High-accuracy Measurement of Free-form Surfaces on Cut Products

Differences between a three-axis machine tool and five-axis machine tool

The inside of a conventional three-axis machine tool consists of a table for fastening the workpiece and the spindle. The spindle moves left and right (X axis), forward and back (Y axis), and up and down (Z axis) relative to the workpiece in order to machine it.
On the other hand, a five-axis machine tool has two rotating and tilting axes in addition to the X, Y, and Z axes. The table where the workpiece is installed rotates. The two rotating and tilting axes consist of the A axis, which rotates the workpiece around the X axis, and the C axis, which rotates the workpiece around the Z axis. With this structure, a five-axis machine tool can machine all surfaces of the workpiece on the table except for the bottom surface, without the need to remove the installed workpiece from the table.
Therefore, a five-axis machine tool can reduce the work required for changing the workpiece installation angle and can machine complex shapes like free-form surfaces without using special tools. The ability to freely move the workpiece also minimizes tool overhang, preventing problems such as reduction in machining accuracy caused by deformation of the endmill or other tool under machining pressure.

Simultaneous five-axis machine tool

A simultaneous five-axis machine tool performs machining while operating all spindle and rotating axes. Typical five-axis machine tools perform processing by turning the A and C axes to the specified angles and cutting the workpiece, then changing the angles and cutting again. The A and C axes stop rotating while the spindle is moving. This is called indexing five-axis machining.
On the other hand, a simultaneous five-axis machine tool performs machining while moving all five axes at the same time, enabling machining of smooth free-form surfaces. Some parts such as impeller blades, bladed disks, and the inlet and outlet ports of reciprocating engines can only be machined using simultaneous five-axis machine tools.

Important points concerning five-axis machine tools

Five-axis machine tools are superior to conventional three-axis machine tools in various aspect; however, caution regarding the following points is necessary when introducing and operating such a tool.

Introduction cost:

With higher performance and more functions, five-axis machine tools are more expensive than three-axis machine tools. Because they require 3D CAD/CAM and NC programs, the high costs of introducing these elements can also be considered a disadvantage.

Machining accuracy:

When the accuracy of the axis movement is the same as that of a three-axis machine tool, the machining accuracy will be lower due to the larger number of axes. As five-axis machine tools have complex mechanisms with many moving parts, machining accuracy may also decrease if these parts lack sufficient rigidity.

Man-hours required for setup change:

Once set up, a five-axis machine tool can perform machining at high speeds with high accuracy. However, preparation before machining a new workpiece involves development of an NC program, setup of each axis, and other work that requires large numbers of man-hours.

Deformation caused by external forces such as clamping force and machining pressure, and countermeasures to it

The force that fixtures a workpiece in place during machining is called clamping force. The force that is applied to the workpiece during cutting or other machining is called machining pressure. Both of these can cause deformation.

• Clamping force

Clamping force deforms a workpiece when the fastening force used to fixture the workpiece in place exceeds the strength of the section that is fastened.
The most effective way to prevent this is to change the position or orientation in which the workpiece is fixtured. If the position or orientation cannot be changed, deformation can be prevented by fastening easily deformed sections using support jigs.

• Machining pressure

A workpiece can become deformed by machining pressure when the workpiece is machined by a cutting tool. Effective countermeasures to prevent deformation include changing the workpiece shape by increasing the thickness at the deforming sections, reducing the amount of cutting, or lowering the hardness of the material. If the workpiece shape cannot be changed due to specifications or functional reasons, deformation can be prevented using a method called warpage removal. This is when a workpiece is roughly machined with extra thickness and is later finished by a separate machining process.

Deformation caused by cutting heat (working heat) and countermeasures to it

Cutting heat is generated by friction between the cutting tool and workpiece during cutting; it is said to reach 600 to 1000°C (1112 to 1832°F). Cutting heat is particularly higher when the workpiece is made of materials that have low thermal conductivity, such as stainless steel, because the heat generated by the cutting tool and workpiece cannot escape. Cutting heat causes the workpiece to expand. When the workpiece expands, the cutting depth becomes larger than the set depth; this results in a failure to produce the design dimensions. Effective countermeasures to prevent deformation include changing the cutting oil, using a combination of machining methods that produces less workpiece deformation (such as electrical discharge machining), or changing the workpiece materials.

Deformation caused by residual stress and countermeasures to it

Stress remaining inside the workpiece even after the removal of clamping force, machining force, and all other external forces is called residual stress. The force acting inside an object from the inside toward the outside is supposed to be balanced with the force acting from the outside toward the inside. However, when this balance is disrupted by machining pressure and cutting heat during machining, warpage and strain can occur. Thin materials are particularly susceptible to warpage, and hard materials are particularly susceptible to warpage and strain. Even when the shape of a workpiece is normal while it is being clamped, it may deform after it is released from the clamp. To prevent this, the process known as annealing is performed. A workpiece is annealed before cutting to soften the workpiece making it easier to machine. It also equalizes hardness to prevent uneven machining and minimizes variation in workpiece hardness.

Problems in measurement using a coordinate measuring machine

A coordinate measuring machine is typically used when measuring a free-form surface by using a probe to contact particular points on the target measurement surface. The probe needs to accurately contact the predetermined points.
When the measurement area is large, accuracy can be increased by increasing the number of measured points to collect more measurement data.

However, using this method for measurement of free-form surfaces involves the following problems.

• A large number of points need to be collected in order to measure the thickness of an impeller blade or the shape of a gear tooth surface. This requires time, and even when it is done, it is still impossible to identify the entire detailed shape.
• Comparison with CAD data takes time and effort due to difficulties in matching the positions in the measured data with the CAD data; it also requires a lot of work to input design values and tolerances.
• Because profiles and other shapes are measured by profile measurement, it is difficult to gain a picture of the entire surface.

Problems in measurement using tooth thickness calipers or a tooth thickness micrometer

When gear tooth thickness is measured using a handheld tool such as tooth thickness calipers or a tooth thickness micrometer, the over-pin measurement method is used.
With handheld tools, measurement conditions such as the contact force (measuring force), differ depending on the operator as each point needs to be selected and measured by hand. This results in variation in the measurement values and makes it difficult to obtain quantitative measurements.
Users also have to cut their sample in order to perform cross-sectional measurement, and it impossible to identify cross-sectional shapes through point measurement.

Advantage 1: Visualize differences from the 3D-CAD data in color.

It is possible to compare a product’s 3D-CAD data with the acquired measurement data for color visualization of any differences between the actual product and the design. This can dramatically reduce the number of man-hours required to analyze the shape of products.
The 3D data can be rotated in any direction, enabling specified locations to be measured, such as the pitch and curved shapes of impeller blades. Additionally, longitudinal and transverse cross-sections can be measured without destroying the workpiece.
The entire shape can be identified and the dimensions of specific locations can be measured by simple click operations, enabling quantitative measurement that is not affected by the skill level of the operator.

• Download Catalogs
• Contact/Inquiries

Advantage 2: Easy measurement of over-pin dimensions. Cross-sections can be clearly identified without the need to cut the target.

Over-pin dimensions of a gear measured using tooth thickness calipers or a tooth thickness micrometer lack reliability and stability. This is because the measurement points vary depending on the operator. Along with that, the jaw and spindle may be unable to actually reach the measurement points. Because measurement is based only on individual points, it is difficult to measure 3D shapes.
With the VL Series, you only need to place the target on the stage and scan it. The 3D shape of the target can be captured by non-contact means and no positioning is required. It is possible to display the entire target or to measure the profile at any location. This makes it possible to visualize and identify the locations and precise numerical values of shape defects.
The VL Series can also measure cross-sections when comparing the acquired data with 3D-CAD data without the need to cut the workpiece. Over-pin dimensions and tooth shapes can be visualized in color using the acquired 3D data. This enables cross-sectional measurement to be performed easily, without contacting the target. By checking for misalignment of cut profiles, it is possible to rapidly detect and enact countermeasures to machining deformation and other defects.

• Download Catalogs
• Contact/Inquiries