Limit/Home switches

[vincemulhollon]

I am planning to add some limit/home switches to my CNC mill.

Sherline mill, Gecko 540, EMC2 controller...

Any specific comments about adding switches to a Sherline? How you you keep the Y axis switch clean? How do you mount the Z axis?

Any general comments about switches?

What is the current state of the art in limit/home switches? Does everyone still use mechanical microswitches or have people moved on to optical things or maybe hall effect sensors and little magnets covered in swarf (Err, forget I suggested hall sensors and magnets)

What kind of repeatability do you guys measure off your home switches? I'm guessing hundredths but maybe it's better or worse? I'm guessing repeatability is controlled half by the switch mounting and the other half by the quality of the switch?

Is the market for switches primarily controlled by repeatability or by durability or by water/coolant proofing or oil proofing or chip proofing? In comparison, the market for steppers is pretty much controlled by torque, for a given NEMA size and shaft diameter.

Have lots of problems with chips wedging into your home switches?

Just check out the "mouser.com" phone book sized catalog on my desk and look for microswitches, or are there "special machinist suppliers" whom sell "special switches"? I'm guessing I can just stroll thru mouser.com?

For all things in life there is a minimum price below which you are wasting money on junk and above a different price you are making a middleman rich. What are typical high and low prices for limit switches? I'm guessing for the price of one stepper I should be able to handily outfit my entire mill with switches?

Is this the kind of thing I will be replacing often like an endmill or something thats good for the life of the machine (barring accidents) like a power switch? I'm guessing they last forever if they're coolant and oil proof?

[S_J_H]

I just use regular micro snap switches on my x3. Really never had a problem with them and they see plenty of oil, grit, chips etc.But no flood coolant.. I replaced one in 2 years. Not the most exact device but I rarely ever ever need them for that. Home once and then basically they just are limits for the way I work.
Sort of depends on how much exacting use you'll depend from the switches I think.

Steve

[macona]

On the sherline cnc we have at work it just uses small round plunger style micro switches. The head Z switch is mounted to the bed on the front to come in contact with the apron. The tail z limit is mounted on the tail stock so that when the carriage comes in contact with the tailstock it stops. The X (There is no Y on a lathe) limits are mounted under the cross slide on the saddle. The two microswitches are mounted on the same screws except one is flipped 180 from the other so that the plungers are not inline. On the cross slide itself there are two dimples, one for the end of travel for each direction. When the cross slide reaches the end of travel the plunger goes in the dimple and the switch activates.

[Paul Alciatore]

There are tens, probably hundreds of thousands of switch designs out there. Prices from pennies to hundreds of dollars or more. You can probably find any spec switch you can possibly imagine.

Standard microswitches are quite robust and have very repetable trip points although it may differ from one switch to the next. Be sure to put an adjustment in the mount. Also probably a good idea to put some spring action in the system to allow for over run so the switch does not get broken. Some microswitches have spring/levers built in on them but this design decreases the accuracy of the set point.

Plunger types are also good and can be purchased in an oil tight case. They also can have a generous overtravel to prevent breakage.

Push come to shove, you could easily improvise a simple switch mechanism. Most logic switches work by connecting to ground. An adjustable screw on the movable part (electrically connected to it) and an insulated contact on the fixed one with a single wire and you are done. You do need a ground connection from the electronics to the machine. All in the open and no complicated mechanism to get clogged up with fluids or swarf.

[SGW]

At the Mach3 presentation at Cabin Fever, Brian said he could get repeatability within a thousandth off home switches.

During the discussion, there was some talk about protecting switches from chips by mounting them inside a protective slotted shield, with the switch mechanism down so chips fall away from it.

The latest Mach3 install/config manual http://www.machsupport.com/docs/Mach...all_Config.pdf gives some suggestions for mounting home and limit switches and which types one might use.

[vincemulhollon]

I am planning to add some limit/home switches to my CNC mill.

Thank you all for the ideas.

I ended up buying some Omron D2JW-01K31 subminiature microswitches from Mouser. Regarding the oil comment, this switch is totally immersion-proof. Regarding the switch mounted so chips fall out/away, I mounted them in spots where that'll happen or at least where they're highly visible and easy to see. I figure I'll only home a clean mill? Regarding the comment about handling some overrun I bought "simulated roller lever" switches which can smoosh down at least a tenth after tripping and they spring right back.

On my sherline mill I ended up mounting the X axis on the mill table so the lever smooshes into the saddle. On the Y axis I mounted it kind of flat on the ways so the saddle smooshes into the lever. On the Z axis I mounted it on the large aluminum block that attaches the Z ways to the base, when the spindle assembly gets too low the spindle assembly smooshes into the switch.

Its a metric switch, but a 2-56 cap screw 3/8 long fits nicely. Now I don't want to discuss the tapping operations, but its all OK after much suffering. The same tap that tapped some cylinder head cover holes so easily in 360 brass, wasn't so easy in "sherline aluminum". Tough strong gummy stuff. Quarter turn and it tries to bind. Tap-free or tap magic or whatever it's called helped a little. Was something like a #51 drill hole, maybe a little bigger. Overall, I don't enjoy tapping tiny holes in Aluminum.

Mechanically it's all good and tests OK with my continuity tester. Maybe I'll wire it all up next week.

[Swarf&Sparks]

I prefer NC switches myself, home-brewed from relay springs.
As long as they open when the carriage hits em and you use a resistor to ground.
Bend em, you can straighten or replace em for cents.
Easy enough to shield from swarf or suds, that depends on your machine and your ingenuity though.

For real precision, I've used slotted optical switches. The secret is to use an op-amp comparator on the output to give you a constant "hard" cut off. I've got better than 5 micron repeatably, the limiting factor being vibration.