019, 12:32 AM #9Most Valued Member
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I still have a single phase motor on my lathe. The VSD is a "one day" project.
What I was trying to get at is some people want the E-stop to isolate the VSD "in case the VSD is commanded to stop but doesn't". Which is fine but, it doesn't take much of a brake to stop a lathe even if the motor fails to turn off(I think).
But a mechanical brake can stop a lathe as fast or as slow as you like. An e-stop that isolates the vsd doesn't do that.
Certainly you wouldnt want to be jumping on the brake pedal every time but that's a choice you get to make each time you use it.
Sparing the graphic details of an industrial accident told to me over 30 years ago. When it hit the fan one operator ran past a E-stop button(at least one) to get to "the button he always used" to shut the machine down. Things did not end so well for the other operator. On my "not very big lathe" I can be about 1200mm from the E-stop button but short of standing behind the lathe I can always reach the brake pedal. Just saying when it hits the fan..... where possible.......I'd prefer not to be looking for a button I haven't used in 6 months, of course sometimes you don't get choice and there are many ways to skin a cat.
Might have to do some tests on my lathe and mill to see just how will the brakes work with the motor on.
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14th Jul 2019, 12:44 AM #10Using Tapatalk
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14th Jul 2019, 06:42 PM #11Member: Blue and white apron brigade
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Yep, given I can stall a 4HP motor with a simple (hand) brake on my HP measurement rig. Because they are connected to a VFD during the braking the VFD eventually trips but it's not quite as quick as you think.
My original idea for the E-stop was only for electrical isolation - it's just fortuitous that it also stops the lathe in ~1s. It's not a brake but its better than nothing and quicker than the standard 3s deceleration to stop.
But a mechanical brake can stop a lathe as fast or as slow as you like. An e-stop that isolates the vsd doesn't do that.
Certainly you wouldnt want to be jumping on the brake pedal every time but that's a choice you get to make each time you use it..
Sparing the graphic details of an industrial accident told to me over 30 years ago. When it hit the fan one operator ran past a E-stop button(at least one) to get to "the button he always used" to shut the machine down. Things did not end so well for the other operator. On my "not very big lathe" I can be about 1200mm from the E-stop button but short of standing behind the lathe I can always reach the brake pedal. Just saying when it hits the fan..... where possible.......I'd prefer not to be looking for a button I haven't used in 6 months, of course sometimes you don't get choice and there are many ways to skin a cat.
Might have to do some tests on my lathe and mill to see just how will the brakes work with the motor on.
1200 mm is definitely too far for any E-stop.
On reflection I agree that it's better if the switch you normally use to turn off the machine is the same as the E-switch but there are some other factors to consider.
Firstly, no one can say how they will react when different proverbials hit he fan.
If the hand normally used to turn off the lathe is caught in the machine, how will this affect the reaction time and E-switch reach-access of your other hand?
Foot/leg reaction time is known to be slower than hand reaction time.
You might also be caught standing with the" wrong foot forward", and try to readjust your stance to apply the brake.
There's even a suggestion that several E-switches/brakes be available - ie a foot and a hand activated brake.
Even then, this still no guarantee an operator will use them.
BTW The main reason for the mechanical brake on my BS is not for emergencies - it's actually not that well placed for emergency - its there for another safety aspect and that is because of the long coast time (120s) after the motor is tuned off. About half of this is silent ,or almost silent, and I have come very close to making contact by hand or other things that would strip the teeth of the band.
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15th Jul 2019, 01:46 PM #12Mechanical Butcher
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15th Jul 2019, 02:25 PM #13
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15th Jul 2019, 05:32 PM #14Member: Blue and white apron brigade
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15th Jul 2019, 08:03 PM #15Most Valued Member
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I wasn't meaning to suggest the tripping of the VSD would be quick, just that a brake can easily be large enough to over power the motor and stop the lathe even with the motor on.
Yes I've not meant to say it was a bad idea.
I'm not so keen on deceleration (at least not the way my VSD handles it), but that would explain why you have set up your E-stop the way you have.
Absolutely. My less than stellar writing skills coming to the fore.
Thats how my lathe brake is set up. I just meant to test a "what if" the motor didn't turn off.
I wasn't meaning to imply I would act any differently to the operator. Monkey brain says "go for the one you use all the time", so where possible make that your E-stop. Of course it isn't always possible.
No guarantees in this world, you make your choices and hope for the best. I still have the standard lever and E-stop.
One issue I found, though happily with no damage to myself or the machine(not so sure about the part). The fwd/off/rev lever can be moved "through the gate" from fwd to rev. Would be bad enough with 3 phase motor, though it may save the situation, but with single phase the motor it just keeps running fwd.
Could be added to my brake/carriage switch with little trouble. Though I do think using a VSD in this case gives you far more options.
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