# Using a photocell as a carriage stop



## The Landshark (Jun 10, 2014)

The storebro lathe i'm rebuilding isnt equipped with a carriage stop,
i was thinking of mounting a photocell on a movable bracket attached to some round stock running parallel to the ways and the associated reflector mounted to the rear of the saddle,
the photocell would be wired to declutch the motor drive when the beam makes contact with the reflector.
would swarf be enough of a problem to falsley trip the cell, obviously the reflector would be better at the right hand side of the saddle.
Has anyone else tried this and how did it work?


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## toolman (Jun 10, 2014)

I don't know anyone who has anything like this, but I would love to hear how it works for you. Best of luck to you!


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## Pmedic828 (Jun 11, 2014)

Made a similar one by using micro switch and it worked great - updated to use hall effect sensor so your photocell should work also.


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## DMS (Jun 11, 2014)

Rather than a photocell, I would recommend a slotted interrupter. Less chance of room light effecting it. If you arrange for the slot to be oriented down, then swarf should not be much of an issue. I have a similar setup on my CNC mill for the home and limit switches.


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## Marco Bernardini (Jun 11, 2014)

A carriage stop must have at least a 0.01 mm precision and be repeatable.
I don't know if a photocell can do that (but you can try).


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## DMS (Jun 11, 2014)

I was getting within 0.001" (0.025mm) repeatability on my machine. I didn't try to tweak it beyond that. Quite satisfied with it. With just a plain photocell I think you would be hard pressed to get that close without a bunch of additional complexity (both mechanical and electrical)


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## The Landshark (Jun 11, 2014)

i'm not hugely worried about precision stopping, i'm more worried about running the tool into the chuck, maybe i could have phrased the question better, 
i was also thinking about using a hall sensor, but i think swarf would be more of a problem here


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## Marco Bernardini (Jun 11, 2014)

Then something with the same scope of a train station buffer:




(source)

At this point a passive mechanical stop would be safer, IMHO.


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## John Hasler (Jun 11, 2014)

Marco Bernardini said:


> Then something with the same scope of a train station buffer:
> 
> At this point a passive mechanical stop would be safer, IMHO.



Not necessarily.  If you stop the motor all motion stops.  If you forcibly stop the carriage and the motor keeps running something may break.


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## Marco Bernardini (Jun 11, 2014)

John Hasler said:


> Not necessarily.  If you stop the motor all motion stops.  If you forcibly stop the carriage and the motor keeps running something may break.



Good point: I forgot on lathes there is a single motor moving everything.


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## The Landshark (Jun 11, 2014)

John Hasler said:


> Not necessarily.  If you stop the motor all motion stops.  If you forcibly stop the carriage and the motor keeps running something may break.




Especially as theres no slip clutch on this machine


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## DMS (Jun 11, 2014)

I have seen modifications to the 7x series of import lathes that used a mechanical linkage to disengage the feed lever, so it is possible even with a single motor driving the system.

Take a look at Gadgetbuilder's site

http://gadgetbuilder.com/ThreadingTools.html#AutoStop


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## stupoty (Jun 12, 2014)

DMS said:


> Rather than a photocell, I would recommend a slotted interrupter. Less chance of room light effecting it. If you arrange for the slot to be oriented down, then swarf should not be much of an issue. I have a similar setup on my CNC mill for the home and limit switches.



Yeah sloted sensor is probably best for an optical meathod, and their used as inter locks on lots of small equipment.  As you say they dont seem to be affected by ambient light.

Stuart


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## hman (Jun 12, 2014)

DMS said:


> Rather than a photocell, I would recommend a slotted interrupter. Less chance of room light effecting it. If you arrange for the slot to be oriented down, then swarf should not be much of an issue. I have a similar setup on my CNC mill for the home and limit switches.



+1 on this one!

I've used a *huge* number of Omron EE-SX670 series slot-type sensors on various assembly line machines I've built over the years, and would suggest this as very good choice.    Datasheet:  http://datasheet.octopart.com/EE-SX670-Omron-datasheet-5390888.pdf

Note that there's a large variety of shapes and orientations available.  The active sensing area is 0.8 x 2.0mm, and the actual transition point is *very* reproducible.  I once checked a (clean, new) one and found it to be good within .001"  That said, your results will of course vary if there's any oil, swarf or whatever in the slot area.  Orienting it downward will help, and you'll want to be able to access the slot area so it can be cleaned with a Q-tip or similar.

One of the very nice things about the Omron (and makes it worth the higher cost than most other slot-type sensors) is that it has a built-in power supply for the LED and an amplifier for the sensor.  All you need is 5-24 volts DC, no external components.  The output has a current capacity of either 50mA (PNP type) or 100 mA (NPN type).  There's also an indicator on the device, so you can verify that it's operating and sensing.

Hope this helps.


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