I am still looking to buy a shop sabre but I am wondering about the price and importance of the home limit switches. They want $750 for the xy and z switches this seems rather expensive. Are home limits that important? What do they do besides tell you that you are at the left front and top of each axis? I can buy and install crash limits for a heck of alot cheaper.

Home and limit are different things. Home is an absolute reference for the machine and limits are just that. Some use the home switch as one of the axis limits but I do not think that's such a great idea - I guess it depends on the individual.

That does seem a little steep for home and limit switches.

But yes - they are needed. You can get by without them but this is not a good idea. Things go wrong and the limit switches can prevent damage to your spanking new machine.

Scott

To bad you couldn't buy the machine and have them knock off the cost of the overpriced control! Then set the machine up with Mach3!

Why would anyone sell a cnc machine ($8 - $13,000) without limit switches (How generic can you get!) I couldn't find anything about limit switches on their site. Did they tell you they are not included ?

If your buying the machine anyway, just buy it without the limit switches. Then add your own (Yes you have to have them!) On the "Shop Sabre" site, they claim you can add custom "M-Codes". So add a hardware switch, and a soft limit switch.

The $750 is what we call price gouging! Thats about like the Bozos that try and charge close to a grand for a floppy drive! :bs:

Sounds like the correct idea, I'm an electrician so the hardware and wiring are a snap
it's the connecting it in with the software that I will be stuggling with. For $750 it sounds like a worthwhile learning experience.

I have a shop sabre (without limit switches) and I am looking to install some; already ran the gantry off the table once.
Can anyone help with the wiring? The paralel connector in the control box (that runs to the computer i/o board) only has about 6-7 wires connected (for the motors). My plan was to wire the rest of the pins to a bus block and connect limit switches, emg. stop, etc. to the bus. I am running wincnc. Is there anything I need know before doing this? Any help would be greatly appreciated.
As an aside, I am very happy with my shop sabre, its well built and rugged, however support from shop sabre has been lacking. I have asked for schematics and instructions on upgrading the machine several times and haven't gotten a response from ss.
Jon

Jon,
Try wincnc.net they are very helpful and there tech named Troy knows the cdontroller inside and out.

For some real nice limit switches, check out the Microswitch line supplied by Honeywell.

They are NOT cheap but they are nearly bullet proof. Deals can be scored on E-bay. THey hold up in wet, sloppy environment and just flat WORK.

Yes, you can get micro switches and other cheaper ones from Digi-Key or Mouser but the Honeywell's are way, way more robust and worth the $$$'s all things condidered.

I first discovered them on my Fanuc/Cadillac CNC lathes and they were factory installed in ~1979 when lathes were new. They're still functioning even though the wiring long since had to be redone.

Would limit switches be just as important on a machine running servos's? I realize the problems that can happen when running steppers, but with servo's there is a less likley chance of running off the edge and causing damage or am I wrong here..??

More important with servos. If there is a problem with the encoders, servo will run at full speed with nothing to stop them. That's what limits are for.

Do they just cut the power to the servo's or are they connected to the softeware?

That's up to the user.

Do they just cut the power to the servo's or are they connected to the softeware?

In practice they will probably always have to be connected to, or through, software. Your servo power is controlled by the software and you cannot cut it directly. Definitely the limit switch which is used as the home switch works through the software and you want it set up so that you can run over the switch and then back off to set the home position. In the opposite direction if you are running servoes and encoders mechanical limit switches may be unneccessary because you can have soft limits in the form of maximum allowable encoder counts. This way your software can predict that a move will exceed the limit and not even start.

The limit switch connection scheme depends on the system design.

The servo amps in my CNC mill have internal shut downs that address overcurrent/overvoltage/overtemps. If these trip, they "tell" the interface card that the system is shut down due to a driver fault.

The limit switches go direct to the PC control card. They are "or'd" so that if you hit ANY one, the whole system shuts down.

The amps also have "inputs" for limit travel inputs to shut down the amp exclusive of the PC controller but the system was NOT wired to use the feature this way. The mill has the limits going right to the PC input terminals that the software deals with.

My CNC lathes have X+, X-, Z+ & Z- limits again or'd right to the controller. THere is a bypass button which enables you to back off the limit should you over run the travel. Again, whole shebang shuts down at overtravel fault...

Your driver cards and software should offer ways and suggested methods for overtravel limit switch incorporation. It is FOOLISH to NOT use them.

As far as cost goes for the option, the 914 Honeywell switches mentioned previously retail at about $75 EACH and they are bullet proof.

Do 4 limits (lathe) or 6 limits (mill) plus 3 "home" switches and you could be looking at as many as 7 switches which knocks the daylights out of $525.

Add time and material for to wire them and do all the logic programming and $750 really isn't that expensive for a commercial system - especially if it prevents a runaway whatever from hurting somebody.

The emergency room cost alone will probably dwarf the $750 expense as would the higher subsequent insurance premiums.