Positioning Accuracy and Repeatability of CNC Machines: Why They Matter in Manufacturing

Positioning accuracy and repeatability are two important CNC machine quality indicators you cannot ignore. Whether in the aerospace, automotive or medical industries, producing parts within tight tolerances is critical in maintaining quality, reducing crap and increasing ROI.

This guide explains all fundamental aspects of positioning accuracy and repeatability, and why they matter for a superior CNC machining process.

What is Positioning Accuracy in CNC Machines?

This is the difference between the target position and the actual position of the CNC axis once movement stops. Position accuracy, therefore, measures machine tool accuracy by gauging how precisely a single movement gets to the tool’s target position.

For example, if the targeted dimension value was 50mm but the machine ends up producing a part that measures 50.03, then the positioning error is 0.030 mm.

· Why Positioning Accuracy Matters

Positional accuracy in CNC matters for various reasons, including:

• This feature measures how close the finished parts’ dimensional accuracy and tolerance are to the targeted dimensions
• High – precision machining, which is vital for some industries, is more possible by getting a machine with the highest positional accuracy.
• This feature allows for precise manufacturing, ensuring there is consistency in each batch throughout the manufacturing process
• A higher value translates to poor accuracy, which in turn not only causes variations in the dimensions of the batches produced but can also lead to geometric distortion.
• Poor positional accuracy raises the cost of production by causing a lot of materials to be scrapped and lengthening the production time as the process is redone.
• Machines with poor precision tolerance also tire operators as they will need to make adjustments to produce the required dimensions constantly.

With poor positioning accuracy, there is a high chance of machining undersized or oversized parts. Besides, there is a high possibility of geometrical distortion that will ultimately lead to parts rejection or costly rework processes.

· Examples of Positioning Accuracy Errors

Position accuracy error is largely determined by the quality of the CNC machine. These machining errors directly impact the final dimensions of the produced parts. They include;

• Hole placement errors
• Dimensional deviation in machined parts
• Geometric distortion
• Cumulative error across travel range
• Thermal expansion errors
• Servo overshoot/undershoot
• Ball screw pitch error
• Load-dependent positioning error
• Reversal error
• Batch inconsistency

What is Repeatability in CNC Machines?

The ability of your CNC machine to repeatedly get to the same position after going through the same set of movements is referred to as repeatability. Repeatability in manufacturing is simply a gauge of your machine’s ability to produce identical results consistently under constant conditions.

For example, if the desired dimension is 50.000 mm, then the CNC machine produces parts with 50.020 mm in every machining cycle, which demonstrates the machine has higher repeatability. However, this does not imply that the machine is accurate.

· Why Repeatability is Critical in Production Runs

Repeatability directly impacts production consistency, which in turn influences the mass production quality. It is critical in production runs for the following reasons:

• It ensures consistent quality in every part manufactured during batch production.
• Consistent in- tolerance productions help you save on production costs as it reduces the scrap produced during the manufacturing process.
• Uniform products help you lower the cost of inspection, as a sample produced is a representation of all the other parts.
• Fewer inconsistencies in the parts you produce foster goodwill with your clients, as they are guaranteed satisfaction with each purchase.
• Uniformity in all the parts allows you to mass produce, as all the parts produced will be identical to one another
• It optimizes production as yields and cycle times are predictable and therefore allow you to plan your production.

Remember, a CNC machine with poor repeatability simply implies the machine produces different parts in every production cycle.

· Common Repeatability Problems in CNC Machines

Repeatability in the final product appears as part-to-part variation. Inconsistent parts are often seen as a limitation in quality control, and they can influence your clients’ perception, as it lowers your production reliability.

The cause of repeatability problems in CNC machines

• Ball screw and bearing wear
• Thermal expansion
• Environmental changes
• Poor fixturing or workholding
• Backlash in the drive system
• Control or compensation issues
• Inconsistent cutting forces
• Backlash-related reversal error
• Loose mechanical couplings
• Vibration during cutting

Difference between Positioning Accuracy and Repeatability

How to Measure Positioning Accuracy and Repeatability

· Positioning Accuracy

Positioning accuracy is obtained by taking the difference between the target position of the axis and the actual distance it covers.

• Use a high-precision instrument when measuring these two positions, such as a laser interferometer or laser tracker
• First set the coordinates, and this value will serve as the target position
• Run the machine to move to the set target, and then measure where the machine stops
• Let’s say that the set target was 200, but the machine stops at 199.985; the difference between the two values, which is 0.015, is the error.

· Repeatability

Repeatability measures how often you get as close as possible to the same value by repeating a process under the same conditions. High repeatability is determined by how little a spread happens, and as long as it stays within the standard deviation range.

• When measuring for repeatability, first establish constant conditions. That is, the location, operator, tools, environment, and your measuring tool all need to remain constant for accurate results.
• The next step is to run the machine again and again for a good range to gauge the spread.
• If the machine has a spread of 0.001 per run, then it falls within the standard deviation, and that is acceptable.

Ways to Improve Positioning Accuracy and Repeatability

Positioning Accuracy

• Check the machine alignment and leveling and properly configure it before initiating the production process to ensure that it doesn’t slip and result in errors
• Regularly calibrate your CNC machine using high-precision equipment to ensure that you can easily identify any position errors and correct them
• Positional errors that repeat through the production process can be corrected mathematically by applying error compensation through your controller
• Thermal expansion often leads to errors in positioning when working with temperature-sensitive materials. You can minimize these effects by ensuring that you work in a temperature-controlled room and that the machine is heated up before
• If your machine is prone to having positioning errors as a result of temperature drift, then you can schedule set times during which the machine is calibrated
• The machine parts and tools should be of high quality, as high-quality parts are standard and are less likely to have any mechanical errors
• Backlash, wear, and looseness in couplings, bearings, and transmission parts should be minimized as much as possible
• Components used to fixture or for workholding should be kept stable throughout the production process to prevent any shifting that results in errors
• The foundation of your machine should be very stable and absorb vibration to minimize any instability while the machine runs
• The machine should be properly and regularly maintained to keep it optimally functional. Maintenance involves cleaning, lubricating, and replacing worn parts.
• Ensure that production takes place under consistent conditions to prevent variation as a result of changes in the environment during production.

Repeatability

• Ensure that you operate in a temperature-controlled room to minimize the effects of thermal expansion, which could increase the spread, reducing repeatability.
• Proper maintenance should be done to minimize mechanical wear
• High-quality sensors and encoders should be used to keep track of changes as they occur in real time.
• Ensure that your system is properly tuned and stable at all times to prevent variations from cycle to cycle.
• Transmission system and drive backlash should be greatly minimized
• Ensure that your machine is regularly calibrated and apply compensation if required to minimize errors.
• Fixturing and workholding should be stable to prevent parts from shifting.
• The foundation the machine is placed on should be stable, and the operating conditions should also be kept constant

FAQs

What are the industry standards for accuracy testing?

The primary industry standards for CNC accuracy testing are: ISO 230, ISO 10791, ASME B5.54

Which industries depend on high accuracy and repeatability?

• Aerospace
• Medical device
• Automotive
• Defense
• Semiconductor and electronics
• Precision mold and die making

What are typical accuracy values for CNC machines?

CNC standard accuracy values ±0.005 mm to ±0.04 mm.

Which is more important: accuracy or repeatability?

Repeatability is often more key in production.

What is return-to-origin accuracy?

Machine’s ability of its return to its original position after moving away

Can software improve CNC machine repeatability?

Yes, software can improve repeatability.

Partner with TSINFA for High Precision Machining

At TSINFA, our team provides advanced machining solutions that are custom-made to meet the dynamic needs of every industry. From 4-axis CNC milling machines and 5-axis machining centers, to universal machining centers, we have diverse CNC machining solutions for every industry. – Request for a custom quote now.