This article first appeared in Practical Engineering 1940 Vol1 No20. The information contained within the article is therefore correct as of 1940. The article provides news of developments in Light Engineering at the time.
Here we describe an adjustable cutter for recessing bored holes. It is adjustable as to both the diameter of the hole which is to be recessed or counterbored and the diameter of the counterbore. The shank is of Bessemer steel (mild steel), and there are two cutters. These are identical in size and shape and they overlap each other. The tool acts on exactly the same principle as the boring bar with flat cutters, but by duplicating the cutter we may make it adjustable for different diameters.
The cutters are held in a slot which is square at the top and at the bottom is inclined to each side from the central line to the same angle as the angle of the cutters, so that the cutters can have their bottom cutting edges ground to the usual angle all along their length. Each cutter has a central slot of a width equal to the diameter of the pinch screw which passes through them. This slot is square with the axis of the tool.
The end of the slot at the cutting end of each cutter is so placed that when the cross screw is through the cutter, the cutting edge is just proud of the side of the mandrel, and when it is at the other end of the slot the cutter is set to the maximum diameter.
Into the top and bottom of the slot, and central with it, are two fairly wide saw cuts which extend some distance from the main slot which holds the cutters. These saw cuts should be a good length. They are used to give the mandrel enough sideways spring to ensure that the screw can press the sides very tightly upon the cutters to keep the cutting edges outward. The cut will be downward and the cutting edges square so that there is no pressure tending to force the cutters inward.
The Pilot End
The screw is parallel from head to the side opposite that into which it screws. The head is recessed and rounded to the same radius as the mandrel. Below the cutter slot the mandrel is reduced in diameter and forms a pilot. In use, the pilot should fit exactly the hole to be recessed and be a close running fit. For larger holes, a series of sleeves is provided, the inside bore of the sleeves being such that they fit the pilot end of the bar or mandrel a tight push fit.?
The outside diameter of these sleeves will vary according to the diameter of the holes to be recessed. They can be made up in quantities to fit the pilot end, and then as jobs arise they can be turned down to the required outside diameter. They can be made from Bessemer bar. The longer ones may be drilled to take a grub screw, and a grub-screw hole should be drilled and tapped in the solid pilot. This will hold the pilot sleeves up to the shoulder on the bar and at the same time prevent them from turning on the bar.
The forming-tool holder saves the material and time cost of forging individual tools for small forming operations. The holder accommodates small shaped forming tools for repetition work, the tools themselves being made of cast steel bar 7in. thick. From this stock the tools are cut off, shaped on the cutting edges and hardened and tempered. They fit the tool holder very tightly by a wedging action, the wedge being driven by light hammer blows on the wedge or cutter.
The holder is a square bar of cast steel of the maximum size to fit the lathe tool post. It is jumped up at the end to form a bigger square in cross section than the cross section of the shank accommodated in the tool post of the slide rest. This is slotted along each side to leave an upstanding lug, through which there is a cotter hole. The axis of this hole should be 1/8in. above the surface and the hole should be at a slight angle relative to the top and bottom surfaces of the slot. The side slots are cut by milling in the lathe. The cotter axis is thus at an angle with the top of the forming tool and the surface of the tool holder.
Details of the Tool
The tool is 6 in. thick and is slotted at the rear end so that when fitted in place its back edge lies close up against the end of the lug, while the two projections—one each side—formed by the slot in the tool, hold it sideways against the outer sides of the lug. The tool holder should be forged with plenty of top metal to take the pressure of the cotter, which is driven in lightly after the tool has been inserted in the holder. The pressure of the cutting resistance keeps the inserted tool tight up against the end of the lug in the holder, and the side projections locate it, while the cotter locks it tight.
The cutter is shaped to the job to be done. One example is for use in forming a small turned handle or knob, but it is obvious that an infinite variety of shapes may be adopted. The cutter is sharpened by honing it along the top along that part only which projects beyond the holder.
Only as much of the cutter as is needed to give the full form to be cut should project beyond the bottom front edge of the holder. The perspective view shows the holder without the cutter. It will be noticed that the driving of the cotter does not have any effect other than that of forcing the cutter down on the surface. The parts on each side of the lug being cut away, the cutter rests on a flat surface. The sides can be cut with a fly cutter in the lathe, the stock being held in the tool post and traversed for cut by the slide rest. The cutter is made dead hard and then let down to a full amber straw colour at the cutting end, then quenched.