Short Slot Drilling Without Headaches

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Short Slot Drilling Without Headaches by the Technical Service Department, TCT Group Head Office, Carbide International Co., Ltd. Taiwan (This article first appeared in the RUWEL-Journal No. 06/2003 and is reproduced here with the kind permission of Ruwel AG.)

The TCT Group is the first Taiwanese company that has invested in PCB industrial cutting tools. And TCT is dedicated to R&D, not only in production techniques but also in the manufacturing of high-quality and long-life tools in order to cut costs and increase productivity for customers.

To meet the great demand of the market, TCT is increasing its capacity continuously with ISO 9002 certification and now has branches all over the world: Hong Kong, China, Korea, Japan, South-East Asia, USA and Europe. The TCT Group is moving successfully toward internationalization.

TCT's policy TCT set up its TCT Tech. Center (including Tech. Service Dept. and Tech. R&D Dept.) equipped with drilling machine, routing machine, hole position checking machine, cross section equipment, etc., to provide technical support for our customers. Through discussion and communication, TCT has the ability to customize special tools to improve the quality of drilling and routing and reduce the cost for our customers. TCT has adopted a flexible manufacturing structure to allow quick response to market demand.

Solution of Short Slot Drilling

Requirement: A problem that we frequently face in short slot drilling is the oblique slot shape (See Figure-1). This problem always gives the engineers in our drilling process division a headache. Firstly, TCT would like to give their definition of the short slot: when L < 2 * D, the slot is a "short slot" (see Figure-2, L: slot length, D: slot width). It is easily realized that the shorter the slot length, the greater is the oblique slot problem. The normal slot drilling sequence is extensively adopted as shown in Figure-3. Therefore, if the slot length is equal to 1.5 * D (slot width), there is no material for positioning the next drilling operation (see Figure-4). This situation will lead to a deflection problem. That is to say, the main reason for the oblique slot shape in short slot drilling is the slot geometry limit.

Concept: Following the general slot drilling concept shown in Figure-3, TCT developed a drilling method to overcome the oblique slot problem. The basic strategy is to pre-drill a smaller hole. The hole size is smaller than the slot width. Normal slot drilling is then performed. The smaller hole will constrain the slot drill bit so that it will not deflect so much on the next drilling. That means no material for the positioning purpose of the slot drill; the straight slot profile had already been decided.

Method: An example is introduced in detail, step by step. From Figure-5, the required slot length = 1.1mm, slot width = 0.7mm. (a) First, a pre-drilled hole performed by T1 (normal drill, 0.4mm) is tangential to the slot profile. (b) Then the starting point for slot drilling is drilled by T2 (slot drill, 0.7mm) on the opposite side. (c) And finally slot drilling is executed continuously.

Necessary Definition: The size of T1 is determined by the difference between slot length and slot width (FT1 = L - D = 1.1 - 0.7 = 0.4mm). The position of T1 is determined by the condition that the circle of T1 is tangent to the slot profile, and T1's center (C1) is located on the slot centerline (see Figure-5 (a)). Of course, the position of T2 is determined by the condition that T1-circle, T2-circle and slot profile are tangential to each other (see Figure-5 (b)).

Performance (Test Result):Figure-6 (a) shows the drilled slot performed by the normal slot drilling process without pre-drill. It shows a very serious oblique slot shape problem (sometimes called the peanut phenomenon). However, Figure-6 (b) shows the slot shape drilled by normal slot drilling with pre-drill. Compared to Figure-6 (a), the slot shape is dramatically improved.

Discussion: In fact, slot drilling with the pre-drill still cannot achieve a perfect slot shape. Figure-7 shows the "Nipple Effect" which is caused by deflection of the pre-drilled hole and the positioning problem of the slot drill. It has been proved that the "Nipple Effect" would occur using the pre-drill method even if the operating parameter (infeed) is adjusted faster or slower. Anyway, the pre-drill method can almost avoid the oblique slot shape problem.