Applied Research for Mini-Finger-Joint

Summary

A machine has been developed which utilizes a mini-finger joint to salvage “waste” wood from a pencil slat making operation at our Company. In the main plant, we produce approximately 5,000,000 incense cedar slats per day of dimensions 4.8 mm × 70 mm × 184 mm. These are used around the world for the manufacture of wood-cased pencils. Prior to the invention of our mini-finger plant, we produce approximately 5,000,000 incense cedar slats per day of dimensions 4.8 mm × 70mm × 184mm. These are used around the world for the manufacture of joint machine, many slats which contained defects such as worm holes, knots, rot, or bark, within a portion of the slat were chipped for fuel, or, in some cases, recut to shorter length slats to fill a limited demand for making short pencils. With our present finger-joint operation, many of these nearly valueless slats have been salvaged to make good wood for pencil making.

A great deal of research and development went into the creation of our mini-finger-joint machine. The fact that our Company produces a rather unique product posed special problems, and in some cases, severe limitations on the design features of the machine. For instance, the slats which we manufacture with a mini-finger-joint must meet the same standards and tolerances for dimension in thickness and width which we guarantee our customers for ordinary solid wood slats. Since we make the finger-jointed slats from ordinary solid wood slats (except that they have defects), we have no excess dimension for sanding or planing to “finish” the mini-finger-jointed slat. Therefore, a major design consideration for the machine was one of inventing a method to produce very precise joints. This meant building the machine with the capacity to hold the wood in precise alignment during cutting of the mini-fingers and also during gluing of the mini-finger-joint. A special feed mechanism which provides for the creation of 50 matched pairs of mini-finger-joints to be cut in a single pass of the machine, combined with a unique glue spreading apparatus and hammer device, have resulted in a machine having the ability to meet the standards we set for dimension and smoothness in the mini-finger-jointed slat.

In consideration of the intended use of our product, the machine had to be designed to produce a very small, tight, durable joint. Such characteristics have been achieved by our mini-finger-joint which uses 8 mm teeth, spaced 2.5 mm apart.

In the selection of a suitable adhesive for our mini-finger-joint, we faced certain limitations imposed by our customers. The most popular adhesive used in the pencil industry is poly vinyl acetate. In order to insure customer acceptance, especially at the onset of our introducing finger-jointed pencil slats, we had to use the same PVA glue already accepted throughout the pencil industry. This choice has caused us a variety of problems, mostly related to the lengthy setting and drying times associated with PVA glues.

The speed at which the operation had to be run in order to be profitable was determined by estimating the manpower which would be used in the operation, the other features of overhead for the machine, and the values of the raw materials and finished product. The woodworking characteristics of the species to be cut, libocedrus decurrens, was also a consideration when predicting the machine efficiency possible and the speed at which the operation would have to be run. For example, the variation in moisture content of the wood, the presence and proportions of various defects through which the saws and delicate finger-joint head has to cut, the tendency for the wood to chip, tear, or “plug” the finger-joint head, all of these factors had to be taken into consideration before and during development of the machine.

We are now approaching completion of our mini-finger-joint development project, it will soon be ready to turn over to a full production schedule. It has been 4 years since we started the machine. Over this period, we have made many modifications and improvements. In its present form, the machine meets the criteria we set for speed and accuracy in the cutting and gluing of mini-finger joints in pencil slats. There are still a few improvements we foresee which we hope will provide for better machine efficiency.

The acceptance of the mini-finger jointed slat for making pencils has been very good. We do not discount the price of our slats for having the finger joint. In fact, because we have the versatility to make special sizes (lengths) of slats, a major portion of our current, experimental production is actually sold for more than the customary price of ordinary solid wood slats.

To insure the dimensional accuracy of the finger-joint and assure a strongly bonded joint to our customers, we have adopted a system for quality control over the finger-joint operation which includes monitoring of not only the finished product, but also the machinery, saw, and finger-joint head maintenance programs. The current production rate for our mini-finger-jointed slats is between 150 and 200 per minute. The outlook for the operation is profitable.