Chinese 7x Lathe Safety Issue

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Cogsy

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Last night I was happily turning away on my Chinese mini-lathe and I needed to do a quick check of the part.

I spun the speed control to zero and the workpiece stopped, but I left the direction switch in 'forward'. Then I grabbed the bottom of the spindle with my left hand, just to rotate the piece a little, so I could see what I needed to. I've done similar things a hundred times before.

This time, the spindle had an uncommanded high speed start up, the chuck grabbed my hand and rotated a half turn before it stopped again. It wasn't a slow spin-up like when you turn the speed control - it made a 'thump' noise like it started at something ridiculous like 700+ RPM. If it hadn't stopped itself so quickly, I believe I'd have been off to the local hospital for repairs.

Obviously my lathe has a fault with something, but I'd be very hesitant to go anywhere near the spindle/chuck or workpiece on one of these lathes, without at least turning the direction switch to 'off' and I'll be pressing the e-stop as well.
 

Herbiev

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Hi Cogsy. I would bring this to the attention of the manufacturer. A recall might be in order before a serious injury occurs
 

Cogsy

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I'm embarrassed to say that I did suspect something like this could happen as my lathe has been doing a few 'funny' things, so I have contacted the seller, who in turn contacted the manufacturer.

Their response was to say that the PC board may be faulty, but as it is out of their 30 DAY warranty, they just offered to sell me a new board... I'll be contacting the relevant authority on Monday and seeing if I can legally return the thing and get my money back, but at least report the safety issue so nobody gets injured. I'll also never deal with supposedly 'reputable' seller again, as they are just completely ignoring my emails now.

I should have taken extra care but I literally only wanted to advance the chuck 30 degrees and I thought it'd be ok. Now I know better.
 

skyline1

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Hi Cogsy,

Sounds like a controller fault to me, but some of the cheaper ones are very sensitive to mains spikes and the like, if you have any high current switching nearby that might be the cause. I once had a 1.5Kw three phase industrial one that kept glitching like you describe when someone turned a nearby fluorescent light on. I traced it to poor wiring in the end, some idiot had run the speed control pot wires in an unshielded cable in the same conduit as the lightswitch so every time you turned the light on the induced voltage would cause the drive to "kick". Any thyristor drive can have this problem but more expensive ones. are better protected. Big ones have a mechanical interlock relay or contactor to prevent this sort of thing from happening.

But a 30 Day warranty on an important and safety critical component is stupid. It is unfortunately rather typical of Chinese companies, they are not all bad, there is some good stuff coming out of china, but they do have a "throwaway and buy new improved" philosophy. After sales support with some is non existent.

I do think you have a valid safety concern though. I don't know about consumer laws in Oz but in Europe they would take a pretty dim view of of it.

This is why manuals nowadays are at least 50% safety warnings and very little by way of operating instructions. It's much cheaper to print pages and pages of safety Warnings than design the thing right in the first place.

Regards Mark
 

MuellerNick

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Even a mini lathe should have a chuck guard. And if that one is opened, there should be no way to start the spindle.
I always open the guard at both my lathes when measuring etc. I could throw the spindle switch with my hip on the manual lathe, and I repeat that habbit of opening the guard on the CNC-lathe.

There are morons on this planet, that remove that safety feature …


Nick
 

Cogsy

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It does have a chuck guard, and I have left it installed, but... I reached under the chuck. My guard only goes over the top. I admit I was doing something very foolish and bad practice, but it still shouldn't have started on it's own.

It's been occasionally fluctuating it's speed on the fly as well. I'd hate to have it go from 80 RPM to 1000 RPM while I'm threading to a shoulder. I expect the crash would be nasty.

I don't know much about electronics, but my shed is separate from the house and the only things I run when the lathe is on is a light and small radio. Thanks for the help though.
 

gus

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Last night I was happily turning away on my Chinese mini-lathe and I needed to do a quick check of the part.

I spun the speed control to zero and the workpiece stopped, but I left the direction switch in 'forward'. Then I grabbed the bottom of the spindle with my left hand, just to rotate the piece a little, so I could see what I needed to. I've done similar things a hundred times before.

This time, the spindle had an uncommanded high speed start up, the chuck grabbed my hand and rotated a half turn before it stopped again. It wasn't a slow spin-up like when you turn the speed control - it made a 'thump' noise like it started at something ridiculous like 700+ RPM. If it hadn't stopped itself so quickly, I believe I'd have been off to the local hospital for repairs.

Obviously my lathe has a fault with something, but I'd be very hesitant to go anywhere near the spindle/chuck or workpiece on one of these lathes, without at least turning the direction switch to 'off' and I'll be pressing the e-stop as well.
Hi Cogsy,

Am glad you were not hurt. This is bad PR for the supplier and manufacturer.
It is up to both to restore our faith in their product.

Gus


Gus.
 

gus

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Hi Cogsy,

Please post foto of lathe plus a close up shot of the controls. I have to alert a mate of mine in USA.I bought him a Chinese Mini Lathe.
 

Tin Falcon

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I spun the speed control to zero and the workpiece stopped, but I left the direction switch in 'forward'. Then I grabbed the bottom of the spindle with my left hand, just to rotate the piece a little, so I could see what I needed to. I've done similar things a hundred times before.
I may be playing devils advocate here but the speed control is a speed control not an on off switch . It does not cut power to the motor. turn the machine off before working on it. Do not blame the lathe for operator head space errors.
You are the operator . you are the one with the brains.
Good safety practice is important. sometimes we can break the rules a hundred times and be OK. 101 and all H*** breaks loose.
And yes I am very glad to here you are not hurt.

SAFETY IS YOUR RESPONSIBILITY
That said,there may be some fault in the circuitry in that the speed of the motor should have been slow not stopped. These boards are notorious for minor failures do to cold solder joints. You may want to remove the board examine with a magnifying glass and re-flow the solder if needed. I would expect these boards are dip soldered at the factory. IMHO QC is not that great in Many of the Chinese factories.

In your case it sounds like some energy was stored in a capacitor then cut loose.


Tin
 

SmithDoor

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Tin Falcon is right
If this was my lathe I would put a swicth with a guard. SAFETY FIRST
I have had I lot of machine tools in my life I did not go to the MFG I just fix it.

Dave
 

Rocketcaver

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My mini lathe will often start up again after I turn it "off" with the speed control. Early on I developed the habit of always hitting the "E-STOP" every single time after turning it "off".
 

Cogsy

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I appreciate what you guys are saying, and I definitely did the wrong thing by grabbing the chuck at all, but I know it's not right to be supplying a lathe that can randomly start off or massively change speeds by itself.

The speed control knob stops the spindle turning long before it hits it's zero mark, so I have just been relying on that when I go to take a measurement or check something (I know now not to do that again), but the way this thing is changing speeds and the suddeness and violence that it started spinning on me is a concern. I hardly know anything about electronics and I don't want to fiddle with something, not be able to fix it and then get accused by the supplier of causing the issue in the first place.

So just to clarify, when you guys stop the lathe to take a measurement, etc, you just turn off the direction control or do you power the lathe off entirely?
 

Anko

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my mini-mill does the same thing, when I pull the speed control knob totally off, it pull a switch that makes "click" that supposedly turn off the machine, but with a little rotarion by hand of the spindle, its start to rotate at very low speed...

I have gotten used to this fact, but I have found that the rotation after the off is very sensitive to te position of the speed control knob, the more close is the knob to the "click", the machine will not start rotating, but if y pass the "click" when I rotate the knob to tur it off, it will hapend.

Thats my experience, hope to help

Saludos
 

hacklordsniper

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i never heard about 30 day warranty, this is illegal in most parts of the world.

When i want to check my piece i turn the speed pot to minimum, put direction switch in neutral, stop the lathe off on start switch and turn off main switch which completely kills the power. This way im 100 % sure nothing bad can happen.

Im not a machinist but i spent last few years repairing/building high power switching supplies and some high current motor drivers and i can give you an advice:

Never, ever trust any electronics especially ones on Chinese machines. Always use a double pole switch to switch off electricity before doing anything on it.
 

Tin Falcon

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So just to clarify, when you guys stop the lathe to take a measurement, etc, you just turn off the direction control or do you power the lathe off entirely?
I have had mine open and had to replace switches so know how things work.
if you put the direction switch in the middle you are turning power off to the motor. The power board still has power to it. . if you turn off the main power switch you discontent power to the board. to be safe turn off both.
In practice I just put the direction switch to the middle. knock on wood I have never had it start unexpected.

I say again turn the machine OFF before putting your hands or measuring tools near the chuck or work. You were not turning the machine OFF!! that is not the MACHINES fault.

The mini mill has a on off switch built into the speed control I usualy engage the e-stop to keep it from starting.
Tin
 

hacklordsniper

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Here is an accident with my lathe, i just remembered i have a picture:

I was making a cut off with a carbide insert trying to cut a piece of hardened steel. It went fine until the index "digged in" the material jamming the lathe. Immediately i punched the safety switch which fell apart and lathe was still powered.

I replaced the switch with good quality one, but luckily this time the worst thing that could happen is that i would break the insert. Luckily i was not in situation that the chuck was chewing my fingers.

Opti (12).JPG
 

MuellerNick

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but the way this thing is changing speeds and the suddeness and violence that it started spinning on me is a concern.
That is right, it should not happen.

I already described how I switch off my lathe while measuring etc. The reason why I use the chuck guard is, that, while bending over the lathe to measure, I might easily touch the lever (sitting on the saddle) that turns the spindle on.
I could even accidentally switch it on and close the chuck guard ... but nothing will happen. The chuck guard has to be closed BEFORE you can switch on the spindle. I verified that, it is a safety requirement.

It doesn't make much sense to switch off all switches on your lathe, unplug it and cut the mains to your house after calling the power plant to switch off the nuclear reactor. You just have to use a safe procedure that you can not accidentally turn the spindle on.


Nick
 

skyline1

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i never heard about 30 day warranty, this is illegal in most parts of the world.

When i want to check my piece i turn the speed pot to minimum, put direction switch in neutral, stop the lathe off on start switch and turn off main switch which completely kills the power. This way im 100 % sure nothing bad can happen.

Im not a machinist but i spent last few years repairing/building high power switching supplies and some high current motor drivers and i can give you an advice:

Never, ever trust any electronics especially ones on Chinese machines. Always use a double pole switch to switch off electricity before doing anything on it.
Sound advice, I too have worked on big machine drives, (Biggest was a 1MW 4 Quadrant, impressive noise changing direction !).

We were always taught never to rely on semiconductor devices for Isolation always a physical switch or contactor, (preferably both).

That having been said it sounds like the controller board is faulty, it should not be starting and changing speed of it's own accord. another possible cause is the speed control potentiometer itself, unfortunately the only way to test it is with an Ohmeter.

It may be a one off, but could equally be a generic problem and I am surprised the manufacturers are not taking more notice, (they should be).

Regards Mark
 

bret4

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I used to have one of these lathes. Sometimes when I would turn off the spindle using the speed control switch it would run a little after the switch was off. I opened up the speed controler and made some adjustments to some of the pots inside and never had that problem again. I couldn't tell you just what pots I adjusted but for these lathes I doesn't hurt to go over these adjustments once in a while. If you do this be really careful not to short anything out and don't get zapped. Take it slow and you should be able to stop the run on problem. Like others said, it's best to turn the main switch off when touching the spindle. Lucky it didn't happen with the key in the chuck.
 

Herbiev

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Cogsy. You are doing the right thing reporting this to the Consumer Affairs authority. There could be dozens of these faulty machines floating around and it is just a matter of time before a serious injury happens. I would definitely ask about the legality of a 30 day warranty also. One year is the norm in Oz
 
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