CENTER LINE LATHE MARKING TOOL
A tool post marker or scribe is a great little tool that will mark positions radial to the center point of a faced surface to within .005". Simple eccentric cam fabrication requires that an offset hole be accurately located at one half the stroke motion of the piston or valve it's operating. I suppose a good machinist's ruler can be used to locate the offset point but it certainly won't be within .005". Since the cross slide on any lathes travels along the Y axis by turning a graduated dial, you can use it to position a tool post holding an engraving point at the lathe center height to mark or engrave lines to any distance within its range.
If you own a Taig, make a tool post out of 1" square material. Aluminum is the cheapest and probably the easiest to find as scrap at a metal scrap yard. I find it in random lengths as short as 2"-3". The mission here is to duplicate the stock Taig tool post which is also 1" square and about 2" high, with the only difference being that they are anodized to make their surfaces more durable. We are not going to be able to anodize ours but with a bit of care during use, it should be able to last for years.
The best way to machine square stock when extreme accuracy is not required is to use one of the self centering four jaw chucks sold for mini wood working lathes. They are spindle relatively cheap to buy ( under $70 ) and they have a 3/4-16 mount so it will fit the Taig spindle perfectly. By scrolling the chuck with a lever, you can open or close all four jaws in unison. Grip a 2" plus length of 1" square aluminum and face both ends to bring it to 2" long. Center drill and drill through with a #11 bit. Counter bore with a 1/4" to a depth just a bit deeper than 1/4". Chuck a 1/4" end mill to the tail stock chuck and carefully square up the conical bottom left by the twist drill. This counter bore will accommodate the head of the 2" long 10-32 cap screw used to attach it to the cross slide. Counter sink both ends of the hole slightly. Pass the cap screw through the counter sunk hole and screw a 1/8" x 3/8" 10-32 square nut to the tip of the screw and slip it through the cross side "T" groove closest to the head stock and lightly tighten. Screw a faceplate on the head spindle and use its surface to square up one of the sides of the tool post flush against it and tighten it without disturbing it. Back up the carriage and install a drill chuck and arbor to the spindle and drill a hole a few thousands under .125" to a depth of 1/2", located 1/4" in from the front vertical edge of the post so it won't interfere with the mounting screw hole. Ream to .125" and remove from the cross vice. The hole you just drilled and reamed is exactly placed at the lathe center height and it is parallel to the lathe horizontal axis. The scribe is best made from a small diameter carbide micro bit. These tend to break quite often and they can be re-worked to a sharp point and used. I begin by breaking off any remaining portion of the twist bit. The smallest the bit number the better it will work. A 1/64 " tip is best. The 1/8" shank tapers as a 20o cone just behind the twist bit so if you break it so only about a 1/32" remains, it can be worked to a very sharp 40o or so point by chucking it on a collet to the lathe spindle and honing it to a sharp point with a small fine grit diamond hone (about $10.00 ). The finished scribing point is inserted into the hole. No glue will be needed as the reamed hole should provide a nice snug press fit.
Applications for this tool are as follows. You have a faced and turned eccentric cam or crank disk that requires a hole to be drill at a very specific distance from center. You can advance the tool post with the pointed scribe toward the turning's center. You can easily see the center if you faced it off. The concentric lines form circular patterns that end at the very center of the part. Once you have the very point of the scribe touching the tiny dimple on the center, you advance the cross slide toward the front by directly reading the distance on the graduated cross dial. At that point I move the carriage toward the work so as to bring the scribe point into contact with the work and give a light tap to the rear of the tool post with a soft head hammer ( brass ) so the point makes a punch mark into the work. If you still need a central hole drilled as in the case of a crank disk, you can do so now in the regular manner by using the tail stock drill chuck. Do not drill the central hole first or you will loose the central dimple with which to line up the scribe point.
Another use is to mark out three or four equally spaced hole locations such as when drilling round openings on a model engine flywheel. To do this without the use of a dividing plate, you rely on the jaws of the chuck to index out three or four points. Take a piece of scrap or a large enough keyway and lay it flat on the lathe bed so it sits directly under the chuck jaws. Rotate the spindle so one of the jaws is trapped and kept from rotating by the metal block underneath. With the pointer at the center of the part to be indexed, scribe a horizontal radial line from the center toward the rim of the flywheel. Rotate the chuck so the next jaw is trapped as before and repeat the scribing process. Finish scribing as many lines as there are chuck jaws. Measure out from center as you did in the first example, where you want the holes to be centered, bring the pointer against the work and rotate the chuck to make a circular scribe mark so it intersects each of the radial lines. Punch and drill each of the intersections and proceed with the drilling and or boring.
You can also rotate the tool post so it is square to the long axis and use it as an engraver of horizontal lines either on dials or shafts.