CNC controller, Step 11.

The CNC controller is the brain of a CNC system. A controller completes the all-important link between a computer system and the mechanical components of a CNC machine. The controller’s primary task is to receive conditioned signals from a computer or indexer and interpret those signals into mechanical motion through motor output. There are several components that make up a controller and each component works in unison to produce the desired motor movement.

The word “controller” is a generic term that may refer to one of several devices but usually refers to the complete machine control system. This system may include the protection circuitry, stepper or servo motor drivers, power source, limit switch interfaces, power controls, and other peripherals. Owners, operators, designers, and builders of CNC devices should understand the tasks performed by these components and how they affect machine performance.

The following sections will discuss the primary task of each component in the controller and how they work together to create a complete CNC system.

CNC Controller Sections
CNC Controllers background
This section covers the history of controllers and where they have advanced to today.

Controllers Components
This section covers the components that make up a CNC controller system and how they work together to control the machine.

Open Vs. Closed Loop controllers
This section covers the difference between open and closed loop CNC systems

CNC Control Signals
This section covers how the control signals used by a control system to operate

Breakout Boards
This section covers the function of a breakout board and how it integrated into a CNC system.

Communication Port pin information

Stepper Motor Drivers
This section covers stepper motor control. The different types of stepper motor drivers will be compared.

Uni-polar vs Bipolar Drivers
This Section covers the differences between unipolar and bipolar stepper motor drivers.

Stepper Driver Step Modes
This Section covers the different step modes such as Full, Half, and microstepping.

Buying Stepper Motor Drivers
This section covers the features to look for when buying stepper motor driver.

Servo Motor Drivers
This section covers servo motor control. The different types of servo motor drivers will be compared.

Enclosures
This section covers enclosures for your controller hardware. Often overlooked, the enclosure is an important part. Bad enclosures can cause overheating or shorts due to debris.

Controller Prices
This section covers generic controller prices. What you should expect to pay and also some tips for saving money.

Where to buy controllers?
This section list a few good resources for buy controller parts and kits.

Controller Power Supplies
This section covers the power supply. Choosing the right power source is crucial for proper machine function.


ACME Lead Screws and Associated Loads

When using ACME lead screws in your CNC router design, it is important to understand the proper way to load the lead screw assembly. Improper loading may cause decreased lead screw efficiency, life, or failure. There are five loading types and two load classes that will be covered in this section;
Classes
-Dynamic
-Static

Types
-Side
-Moment
-Thrust
-Tension
-Compression

Load Classes

Dynamic Load- is the load applied to a lead screw in motion.

Static Load- is the load applied to a lead screw that is not in motion.
Side Loads (Avoid) lead screw to sideload

Side loads or radial loads are loads that are applied to the nut radically, perpendicular to the lead screw axis. Reference the image. Side loads should be avoided. One way to sideload a nut is to not properly align the ends supports and the nut attachment point. Sideloading will reduce the critical speed and may bend the lead screw permanently.

Many CNC builders uninvitingly sideload the ACME nut by miss aligning end support bearing or by allowing some of the weight of the y-axis carriage or the z-axis carriage to be applied to the lead screw. This should be avoided. The linear bearings should hold all of the carriage weight. Keep in mind it doesn’t take much misalignment to force side loading.

That is why it is important for the end support bearing blocks to be adjustable in position and not hard set. This way any misalignment can be adjusted without reworking the whole machine.

lead screw moment loading Moment Loads (Avoid if possible)

A moment load or off-centre load is a load that is applied to the nut off of the acme lead screws axis. Reference the image. As you can see in the image, a moment load will unevenly load the nut which can cause binding and uneven wear on the nut.

It is often hard to completely avoid moment loads when using lead screws in CNC routers. However, there are ways to reduce the overturning effect of moment loads.

1.) Reduce the length of the moment arm. Always try and attach to the nut as closely as possible. This will reduce the moment force, therefore reducing the overturning effects.

2.) Make the moment and attachment point very rigid. If the arm or attachment point is weak, there will be more of a tendency to flex causing the nut to overturn.

Of course, the best way it to overcome moment loading is by keeping the load point in line with the lead screw axis, which is called thrust loading.

Thrust loading (Preferred) thrust loading lead screw

Thrust or axial loading is when a load is applied parallel to and along the lead screw axis. Reference the image. This is the best form of loading for a lead screw. When designing a CNC router, it is best to try and design your system to achieve thrust loading.

Tension and Compression Loading

Tension and compression loading is illustrated in the image below. Both of these loading types are acceptable loading types to a degree. It is always better to have tension loading than compression. Just like a rope or cable, it is far easier to pull than push. However, for a CNC router, you must utilize both compression and tensile loading unless your system utilizes a rotating nut design. With a rotating nut, the linear screw is always being pulled upon.
compression and tension loading

For everyone else, make sure that your lead screw has sufficient column strength for compression loading. If the compression loading exceeds the column strength, the ACME lead screw may buckle. This will be covered in greater detail in the ACME lead screw design considerations section. You may also check out the calculator’s section. There you will find a column strength calculator.


Back Lash and Back Driving

Back Lash and Back Driving

Backlash and back driving are both concerns for the CNC router builder that need to be understood and addressed. Let’s get to it.
Back-Lash

The technical definition of backlash (lash) is the amount of free movement between the nut and the lead screw without rotation of the nut. In short terms, it is the “play” between the nut and lead screw. If you take a nut and thread it on a bolt or threaded rod, you should be able to pull and push on the nut along the bolts length and feel slight movement. The lash is the factor that may reduce repeatability and accuracy in a machine.
CNC backlash

The lash is usually referred to as backlash because of the when the lash affects the machine. If the CNC machine only moved in one direction, the gap between the nut and lead screw threads would always be compressed and would not affect the machine. Reference the image to the right.

In normal operation without an anti-backlash nut, the lash allows the lead screw to rotate slightly without engaging the nut. This means that the desired position is lost.

The lash between a nut and lead screw will increase as they wear. This is why most acme nuts used on CNC routers are where compensation anti-backlash nuts. This will be cover in the ACME nuts Section.

There is also a radial or transverse backlash. This can be noticed by trying to rotate the nut perpendicular to the lead screw axis. Again, this may be minimized by anti-backlash nuts. Also, increasing the length of the nut will help.

Back Driving (Creep)

Just as lead screws can convert rotary motion into linear motion; it may also convert linear motion into rotational motion. This conversion from a linear force into rotational force is what is known as back driving or creep in the CNC industry. ACME screws are often considered to be self-locking, meaning back driving is not an issue. However, if the efficiency of the lead screw is sufficient, then back driving may still occur. Generally, any linear motion system with efficiency greater than 50% may back drive or creep.

Furthermore, vibration may assist the system to back drive. Systems that typically would not creep may if vibration occurs.

For the CNC builder, this primarily affects the z-axis system. The combination most attributing to back drive is a z-axis servo driven system with a ball screw or rack and pinion. Because gravity affects the z-axis system, counterbalance or springs are usually used to deal with back driving issues. A very efficient system will drive itself into the bed or piece if power is lost and there is no braking or counterweight system.

Also, back driving may occur on a smaller scale only causing enough position loss to ruin your project. It is vital to keep this in mind when designing your CNC router. Even if using acme screws, leave some room in your design for an anti-back driving system (springs etc). The simplest approach is to use a low lead acme system for the z-axes.

Continue To: Loads Associated with linear Screws


ACME Lead Screw Efficiency and Life

For the CNC router builder or buyer, some of the utmost concerns with the ACME lead screw are efficiency and life. Both of these aspects can make or break a quality machine. Unfortunately, it is impossible to give an exact life or efficiency for ACME screws, but we can make some decent estimation on both. The best way to obtain this information is from the manufacturer.
Life

The life will depend on maintenance, environment, lubrication, friction, and proper usage. On many CNC machines, the ACME nut is made of a high strength polymer plastic increasing the life of the screw significantly. With proper maintenance and lubrication, the lead screw will probably outlast many other parts of the CNC router. If the nut is bronze or steel, the wear on the screw will be much more significant, but this is rare in modern machines.

Screws with plating may have less life as the plating can chip and wear. However, most screws used in CNC machines are polished, not plated. For the CNC router builder, keep this aspect in mind. In most cases, the ACME screw will outlast the ball screw. If you like, you may use this lead screw life estimator, but keep in mind it is the ONLY estimation. The metric version may be found in the calculator’s section.
Efficiency

The efficiency of ACME thread form has increased significantly over the years and will continue to improve as new materials and thread forms are created. The efficiency of a screw and nut assembly is determined by the lead, nut material, and friction. There is a wide range of acme screw efficiency percentages, it can be anywhere from 10% to 80%.
For the CNC Router Builder

To maximize the efficiency of the assembly you choose, try and design your system with the highest lead as possible. Increasing the lead greatly improves efficiency. For a Stepper Motor system, a high lead setup also maximizes power output as most torque is at low RPM.

Also, always choose a plastic nut when possible. This will help reduce friction and increase screw life. If the efficiency of the screw is not provided by the manufacturer, use the Efficiency Estimator

The calculator is based on data of over 200 ACME screws and their efficiency percentage. The formula used is based on diameter for friction estimations and lead. It assumes that the lead screw is steel or steel allow and that the finish is of good quality (centralizing threads).

Continue To: Back Lash and Back Driving Definitions


The Lead Accuracy and Straightness of Acme Screws

For the CNC builder and buyer, the lead accuracy, tolerance, and straightness of acme screws are very important considerations. The following section will cover these aspects of lead screws.
Lead Accuracy

Lead accuracy refers to the differential of the theoretical and actual linear distance travelled by the nut based on the lead. Lead accuracy is given as deviation per linear measurement. For example, the lead accuracy of a lead screw may be 0.005 inch/foot.
Eaxmple:

Lead accuracy = 0.006 inch/foot
Lead = 0.25 inch

So if the acme nut travelled 1 linear foot or 48 revolutions the deviation would be plus or minus 0.006 inch. The actual length travelled could be 11.994 inch or 12.006 inches. You may like to check out this simple acme screws accuracy calculator.

For estimating the machines capability, it is important to know and understand the lead accuracy of the acme screw. Sometimes the lead accuracy is not given and must be acquired by contacting the manufacturer. Each manufacturer has standards for lead accuracy tolerance.
Lead Screw Straightness

Lead screw straightness refers to the tolerance of straightness of the screw. Straightness is also given as deviation per length, such as 0.001 inch/foot or 1 mm/meter. The straightness of a lead screw is important for maximum rpm capabilities.

It is important when handling lead screws to always keep them supported evenly as possible to maintain the straightness. In general, quality rolled and milled lead screws of the centralizing type will maintain a 0.010 inch/foot tolerance. However, this is again up to the manufacturer. Ground lead screws usually hold a higher tolerance than rolled or milled, as low as 0.001 inch/foot tolerance.

If you where end machining the acme screw, you must support the lead screw throughout its length in order to maintain the straightness while milling. If you have the end machining done elsewhere, request that they straighten the lead screw after the machining is performed.
Continue to: Life and Efficiency of Lead Screws


Why the Acme thread form is used for lead screws.

The Acme thread form is most often associated with lead screws or CNC drive systems and for good reason. These threads are broader, stronger, and squarer than standard V-shaped threads. This makes them ideal for power transmission and carrying loads. However, there are other thread forms used in CNC drive systems as well. Reference the image below.
CNC lead screw

The acme lead screw is probably the most widely used in the united states because of the availability. Also, the availability of acme nuts is far more readily available than worm screws. ISO Metric Trapezoidal is often found on European CNC systems. Some of the mini lathes use this thread form.
unc thread form

Standard threads have been used in hobby CNC routers with some success, but it is not recommended for machines larger than about 12 inches or so. It is also not recommended for machines designed for large loads. In any case, it is usually worth the investment to buy at least general purpose or stub acme thread lead screws, but centralizing or higher grade acme screws are always recommended. If typically threaded rod is used, be sure to use an oil-based grease to diminish friction and galling.

Overall acme screws have much better wear properties, load capabilities, and tolerances, than standard threaded rod. Since the threads are thicker and wider, they operate better in environments with dirt and debris as well.

Continue to: Acme Lead Screw Straightness and Accuracy


The ACME Lead Screw

The following sections will discuss the terms associated with the ACME lead screw. As with any piece of hardware it is best to know the terminology associated with that product before using or purchasing.

Major Diameter

The major diameter also referred to as the land or outside diameter, is the outer most diameter of the lead screw. For a ½-10 ACME lead screw, the major diameter would be 0.5 inches nominal.
acme lead screw major diameter

Minor Diameter

The minor diameter or the root diameter of a lead screw is the innermost diameter of the lead screw. The minor diameter is the diameter of the lead screw between the threads.
Thread Helix Types

There are two types or directions of the thread helix, left hand and right-hand threads. Most Common threads are right-hand threads. Always be sure to check if the lead screw is right hand or left-hand threads before purchasing. Right-hand threads are denoted (RH) and (LH) for left-hand threads. The saying “righty tighty lefty loosey” describes a right-hand thread and nut, where turning the nut clockwise or right moves the nut further along the lead screw. Left-hand screws are opposite. The image below illustrated how to identify left and right-hand threads.
left and right-hand threads

ACME Thread Forms

ACME threads are different than threads found on the standard threaded rod. The trapezoidal shape of ACME threads lends it well to power transmission. ACME threaded rod vs UNC or “typical” threaded rod.
ACME Thread Classes

Among ACME threaded rod or lead screws there are three major classes; stub, general purpose (G), and centralizing/centering (C). The stub is the rawest form of acme threaded rod. Although it is still possible to make a CNC drive system from this type of ACME rod, it is usually worth paying the extra price for a general or centralizing ACME thread form. ACME stub threads do not have the straightness or tolerance of other classes. The efficiency of this class of rod is significantly less.

The general purpose is a step above the stub. General purpose ACME threads can be finished and have decent straightness but depend primarily on the manufacturer. General purpose ACME threads may need to be de-burred before use.

Centralizing ACME threads have tighter tolerances on the major diameter than the other types of ACME lead screws. Thus these are less likely to bind under side loads. Centralizing threads are most common for power transmission purposes. Also, this type of thread will have a nice finish and will be the most efficient of the three.
Pitch

The pitch of a lead screw is the linear distance between the threads.
Lead

Lead is the linear distance the nut travels and the lead screw for every one revolution. The lead is equal to the pitch times the number of starts.

Lead = Pitch x Starts

lead screw starts

It is important to understand the lead of a lead screw as it is one of the most important aspects when choosing a lead screw. Check out the lead and pitch calculator here.

Screw Starts

There may be more than one thread “strand” on a single lead screw. These are call starts. Multiple start lead screws are usually more stable and efficient at power transmission.

You can identify the number of starts of a lead screw by viewing the end of the lead screw. The figure below represents the most common types, single, double, and four start lead screws.

Lead Screw Notation

Lead screw size and TPI are usually the first specifications given. TPI refers to the threads per inch. For example:
1/2″-10

This lead screw is ½ inches in diameter and contains 10 threads per inch. The information above does not specify the lead or number of starts. The number of starts should be stated elsewhere. This format is the most common.

Caution: Some manufacturers lead screw information is given by diameter and turns per inch, NOT threads per inch. It does not matter when discussing a single start lead screw. However, the threads per inch and turns per inch are NOT the same when discussing multi-start lead screws. The simplest way to check is to verify the pitch and lead. Always be sure to check.

Continue to the next section: The ACME Lead Screw Lead Accuracy and Straightness

Or Visit: Acme Threads vs Typical Threads


Y-Axis Frame Design, Step 3.

Now that we have looked at the X-axis frame designs and considerations in a do it yourself CNC router design, let’s look at the Y-axis gantry assembly.

The gantry design is the most popular design in the do it yourself CNC router community. It is popular for a reason, it works. When you build a CNC router, it is important to keep the design trade-offs in mind. No matter your budget, the parts you have, or the material you use, there is a design that is best for you.

The gantry design is a proven design for “do it yourself CNC routers.” However, there are still many things that you should be aware of.

From a design standpoint, you want your gantry to be stable and balanced. Design the CNC gantry to meet the forces that it will encounter. This will prevent excess stress and strain on your bearings, lead screw, motor, etc.

In order for you to be able to design and build your gantry to meet the required forces, you first need to identify and understand the forces involved.

Let’s take a look at the forces evolved with a do it yourself CNC router gantry.

The above image illustrates a side view of a typical do it yourself CNC router gantry.

Take a minute to look over the image, there is a lot there. Now let’s discuss what is happening. It may seem confusing at first but it’s rather simple once you understand what is taking place. We will discuss.

Center of gravity/mass
Forces
Moment
Y-Axis

Let quickly identify the labels above:

D1 = the distance between the cutting tool (the router bit) and the centre between the two Y-axis linear bearing rods/rails (D3).

D2 = distance between lead screw/ linear bearings and the bottom Y-axis linear bearing rail/rod.

D3 = distance between the lower and upper Y-axis linear bearing rods/rails.

D4= distance between the 2 linear bearings that sit on the X-axis linear bearing rods/rails.

Now we will look at the forces evolved.

The Technical Explanation: CNC Router Forces
(scroll down for the short Version)

The image above illustrates a gantry that is moving from left to right as you look at the screen. It is being pulled or pushed by the CNC drive system at the bottom. Now, the router spindle at the bottom. Now, the router spindle is lowered and it starts cutting.

The cutting action opposes the movement of the gantry resulting in a cutting force. The cutting force varies according to the gantry acceleration, spindle RPM, and the chip load. The chip load depends on the bit you use, the RPM, and the material. We will get into these details when we discuss the CNC router spindle. For now, just know you have a cutting force opposing the movement of the gantry.

Just so you know, a force is equal to the mass of multiplied by its acceleration. The units of force are lb-f (pounds of force) in the English system or the Newton in the SI system.

The cutting force results in a moment, which is moment A in the figure above. A moment results when you have a force applied at a distance. A moment has units of lbf-in or N-m, we usually call a moment force torque.

Moment A, in the image above, is the result of the cutting force being applied at the distance D1.

Moment A = D1 x Cutting force

Example:
If the distance D1= 12 inches and the cutting force is 5 lb of force. Then the Moment A would be 5lb x 1ft = 5 ft-lb of force. ( I converted 12 inches to 1 foot) You can see that even if the cutting force remains the same, the longer the distance of D1 the larger the moment will be.

Moving on, the Moment A results in 2 forces on the Y-axis linear bearing rods/rails.
These resulting forces are forces A and B in the figure above. Force A and Force B are equal to each other Force A = Force B.

Force A = Moment A divided by 2 divided by ½ of D3 this equals to

Force A = Force B = Moment A / D3

You can see that as the vertical distance between the two linear rods/rails (for the Y-axis linear bearings) grows, the resulting forces A and B shrink which is good. Why is this good? It reduces the amount of centralized torque that is on the gantry itself.

Moment B will decrees as force A decreases.

Moment B = D2 x Force A

Moment B is what causes the whole gantry to rock or want to rotate due to the cutting force. This is not a good thing. You want to decrease Moment B as much as possible. Why?

You want to make have equal amounts of force on your set of linear bearings as possible. This will reduce deformation and chatter in your machine.

There are two ways to reduce Moment B.

1) Reduce Force A
2) Reduce the D3

A well-designed machine keeps force C and force D to be as equal as possible. And that is the goal.

Force C and D are the sums of the weight of the machine and resulting forces that occur due to moment B.

We also need to consider the weight of the gantry and try and calculate or guess where the centre of gravity will be and keep that directly in the centre between the two separated bearings (½ D4). The centre of gravity is the point at which the machine would balance.

That is why you often see the gantry upright side arms slanted backwards an a do it yourself CNC router. This compensated for the weight of the spindle which hangs our over the Y-axis linear bearings. When you build a do it yourself CNC router, you want the centre of gravity of the whole gantry assembly to be directly between the two linear bearings. Or if you have a stationary gantry and a mobile bed, you want centre of gravity to be in the centre of the bottom of your gantry side arms.

This assures that your machine is balanced and could stand own its own. This applies equal load on your bearings.

The short answer (summary)

When you design or build a do it yourself CNC router, keep the following in mind:

Try and keep the distance between the X-axis lead screw and linear bearings, as close as possible to the bottom Y-axis linear bearing rods/rails. Or as close to the centre distance between the top and bottom Y-axis linear rods/rails. (Minimize D2)

Keep the spindle plunge arm on the Z-axis assembly as short as possible and make that arm out of rigid material to prevent flexing. A normal Z-axis arm travel is anywhere from 3 to 6 inches. (Minimize D1)

Calculate or estimate where the centre of gravity of the gantry will be located, including the spindle. Design your gantry side arms to compensate and place the centre of gravity (CG) between the front and back X-axis linear bearings per arm. (CG should be located at ½ D4 and as close to X-axis lead screw as possible)

Maximize the distance between the upper and lower Y-axis linear bearing rods/rails but still allow for clearance under the bottom rod/rail for your max Z travel. (Maximize D3)

Other considerations

A good gantry design is one of the most crucial factors for a quality do it yourself CNC router. As with all DIY CNC routers, the budget is a concern which means the material is also a concern. Try and visualize and estimate the forces evolved and make your do it yourself CNC router design work with the materials you have.

If you would like a more thorough analysis you may consult us. We offer free engineering design analysis of your machine. We can help you find:

Specific CG location
Material stress and strain analysis
Dynamic simulation of your machine
Material selection
And more

Remember, we will discuss more on the gantry design in a later section. Topics such as lead screw placement, motor placement, linear bearing attachments, etc. Which are all important consideration with a do it yourself CNC router project?

Now let’s take a look at Step 4: the Z-axis assembly design for the do it yourself CNC router.


CNC Router Table, Step 8

CNCSo after picking all the moving parts designing your axis and making a very sturdy machine you need to compliment it with a very sturdy and well thought out cutting bed.

Get it right and it will make your CNC’ing experience enjoyable.

Most people that by second-hand machines overlook the cutting bed and always an underestimated part of the machine, when designing your machine it is important to understand what you will be using your machine for as there will be a bed style that fits your machine best.

For example, if you are making a bar length machine you will be better using a fixed clamping setup.

Or if you are building a machine for prototyping you will probably be working with materials of all shape and size, this would probably be best suited to a T-slot style bed, which offers many different clamping solutions, then again you may be making the same product over and over in that case you would get away with a flatbed with preconfigured holes for clamps, so their variety of different pieces or same product repeated.

On modern tables, you may find they have a combination of cutting beds while this is good its also a lot of work to get right and consumes a lot of space.

If you are designing and building your own CNC machine, your options are unlimited the only limit you have is your imagination and budget to contend with.

With that said we have 3 main cutting beds to cover for the DIY builder.


cncT-Slot Table


CNCThe Vacuum Table


CNCThe "Disposable" Table Top


CNC Motor Selection, Step 7.

CNCCNC motors are literally the heart of the CNC machine without them we would not be able to drive the axis back and forth and your spindle motor would just sit in one position.

The type and size will define the machines accuracy precision and speed.

There are two main types of CNC motors these are servo and stepper motors, and within these two motor types, there are 7 good options for a CNC build.

Both stepper and servo motors have advantages and disadvantages and selecting the right one to make your machine perform well and efficiently, picking the right one will be covered in the articles below.

In the following articles we will be covering aspects of all types of CNC motors, selecting the motors goes hand in hand with selecting the right CNC controller and selecting the right drive components that we covered in the last section.


cncTerminology linked to Motors


CNCStepper Motors Vs Servo Motors


stepperStepper Motors


servoServo Motor