MAKING A GRADUATED LEAD SCREW FOR YOUR TAIG MICRO LATHE
When I last left you, I tried tantalizing you with the idea of a graduated lead screw that could be used to operate the Taig carriage toward the headstock during turning and boring cuts. Although, turning the carriage crank very steadily by hand will produce a smooth, even finish, a lead screw can produce and even better surface finish. The secret lies in the fact that you end up with a reduction. In other words, for every ten to twelve turns of the lead screw crank, the carriage will advance the equivalent of one full direct turn of its handle. The almost 12:1 reduction enables you to make extremely even and finely spaced cuts without too much effort on your part. Because the lead screw I designed does not directly connect via a nut to the carriage, you are not a slave to it like you are on a Sherline as well as other lathes. As it advances the carriage, its tapered tip merely bears against a small square of brass sheet attached to the side of the carriage. The system works great. Unlike a common lead screw, this one will also allow you to directly measure the amount of carriage advance by simply reading its graduated collar as you turn it. It reads in thousands of an inch and provides .050" of advance per turn. Once you are finished using it, you simply crank it back toward the tail stock and out of the way so you can once again use the carriage crank. I use mine to bore as well as turning shoulders to specific depths. You might be tempted to continue relying on a dial indicator but I have found it to be accurate to within .001 in 4" of travel. If this sounds good to you, then lets go ahead and build one.
The heart of this device is simply a length of ordinary threaded rod. You do not need an acme thread for this low stress application. I had two choices for the rod size. The easier route was to just visit my local hardware and pick up a foot of 1/4-20 steel or brass rod. The second choice involved placing a special order through places like MSC for some 5/16-20 rod which is the fine thread option for this diameter. The bigger rod would have produced a more robust lead screw. Not that my 1/4-20 choice lacks in strength. Its just that if I was doing it again I would opt for the later. Use of a twenty pitch screw will give a .050" linear advance rate per full revolution so this proved to be a good choice. If you own a lathe capable of cutting threads, you may want to experiment with 10 or even 40 pitch to give you either .100" or .025" advance per turn. Your choice would depend on the advance rate desired.
I cut the 12" piece of rod to a 10" working length and chuck it to the lathe in order to create the tapered tip. After facing off the cut end, I set the Taig compound to approximately 45 degrees away from the spindle axis and carefully begin to cut the taper. You want to continue cutting until the flat of the tip is about 1/16" diameter. Polish the cut with a couple of grits of silicon carbide paper to remove burrs and such and you are done with the screw. Instead of a nut, I use a threaded block through which the screw can pass. If you have seen my Basic or Advance Taig Micro machining Videotapes, you can clearly see what I am talking about. The block sits directly along the front side of the lathe bed on the mounting board. It is either bolted or like I did, just epoxied in position. The positioning is such that the tip of the screw bears against the side of the carriage at a height more or less equaling the height of the toothed carriage rack located along the front side of the lathe bed. By being set at the same height as the rack, it eliminates the possibility of lateral jamming as the screw begins to apply lateral force against the carriage. Take a 2" length of 3/4" x 1-1/2" of aluminum or brass, ( aluminum is cheaper ) and clean the saw surfaces either by side milling or by mounting it on the four jaw and facing the ends off. Locate the vertical center line along both ends and locate the location for the hole by measuring from the mounting board surface to the top of the teeth of the carriage rack. Mark this height on both ends of the block to locate the positions for the two holes that will allow the screw to pass through. At first, I was just going to drill and thread the hole directly on the block but that would have created a loose fitting situation with too much backlash of the screw. Instead, I decided to drill the through hole oversize and make two brass threaded bushings that would press fit into the larger hole on the block. To totally eliminate backlash, you just turn one of the bushings in one direction until the screw can turn backwards and forwards without any backlash. You can actually lock the screw if you turn the bushing too much. I made my bushings out of a short piece of _" brass rod. Chuck it, face both ends and begin drilling a through hole to the 1/4-20 tapping size and finish tapping with a tail stock held tap. Part the stock in half to create two bushings. Re-chuck and clean up the parting cut surfaces and chamfer the edges nicely. Machine one end of each bushing to create a portion that will press fit and a square shoulder that will sit flush against the side of the block when pressed home. Press the bushings in place and align one so you can thread the screw through both of them. Now you can adjust one of the bushings to give the desired amount of play or backlash. Mine gives zero backlash. Screw it almost all the way through the block so you have about 1" of the untapered end stick out of the block and set it down on top of the mounting board. Move the carriage all the way toward the head stock. Align the block so its parallel to the lathe bed and it is almost touching the side of the bed base. Locate where the pointed end touches the side of the carriage and epoxy or CA glue a small square of brass sheet to act as a bearing surface and protection against the softer aluminum carriage casting. Mark the block position on the mounting board and epoxy the block to the board.
The next project will be the graduated collar.