The super-precision index device released by Dai-ichi Sokuhan Works in 1989 is an indispensable tool for manufacturing products in compliance with standardized specifications (such as ISO or JIS). Its structure of two thick steel discs are fitted in very close contact with each other with 720 serrations provided radially engaged on both discs. Since all serrations come into fully uniform contact, the device can achieve astonishing index precision at any angle while maintaining high rigidity and mechanical stability. The company’s traditional lapping technology was employed to give these serrations a fine finish.
This technology involves the use of minute particles called “powder” to level out the infinitesimal irregularities on metal surfaces. This device manufactured by employing a dedicated processing machine and craftsman expertise realizes a minimum indexing angle of 0.5 degree and indexing precision of less than 0.2 second. Institutes involved in establishing standards both in Japan and abroad use this device as high-level standard equipment.
Dai-ichi Sokuhan Works Co. http://www.daiichisokuhan.co.jp/
Originally established in 1944 under the name of Ojiya Kouku Seiki (meaning “manufacturer of aviation precision equipment”), this company has grown to be one of the few major manufacturers specializing in precision measuring instruments in Japan. The company develops and manufactures a series of gauges such as screw gauges for which the company occupies the top share of the domestic market, such measuring instruments as pneumatic micrometers, and precision mechanical parts. Their super-precision processing technology enables the strict enforcement of standards, even contributing to revisions of the Japan Industrial Standard.
The 720 serrations carved on the indexing device. The finish of these teeth determines the precision.
An example of angle indexing for a polygon mirror. The device equipped with a tube is an auto-collimator.
A metal surface to which manual lapping is applied for a mirror finish.
Scales to measure length or weight might be easily neglected in our daily lives. These devices are, however, controlled according to strict standards set in the industrial world. “Angle” represents one such standard.
“The polygon mirror used in a laser printer or copier machine is one of the parts for which angle precision is very important. This mirror forms a multi-angular prism, and the faces of this mirror should be arranged in an exactly equiangular manner.”
The polygon mirror scans a photoconductive drum rotating at high speed by reflecting laser beams. This requires a considerably high level of precision since any deviation in angles between the faces can distort the images. To check the precision, an instrument called an auto-collimator is used to measure the mirror when mounted on a fixture. For an octagonal prism, for example, the precision of angles can be checked by rotating the fixture 45 degrees to measure each of the eight faces. The “fixture” used for such measurement is the super-precision indexing device which has virtually zero error in its angle indexing precision.
“A general-use indexing device is also installed in NC machining tools, but its precision is about 10 to 15 seconds plus or minus. The super-precision indexing device is also used to calibrate such general-use indexing devices; that is, it provides a “primary standard of angle” for confirming the angle precision of other devices. The primary standard indexing device usually requires about 10 times the precision as that is needed by general-use devices. In the West, such high precision indexing devices have been used for a long time.”
Strangely enough, however, there were no indexing devices made in Japan, the leading country engaged in the precision instruments industry. Against this background, Mr. Ohdaira and his colleagues rose to the challenge of working on the development of such a device.
The structure of a super-precision indexing device is really quite simple. The fixture is composed of two thick steel discs stacked on the top and bottom. The contacting areas of these discs are provided with teeth called serrations 5 mm in width and 20 cm in diameter in a radial form. There are 720 serrations that measure 0.5 degree per tooth, and which come into full contact without any gaps wherever the discs are positioned so as to allow the device to index accurate angle information. What enabled the precision processing of the device is the company’s traditional “lapping technology”—the precision polishing of metals. Using minute grains of diamond or ceramics powder, the metal surfaces are gently polished as if being stroked by fingers. It is a quite delicate manual process. Given the tooth width of 1 mm, however, it is impossible to literally use one’s hands.
“Thus, we initially built a processing machine exclusively for that purpose. This machine first carves the 720 serrations. This process called “grinding” is conducted by blades attached with a grinding stone, but this process leaves the marks of the grinding stone on the processing surfaces. These minute irregularities are later smoothed out in the lapping process.”
Still, the processing machine is just a tool. The expertise of skilled lapping craftsmen was subsequently necessary to determine the best powder materials to use, proper duration of the lapping process, and other factors.
“We determine the duration in advance because once you start operating the lapping machine, you cannot stop it to see how things are going. That’s because stopping the machine changes the condition of the teeth. Careful control is also necessary since even a small change in vibration or temperature during machine operation will affect precision. When the process is complete, the lapping craftsman visually checks for residual traces of powder left on the teeth to determine whether lapping was conducted uniformly on all teeth. What you need to do while checking is to observe the overall finished condition rather than giving too much attention to details. Thus, the craftsman does his work not only by using his hands but also his eyes.”
Finally, the top and bottom discs are rubbed together in a process called “free lapping” whereby the marks of the finest powder can be removed. Then, a measuring instrument is used to inspect the device. However, there are limits to the resolution performance of measuring instruments, and therefore, they encountered a problem of not being able to determine whether the error indicated was attributed to the device or the measuring instrument. Following the above procedure, they spent three years to complete the production of a super-precision indexing device capable of achieving indexing precision of 0.2 second for the minimum indexing angle of 0.5 degree, that is, 360 degrees divided by the number of serrations provided. The device has been recognized domestically and abroad as the “primary standard of angle indexing.” However, what Mr. Ohdaira is most proud of as an craftman engineer not the device itself.
“What I really want everyone to know is this wonderful skill of lapping and monodzukuri using traditional craftsmanship. Such expertise has been cultivated by the persevering character of people living in a land of deep snow, Ojiya.”