A New Frontier in Surface Finish Control

Source: Mahr Inc.

Surface finish measurements on precision components include options where the part is brought to a gage or where the gage is brought to the part. For example, automotive parts such as pistons, connecting rods, camshafts and transmission gears are all small and easy to bring to a dedicated gaging station where the operator can quickly assess the surface finish on the part. These gages align the part to the probe and create a foolproof operation to properly measure it.

On the other hand, there are large parts that aren’t that easy to move. Take a marine diesel engine block, helicopter transmission housing or some other large precision part that would be difficult to remove from the machine, make a measurement and then, if needed, return and realign that part for another finishing pass. Surface finish in these applications is critical. There are thousands of dimensional checks to be made, along with many critical surface finish callouts to measure. For example, engine block surfaces specify certain surface finish measurements, which could include any mating surface where a seal is created.

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In previous columns, I have discussed making surface finish measurements at the point of manufacture several times. The reasons for making surface finish measurements are numerous. Surface finish is becoming a larger component of shrinking tolerances, as it affects the function and appearance of manufactured parts. As machine tooling wears, surface finish will be affected, and thus, it becomes critical to measure a part’s surface finish regularly — or in the manufacture of very critical components, every part.

Handheld portable surface gages are extremely easy for the operator to use — often with single button and error-free operation through an automatic cutoff selection that ensures the correct measurement results, even for non-measurement technicians. However, despite the portability and ease of use, having the operator position the surface finish gage on hundreds or thousands of parts during the day at the precise location of the required measurement can be challenging even for the most skilled users.

But the reality is that more and more manufacturing facilities want to run lights out, 24 hours a day, where there is no operator involvement in the machining process. No one is there to stop the manufacturing process and place a portable surface gage in the machining center to make the critical surface check.

This situation will involve a new type of surface gage that is actually part of the manufacturing process. The technology employs a tactile surface finish probe that drives into a standard machine toolholder, which can be placed in the toolchanger just like any other cutting tool.

The advantages of this concept can be important when high volumes of large, high-precision parts are being manufactured. Obviously, this automated roughness measurement eliminates manual intervention by the operator and improves process reliability by eliminating manual checks after the part is taken off the machine.

It is an automated process with no operator intervention, thus perfect for unattended machining applications. The surface gage is kept in the machine’s toolholder; it is employed just like any other cutting tool during the manufacturing process and controlled by G codes within the normal machine controller. Each surface check is then automatically documented in a standard-compliant (secure) manner for quality assurance. The process is done within the machine, measuring surface characteristics without expert knowledge.

The gage is placed back in the holder if the surface is good, and the next machining process begins. If there is an issue, the previous process can be repeated and re-evaluated.

Also, these surface checks can be stored and tracked, allowing this in-process automated surface checking to potentially increase the service life of cutting tools through wear monitoring.

Placing a delicate precision surface gage may seem contradictory, with all the vibration and contamination naturally associated with a machining process. However, with the proper design concepts, these challenges can be mitigated. Air can also be used to help clean contamination from the part and the surface probe.

For example, the entire surface roughness system can be made compact and small enough to fit within SK, HSK, BT and Capto toolholders. The design enables the long-range (15-mm) tactile skidless system to be vibration-resistant due to a special support point (support on the workpiece) while still providing Rz measurements to 1 um.

Within the toolholder is the measuring system, which is connected wirelessly to the machine tool via a separate controller. This controller is the interface to the machine tool and controls and analyses the surface measuring tool, sending qualification information to make decisions about the process. This is all seamless to the machine tool, making the addition of this feature unintrusive to the machine.

In the past 30 years, surface finish measurement has moved from expensive and complicated measurements performed in a precision quality laboratory to small bench systems and, most recently, to powerful handheld systems. Now, the final stage is near, where we will begin to see surface roughness measurements captured and analyzed truly at the point of manufacture: right in the machine tool.