KNURLING ON THE HARBOR FREIGHT MINI LATHE
I began machining ( not counting my very early experiences in school ) with a very small Taig microlathe which I find entirely adequate for most of the small work I do in my shop. With it, I have built complete steam powered stationary model engines. Many times as I am machining a small knob or a special screw, I feel the need to knurl the otherwise plain surfaces. It not only provides a very attractive finish on them but also produces a perfect gripping surface on all knobs, screws or levers. There are a few problems encountered when one attempts to knurl on a Taig. First of all, Taig Tools does not make a knurling tool for it and besides, the lowest speed of around 550 rpm is simply to fast. Even if there was a knurler for it, it would have to be of the very expensive scissors type, as the "push into the work" type tools are simply too stressful on the spindle bearings and the cross slide lead screw nut. Repeated use of this type of tool would lead to the early demise of both of these components. Sherline lathes have designed a very ingenious knurling tool which when clamped to the cross slide table ( "T" slotted like that on the Taig ) acts a clamp around the rotating work and absolutely no lateral thrust is applied to the spindle bearings or cross slide nut.
In a larger more stout machine, you are able to use the more simple and much less expensive style of knurling tool without much fear of damage to those two components. You are still asked to progress through the knurling procedure with care and without any hurry whatsoever. I was able to locate a trio of Asian made knurling tools which fit perfectly on my lathe tool holder. The first tool is a simple straight knurler and basically as the name implies, it produces a set of horizontal, parallel triangular faceted lines on the surface of the work. Much like that found on the edges of common coins. Thus the name "coining" by which it is also known. The second tool produces a diamond pattern of pyramids that can also be very attractive on the edge of a workpiece. The last is a tool with a rotary head containing three matched pairs of diamond knurls in coarse, medium and fine sizes. The process of knurling does not generally involve any actual cutting but is more of a deformation of the metal surface. As the points of the knurled are pushed into the work, they cause the metal to be displaced until the complete pattern is formed. In order for the knurls to track and register on the previous marks, the diameter of the workpiece must be tailored so you do not get an overlapping pattern developing. If this starts to occur, just take a light diameter reducing cut across the surface of the part being knurled and make a second attempt. Eventually, you will arrive at what appears to be the perfect diameter as you will get a perfect meshing of the knurl pattern develop revolution after revolution. If the edge being knurled is narrower than the width of the knurling wheel, you can just leave the carriage in place without moving it. If the area is longer, you will have to advance the carriage across the length of the work until a pattern has appeared along the area you want. Long work will need to be supported by the tail stock live center. Continue to move the carriage back and forth along the length of the work and advance the cross slide into the work about .002" to .003" per cycle, while liberally applying a cutting made for the type of metal you are working on. The knurling process is considered finished when the peaks or points of the diamonds straight lines are fully sharp. Further advancing of the cross slide is futile at this stage as the pattern of the knurl has penetrated as far as is ever going to penetrate. If you feel that he sharp point on the top of each knurl is too abrasive on the fingers, you can knurl only as deep as you think need to go. The sharp points can be leveled down a bit by taking a turning cut across them just enough to create a tiny flat on each of the diamonds.
Use the auto feed carriage feature of the lathe as it makes the whole job go a lot easier. My Harbor Freight Minilathe has a reversible electronically controlled motor that makes the whole job of knurling a heck of a lot more relaxing. You just make a pass across the work and return the carriage back to the starting position by just flipping the reverse toggle on the control panel. Advance the tool a few thou and repeat the process. I hope this takes some of the fear and or mystery out of process knurling.