Limit switches and homing switches

[AeroKam]

Hi all,

I am at the stage of fitting limit switches etc to my home built Router table project. What is the best type switch to use for homing switches. Are optical switches suitable or should I stick to using lever roller type with a ramp?

thanks,

Bruce

[turmite]

Hi Bruce,

Maybe this will answer your question! I'm in the process of taking roller and ramp switches off my machine and putting opticals on! I'm having to build enclosures to keep the dust free, and my enclosures won't be liquid proof, but could be.

Mike

[NC Cams]

If you use the inexpensive Radio Shack or other inexpensive "electronic" switches, they are quite suseptible to moisture and dirt induced malfuntion.

HOWEVER, if you buy some GOOD, high dollar commercial limit switches that are made/suited for harsh environment (IE Honeywell 914CE's), you'll pay a lot for them but NEVER have to deal with them ever again. These are darn near bullet proof rolling ramp type and they are VERY well sealed and VERY, VERY robust.

Check E-bay as they can show up there from time to time.

[AeroKam]

I guess a compromise could be to use roller ramps for homing and accurate opticals for datums. Although accurate homing switches would enable them to be used for racking correction on on my master slave X Axis.

[martinw]

Dear all,

Do you want accuracy? If so, no matter how good the specification of the switch, ( be it optical or mechanical) , the dust will get you, even if the switch is sealed. The simple reason being that it trips not against an ideal reference surface , but an ever-changing swirling dune of dust or swarf laid on that surface.

Actually, it might not matter.

Best wishes

Martin

[turmite]

Martin the kind of sealed I'm talking about is inside an enclosure with gaskets and seals. There will be no dust.

Mike

[Greolt]

Originally Posted by AeroKam I am at the stage of fitting limit switches etc to my home built Router table project. What is the best type switch to use for homing switches. Are optical switches suitable or should I stick to using lever roller type with a ramp?

As you can see you will get a range of opinions about switches.

I have optical sitches on a mill and am very happy with their performance. They are well protected from coolant and swarf.

However on a router dust is going to be an issue. I would not use optical on a router. Not saying you can't, just I wouldn't.

Now bearing in mind that this is a router and you want a level of accuracy matched to the
type of application and trouble free operation, I would go for something like this roller type switch.

Ebay number 7621352713 . This seller also has a range of other limit switches.

Reasonable cost, pretty good dust protection, much better than a radio shack micro switch, reliable and good repeatability.

It is desiged as a limit not just a general pupose switch. For a lot less money than some.

Would I use these on a squillion dollar machining center? No. But they are a good match to your purpose.

Greg

[Geof]

I think you should look at the 'similar threads' that are shown at the bottom of the page; you will find this topic has been discussed many times.

Using a switch; mechanical microswitch, optical switch, proximity detector, magnetic reed switch, whatever, is okay for a limit switch because you are not worried about good precision. Using any of these for homing and relying on the accuracy of the switch actuation for the home position is probably not okay because no matter how good the switch they do not operate with an accuracy of 0.001" or less which is really what you need for homing.

Commercial machines home using a combination of a switch and an encoder which gives a signal at a specific point in the rotation of the encoder; I think this is called the Z channel pulse or signal. The sequence is that the controller runs the axis slowly towards the limit/home position until the home switch opens. Then it reverses direction so the switch closes. As soon as the switch closes the controller starts looking for the Z channel signal and when this is detected that is the home position. This means the position is determined not by the linear movement of the axis causing a switch to open or close but by the angular movement of the feedscrew triggering a signal. This is much more precise; the switch operating position may fluctuate by many thousandths of an inch so the home position would also would fluctuate the same if it depended on linear motion, however when the home position depends on angular motion of the screw even if the actuation fluctuates by a degree or two this is only a fraction of a thousandth of an inch of linear motion depending on the pitch of the screw.

With a stepper driven system an encoder is not present but the homing sequence described above can be replicated using two micro switches. One switch is actuated by a cam mounted on the screw the other is actuated by linear motion. Homing is done by having the axis move toward the linear operated switch until it opens. As soon as this happens the direction is reversed until the linear switch closes then the controller (computer) looks for the cam operated switch and when this actuates that is the home position.

A search on the topic of Homing should find a number of threads and somewhere in them are links to an off the shelf solution that does just this sequence.

[Greolt]

Geof I think you might have missed the "my home built Router table" bit of Bruce's question

Greg

[Geof]

Originally Posted by Greolt Geof I think you might have missed the "my home built Router table" bit of Bruce's question Greg

No.

It would not be at all difficult to incorporate the two switch homing; one linear one angular into a homebuilt router. Just because something is homebuilt does not mean it cannot be accurate/precise.

[martinw]

Originally Posted by Geof Using a switch; mechanical microswitch, optical switch, proximity detector, magnetic reed switch, whatever, is okay for a limit switch because you are not worried about good precision. Using any of these for homing and relying on the accuracy of the switch actuation for the home position is probably not okay because no matter how good the switch they do not operate with an accuracy of 0.001" or less which is really what you need for homing. .

Dear Geof,

I've used a miniature button microswitch as a home switch in a gadget for accurately locating a moveable probe. The probe was fixed to a linear stepper motor that moved 0.033 mm per step ( 0.0013").. Even though the data sheet for the switch stated a switching differential of 0.1 mm, the home switch tripped consistently within one step. I was very surprised.

Best wishes,

Martin

[Geof]

Martin; Yes it is impressive sometimes how good the switches can be but depending on how fancy your machine is +/-0.0013" may not be acceptable for homing.