Diagnostics - The NOT so arcane science

On Different forums that I have active been with for going on 10 yrs; I have come across this issue time and again. I have had a good number of clients that primarily pay me to diagnosis issues with their machines and fix them.  What I have seen is a severe lack of some basic understanding by many folks of how to do this.

HAVE A PATTERN

The Biggest issue I see time and again is folks not having a pattern with which they check for errors. This leads to missing rather simple issues (I earned this T-shirt years ago and still keep a list of my pattern). By having a Pattern to your checks and keeping a list of them you can be sure you eliminated an issue or at least limited it's ability to be part of the problem.

Keep detailed Notes

Keeping a detailed build, maintenance, and repair log also let's you keep a much better grip on what an issue can be and track it down in a much more ordered pattern.

Learn errors (thereby learning from them)

This sounds strange but there are different types of errors/faults that are classically electrical in nature and there is also those that are structural in nature and also those that are mechanical. Learn the ground rules of each type. Document, this means you start to know the general area of where to look when an issue comes up and can shorten the amount of time finding and fixing a problem will take.

Electrical errors can be intermittent and often are. Structural errors will induce the problem at the same point each time and most often will affect motion. Mechanical errors also repeat at the same point regularly and can be mapped as such. These are but some of the primers to finding errors in each area. There are issues that cross over.

Document

Document and document. Find resources on line that cover areas you don't know and learn from them and document. By documenting what is going right (print/cutting [Feeds &Speeds] settings and results) as well as what has gone wrong (print/F&S settings and failures [lessons]) you again create a body of knowledge that you can go back to when what you think you remember does not match what is in front of you. By keeping the documentation and what happened and why you improve your product shorten time to get production done and improve your best practices. While at the same time shortening any down time and developing a list of what parts are most likely to break or have issues so that your back up stock is much more targeted instead of having a spare machine around to take parts off of.

This in turn will save back the time you took to develop the knowledge base. It is one of the main reasons machinists and CNC folks guard their personal feeds and speeds log books like pure gold and it is extremely difficult to find reference documents on feeds and speeds as they depend on the material and so many other variables (I know that there is a big push to generate these bodies of data but they are still running into the issue that variables must be dealt with and the options to do so with some ease comes at a very high price tag in sensing equipment).

So Good luck and start your log book/maintenance journal today if you have not already done so. Remember to review the logarithm to insure it is functioning and that you (we all make mistakes) did not miss a step when setting it up for your machine. A simple one follows:

Issue: Does it show obvious mechanical or electrical issues?

No-Start check list  // Yes- Go to that section and start check list

Standard check list start with power and cable checks are they all their and properly connected?

Yes good shut down so that unpowered mechanical checks can be eliminated. move on from there.

Area check list go to section and start checks that are appropriate for that section and error type. whether this is powered or not will be determined by the section of the check take proper safe guards.

That is a start point, you add in what systems need to be checked and create a sheet that you can check them off with. This saves a good bit of time in the long run and also a version can be set up for daily/weekly/ monthly/etc. maintenance.

Again good luck. 

Comparing Accuracy and Resolution.

Why Compare?

Okay, why compare the two as some will say they have little to do with each other and other say they are functionally the same. Before we go on any further I am going to go for definitions so that we can get on the same page:

Accuracy: technical the degree to which the result of a measurement, calculation, or specification conforms to the correct value or a standard.

Resolution: technical The amount of detail which can be shown or created by an object or tool

Okay so now we have those, how does it apply to either CNC or 3D printing and can or do they affect each other? Yes and No is the answer and let's look at that.

Accuracy in a CNC and 3D printer is dictated by the type of drive system used. These are chosen for many different reasons to include cost, ease of construction, accuracy, and Load bearing to name some of the points that come into the process of choice.

Coupled to this is either a stepper motor (which we will be using in this discussion as base) or servos. Servos bring a whole new level to accuracy and an associated cost in equipment and complexity. They the superior choice in the area of both accuracy and repeatability.

The other areas of accuracy that is often over looked (and can affect both repeatability and resolution) are what type of support system are you using and what is the accuracy/tolerances of that. How true and ridged is the frame the frame and mounting points on the machine to insure that you don't have flex or induced misalignment.
You will hear many arguments over types of drives and why. Each have their function, I would strongly suggest NOT falling into the one is always better or superior than another. That debate is for another blog and they why and how of it applying to DIY or small business machines is for another day. back to the discussion on hand.

So Accuracy and repeatability directly affect resolution. Some would say that is the only direction that the effect goes. Not exactly. A stepper motor set at whole steps only will have a direct effect on accuracy and repeatability by the fact of the limits of what it can move. So no micro stepping becomes important as that allows for smaller segments and more use of the accuracy that a given drive system has within it. In turn that allows for higher resolution of a model, fine forming of arches (Which are broken into straight lines in most CAM and Slicing software.

So following that logic microsteps of great than 1/64 are great as they allow you to create extremely fine points of movement. IF only it worked that way. As you increase microstepping you decrease available torque. Unless you are running rather advanced logarithms in your firmware and drives that will be a full time loss (which means you don't get it back as you are slowing down to change direction of a cut or extrusion). So the knife cuts both ways. There is a medium point though.

A industry standard distance for measuring accuracy is 300mm (which is rather close to a foot some might take note). If you have a drive train that has a given accuracy (Accuracy trumps repeatability in that you have to get the wanted position before you can repeat it). Okay we have let's say 0.185mm/300mm (.007in/FT) accuracy and a repeatability of 0.0529/300mm (.002in/ft). This coupled with how much error we induce via either under powering the motors (most often with to little Voltage) or by trying to use too many microsteps and exceeding the usable torque curve, and lastly try to use to much acceleration with a motor and causing issues that way. Let's be kind to ourselves and say that we did the math and are powering our steppers at 75% of maximum voltage (That maximum is without cooling of the motors and can go higher with cooling or if the motors are not run for long periods of time under that full load). We are using 1/16 microsteps which with a TR1205 screw we get a per step resolution of 1.5625 um/step. You might notice that our per step resolution is well beyond what the machine tolerances will give us. The science that will give us the exact accuracy and resolution is why FEA programs exist unless you like doing large equation math problems (I don't).

The crux of the issue is that often we exceed the structural limits in an attempt to improve via software or other means. When designing and using your CNC or 3D printer take the time to learn what the capabilities really are and what the accuracy truly is this will help you in not only knowing what you need to do to improve product out come (professional/client/personal) but also to better understand where you can fudge and where you really can't. While technology is moving forward much of what it written here is still directly applicable and most likely will be for years to come.
So in closing on this article as I have rambled a bit. Insure you have the resolution to make the accuracy something more than a hoping that you can over come the physical limits of a machine.

With that best Luck in your business or hobby and for designers, good luck and yes I have set you a challenge.

Let's Look at terms, as words ARE important

Terms (words we use to define)

In going back over my writing and having folks ask questions I have come across the problem of terms being used for things other than what they actually mean. This can cause all sorts of headache and fray the patience of many well meaning souls who are really trying to help others but due to mis-communication  you end up with the wrong idea getting across or not understanding what an issue is for some one. So below I will be listing terms and what they mean and a bit of how they apply to different process'. Hopefully folks will find this useful and will help reduce headaches for everyone.

Rigidity:  The ability to resist flexing or forces that cause distortion. This helps in the world of CNC and 3D printing in a couple of ways. increased ease in forces going into motion and not being absorbed by the machine. Decrease in some forms of resonance. it can also aid in calibrating a machine due to both of the above proprieties.  Too rigid, or too much rigidity can cause issues of binding, and stress points that will fail. Though too much rigidity is a had point to get to in many types of machines.

Microstepping: Is a process in CNC and 3D printing where by either software or circuitry you intentionally split the full step of a stepper motor down into fractions of a step. Common is 1/2, 1/4, 1/8, and beyond. Use has multiple benefits and some down sides and requirements. It is not positioning though it can affect positioning (increasing microstepping to where you are exceeding the maximum steps/sec will induce error that will be cumulative).

Drive: When dealing in the CNC & 3D printing world this refers to how you are moving the tool module along an axis (not all machines use the same drive on all axis, an Prusa i3 uses 2 belt drives and 1 dual screw drive. What a drive runs on can be many different items from linear rails, rollers, and assorted options.

Steps per: This is an area that can have more than one use. Steps per unit of measurement or unit of measurement per step tells you the resolution of an axis. Steps per revolution tell you how many steps a motor makes in order to move 360°. That number also affects resolution as it is used to arrive at the measured distance per step. It is also the starting point for how much microsteps can be reasonably used

Drivers:  The hardware that controls the electrical movement of a motor whether that is a Servo (closed loop) or a Stepper (Open and closed loops) that depending on type will limit current and possibly voltage. They come in digital and analog types and are mostly voltage rated with controls to limit current. They can be found in either on board, pull up or stand alone designs. They also can have safety measures against stalling, back EMF, and other features.

Commissioning: When dealing with machines industrial or light industrial, commissioning refers to the process where set up, intial run checks and first servicing is done to the machine to insure proper functioning. This can also be applied to the hobbyist/small shop environment as it is a good practice and helps to insure proper functioning.

To Be Continued......

Voltage and Stepper motors, Thought and why

Part 2

Okay, having gone over part one and had some folks ask questions that have lead me to bring part two a bit forward in getting it out.

Folks have asked am I saying that that is maximum speed a steeper can go at a given voltage. No, I am not. What I am saying is that ACCURATE motion for the voltage is limited within that area. As you go beyond that speed (steps/sec) you are going to increase risk of error. Maximum speed is the equation below

Using this equations:

Max Speed=V2LImax⋅spr

Minimum Time per Step=2LImaxV

Pmax=ImaxV$$$$
$$$$

Where: V = Applied Voltage, L = the inductance of the motor, I_MAX = equals maximum current, and spr = Steps per revolution

Source for these equations is: HERE

Notice how once again voltage plays a major part in the equation. So we come around again to what is maximum and why voltage is important for stepper motor usage. The steps per second gives you a direct relation to the amount of accurate micro stepping you can do before inducing error.

So recapping from Part 1 and Part 2 we have this:

Stepper motors in most usages of CNC and 3D printing (AM) are open loop systems. Open loop systems need to operate near optimum function to insure accuracy and reduce possible induced error.

A stepper motor has a maximum Voltage where going beyond that voltage will start inducing damage to the motor unless actions are taken to deal with heat and EMF that will be an issue.

At or near that Voltage the motor will be able to have the largest torque curve available to it.

That torque curve dictates many diverse issues with the movement of the axis to include but not limited to smoothness of motion and accuracy of motion (protecting against loss steps) which also helps with the ratio of micro stepping.

Steps per second are directly effected by the amount of voltage available to the motor. which directly affects the amount of micro stepping that a motor can do accurately (not inducing error).

So stop for second and do the math, I would love to see more drives like the Bigfoot series that can handle 60V so that you can safely run 48V motors and insure no issues with EMF (though that is more a CNC issue than AM issue; it is still an issue.

Best Wishes and hope this helps

Voltage and Stepper motors, Thoughts and why.

Part 1

Working with CNC and 3D printing I have often run into folks who disregard the need of proper use of voltage. It is a lesson I learned while refining machine function in CNC and I find that it applies to not only CNC but 3D printing as well.

So why is voltage important at all? I have heard time and again from online experts state that only current is important for stepper motors. That too much voltage is more trouble then it is worth.  While high voltage is something one needs to be aware of and deal with safely the greater risk is the current in a line for health and life reasons. Voltage on the other hand is very important to a stepper motor.

The rating on a Stepper motor has the base voltage which is the voltage needed to rotate the stepper on step. It has the current for the stepper, which will be different may be different if it is possible to wire it in series or parallel. The resistance will be per phase and at what temperature that resistance was measured (and the tolerance of the measurement). Lastly but not least it will have the inductance per phase (again with tolerance for measurement).

So going over basic electrical theory we have these laws to deal with:

V=IR   Where V = Voltage, I = Current in Amps, and R = resistance in Ohms

That allows us to extract other needed equations dealing with current and resistance. The next law also has a very important effect on the motor and is what causes the direct link between I and V in functional usefulness.

The next equations allow us to deal with the inductance and how that effects the voltage and current usable to the motor. First is getting the back emf off of a motor for a set value:

V_l = -L(di/dt) Where V_L equals the back emf voltage and -L equals the negative effect in Henries multiplied by the change in current (di) divided by the time it takes (dt)

From here we can add the following equation that will effect what voltage we need.

L = V_l /(di/Dt) Which is an extraction of the equation above.

Now inductance is an issue for a stepper motor as it the RESISTANCE to CHANGE in CURRENT. That will directly effect how well a stepper motor is able to deal with steps per second and the amount of torque that can be applied by the stepper via said force.

Now it is also known that a stepper motor has a decay rate and a maximum rpm. That there is a point at approximately 40 to 60% of maximum rpm where you lose enough torque to significantly increase the risk of stalling and loss steps. So the inductance tells us what Voltage a given motor needs to be able to get the maximum effective torque for the minimum amount of time. As this voltage will inhibit loading of the coils and insure that the current arrives and drops as needed. The equation for that comes in two forms depending on where in the world you learned it. They are as follows

32 * √L (disregarding the mH and using the number without accounting for the mH). An example is a stepper with 3.2 µH, the equation would read 32*√3.2.

The other option with this equation is:

1000*√L (accounting for the micro). So this equation would look like 1000*√0.0032.

Realize that any voltage above to the base and below the optimum can be used and the motors will function. What will happen is you will lose torque and have a quicker decrease in the amount of micro stepping that can be effectively achieved.

For CNC this directly effects the ability to cut and effectively use tooling at optimum speeds and feeds. Which directly effects what materials you can work and the finish you will from the cutting tools.

For 3D printing it directly effects micro stepping and accuracy/smoothness in the motion of the printer. Which directly effects surface finish and accuracy of material placement. So both sides stand to win a good bit by taking the time to give the motors the voltage that will allow them to run smoother. the interesting point is that running a motor at this voltage I have found that unless under heavy or sustained loads they tend to run cooler than running at lower voltages. The most defined difference I have seen is going from less than half optimum voltage to within 10% of optimum.

Let's look at three different stepper motors and the equations that will govern optimum voltage and what that means to either CNC or 3D printing.

Motor number one is common to CNC and has the following information:

3Nm Nema 23 Bipolar motor

Bipolar wiring with 2 phases
Degree per step: 1.8°
Rated voltage: 2.73V (Bipolar, Parallel), 5.46V (Serial Bipolar), 3.9V (Unipolar) (Minimum Voltage to cause function of the motor)
Rated current: 2.1 Amps per Phase (4.2 Amps when wired in parallel bipolar) 3A Unipolar
Resistance: 0.65 Ohms
Inductance: 12.8 microHenries (Serial Bipolar) and  3.2 microHenries (Unipolar and Parallel Bipolar)
Nm : 2.15 when in serial and 3.0 in parallel.

Now comes an interesting point on Voltage by numerous sellers is recommend to be between 48-80V.

Let's get into the why of that and from their we can build a picture of the uses intended and shift that to a 3D printer which will have some different demands but a stepper motor still deals with step and direction as it's main function.

Using either 32*√(L) or 1000*√(L). Voltage of a stepper motor) we get the following optimum voltages.
Serial Bipolar then has a required Voltage of 32*√(12.8) or 1000*√(0.0128). Which gives us the spread of 113.137 to 114.4867 Volts.  With a current of 2A So that sets up one option.

Now for Parallel Bipolar or Unipolar set up we have a different set of numbers 32*√3.2 or 1000*√0.0032.  This gives us a spread of 56.57 to 57.24 Volts. with a current of 3A for Unipolar and 4.2A for bipolar parallel. Remember this current is per phase so you are shifting through the arch when dealing with current, this is addressed below.

The most popular option in small CNC and DIY is running in parallel bipolar as it give you the most torque for the energy supplied.  Many in 3D printing will tell you that is not needed for 3D printing as you are not dealing with the resistive forces that cutting creates.  I would point out that same torque allows smoother changes in direction and real access to micro stepping without the increased risk of loss steps which translates into misalignment of layers loss of integrity of the model at worse. 

Now let's look at how that Voltage effects both systems directly. That area is steps per second available at a given voltage with a constant current and impedance. The equation for that is T = IL/V.

Where: T = time per step in seconds or parts of.
             I = 2*Imax (Imax is supplied current and doubled to account for a full cycle)
             L = Inductance measured in microHenries
             V = Voltage applied

T1 = 1/T and gives you Steps per second. So let's for now work with the 56 to 57 V power range and see where using different voltages will get us.

12V which many folks entering DIY or trying to get into light industrial on the cheap will play with, can be our starting point for this exercise. So here it goes:

T = (8.4*3.2 E-3)/12; T = 2.24 E-3; T1 = 446.43 Steps/sec = 892.86 Pulses/sec
T = (8.4*3.2 E-3)/24; T = 1.12 E-3; T1 = 892.86 Steps/sec = 1,785.71 Pulses/sec
T = (8.4*3.2 E-3)/48; T = 560.0 E-6; T1 = 1,785.71 Steps/sec = 3,571.43 Pulses/sec
T = (8.4*3.2 E-3)/56; T = 480.00 E-6; T1 =2,083.33 Steps = 4,166.66 Pulses/sec

Now how does the above directly effect production and movement?

Available motion is measured as a percentage of maximum rpm. Due to the decay and torque cut off point. Also at this point the mass of the drive system comes into play and the effects of backlash start becoming much more an issue

At this point I am not going to go into the physics side of the mass of the screw and/or loading stretch with belts and the different levels of backlash each system has. That is for another discussion and is widely debated on the internet.  Though among mechanical engineers and those who work with drive systems daily the normally accepted order of hierarchy for tolerances and accuracy in descending order is Ballscrews, Leadscrews (trapezoid/ACME not threaded rod), Belt drives. Realize that all of these drive trains have their uses and are used as different system benefit from each depending on cost, accuracy required, levels of maintenance and lifespan desired, etc.

Now take the fact that most motors work best between 40-60% of their maximum rpm and you can rapidly see why using voltages closer to optimum yield much better control and finer motion. as the difference in real motion ability between 12V and 48V is 357.144 steps/sec or 833.33 steps/sec. Consider that a stepper has 200 steps per rotation and you can see that lower voltage directly effects both speed and micro stepping. 


The next motor has this information (which is a very common motor in 3D printing):

54.374 Ncm Nema 17 motor
2 phase
Degree per step: 1.8°
Rated voltage: 2.77 Volts (V)
Rated current: 1.68 Amps (A)
Resistance: 1.65 Ohms
Inductance: 2.8 mH/Phase

Optimum voltage is between 52.915 and 53.546V. From this it is easy to see that 12V to 24V PSU to the motors is going to cripple the motors in not only removing the ability to functionally do microstepping effectively, but also increasing the risk of step loss at any serious level of motion. A 48V power supply with a healthy smoothing ability to insure constant and clean current to the motors would more than pay for itself in improvement in speed and accuracy.

The last motor is to help show that this is not only 1.8° step motors that have this issue.

40 Ncm Nema 17 Motor
2 Phase
Degree per step: 0.9°
Rated voltage: 3.1 V
Rated current: 1.7 A
Resistance: 1.8 Ohms/Phase
Inductance: 4.0 mH/Phase

Optimum voltage for this motor ranges between 63.25 an 64 Volts. Add in that you have 400 steps per rotation and the need to get closer to the optimum voltage becomes even more important.  With even higher benefits as well as the decay arch is sharper with faster loss as these motors often have a torque curve that stops below 40% of maximum.

Also remember that you extruder motors are using torque to move the filament and that those with reduction gears are even more dependent on being at or near optimum voltage as that is where the greatest amount of torque for them to the job is located.

Hopefully this has helped folks and will go into some more application for both 3D printing and CNC in Part 2

Additional reading for those who are interested:
From Gecko Drivers

Hello and Welcome.

What is here?

Well this is my blog about running a small shop with traditional hand and power tools as well as CNC and 3D printing. This is part of my business and I hope it becomes a resource for folks to understand some of the basics and not so basics of small shop light to medium industrial type work.

My back ground is a good bit varied with a Doctorate in Chiropractic, served 16 years in the reserves of the US Army, and experience in many different fields over the path of life. Currently owning and running my own business of Marino Customs as I continue to recover from cancer and some secondary health issues that came with that. The goal of getting back into practice some day.

I tend to grab a hold of a subject with both hands and try to learn it as best as I can. Knowing that I will have to continue to learn and that any subject can have many different wrinkles in the subject matter. I continue to be reminded of that in both my hobbies of amateur radio and in working with musical instruments (both playing and working on them).

So the goals here is to go over some of the points that I have found on my own path that are not well covered in forums online nor obvious unless you come from and engineering background and even then can be esoteric.

My hope is that this will help others as I was helped.

What are some things I will be looking at?

CNC and 3D printer design, stepper motors, Shop design and layout, tool usage and getting focused on things that help a shop move forward. The benefit of this for me is to remind myself of these important items and to help other see the reasons for them and the long term benefit as well.

Well until next post take care