[CJL5585]

Guys,
Here is a wiring diagram for a Generic breakout board, the home switches, the limit switches, and the generic drive interface.

This should answer a lot of questions for the new guys who want to figure out how these things work, and also for individuals who are building a machine.

If you need this diagram, I recommend that you download the pdf to your computer, as it may disappear into the pile of threads and posts.

Jerry

[smarbaga]

nice drawing, thanks.

[pminmo]

Excellent drawing. This is the first I have seen all the limits in series. Since that is not an expected even, wouldn't it be wiser to use an input per axis? Home is an algorithmic event. i.e. it's primary use is commanded, so it can be logically found per axis, so they could be parallel.

[CJL5585]

Excellent drawing. This is the first I have seen all the limits in series. Since that is not an expected even, wouldn't it be wiser to use an input per axis? Home is an algorithmic event. i.e. it's primary use is commanded, so it can be logically found per axis, so they could be parallel.

The drawing is based upon NEC and OSHA regulations.

Since limit switches are a required safety feature on industrial machines, and only are used to stop a runaway machine tool, they are required to be wired fail-safe, which means that if ANY WIRING OR SWITCH BECOMES BROKEN OR FAILS, the machine stops ALL movement. This could be a life or death situation in a factory, OR AT SOMEONE'S HOME SHOP.

This is why they are wired in series, and also why they use ONE input. Any other method using normally open switches which close, or different inputs for different axis, do not meet the safety features of the fail-safe mode.

The reason that the home switches are wired in parallel, is that if one uses the g-codes to home the axis, one has to know which axis switch was activated. There is not a way to indicate which switch was activated with the switches wired in series.

I have read posts on this site of people asking for information on the wiring of breakout boards, limit switches, and home switches. As a result, I did take the time to make a drawing taking into account the safety features and code issues and formulated them into a drawing.

I hope that the drawing is useful, and is used.

Jerry

[ger21]

Why are the home switches normally open? I'm guessing you have a limit switch behind the home switch if the home switch fails?

[CJL5585]

Why are the home switches normally open? I'm guessing you have a limit switch behind the home switch if the home switch fails?

You are correct. That is the normal configuration on a commercial machine.

In performing field service on CNC mills, lathes, robotics, etc. I never saw any step and direction systems; only servoed systems. These commercial systems can achieve feedrates of 1,000 inches per minute or more, and are very dangerous if any control feedback is lost, wires vibrate loose, bolts loosen up, or any number of things which could be life threatning.

I realize that for step and direction hobby machines, that most don't probably need limit switches. If they go to the end of travel, the steppers just stall out.

However, there are some folk who are building or re-building powerful machines and need to know how to correctly wire their systems to be safe, and to be able to pass a safety inspection without getting written up. It is for these folk that I included the home and limit switches.

Jerry

[pminmo]

The drawing is based upon NEC and OSHA regulations.

Since limit switches are a required safety feature on industrial machines, and only are used to stop a runaway machine tool, they are required to be wired fail-safe, which means that if ANY WIRING OR SWITCH BECOMES BROKEN OR FAILS, the machine stops ALL movement. This could be a life or death situation in a factory, OR AT SOMEONE'S HOME SHOP.

This is why they are wired in series, and also why they use ONE input. Any other method using normally open switches which close, or different inputs for different axis, do not meet the safety features of the fail-safe mode.

I have read posts on this site of people asking for information on the wiring of breakout boards, limit switches, and home switches. As a result, I did take the time to make a drawing taking into account the safety features and code issues and formulated them into a drawing.

I hope that the drawing is useful, and is used.

Jerry

Great, I've never had much luck finding specific OSHA requirements relating to limit wiring. But would agree, safety is paramount, probably something most DIY'ers don't pay enough attention too. And seriesing limit switch's is a better failsafe solution. I would wonder though if it still might be acceptable to series switches per axis.

Can your give us some more specifics on the OSHA and NEC requirements? i.e. part-subpart....details I made a career of having to read/interpret federal and company regs, but financial and organizational...Oh the joys..... technical....

[CJL5585]

I have given away my National Fire Code and NEC manuals since I took early retirement. I did not plan on ever using them again, So, I gave them to some associates for reference.

As for OSHA, They have a web site, but it is very difficult to zero in on anything particular. They tend to run one around in circles.

I do not remember which sections, sub-sections, etc. which related to machine tools. Sorry.

You might be able to reference something to the Emergency Stop regulations, as I think that they are related. The reason being that some robotic installations have secure perimeters in that if one breaks a security beam, or enters an area while the robot is in motion, that everything comes to a screeching halt. I am not sure, and have tried to forget everything about work, as I have been away from all systems for 25 months.

I think it doesn't really matter about the limits on a small stepper driven system as the stepper could be stalled easily, but when dealing with powerful commercial machines and equipment with servo's, it's a different ballgame. If an encoder fails to send pulses back to a 50 HP servo, then the system sees it as the system has not moved, and more voltage and/or current is supplied to the system so it will move. When an axis crashes at 500 IPM, I believe that the other axis would totally destroy the million dollar machine if they were still allowed to continue on with the machining program. I have seen a similar accident happen with the operator's hand on the E-stop pushbutton. He pressed the E-stop, but his reaction time was too slow. Just an observation.

.

[CJL5585]

If one really thinks about it, who benefits from limit switches?

Is it the operator?
Probably not. The company is concerned that he could get killed by flying debris from a wrecked machine, and they would have to do a lot of paperwork, etc., however an operator will not have enough time in many cases to stop the machine, even if he knows it might crash.

Normally, when a machine malfunctions and crashes, it depends upon the speed of the various axis how much damage is done. Slow moveing machine, slow wreck. Fast moving machine, bigger wreck with flying parts.

So who benefits?
Probably the insurance company who insures the business. The insurance company pays out less money if the limits work and the machine does not totally destroy itself.

Bottom Dollar:
The company investment in the CNC equipment is somewhat protected, and the insurance company could save big dollars on repair of an expensive machine.

Seems like insurance companies control every aspect of human existence in one way or another.

Jerry

[bigz1]

Jerry being clueless about electrics this is a great help. Thanks.

If I were to put another homing switch in parrallel on the X axis could I have 2 'origins' on my machine, with the option to ignore 1 if I want to use the full cutting area?

[smarbaga]

i think that a limit switch should be the extreme end of the line and only be used when something goes wrong. home switches should be used as a zero reference.
these could be on a slider and adjustable if one so desires.
i think limit switches should be mechanical and be hard wired to kill the motor power, perhaps releasing a relay that is set on power up.
all the limit switches should be normally closed and in series with eachother.
this is my opinion, my practice is often different.

[CJL5585]

Jerry being clueless about electrics this is a great help. Thanks.

If I were to put another homing switch in parrallel on the X axis could I have 2 'origins' on my machine, with the option to ignore 1 if I want to use the full cutting area?

As Smarbaga suggests in the next post, many home switches are mounted on a slider or mechanism where they are moveable if desired. In commercial applications the home switches are sometimes adjusted to a new position with each job as the machine setup changes.

You can wire two homing switches in parallel if you so desire, but the machine will home on the first switch it sees. However, if you have a means to switch the closest switch out of the circuit, the machine will home on the second switch.

[lgalla]

Jerry thanks for the information,This has been a question on my mind.I have seen on the zone people not using limit switches. Never understood this.If you have a problem running steppers they will just stall??????Servos will continue until they burn up or something breaks ?????Is their any machine controlers that detect a problem and shutdown the machine?I thought I read somewhere Mach3 can do this.Again Thanks Jerry
Larry

[CJL5585]

Jerry thanks for the information,This has been a question on my mind.I have seen on the zone people not using limit switches. Never understood this.If you have a problem running steppers they will just stall??????Servos will continue until they burn up or something breaks ?????Is their any machine controlers that detect a problem and shutdown the machine?I thought I read somewhere Mach3 can do this.Again Thanks Jerry
Larry

Normally, most stepper motors will stall if they reach a position where they cannot move.

On the other hand, servo controllers are looking for pulses from the encoder. No pulses from the encoder indicate no movement, so the controller cranks up the voltage and/or current. If the drive does not have some electronic safety features such as overcurrent protection, etc. the loop continues until the weakest link fails.

Some of the stepper software such as Mach can utilize a software limit setup whereby the user enters the max table size dimensions, and so forth. The software uses this information to track the positions of the various axis relative to where the limit switches would be placed, and sends out the step signals accordingly. Some software can also use the home switches as a reference to shut down the machine axis once it has moved too far past the home switch position.

Hope this helps.
Jerry

[lgalla]

Hi Jerry
Thanks,lots of useful information.I noticed in pictures of Zone members machines that they probably use more than one circuit.I guess for safety there should be a deicated separate power feed and panel for all the machine functions.I am guessing E stop should be to a magnetic starter?????In Canada industrial voltage is 575 3 phase.All my machines have maginetic starters.I am not good at explaining the benefits of starters but understand their function.Perhaps you could explain to the members their function.I had a 5HP compressor in storage and when I decided to commission it, the starter had mild corrosion on the terminals.You could imagine the shower of sparks and minor explosion at 575 volts.Motor was saved destruction but had to purchase new starter at 500bucks.You have the knowlege and capability of explaning safety issues.I respect that you are retired and may be dont want to think about work type issues but saving some one a serious accident is a very useful pastime.Hope you will continu to provide info on safety issues.
Thanks
Larry

[CJL5585]

Hi Jerry
Thanks,lots of useful information.I noticed in pictures of Zone members machines that they probably use more than one circuit.I guess for safety there should be a deicated separate power feed and panel for all the machine functions.I am guessing E stop should be to a magnetic starter?????In Canada industrial voltage is 575 3 phase.All my machines have maginetic starters.I am not good at explaining the benefits of starters but understand their function.Perhaps you could explain to the members their function.I had a 5HP compressor in storage and when I decided to commission it, the starter had mild corrosion on the terminals.You could imagine the shower of sparks and minor explosion at 575 volts.Motor was saved destruction but had to purchase new starter at 500bucks.You have the knowlege and capability of explaning safety issues.I respect that you are retired and may be dont want to think about work type issues but saving some one a serious accident is a very useful pastime.Hope you will continu to provide info on safety issues.
Thanks
Larry

Larry,
I will make an effort explaining a magnetic starter and how it works. However, I have not designed, wired or troubleshot any starter circuits for several years, but the design (to my knowledge) remains the same.

The basic form of a 3 phase magnetic starter is a multi-pole contactor or relay that is normally open and completes the 3 phase circuit when the coil for the unit is energized causing all contacts to close at almost the same instant in time. These starters also have the provision for motor overload (current link type fuses) protectors. Various sizes of protectors are available for the various voltage and current ratings.

Most starters that I have seen, also have the provision so one may add auxillary contacts to the main contactor. These auxillary contacts are available in normally open or normally closed configurations. These auxillary contacts are usually used to also start and stop other motors, conveyors, or machines when several functions must be synchronized togather. (Also can be used in conjunction with limits on other axis of machine to stop same with safety or limit condition.)

Normally the wiring of a magnetic starter is basic and not very difficult. The coil for the contactor or relay is usually available as a standard DC voltage (for DC control of the starter), or as a standard AC voltage (Usually as the voltage of a single phase of the 3 phase configuration).

In usual practice, the magnetic starter is energized thru the use of an "ON" switch in series with a normally open start switch. Also in series is a stop switch and a Normally Closed Emergency Stop switch, (local or remote locations) and a latching relay. With power on, when the start switch is pressed the voltage feeds thru the stop and E-stop switches to the latching relay coil. The latching relay has a normally open (de-energized condition) contact which is wired in parallel with the start pushbutton. When the latching relay is energized, the contact by-passing the start switch is completed (same as holding down the start switch) allowing constant voltage to be applied to the coil of the magnetic starter, allowing the 3 phase motor, etc. to operate.

The safety circuits (whatever are required such as limit switches, overspeed, overcurrent, etc) are inserted into the circuit USUALLY AFTER THE E-STOP AND BEFORE THE LATCHING RELAY, so that any condition stops all motion by removing the control voltage. If the latching relay was still energized, removing the condition that stopped the machine would allow it to instantly start movement again, causing a very hazardous safety condition.

Hope this gives some insight into a magnetic starter and how to wire in the safety circuits such as limit switches.

Jerry

[smarbaga]

hello:
below is one method to hook up a power on starting system.
i think it really depends on your system how one will have
the power applied.

[NC Cams]

Jerry

The drawing should serve as a template for the newbies, especially those electrically challenged.

The NO/NC issue on the switches can be daunting to the neophyte. This gets even more complicated if PU versus PD resistors are involved. This is a good "primer" diagram to get most guys going.

My Fanuc wired all the limits in series. They also have a momentary "easy" button shown as viable on the wiring diagram (it was not wired in, however) to enable the user to back the axis off of stop if you drove into an axis.

The wiring on this Fanuc machine also has a "near limit" switch which is cam operated that stays engaged while in the zone. This puts it into a real slow operation "deceleration" mode to prevent violent crashes of the machine into the end stop by servo motors. I dunno if such a feature could be used on a stepper but it would be nice for similar reasons.

[bigz1]

I am coming to the end of my build(3 axis MDF machine with steppers) and am about to install the electrics.

With the homing switches would it be acceptable to use them as limits aswell? I haven't decided which software to use yet.

Also it appears with the homemade machines the Z height is set by setting the tool from a block. In this case would it be better just to have limit switches either end of the travel rather than a homing switch and a limit switch?

[CJL5585]

I am coming to the end of my build(3 axis MDF machine with steppers) and am about to install the electrics.

With the homing switches would it be acceptable to use them as limits aswell? I haven't decided which software to use yet.

Also it appears with the homemade machines the Z height is set by setting the tool from a block. In this case would it be better just to have limit switches either end of the travel rather than a homing switch and a limit switch?

First of all, home switches and limit switches are not a requirement for a homebuilt stepper based CNC system. The home switches are for convenience, and the limits are for individuals who would like to protect their investment in their homemade router or mill.

Some software packages can use a single switch for both a reference home switch and a limit switch. Mach software is one of these. Check the software that you wish to use to see the support for limits and homing functions. There is not really a standard among hobbiest CNC software packages, so it is your obligation to research all software available for something that will work for you.

One can choose to have a single home switch, two home switches, three, or none at all. Same goes for the limit switches.

Wire up whatever configuration that turns you on. It's your machine and your decision as to what your needs are.

Jerry