Drill Length - Smaller the better!

Many times there is a necessity of drilling holes that have a long length. The operation becomes uneconomical and also time consuming if the diameter is small and length of the drilled hole is large e.g. crankshaft oil holes can typically be 7mm diameter X140 mm long.

Experiments have shown that managing to reduce the overall length of the drill serves two purposes,1)Cost of the drill reduces and 2) Drill is less prone to a breakage (buckling effect) at higher feeds.

Examples mentioned below will show that by redesigning the holding element or guiding elements the performance of the drill could significantly be improved.

These drills (from HSS) are also specially made for deep hole drilling application and have a parabolic flute, thicker web and a high helix angle. Gun drills are common and so also are Ejector/BTA drills for higher sizes. Now solid carbide drills with internal coolant holes that use minimum quantity lubrication are becoming quite popular.

By increasing the gun drill adaptor length The length of the drill was reduced from 600 mm to 470 mm. It worked better at 20 % higher feed rate and was also cheaper.	Guide bush was removed for this oil hole drill in differential cover and a stub drill was introduced that was cheaper by 50%.
Guide bush was redesigned by cutting a slot in the original bush and mounting a smaller bush as shown above. Drill length became 200 from the original 250 mm. It performed better and was also cheaper.

By Surendra Datar
The opinions expressed here are solely of the author and are not of the company where he works