Wiring Diagram for Generic Breakout board, home and Limit Switches, Generic Drives.

[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]

Originally Posted by 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.

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]

Originally Posted by 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?

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]

Originally Posted by CJL5585 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]

Originally Posted by 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?

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.