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Step drill bit hex shank adapter

Why

In many cases, a fairly large hole has to be drilled into a thinner or softer material. A step drill bit is often employed here.

Softer materials, like wood, thinner aluminium, or plastic, can be easily drilled with an electric screwdriver, using drill bits with a 1/4" hex drill bit shank. Work with such drill bits is comfortable. The limit is that the sets often have diameters ending with 6 millimeters.

A step drill bit can be often used in hand, for reaming a hole; often this is useful for making a hole for a switch or lamp or other peripheral in a plastic box. Holding it is however rather difficult and somewhat painful.

A hex-shank step drill bit would be a nice compromise, if it could be bought easily. As it is not, it had to be made.

How

There was a choice of two approaches - additive and subtractive. The subtractive method would use an angle grinder and material would be removed from the top part of the drill bit shank to form the 1/4" hexagon. This was argued against due to the requirements for accuracy that were not sufficiently easy to achieve manually. An additive approach was therefore chosen.

A set of screwdriver bits was obtained with an el-cheapo electric screwdriver. One bit was chosen as a donor of the hex part.

The tip was cut off from the bit. The cut surface was machined flat on the lathe. The steel turned out to be less inferior than expected; the saw blade survived but probably was not happy, the carbide lathe tool did not mind. Attempt to drill a coaxial hole for centering the hex bit on the shank did not succeeded due to the steel hardness and a desire to not damage the drilling bits.

The hexagon and the step bit were then held in a vise, and were welded together using a conventional 2mm rutile-clad electrode at about 60 amps current. The welding time was kept short, in order to not overheat the business end of the drill bit and keep the parameters of its steel's heat treatment.

The weld was then ground to shape using an angle grinder, so no extra material protrudes beyond the diameter of the shank (in the shank area) and beyond the profile of the hexagon (in the hex area).

The coaxialness of the shank was not compromised during the operation; chucking it in the drill press chuck did not reveal any tendency for excentricity.

The hex bit was welded on with a slight angular inaccuracy. Due to small angular deviation the drill bit shows some amount of excentricity. It was however not of significant value and it turned out that most of the excentricity was caused by the uneven grinding of the weld between the hex and round shank. Machining this interface into a conical shape on a lathe made this problem go away.

TODO

Images


Step drill bit, screwdriver bit

Step drill bit, cut screwdriver bit

Step drill bit, machined screwdriver bit

Welded shank

Welded shank

Finished item

Finished item

Conical-machined shank

Normal hex-shank-with-lock-groove drill bits with the step one

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