Lapping and Sharpening Jig
J. Malcolm Wild depicts a tool in "Wheel and Pinion Cutting in Horology" that is intended for lapping or sharpening small tools, such as fly cutters, gravers, drills and other small bits. There are several photos of the tool as well as a description of its use in this book and there is what appears to be a more refined version of the tool on his website as item JMW 1587. I will be attempting to follow the design of the tool as depicted in the book. Dimensions are not given, so a drawing was started. The primary components of the tool were based on brass stock that I had on hand. The base has a bore for a tool-holding mandrel and has pivots for a bracket that acts as a guide to the lapping tools. This arrangement allows both the tool to be sharpened and the laps to be rotated in different axes, which can accommodate a variety of angles.
The base of the tool was made from 5/8" x 1-1/4" brass bar (type 360). A roughly 1-1/2" long piece was sawed off, as shown above, and the sawed edges milled to help make them flat and square to the extruded sides of the brass stock. A step was milled on either side that is about 1/4" wide and 1/8" deep. These steps are for holding the finished tool in a vise. Afterward, the work was coarsely polished on 400 grit emery paper to remove tool marks and generally smooth the surface and improve the appearance, which can be helpful when scribing out dimensions.
The top face was scribed to mark out where material will be removed, which was then done by milling.
The location for the central bore was scribed and center punched. A divider was used to lay out the approximate diameter of the finished hole. This hole is for the tool holder. The work was mounted in the machine vise and the location found and spot drilled, and drilled through 1/8" first and then opened up to 1/4" (the largest drill on hand of the necessary length).
The work was moved to the 4-jaw chuck and a piece of 1/4" drill rod held in the tailstock to help bring it onto center fairly quickly. The hole was then bored open to pass a piece of 3/8" diameter brass rod with a snug fit. This requires a boring bit with a long reach (at least 1.5"), this one just made it to the other end of the hole. The corner of the bored hole was chamfered using the topslide and a small boring bit, and the face turned to true the surface to the bore but only needed a light cut.
The swinging hone guide bracket was started from 1-1/2" x 1/4" brass bar. A length a little over 1-5/8" was sawed off and the sawed edges squared up using the vertical slide on the lathe with an end mill. One face of the plate was coated in layout blue and the dimensions scribed out. The longer pair of arms were started first since they will involve a more difficult machining step. The 'corners' were drilled first, and a slotting saw used to make the first two cuts.
The sawing was completed by hand with a jewelers saw, which I obviously had very little control over. This rough edge was finished with a 1/4" endmill. The rounded corners were squared and the arms brought to desired thickness with light cuts using the sides of the endmill.
An arbor for a fly cutter was made from a length of 1/2" cold roll steel rod (12L14). This piece was found in the stock drawer, but appears to be just long enough to work. It was mounted in the 3-jaw chuck and supported with the steady rest to face, chamfer, and center drill each end.
The position for the fly cutting bit was drilled through 4.9mm (something slightly smaller would be better) and reamed 5mm. The work was rotated 90 degrees by eye with a length of 5mm drill rod in the reamed hole to help. The position for a set screw was drilled 4.2mm and then tapped M5x0.8.
A fly cutting bit was made from 5mm oil hardening drill rod. The rod was turned to a 45 degree point and the tip milled to half it's width with a 1/8" endmill. A flat was milled for locking the bit in place during use. The bit was parted off and the parted face chamfered using the top slide.
The fly cutter was smoothed and flats polished with an India and Arkansas stone slips. The cutter was hardened by raising to red color with a torch and holding it in a stainless steel small wire basket. The cutting face was repolished, but otherwise used as is and in the fully hard condition. It was mounted in the arbor and the tip extended 1/16" from the surface of the arbor and locked in place with the set screw.
The bracket was mounted in the machine vise and on the vertical slide. The tip of the fly cutter was positioned 6mm from the surface, and fed into the work with 0.25mm deep passes milling each arm with each pass. The fly cutter leaves a 5/16" radius that will mate with the 5/8" diameter tube below.
The pivot arms on the other side of the bracket were cut out next. As was done above for the other arms, the corners were drilled first and the center portion sawed out, and the space then squared up with an endmill. The width of the space is 1 inch, but this should be reduced to 7/8" or so.
The locations for the pivots were scribed out at about 1/8" from each side, but some extra length would be preferred on the end for the set screw threads. The scribed location was found, spot drilled, drilled 0.110" and then reamed 1/8". It would likely be preferable to drill and ream both holes at the same time, but my available drill bits were too short.
The ends are drilled 2.5mm and then tapped M3x0.5 for set screws to lock the pivot centers in place. The ends of arms were then rounded over with "hand" files. Stock M3 set screws that are 3mm in length were used.
The pivot centers were made from a length of 1/8" oil hardening drill rod. The rod was turned using the top slide of the lathe to produce a 60 degree point on each end. The rod was then parted in half, and the parted ends faced and chamfered.
The pivot locations were scribed onto the base and then center drilled with a #000 combination center drill and 60 degree countersink. The bracket could then be tried on the base to check the fit and movement of the bracket.
The guides were made from thick-walled, brass tubing. They could be machined from solid rod, but tubing was available that is listed as low-lead, hard-drawn, 330 grade. The tubing is 5/8" in diameter with 1/8" thick wall, leaving a 3/8" inside diameter.
A tool holding mandrel was started from 3/8" diameter brass rod. A 3" length was mounted in the 3-jaw chuck and supported in the steady rest. One end was rounded over and a short section turned using a graver made from round drill rod to form a radius that will act as a grip for adjusting the tool in use.
The work was reversed on the lathe and the other end faced, spot drilled, and drilled 4.9mm (not shown). At this point I realized my 5mm reamer would not fit within the space available, so the work was repositioned in the vertical slide with the reamer mounted in the headstock. Less than ideal setup, but alignment was preserved as best as I could under the circumstances.
The base was drilled and tapped M4x0.7 for a screw that locks the tool holding mandrel in place. The thumb screw was made from a short piece of 1/2" brass rod. It was turned and threaded M4 and then parted off. An arbor was prepared from another short piece of 1/2" brass rod and the screw mounted. The parted face was chamfered and the face turned with a round graver to give it a concave surface. The headstock was locked and the milling spindle used to mill grooves around the periphery.
The abrasives for two of the hones are made from DMT Flexi-Sharp sheets. They offer 0.006" thickness steel shim that is coated on one side with their diamond abrasive in four available grit sizes. Shown are the 600 and 1200 mesh (also referred to as fine and extra fine).