Frequently Asked Questions

“I’m using a small diameter split drill bushing and I can’t seem to get the drill parallel to the centerline of the spindle. Why?"

• A common cause for this problem is locking the second set screw on the bushing. The holes for small diameter drills do not ex-tend the full length of the bushing. The back end of the bushing usually has a large internal chamfer or a counterbore. This cleared area is under the second lock screw. When you tighten this screw, the bushing may distort, shifting the drill’s position. There may be line contact on which the drill will pivot. The solution is to use the front locking screw only.• A second cause is that the bushing is too short. The first set screw is locking on the back of the bushing, causing it to collapse instead of gripping the shank of the tool.

“I am trying to drill a very accurate hole using a DA-style collet system without any success. Any suggestions?"

• Extremely accurate drilling requires a very good holder and collet. With the considerable flexibility of the DA, TG and ER collets, we tend to forget the cardinal rule for collets – the bore of the collet should be the exact size of the diameter being gripped for the best TIR and gripping force. Therefore, for extremely accurate drilling, the collet bore should be the same size as the shank of the drill.• Second, if you have a choice of holders, it is best to use the ER collet holder over the DA collet holder. The DA holder is a double-angle holder, with a closing angle on the front and back of the collet. The theory is that the collet will close parallel to the centerline of the holder. Unfortunately, it is very difficult to manufacture a holder and collet that accurately mate. For standard drilling tolerances, this usually is not a problem, but can be when doing extremely close tolerance drilling. The ER is considered a single-angle collet because the major closing angle is long and shallow. This results in better concentricity than the double-angle system. The TG collet system is better than the ER system because it has an even shallower closing angle. Unfortunately, this system is usually for larger shanks and most holders that use these collets are for suppliers of vertical machining centers.• For extremely accurate drilling, the bore of the collet should be the exact size of the shank of the drill.

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