Application

Flexible roughness measurement employing an innovative miniature measurement system

Conventional instruments to measure surface roughness comprise of a PC, a drive unit and a pick up. In addition one needs tilting devices in order to align the pickup in space. Herewith we introduce a new concept for surface roughness measurements which is very cost effective due to its flexibility and lends itself for applications which cannot normally be done by conventional systems due to either high cost only or not at all. It is especially suited for in-line use.

The current standard with regard to roughness measurement refers to stylus based instruments only. Optical methods are considerably faster but cannot be used universally due to the given laws of physics. Smoothly ground surfaces are useful for such sensors because they evaluate the scattering properties of the surface. For non-periodic surface structures and above a roughness value of approx. 1 µm they tend to fail. Sensors working to the speckle contrast principle are limited in a similar manner but can be used to a roughness Ra < 3 µm. Particular advantage is the possibility to imitate a ‘traverse length’ of a few mm. Point wise scanning optical sensors often generate artefacts at steep slopes. All optical methods require that the surface needs to be clean and often a production environment is usually detrimental to the use of such devices.

There are small housed roughness instruments on the market but almost all of them use skids. Instruments using a reference datum are more preferable. BMT recently introduced a new miniature roughness measurement system which lends itself for in-line applications and fitted in its matchbox-sized, which houses a reference datum based pick up, the drive unit and electronics. The technical data of this miniature system is essentially the same as the large, costly roughness measurement systems however it can be connected to any PC via its USB interface. The software meets all technical and ergonomic requirements and calculates the various standardised parameters.

Fig 1. Instrument

It was the goal of this BMT development to eliminate known disadvantages of typical roughness instruments as much as possible and offer a device which is optimised for in-production applications featuring; an increased flexibility for different measurement configurations; a measurement time adapted to the production cycle; a high immunity to external vibrations and contamination by dirt. The result is the MiniProfiler roughness measuring instrument having the following advantages:

• Miniature size
• Data acquisition with 24 bit
• Drive unit and skidless pick up inside one housing
• Vibration insensitive due to a very small measurement loop
• Motorised stylus retraction
• Stylus easily exchangeable by the user
• Insensitive for getting dirty by a compressed air inlet
• Calibration piece integrated inside the instrument

An application example is given in Fig. 2; in-line roughness measurement of brake discs.

Fig. 2 brake disc

Two pickups measure simultaneously on both sides of the disc. Due to the miniature size, the instrument can be fitted to the disc, which eliminates all vibrations from the measurement. The traverse length is 12.5 mm. The stylus is retracted during the reverse drive.

Depending on the application, the base plate of the instrument is designed to be work-piece dependent and can be exchanged by the user.

 

 

Examples of adapter pieces are (Fig 3 above from the left) bottom of a hole, engine block bearing, valve seat, plane area, crankshaft bearing. In all these cases the adapter piece is designed such that the pickup is supported by the work piece in order to eliminate detrimental vibrations. The instrument performs measurements using a reference datum, not a skid. In the production environment the instrument can be supplied with compressed air. Experience shows that even after many months of continuous operation no dirt gets into the pickup housing.

In order to measure the microstructure of cylinders in the manufacturing cycle,

Fig 4. Cylinder roughness measuring head

one pick up will not be enough due to the required measurement time. Fig. 4 shows a measurement head for the surface roughness measurement of the honing structure inside liners. This features 9 roughness instruments, which measure simultaneously in 3 levels and 3 angular positions. In this way one can measure the surface in a statistical and meaningful way and can measure the roughness of the cylinder face within 20 seconds.

The measuring modules having different adapters can be stored in a magazine and together they form a very flexible measurement cell with a robot.

Fig. 5. Measurement cell

Using such a robot based measurement device and work piece dependent mini roughness measurement system, it is possible to measure various features of complex work pieces within one measurement cycle. There would be a need for cumbersome retooling actions or to transport the work piece to several different measurement stations using conventional roughness gauges. Roughness and profile measurements can only be done in a sensible manner by using the miniature roughness instruments shown here since they eliminate any requirements for

Fig. 6: Magazine

positioning stability of the robot arm. 

The combination of a small robot and a small roughness measurement system results in a drastic cost reduction for quality insurance and therefore a faster payback on the investment.

 

All data collected by the device is saved by attributing them to the specific work piece.  

 

A trend chart is recorded and displayed for each measurement position. Green: within tolerance. Yellow: Warning threshold, Red: Tolerance limit exceeded.

Fig 7: Measurement data chart