Elite closed loop - can it protect against crashes?

Since the Elite machines have closed loop stepper motors, can they detect and protect against a crash? Example, if the spindle doesn’t turn on, and the z axis lowers the bit in contact with a work piece, the closed loop system knows immediately that the z stepper wasn’t able to make the next step beyond the work surface. So instead of the machine attempting to lower the bit to a depth of 1/4", it would know at probably .005" that it has crashed into something.

I don’t see this feature mentioned.

The same would be true if a toolpath attempted to route at a depth/speed that the current bit cannot achieve. Perhaps the bit has become dull. The first step that the machine takes where it doesn’t immediately get feedback that it succeeded, the machine could shut off and protect itself, the router bit, and the work piece.

As a potential customer, and a new CNC user, it would be quite valuable to know that there are some safeguards built in to protect my investment. Just sayin.

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I do not own closed loop stepper motors, and I am also a relatively new user of CNCs.

The answer I would give is that only you can protect against crashes, and that will come with getting to know the limits of the machine, and careful research and attention into your tooling, feeds and speeds, and toolpaths.

When the motor faults, the crash has already occurred, so it is more a question of how much damage has been done or can be prevented. The Masso closed loop motors will fault for a variety of reasons including when steps are lost due to hitting an obstruction (see video - MASSO Closed Loop Stepper Motor - Episode 30 - YouTube ), but I do not believe it will be fast enough to save a brittle carbide end mill or perhaps even your project, even though one motor fault will automatically end the running program.


Hey UltraPeepi,

in case a spindle doesn’t turn on, if its VFD is correctly wired with a safety circuit, it has already signaled this error condition to the CNC controller which stops the program then. To check if the VFD itself is operating, it can be set to send acknowledgement signals via ModBus, or one of its logic output terminals can be programmed to one of the functions that confirms its operation and this can be wired to the safety circuit as above. In both cases the CNC controller would be signaled the error condition and stop or not even start the program.

As for the closed loop stepper, this is a stepper motor that has a so-called encoder sitting on the rear of its shaft which is a glass disc with a system of scales etched into it that are sensed by a infrared photoelectric sensor and which can hereby be used to track every step and direction of the motor’s movement. The closed-loop stepper driver compares the given step commands and the executed ones and can signal a error condition if the motor did not follow the steps. A small deviation, in case of staying behind an amount of steps, can be corrected by the motor if it is not blocked, but if it lost too many steps or if it cannot execute the step commands anymore, it signals an error and the program is aborted.

The use of motors with encoders brings the benefit to be able to identify an operating condition that causes steps to be lost, e.g. when the feed rate is too high. By avoiding conditions where the stepper driver signals errors, the operator is able to prevent programming the machine in a way that the machine is not able to cope with the mechanical load and also that the tool is in danger to be broken during operation. But it is not able to detect a tool breakage during operation.

A tool breakage detection system is not part of the Elite Series. You find this on rather expensive industrial milling centers, however it is imaginable to retrofit such a system. There exist different methods to realize tool breakage detection, from simplest to most expensive:

  1. Tool length verification.

    With this method, you set up a tool table with the tool lengths as usual, but at positions in the program flow, you check if the tool length is still the same as registered. You need the tool length probe and a custom script to achieve this functionality.

  2. Whisker wand switch

    On automatic tool change, an actuator with a whisker wand detects if the tool is there. Saves time compared to 1.

  3. Light reflection system

    The tool is exposed to light. Electronics determine whether the tool is present by analysing the reflective light pattern from the rotating tool.

    On very expensive machines, a laser measurement can also be used to measure the exact diameter and thus the wear of the tool, which can then be calculated into the milling path.

  4. Smart tool monitoring system

    The machine data are analyzed to determine tool condition and wear with different methods which also allow to predict tool breakage in the future. Example

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Lot’s of good info. I hadn’t consider bit break detection. I was mostly wondering about crash detection. I understand that it is my job to prevent crashes, I also understand that I am likely to make a mistake at some point.

I appreciate that while spindle not powered up should be detected a different way, the core of my question remains.

Let me use a different example (it’s going to be ridiculous - but I just want to make clear the underlying questions) : I have a toolpath set to plunge 1/4", but I have my resurfacing bit in the router. How soon will the closed loop system detect that it cannot plunge 1/4"? Will it detect and shut down? Or will it detect, retract and shut down? Will the error detection logic prevent damage to the rails/ball screws? If it protects the bit or workpiece, great - but that’s not my significant investment, the CNC machine is.

The resolution of a single step is so minute, I suspect that the system, as rigid as it is, will flex somewhere to absorb the single step crash. When the tension is released, I suspect the system will relax back without any damage. The work piece will certainly be marred (if not ruined); and I suspect the resurfacing bit will be undamaged. But again, I am less concerned about the work piece and the router bit.

Here is the ultimate question I am trying to evaluate: is it worth another $800 (plus the additional lead time), for an Elite machine? If the machine has some level of crash protection/mitigation that will save me from an $800+ repair, the answer is quite clear.

I would not get the Elite machine thinking it has any level of crash protection - my feeling is it does not, nor would any closed loop stepper motor/driver, at least not in the way you are thinking. When a motor faults, Masso will stop the running program - how fast and what damage it will prevent will depend on many factors. It will not retract the Z axis as part of this.

I have been using the Masso G3 for many years on my modified Woodworker ATC set-up. I would definitely spend the extra money, and wait, just to get the Touch controller and sensors in the package. I personally do not believe closed loop steppers are needed, or offer any real benefit - at least for the work I plan to do and the way I will be working.

One thing to consider as well is how the spindle/VFD safety parameters are programmed, and how this will be integrated with the Masso Touch on the Elite series safety circuit(s). That is if you plan on getting a spindle. I do not have any details on how, PwnCNC kits for example, accomplish this but it would also be a factor in determining how well your crash protection goals are met.


I do not own these specific closed loop steppers from Masso but my understanding is they are configured to “give up” after losing one full revolution and raise an alarm stopping the machine. With a 16mm per revolution ball screw this would be 16mm of attempted travel before it raises the alarm.

I would echo @TMToronto on the Elite, it is worth the additional $800 for the additional features it offers that will frustrate more advanced CNC users on the Onefinity Buildbotics forked controller. I started with the OF BB controller and like TMToronto I went through many hours of wiring and other work to convert to the Masso G3 (plus about $1500) and it has been worth every penny (I run my Onefinity 40 hours a week)

I do not have closed loop stepper motors on my OF but it is on the todo: list.


Hey UltraPeepi,

I think your question is mainly focused on the open-loop vs. closed-loop stepper.

Besides this difference when buying an Elite model, the Masso Controller gives you an experience much more close to industrial CNCs (read the Masso documentation to see all the capabilities and options), while the Buildbotics.com Controller is an admirable software project of the author of CAMotics.org, and the Onefinity Standard Series Controller that is a hardware and software fork of it, lacks a few capabilities, e.g. adjustable feedrate during a g-code program running. It however implemented largely the entire G-code standard so you can run any g-code program on it, and please do not to forget, the Onefinity Controller is available at a fantastic price for a CNC Controller that works.

Now regarding open vs. closed loop, what running open-loop steppers mean, is what I have here in my lab: A Onefinity Controller with no machine connected to it, no stepper motors. But I can run the g-code programs, and the machine does not even realize the steppers aren’t there. It runs and runs and the movements are correctly reflected on the position, absolute position and offsets columns of the table on the CONTROL page. Even stall homing works, but of course it always “thinks” after a short motor run that it is at home and sets the machine coordinates to zero as you expect. If you know how stall homing works, it’s clear that it works without steppers attached. What does not work, is touch probing, but if I connect the touch probe and in a certain moment put the magnet end and the alumium probe together, you have successfully probed (some value). It is even possible to do manual workpiece probing, since you can use G-code commands in the command entry field of the MDI tab to “move” the carriages somewhere, and then enter G92 to have a workpiece zero correctly set (even if the workpiece doesn’t exist).

So now let’s think what does that mean if you have steppers and an entire machine connected: Imagine a situation where either

  1. you have set a feedrate that, considering the bit and the material, is too high for the bit and/or the machine to cope with it, and you loose steps,

  2. you have heavy EMI in your workshop, e.g. because you have a VFD not enclosed in an earthed control cabinet or you terminated the spindle cable wrongly, and you loose steps, or

  3. your stepper cables have inner invisible cable breaks or just the only tin-plated Molex plugs which you find on the Onefinity may have a bad contact, what can happen especially if your machine has no serious cable management and no strain relief, so you loose steps,

in all these cases, you may loose steps, but the machine is not able to detect it. So after program run, in the best case, some dimensions of your workpiece do not fully correspond to what the program said, and in the worst case it milled totally wrong paths into your workpiece and ruined it (by the way, since you said your investment is the machine, not the workpiece – when building stringed musical instruments, especially large ones like celli etc. you can easily work on workpieces in a week that cost more than your machine).

If you now compare this to closed-loop steppers, the system will detect the above conditions, however not forcibly which of the three causes mentioned is present, it still can be hard to diagnose this, and the machine will have stopped the program. Only when the motor is a few steps behind, it will have a positive effect, where the driver will try to keep up with the lost steps and “save” the workpiece. But in many cases it will simply stop the machine, but in comparison to an open-loop stepper system, you avoid that the machine proceeds with the work, so the damage on the workpiece is limited.

Wow, with an open-loop stepper system, that’s the Makita burning condition. You set your workshop in fire :slight_smile:

If you have closed-loop steppers, it depends. If you use a spindle possibly the VFD will detect mechanical overload and trigger the stopping of the program. But the moment when the ALARM condition of the closed-loop driver will be triggered depends on whether the carriage is blocked or if it still can move.

It will simply stop the program at the point where it was and the Emergency stop state will be active. All motors are stopped. If the VFD is wired correctly with a safety circuit, the spindle is set to STO (safe torque off).

The closed loop stepper driver has an output that is the ALARM output. If it is triggered (the conditions were referred in a previous post), the above actions will be the result if all is set correctly: The program and all motors are stopped. Whether your ball screws or your machine is damaged then, can unfortunately not be detected by your machine, but generally as a CNC operator, you can damage or even destroy your machine at any time.

However you have to know that the vast majority of CNC machines out there, hobbyist and professional, use open-loop stepper motors. This is because the stepper motor is a very reliable and well-researched piece of technology. It is easy to make him reliably do what you expect from it because it has mechanical steps implemented as hardware. As a CNC designer, you can know very well what the limits of your motors are and dimensionate them accordingly. So to use it successfully and without errors, it is sufficient to know what the limits of your machine are.

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Thanks everybody for your responses, and for your very serious answers. It is nice to have your experience to tap into.