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50% of the problems are servo problems. Any one can troubleshoot with basic knowledge. Flowchart with simple YES or NO questions. Text and picture help to answer the questions.

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 This is the go to app to learn the fundamentals of CNC Machines. Do you know that it costs an average of $5,000 for a service engineer to walk into your facility? Using this troubleshooting app you can learn how the machine and its components work.

The Step by step flowcharts included in these two apps will walk you through to troubleshoot your problem. All you have to do is answer YES or NO.  If you don’t know the answer, you have help in the form text that can help you answer the question. The idea is to narrow it down to one or two faulty parts. Sometimes buying two parts is cheaper and faster than getting a CNC engineer to visit your site.

We have alarm and symptom approaches to troubleshooting.

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Visit home page to learn about what is done so far and what else is on the pipeline.

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Who are we: This is a troubleshooting knowledgebase app is brought to you by CNC Onestop, Inc. who are experts on all kinds of CNC and PLC based machines. We also sell parts and we have special offers, free tech support and free restocking privileges for those who buy parts from us. We also give training to people in your location or in our location.

History:   Our Chief Engineer Ven Swaminathan has a Masters degree in Electronics and has over 35 years of 15 hour a day hands on service experience. Even today he is much sought after in almost all of the aircraft and automotive companies in the US. He personally has worked for 1200 companies in North America.  He is experience in more than 100 type of controls and a few hundred models of machines. His passion towards bringing his knowledge to the machine tool owners to do it themselves has evolved into the tool called Share Your Expertise.

Plans: We are launching the first version of this app for free. We will be adding more makes and models of drives, spindles and controls. We also provide help through Apple FaceTime for a fee. We also have support engineers who can come to your place and troubleshoot your machine. We are all about quality and one-stop troubleshooting. There are no machines we cannot fix. When we walk out we will have your machines running.  We provide the parts you need and will be one-stop solution for you. Either do it yourselves or take our help to the extent you like. We want to be the one-stop center for your CNC needs.

Partners:  We are also looking for independent service engineers all over North America. If you are an independent service engineer and if you have what it takes to troubleshoot any machine send us your resume.  If you meet our expectations, you will have an opportunity to represent us. Our engineers work about 3000 hours and wait time is 3 weeks to get one.  We have contract with several OEMS to service their machines all over North America.  Our average is one out of 50 people who apply qualify to work with us. We are for quality than quantity. We look for technical ability, attitude to troubleshoot, maturity to deal with the customer and above all a CAN DO attitude. If you are the best in the business, email us.



Knowledgebase on CNC

4. Different parts of a CNC Control

4.1 Numerical control: Photocell was used to detect the holes punched in a paper tape. NC control used this information and sent a command to the servo drives to make the move. This is how simple the NC machine was in the earlier stages of 1960s.

4.2 CNC Control: After the development of computers, which comprised of memory, input/output mechanism, display unit and a keyboard, it was possible to create, edit and delete a program with ease. Combining the computers to NC machines made it a CNC machine. The following topics will deal with different components of a CNC machine.

4.3 Power supply is the heart of a machine. Without a power supply nothing will work. A power supply  gives out DC voltages for ac input. There are different kinds of power supplies. The common one is Linear.

4.3.1 Linear Power supply: This is a basic power supply. For an ac input it gives out specified dc output. Even without a load it will put out an output. It regulates the output voltage up to a designed load. Some power supplies will blow a fuse if it is overloaded and some will go into a fold back mode putting no output but still saving the power supply. The efficiency is low. It is bulky. The main characteristic in service point of view is, it should put out a voltage with no load. It can have several protective circuits. 

4.3.2 Switching Power supply (SMPS):  Switching mode power supplies are advanced power supplies. The efficiency is high. With higher loads it goes into protective mode. This makes it more reliable and gives a longer life span. It is also possible to design for varied inputs like 100 to 240 volt input for the same output. It is compact and can weigh very less. Most of the laptops has this kind of power supplies.

It has to be tested with the minimum specified load. Though some of them will put out voltage without a load connected to it, the earlier models will need a load to be connected to it.

4.3.3 Checking of Power Supply:

a)   Machine may have more than one power supply. One for the control, one or more for drives one for spindle, one for external source (Mostly 24V). First thing to find out which power supply is being suspected.

b)  Make sure the input ac is available at the terminal of the power supply to the specification. Always measure across the ac terminal than going across terminal to ground.

c)   Check all the possible dc outputs. Dc output should be 1.414 times the A/c input. Be aware the PSU may have its own transformer inside to supply different AC inputs for several possible DC outputs.

d)   Check the dc outputs with no load or slight load

e)   If there are some extra wires other than input they could be acting as switch from external sources. Make sure they are at where they should be.

f)   Remove all the loads and check for resistance across the wires like 5V and 0, 12V and 0, 24V and 0. Make sure they are not shorted, by measuring the resistance across the load. If they have very low resistance or short, then the problem is in the load side and not in the power section.

g)   Once it is decided, it is the power supply, check for any blown fuses in Input or Output side.

h)   Remove all the wires and check for the Input resistance on the power supply and whether it is shorted.

i)     Similar to the above check and make sure the Output resistance of the power supply is not shorted either.

j)    If you suspect only one power supply load side is shorted, Load Testing can be done. Calculate the power output by multiplying output voltage by rated current, and find a bulb with lower than that wattage. Connect it and see if it comes on. A thing to remember is, if it is a switching PS all the outputs should have a minimum load.

k)   Some power supplies might have a terminal for Remote Sensing. This senses the voltage at a remote place where the load is and adjusts the power supply to put out a voltage specified at the remote place. A 5V PS can be 5.2 at the output terminals, where as it could only be 5V at the remote place.

l)    Some power supplies may have a capacitor externally connected. Make sure it is OK.

4.4 Power Conductors: The wires, which are carrying the power supply to different section, have to tight with least amount of drop in the wires as well as in the connectors. Otherwise will behave unpredictably.

4.5 Processor: Processor is the brain of a CNC machine. When this is dead it is like; the machine is in  coma. It can not apply logic. Normally the processor board can be one or more than one boards, depending on when it was made. It takes the information from the memory, processes it, and gives the appropriate command to the axis and spindle. While doing so it takes input from the input board as to what is the status of the machine like whether it is in home or not, has it been homed yet, what is the position of the tool changer etc. Then it processes that information through logic called "ladder" and decides whether it can ask the Axis to move, so that it will be a safe movement. Also it gives output to tool changer, pallet changer, based on the present status of the machine.

 Usually processor boards will have some light indicating that it is running or halted. This gives a good indication about whether it is having a problem or running OK. Lot of machines have motherboard which will have processor board built in with some other features. It is always advisable to read the specific description about the processor board. This information can be found in the control manual.

4.6 Memory: It is same as in human memory. The commonly used memories are RAM, EEROM/EPROM/Core memory and Hard disk/Bubble memory. Core memory was used to store all the information. This kind is not being used because of poor reliability and cost. Most of the recent machines use EEROM/EPROM for storing the system information, RAM for parameters, and hard disk for programs.

RAM - Read and write memory, fast access time

ROM - Read Only memory

EEROM - Electrically erasable ROM

EPROM - Same as above, but can be erased only under UV rays.

Hard disk/Bubble memory - Can be read and wrote but not as fast as RAM, but cheaper.

Usually some memory boards have s few lights indicating whether it is faulty or Ok. Parity alarm is the most common alarm. If the memory gets corrupted or loses the information often, is an indication of memory board starting to go bad. It is always a good practice to have the memory backed up. When this boards goes bad the machine will look like brain dead. The way to differentiate between the processor bad or Memory bad is from the lights on the boards. Some controls will put up the message on the screen if the memory is bad.

4.7 Control Wires: These are the wires which takes feedback as to what is status of a machine and feeds it to input board. Also it gives the command when some movement has to take place. These wires are connected to Input devices and output devices. If these wires not intact or if they are not properly grounded, the system will behave inconsistently, and one will see intermittent faults.

4.8 Input Board: This is most cases is a separate board connected to input devices and the processor. This might take 5V, 12V or 24V based on the make. This board converts the voltage to 5V and sends it to Processor in a sequential manner. Most of these boards have one led for each input, which is helpful in troubleshooting.

4.8.1 Input Devices:

The switch comprises of two parts, one is a switch and the other is a contact. The switch is connected to a contact. This contact can be normally open (NO), or normally closed (NC). For example the wires going out of the contact can be connected in such a way, that it can put out a higher voltage or lower voltage when the switch is actuated.

Pressure switch - Can be set to actuate the contact when the set pressure is reached.

Limit switch - Can be set at a particular place, to give a signal that particular object has reached that position

Flow switch - Can be set to actuate the contact when the set flow is reached.

Float Switch - Can be set to actuate the contact when the set quantity is reached.

Magnetic reed switch - Same as a limit switch, but uses magnetic principle instead of physical contact.

Proximity Switch - Proximity switch senses when an object reaches proximity to the switch. Since it is fully electronic there is no contact. Instead it has a PNP and NPN model. PNP puts out the supply voltage as output when a object comes close to it, and NPN sinks the supply means puts out 0V. Few more things to remember are

a)      Some switches have 3 wires and some has two wires. The extra wire is common.

b)      Some of them will sense only metal and some a few more metals.

c)      They work under 3 different principles magnetic, inductive and capacitive

d)     Some can be mounted flush and some cannot be.

e)  Some have a led showing the change of status. But in some cases though the led may seem to work, the voltage may not all the way go to zero or vice versa. Malfunctioning is common on these switches.

4.9 Output board: This is normally a separate board connected to output devices and the processor. This board receives 5V signal from the processor in a sequential manner and converts them to. 12V, 24V or 110V AC based on the make.  Most of these boards have an LED for every input, and in some cases fuses too. This is helpful in troubleshooting. This board may drive directly drive the output device or it maybe connected to a relay. In turn relay may be used to drive the output devices.

4.9.1 Output Devices:

Solenoid: Most of the machines have hydraulics and pneumatics. This is what is used to actuate the auto tool changer, auto pallet changer etc. Solenoid is the most common form of output device. They work on 12V and 24V DC, and 110V AC depending on the choice selected by the OEM.

Light bulbs: Another output device to indicate the status or alarm.

One speed motor: Coolant motor is an example.

Bigger contactor: A bigger contactor might be connected to an output device to actuate a bigger motor like hydraulics motor.

4.10 Servo Board: This board is located in the CNC Controller. This gives command out to the servo drives and takes feedback in from the positional feedback units. It also performs the same function with the spindle. In some occasions, if the tool changer motor is configured as a drive, than it is controlled by this card too.

4.11 Peripheral Board: This board is part of the CNC control. This board is used to interface with Keyboard, Monitor and peripheral devices like Reader, Punch, RS232C etc.

4.12 Peripheral devices: Peripheral device is used to a) communicate commands to the processor b) load a series of commands in to the memory and c) give output from the memory or processor to a CRT or a punching device.

 4.12.1 Keyboard: Every control has some form of communication to input the commands. Keyboard is one of the commonly used interfaces. This is connected to the Interface board. If it has more switches for specific function, than it is connected to the input board.

4.12.2 Monitor: CRT is the most common form of display unit used in the machine. This is connected to the peripheral board too. LCD is widely used because of its compactness.

4.12.3 Paper tape reader: It is the oldest form of Input device. Though the technology of reading has changed, functionally it does the same function of loading. Paper Tape: Paper tape is nothing but a roll of paper in which holes are punched. Every character has a set of holes assigned by ISO or EIA standard. That is what is punched in these tapes. When a paper tape reader reads this, it writes that alphanumeric character into the memory, as we would type using a keyboard. It is just faster and more accurate way loading the information into the memory.

4.12.4 Magnetic tape reader: This is another device using magnetic tape instead of paper tape. Magnetic Tape: This is another method of storing the information using magnetic principle like any audio or videotape. It has a longer life and is stronger.

4.12.5 Output Peripheral device: It is used to punch out the contents of the memory. Paper tape Punch is the most common form of output device.

4.12.6 Input/Output Peripheral device: RS232C:

This has replaced all the older forms of slow and expensive devices. Also this can function as input/output device. All is needed is a computer with a serial port. Also one computer can be networked to several controls. Involves some setup time.

 Though it is simple, it could be tedious to troubleshoot. Some steps to watch

 1)  Baud rate, start bit, stop bit, parity and handshaking are the important parameters that should be same between the machine and the computer,

2)   The COM port should be set right on the computer. - This can be confirmed with control panel of window settings.

3)     The first character, which each control recognizes, is important.

4)  After all this is set the equipment which is going to receive the file should first be ready. Only after that sending equipment should be started.

5)  After all this if it does not work, get a working computer and make it work with the machine or take the present computer to a similar machine and make it work.

6)   The board in the control can cause it not to work or the board in the computer can also cause it not to work. The best way is to swap and find out where the trouble is.

7)    The cable is an important piece of the pie. Make sure the cable works with some other machine. Floppy Disk: This is becoming one of the widely used devices. It is reliable, fast and no setup is needed. The amount of memory it can hold on a disk is limited. Network Card: The very latest machines have the capability to be networked. This is extremely fast. DNC/Drip feed: This is used in older machine where the memory was not enough. Mostly this was used in Mold components because the size of the program. A computer is connected with the machine and it loads to the extent the CNC can take. It stops when CNC sends a signal to stop and start loading into the memory.

There are several in between devices available to do this task. They act like they are tape reader and take the program from the computer and feed it to the control as it seeks the information. Care has to be taken to have as much information and manuals as possible when you buy one of these devices.

4.13 Hand Wheel: It is constructed like an encoder. This is used move the axis in handle mode. It is connected normally into servo board on the controller side. In the very latest machines this unit's out put is converted into serial and given to the controller as a serial input.

Scope can be used to check the older ones. There are two signals out A and B. They are 90 degrees apart. The output is a square wave with 5v in Magnitude.

4.14 Battery Backup: Most of the machines have a battery backup to store the contents of the RAM. As mentioned earlier RAM is used for storing parameters. 

When the battery low alarm occurs, never shut the machine down. Keep it on and exchange the batteries. The polarity of the battery cannot be changed. Change all the batteries at once. Keep the machine on for a while after the battery is changed, so as to let the battery charge up.

4.15 Executive Tape: After the evolution of CNC controller though, all the system information was put into the tape and loaded in. Whenever the system memory was lost, the entire system has to be loaded back. The disadvantage was the user could not edit it.

4.16 Parameter Tape: Later stages when different kinds of more reliable memories were developed, the system memory was divided into two. One is secure system, which is stored in the form of EPROM etc. The second is variable piece of information called parameter was loaded into RAM or EEROM. This was accessible by the user to edit when necessary. When a memory is lost only the parameters needed to be loaded and not the entire system memory. This tape is called the parameter tape. Now a days it is uncommon to find tapes instead they are backed up in the form of disk through RS232.

Knowledgebase on Drives, Motors and feedback devices

4.17.1 DC Motor: DC motor has an armature and a field, wound over poles (also called stator). The field can be a permanent magnet or a winding which needs a power supply. The armature is the rotor, which is also a winding. Since it is the central rotary part, a commutator is needed to transfer the power from outside. The speed control is achieved by varying the armature voltage or field voltage (if there is one).  Speed increases when armature voltage is increased or field voltage is decreased.

Resistance check: DC motor needs a lot of maintenance. Since carbon brushes are used, they wear out and makes the commutator and the surrounding area dirty. It is good to take the brushes out and blow out with dry air, once in 3 months. Also check the resistance across the armature, and it should be in single digit. If it is more, that means the contact between brushes and commutator is not good.  Also check the resistance between the ground and one terminal of armature (ground resistance). It should theoretically be fully open (Infinity). But it will be Meg ohms. If it is in kilo ohms, it needs cleaning. In case of field the resistance is in the range 100 to 700 ohms. This check gives a quick idea of whether there is problem. But a Meggar check is the foolproof check for ground resistance, since it generates about 400V AC. When doing a Meggar check there is a chance to damage the circuit to which the motor is connected. Always disconnect the motor from rest of the circuit to have a good reading.  There is a good chance there is more than one motor of the same kind in the machine. It is always a good idea to compare it with another good motor. If necessary the motor could be switched, to see whether the problem switches.

Current Check: Check the current using clamp on meter and see if it is abnormal compared to the nameplate specification of the motor. It is not abnormal for a motor to draw up to 8% of its rated current, at standstill. Also check for almost equal current on both directions of movement (Axis which moves against gravity will have unequal current). The axis against the gravity normally draws more current than the other axes.

Heat check: If the motor is warm it is OK. If it too warm or not tolerable by hand-touch, then there is a problem.

Cable check: The cable, which carries current to the armature, should have almost zero drop. Check for any short of the cable to the ground.

Not to do:

1)   Do not take a hard emery sheet and clean the commutator. By doing this more minute pits are made and it makes it worse. Try always to clean with dry air or alcohol using a soft cloth.

2)   IF YOU ARE SENDING THE MOTOR OUT FOR REPAIR SEND IT TO PLACE WHERE THEY HAVE A SOUND KNOWLEDGE OF THE MOTORS. LOT OF PLACES WILL PUT THE ROTOR IN A LATHE AND TURN THE COMMUTATOR AND QUITE A FEW TIMES THEY SHORT THE COMMUTATOR OUT. Lot of them do not balance these after they do turning. By sending it to the wrong place you might open a can of worms. If it is grinding spindle motor, take extra care in sending it out.

4.17.2 DC Drives: DC drive is the electronic control, which provides proper power to the motor. It gets a voltage called command voltage (Normally 0 to +- 10V) from the CNC control. It also gets feedback from a tach generator, which measures the present speed of the motor. The drive adjusts the output voltage based on the above two voltages. It also has some enabling signals. The OEM based on their machine tool design uses these signals. 

Command Wire/Tach wire to the Drives:  

The wires have to be tightly secured, and the ground wires must be properly connected according to the design. Troubleshooting sequence for a servo problem 

a)      Check whether there is input 3 phase or 1 phase.

b)      Check for the fuses in the drive on the power section.

c)      Some machines use a separate 1-phase ac supply to the control section. Make sure they are ok

d)     Check all the voltages like 5V, +12V, -12V are ok on the drive.

e)      Check for enabling signals, if they are used.

f)       Check and see whether there is some DC voltage coming in when it is commanded to move.

g)      Do the Tach-check mentioned in Tach section.

h)      Do the motor check mentioned in motor section.

i)     Balance adjustment: Make sure the motor does not move when Command signal is zero (Put a jumper across the command voltage, after removing the wires coming from control). If it moves adjust the balance potentiometer to stop the movement.

j)     Gain adjustment: If the movement is rough, or the motor overshoots, or if the motor vibrates;  reduce the gain, by adjusting the potentiometer on the drive

k)     If another axis has a similar drive, it can be swapped to see whether the problem switches to the other axis.

4.17.3 AC Motor: All the newer machines have AC motors. Every maintenance person comes across a question whether the motor is bad or the drive is bad. How do you tell if the motor is bad. Can you even tell if the motor is bad. Absolutely. The following tips will walk you through in a simple step by step process.

AC motor is cheaper compared to DC motor and needs less maintenance. It can be single phase (2 wires) or three phase (3 or 4 wires) for the armature. It has a solid rotor unlike DC motor.

Heat check: When the motor gets hot that is an indication either the load is high or the motor is going bad. Heat is generated due to loss of current inside the motor either by really working on the load or leaking inside.  First line of check is to measure the current.

Current Check: Check the current using clamp on meter. Use a true RMS clamp on meter. The new generation of motors will have a high frequency also on the lines. If the meter is not a true RMS meter it will give erratic readings. It is not abnormal for a motor to draw up to 12% of its rated current (See the name plate), at standstill or no load. If the no load current is higher than 25% further checks are necessary. If the load current is in the range of 50% and higher the motor will get hot which is OK.

Resistance check: Next check is to check the resistance across armature. Use a Multi meter to check the resistance. When you check the resistance make sure the motor is disconnected from the drive.  The resistance should be in single digits. Bigger the motor the lower the resistance. It is not unusual for a 50KW motor to have a .3 ohm resistance. Second resistance check is to check the resistance between the ground and one terminal of the  armature. (ground resistance). It should theoretically be fully open (Infinity). But it will be Meg ohms. This check gives a quick idea of whether there is problem.  This means the motor is leaking current to the ground. The third more thorough check is to do a Meggar check. You can pump in a higher voltage into the motor and check ground resistance. There are several companies sell Electronic Meggars. When doing a Meggar check there is a chance to damage the circuit to which the motor is connected. Always disconnect the motor from rest of the circuit to have a good reading. There is a good chance there is more than one motor of the same kind in the machine. It is always a good idea to compare it with another good motor. Some motor manufacturers would want the resistance to be in the 200 Mega Ohms. Anything below 10Mega ohms is a problem.

Cable check: If the readings are not high with Meggar check, make sure the cable is disconnected and the check is performed on the motor terminals. The cable, which carries current to the armature, should be completely open without motor or the drives disconnected to it. If the resistance is 100 Mega ohms or less the cable should be replaced.

Mechanical Check: Make sure the bearing are Ok. The air gap between the stator and rotor is 1mm. There cannot be much play in the bearings.

If you decide to send the motor to a shop, make sure you take readings of ohms before you send it out and check it when it comes back in.

4.17.4 AC Analog drives: AC drive is the electronic control, which provides proper power to the motor. It gets a voltage called command voltage (Normally 0 to +- 10V) from the CNC control. It also gets feedback from a taco generator, which measures the present speed of the motor. The control adjusts the output voltage based on the above two voltages. It also has some enabling signals. The OEM based on their machine tool design uses these signals.

Command Wire/Tach wire to the Drives:

The wires have to be tightly secured, and the ground wires must be properly connected according to the design. AC Digital Drive: Though the overall function is the same as analog drive, none of the signals can directly be measured. Also none of the pots can be adjusted. All adjustments are done on the control in the form of various parameters. The same tach can be used for positional and speed feedback.

 1)    If a drive is suspected just a simple switch which addresses the drive can be changed and find out if the problem switches to another axis.

2)    Some drives do have parameters stored in the drive itself. When a new drive is put in these have to be loaded; or else the drive will not function right.

3)     In most cases the gain and balance etc are adjusted in the control.

4)     VGN is velocity gain which acts as gain in analog drive

5)      PGN is positional gain. This comes in to play when there is a positional problem.

6)   The best way to handle this kind of situation is to have a backup of parameters. It helps to know the default parameter from the book.

7)  If it is humming, there is a parameter to suppress a particular frequency. This will help if the motor is not really being overloaded.

8)    The key is to always look at the load current. If it is within limits then the machine is OK. 

4.17.5 Feedback devices: Velocity and position are two the kinds of feedback. Velocity is for speed stability and this is achieved by the drive. Position is for the accuracy, and is achieved by the CNC control. Each feedback has few devices, which are normally used to achieve the objective. Some of them are analog and some are digital. The way they are connected will depend on the objective and the type of feedback device. Velocity or speed feedback devices: 

Tach - This is a device which is used to measure the speed of rotation of the motor. It is a generator, which puts out voltage based on the speed of the motor. This could be at the end of motor shaft, or it can be at the end of ball screw. It could be directly coupled or through a timing belt.

DC: This is exactly like a dc motor, but it generates voltage.

Check: Checking a Dc tach is similar to a DC motor. The resistance across tach terminal is about 150 ohms. The commutator needs periodic cleaning. The brushes need to be periodically changed and the spring tension needs to be checked. Also another check to perform on the commutator to go segment to segment and make sure that no two segments are open. This will cause the speed to fluctuate and not be steady. If it has a bearing it needs to be checked too. 

AC: This is exactly like an AC motor. Could be 1 phase or 3 Phase.

Check: The windings can be checked for resistance. All the three phase should read approximately same. If it has a bearing it needs to be checked too.

4.18 Spindle: Spindle is used to run the tool on part to machine it. Normally it is the biggest motor on the machine. Recent machines has AC motors, but it was more common to see DC motors. The main reason being is its characteristics to have high torque at low speed. It is common to see a Gearbox due to the reason that it can run at wide range of speed, constant torque and constant horsepower mode.

4.18.8 Regenerative braking: When the spindle is stopped from a high RPM to zero in a very limited time, the regenerated electricity by braking is dissipated in the power resistors. These resistors are big, and care should be taken in touching it. 

4.18.9 Orientation: This is necessary to make the tool changer work. The spindle has to come to the same spot every time when ATC is made to do the tool change. This is done by a mechanical device, and some electronics to make it come to the same spot (could be a Resolver or encoder). This position can be adjusted by electronics or parameters based on the type of drive. It is similar to homing sequence on the axes. 

4.18.10 Threading or non rigid tapping: Whenever these features are available in a machine, that means a Resolver is used to bring it to the exact position, every time. For more details check positional feedback devices. 

4.19 Positional Feedback: Means the feedback to the control to maintain the position (In case of digital there could be just one unit acting as both velocity and positional- This device could be either going to the control first and the drive next, and vice versa). This device is used to achieve the repeatability. In case of axis it repeats the position linearly and in case of spindle it reaches the same angle in a circle thereby able to produce a thread.

Resolver: This is the one of the earlier devices. This is an analog device. It can be checked with a scope. Still, it is hard to tell whether it is OK or not. Best way is to switch it, if it is suspected. One has to keep an eye for the bearing in the unit, which tends to go bad.

Encoder: This is a digital device, which is used as a positional feedback device. It has a photocell and three receiving cells, one for home position and two for incremental position. This can measured with a scope. Look for two square waves. One has to keep an eye for the bearing in the unit, which tends to go bad.

Pulse Coder: This is very similar in construction to the Encoder. This is used to provide both velocity and positional feedback.  The output from this unit can be configured to go to drive first and control next or vice versa. When there is problem, any one of the three areas, drive, control or pulse coder could be causing it and it is difficult to say which one.

Scale: Though there are a few kinds of scales available, Linear glass scale is the widely used one. The scale has to be kept clean. It has a reader moving on it linearly. This has a photocell and three receiving cells, exactly as in the case of encoder. There is home pulse every so often. Also there is a unit inside the scale, which can be moved to assign a home position pulse. The output of the head is connected to a Preamp.

Pre Amp: A Preamp is used to amplify the signals, which are generated in the head of the scale. Normally one would find more than one of these units. If one unit is suspected it is very easy to swap it. This is the easiest way to troubleshoot it.