[rusel]

Hi
I went and put limit switch (See thumbnail) on our mill, worked ok for little while
then started getting false signals. Found out that the coolant was shorting them out.
Question is is there a supplier of low cost water proof switches.
I would also like to get some home switches as well.

Russell

[Bubba]

I use a AAP2T51Z11 limit switch from Automation Direct. Got mine several years ago and use them as combination home and limit. To seal the wiring, I used some poly tubing from the hardware store and silicone rubber to seal it at the switch end.

As I remember, I paid about $12 a piece for these particular switches. They have been reliable and accurate for homing purposes as well of course as limit switches. They have several form factors, but this is the one I picked to work on my machine.

[Al_The_Man]

One of the best is the hermetically sealed Honeywell 914CE2-9 style.
They are pricey, but can be had on ebay often very cheap, Omron also make a version of them.
Al.

[rusel]

Thank for the info
Bubba
You use yours for a home switch. What accuracy do you get?

Al
They are price around $100au plus delivery. Will have a look on ebay. Would you use them for a home switch?

Russell

[Al_The_Man]

Al
They are price around $100au plus delivery. Will have a look on ebay. Would you use them for a home switch?

Russell

They have a micro switch internally so they should be accurate.
Al.

[Bubba]

As best I remember, I was looking at a deviation of less than .001" (0.0254 mm)and I am trying to be conservative here as this was setup several years ago.
The pdf file I have lists it as "0.01mm on the operating points at 1 million operations"

[Valen]

Just spoke to Russell (rusel) and he mentioned this thread to me, he's my father and we are both trying to get this conversion "just so" .
I looked at the datasheets for the switches you guys mentioned and the honeywell is .01mm and the automation direct limit switch is the same.
We are using .001mm glass scales on our mill for the servo loop. I know they are way overkill in terms of absolute accuracy but because we use them for the servo loop they need to be finer than if we were just using them for positioning, and it feels kinda wrong to not try to get the same resolution out of the home switch?

We are making our own tool length probe based on some of the designs we have seen on here and around the net and it seems to be getting ~.004 mm which isn't too bad for a prototype.
http://www.cnczone.com/forums/showthread.php?t=109146 so I was hoping there would be something off the shelf that would hit that sort of accuracy that didn't cost mega $.


A quick intro to our mill

I want to do a writeup on the build for it, what would be the best place on here for it?

[rusel]

Hi
Been think about this :idea: (ouch) I can get some of those limit switches that you guys suggested on one end of each axis, but on the other what if I made a home switch that has a limit switch in the same enclosure. Would that not be beautiful....... One mounting for both.

Russell

[wildwestpat]

Hi Russell

Can I suggest the electronic proximity switches used in the automation industry. There are several types both the physical arrangement for mounting as well as the technology used to 'sense'.

I have found these for sub $10 from china via ebay and been very pleased with them both interms of sensitivity and long term stability for use as 'home' switches. They interface easily to breakout boards PROVIDED you get the right polarity of switch.

Here is a link to the Ormon site as they have the best data sheets to give you an idea.

http://www.ia.omron.com/data_pdf/dat..._ds_csm446.pdf
OR
http://www.ia.omron.com/product/48.html

The main advantage is that these are fully waterproof - pick the sensing technology to suit the swarf / environmental crud that they will meet in your machine. Use some cunning to use only one switch per axis.

I use a wired in glass minature slow blow fuse as the limit switch of last resort and would avoid mechanical microswitches unless there are health and safety considerations which might require non electronic switching. (Safety switches are a complex subject and should not be confused with the other switching requirements.) My 'glass fuse breakers' are arranged so that the mill bed will smash the glass and break the wire which in the slow blow fuse is held by an internal spring which ensures a clean break in the circuit. The only care being to ensure that the metalwork of the machine does not form a continuity path accross the 'broken' fuse. I also made sure that the endo of travel approach speed / inertia were taken into account so that a runawway axis would be protected from a hard grounding.

Hope this gives an alternative view on switches and one that would sit well on your quality build.

Kind regards - Pat

[Bubba]

Russell,
I use 1 switch and trigger it from either side AND use it as a combination home/limit switch!
The problem with trying to have two switches in the same container is the fact that the controller will trip out on limit unless the home switch trips prior to the the limit switch! Using the home as limit scenario, allows using one switch to handle both features. All of the control programs that I am aware of allow this.

[rusel]

Thanks Pat for the info and I've had a look at the sites.
The problem I see with these sensors, if I have read them correctly, is there not that accurate, we are after 0.001 mm. Then there is the possible swaf interference
Please correct me if I am wrong

The glass limit switch sounds a little like calling the fire brigade I will have to think about that some more

Bubba
I was thinking of having some distance between the two switches.
Our system can handle the limit/home switch in one, but what I am after is a high accuracy for the home switch

Russell

[rusel]

This is just a general lay for discussion
When the red actuator is touched the two home plates move, where there is three contact in series running on very low voltage to stop arcing. This break the circuit and the pcb sends a home signal out.
The actuator can keep on moving then the limit switch is then switched.
Total mill move should be stopped by the hard stop before actuator reaches case end.
Thought, comments please

Russell

[wildwestpat]

Hi Russell

I am getting repeatability that is certainly better than 0.005mm (inductive type short range switch). The data sheets are difficult to interpret as the detection is dependent on several factors. I go for the short range switches and set them up so that the detection edge is approached from one direction for consistent results.

It is my experience that detection based on mechanical microswitches have long term repeatability issues as the over centre spring ages. IMHO the long levers and associated bearings are also a source of precision problems that are best avoided by going all electronic - but then I have spent all my working life in the electronics industry so I may be biased!

If you are interested I could try and gauge the detection accuracy but my DTI is not the best and I would hesitate to claim measurement in the sub one thousandth part of a millimeter.

As for the glass break switch of last resort I like to fit these if the machine is unsupervised and has enough axis power to cause damage. There are other ways of protecting the machine. I just happened to have a number of very small wire ended anti surge glass fuse links intended for PCB mounting - only fair to say that having fitted them they remain unbroken several years on. Such is life buy insurance and you get away with it - forget the insurance and regret the scrimping.

I think you will find that the automation industry has switched to electronic switches and the price of the mechanical water proofed microswitch and actuator is increasing as these fall out of use. It is also apparent that the super conservative safety requirements are gradually accepting electronic switching where there is adequate fail safe protection for all possible failure modes. (Safety is a complex topic and one I do not want to get drawn into as this is best left to local experts from the relevant Government Agencies or their official representatives.)

Good luck with your selection be it mechanical or electronic. Regards Pat

[rusel]

Thanks Pat
Just so I can follow
If the sensor is pointing north which way are you moving the trigger martial
south towards the sensor or east to west across it (at what hight?).

If I could ask you to have a look at other build and ask you for your expert comment would be appreciated.
We where going to use the same method in this home switch.
http://www.cnczone.com/forums/showthread.php?t=109146

Thanks
Russell

[wildwestpat]

Hi Russell

Just read from the start of your thread. Here is a design I have used as a cover for the V3 style microswitch to provide some protection. These are simple delrin covers that fit onto a flat surface and are actuated by a plunger that slides infrom either side at the end of travel. The switch block is mounted on a convenient part of the machine and adjustable bullet ended rods (think screws protruding from blocks for adjustment) plunging into the block via the slot to trip the switch. The use of roller actuators copes with over travel and could act as a mechanical stop if made sufficiently strong. But please consider a more elegant solution. Use with microswitches like this one as they are sealed to IP67 but it is still a very good idea to keep them clean and dry as contamination of the operating plunger is important. You could arrange to use just one switch but the over travel mechanics require thought. When I used this design I had a load of the sealed V3 size switches to hand and was more interested in speed of construction and idiot proof assy and maintenance.

A source of switches is here:-

http://uk.rs-online.com/web/search/s...duct&R=6160287

That is if I have not convinced you to go electronic full sealed as these can be had for a similar price.

Regards - Pat

[RomanLini]

Hi, there's a thread here in the open-source forum;
Electronic home switches made easy!

That myself and many other people have contributed to that shows how to make your own high accuracy home and limit switches using hall sensors.

It's worth a read, and yes I know the suggested sensor uses a magnet which could be problematic on a metal mill and would require some construction to keep swarf away from the magnet, but then waterproofing mechanical microswitches and keeping swarf out of the crush area is problematic in it's own way. Anyway that thread may give you some ideas.