Precision and accuracy are the most important characteristics of CNC manufacturing. CNC machine tools are designed to perform to certain standards of accuracy. You can assess CNC machine accuracy in two different ways; dynamic and static modes.
Types of CNC Machines
The type of CNC machine you’ll need depends on the products you plan to create and the materials you work with. There are several types of CNC machines to choose from, each of which use different fabrication methods. The most common machines used in CNC manufacturing include:
- Lathes and Turning Machines: These machines typically rotate materials during manufacturing. The turning features of cut around the outside of materials or on the inside. Lathes and turning can also create threads, slots, and holes. They are often used to produce table legs, screws, cue sticks, and bowls.
- Milling Machines: Milling machines rotate tools while holding materials in place. These are often used to drill holes, cut slots, and create gears.
- Laser Machines: Laser machines use laser beams for precise and detailed cuts. Due to the high heat created by laser machines, they are often used for melting and cutting metal products.
- Electrical Discharge Machines: These CNC machines shape materials with controlled electrical sparks which create heat up to 21,000 degrees Fahrenheit. Due to their high precision, electrical discharge machines often create jewelry, plastic molds, and small engine parts.
- Plasma Cutting Machines: Plasma cutting machines use high-powered plasma torches that produce heat as high as 50,000 degrees Fahrenheit. These are typically used to cut brass, copper, aluminum, and stainless steel.
Things to Consider When Comparing CNC Machines
Whether you’re purchasing your first router or converting an existing one, knowing your CNC manufacturing goals should be your first priority. Reflect on how large your production capacity will need to be.
The last thing you want is a machine with limited capacity if you’re running a high production facility. On the other hand, if you’re running a small shop with lower production demands, you want to ensure that the CNC machine you purchase will give you a fair return on investment.
You must also reflect on how complex your CNC fabrication will be and whether or not your CAD and CAM software supports the complexity and depth of your products. Answer the following questions before making your decision:
- Are you producing items that require little detail or can be completed manually?
- Are you producing 3D parts and products that require sophisticated milling, drilling, cutting, and carving processes?
- Do you plan on prototyping?
- Which materials do you plan to work with and how much precision and power will you need
- What degree of speed and accuracy are you aiming for?
Frame rigidity often impacts the cutting quality in CNC router machines. Rigidity is the heaviness of the machine frame, but other factors also come into play. That includes:
- CNC control features.
- Acceleration and deceleration.
- Tool and part holding.
- Momentum of heavy moving parts.
- Frame weight and load capacity.
There are two types of CNC machine accuracy to consider:
- Absolute Position Accuracy: This is the capacity to reach a certain point within a particular tolerance. This measurement can vary depending on whether it taken on a single or multiple axes, or if a load is placed on the cutter head.
- Spatial Position Accuracy: This depends primarily on encoder resolution, which is the number of measuring units within one revolution of an encoder shaft.
Repeatability is a CNC machine’s ability to return to a point in space whenever a program is executed. It’s often measured on single or multiple axes, and exceeds the capabilities of position accuracy on many systems.
The best way to determine repeatability of a CNC machine is to perform a repeatability test. This involves the following steps:
- Select the measurement function and range.
- Identify repeatability test points.
- Employ the right measurement method.
- Utilize functional measurement equipment.
- Collect about 20-30 samples and look for consistency.
Predictability is when the control portion of a CNC machine works consistently, no matter the program it’s running on. When the control portion works properly, it calculates the tool’s path in advance and detects potential errors.
On the older CNC machines, the computer would command the head to move to a certain place using the X-y axes, but the tool’s path wasn’t always taken into account. Modern CNC machines’ movement of axes work in proper coordination with each other within a specific set of rules, called interpolated path. This gives CNC machines the ability to work around fixtures and maneuver cutters in and out of parts.
Dynamic Mode refers to the accuracy of the axes as they are moving when the machine is functioning. There are two things that may cause dynamic accuracy errors: vibration and a servo system positioning error. One of the main factors defining performance is machining speed. Typically, a high-speed machine will be able to execute a program at around 20000mm/min. Because this feed rate is high, it’s possible that it could lead to a high lag value on the servo system and also high vibration on the corners due to rapid acceleration and deceleration. The cutting force and spindle on the tip of the tool can also cause high levels of vibration that can lead to overall accuracy errors.
This refers to the movement of the machine’s axes to a certain location where their position can be measured externally (you can use an interferometer laser for this) against the standardized values in the CNC. During this procedure, once the axes are at rest, you can use the laser to measure their position. Whatever the difference between this measurement and the command position, is referred to as static accuracy. Mechanical alignment or volumetric, cross, or linear compensation can improve this accuracy.
Get A Free Quote on the Right CNC Machine
Do you have your sights set on a certain CNC machine or need help choosing the right one? At CAMaster, we use machinery, hardware, and software to improve your company’s productivity and efficiency. Our team has a vast knowledge base and we are committed to sharing our experience with you to help you take your enterprise to the next level.
Find out more about comparing accuracy between CNC machines, call us at 1-866-405-7688 today!