View Full Version : radius marking jig

12-29-2009, 02:33 AM
Another little project- took all day though. This is- I don't know what to call it- it's a tool to position a marking point on the face of a workpiece in the lathe. It will position the point a precise distance from the spindle axis, arrived at by turning the lead screw.


This is a view of the workings side. The base is something I made up a long time ago to hold something on the ways of the lathe. It sits on the flat ways and is precision fitted to have essentially no play. The two small holes are for bolts that bring up a plate from the bottom to secure it in place.

Here is a detail showing the construction of the slider assembly. As you can see, I've used some spring pins to help keep it aligned.


Here's a close-up of the lead screw area. I used two nyloc nuts to keep the threaded rod positioned. I can adjust the nuts to eliminate end float, even give it a slight pre-load. The two nuts on the slider assembly have been given a slight upwards push before marking the bolt holes. This makes the lead screw bend upwards very slightly, which in turn pulls the slider downwards onto its baseplate. I've also spread those nuts apart to take out the play in the threads. If the tool gets much use and some play develops there, I can loosen one of the nuts and take out the play again. As it is now, there is zero play anywhere.


This side of the gizmo faces the chuck. What you will see here when it's finished is two or three holes near the top of the slider assy, spaced exactly one inch apart. The precise location of those holes will be determined by mounting a drill bit in the chuck, then bringing the gizmo up against it. With the adjustment set for zero, I'll lean the assembly toward the drill bit, which will mark a hole location. I'll dial in exactly one inch, then make a second mark, then a third. Once those holes are carefully drilled, a sharpened rod will insert from the tailstock side- and that's the entire function of this thing- dial in the required radius on a part, then tap the rod to leave a mark on the workpiece. By having three holes in the slider, I'll be able to mark out radii to the limit of the swing of the lathe.

I should mention here that I have modified my lathe so I can index the chuck. It wouldn't be much use to have this gizmo if I couldn't also space the holes precisely around a circle. So far I can position the chuck every 15 degrees- a future addition will allow me to divide a circle into 100 parts, plus I can fill in location holes every 5 degrees if that would make any sense. I plan to integrate the locking pin with a power switch so I can't accidentally turn on the motor if the spindle is indexed.


Oh, yeah- I haven't finished making it yet- there will be a knob on the threaded shaft marked out with 50 marks, and a bit of a plate of sorts behind it with an index mark on it. The markings will represent 1 thou. In the first pic, the bottom of the slider will get an index mark and just below that will be a corresponding inch layout. Between the two I'll be able to adjust the radius marker pin to anywhere from zero to 4.5 inches, within a thou or maybe less. I've done some experimenting making holes that the marker pin can fit into with zero play, so I'm not worried that there would be slop there- what I do have to ensure is that the point is ground on it perfectly centered.

I could have used this tool the other day when I laid out the pin holes for that blender repair device. It would be good for marking out holes for mounting bolts, for lantern pinions, crankpin locations, etc. My challenge after the holes are marked out on a workpiece will be to drill the part without making the holes wander off center. A future addition to this jig might be a rotatable marking point, a center drill I guess- something to leave more of a dimple to guide a drill bit later.

12-29-2009, 10:12 AM
An easier way is to use a pair of spring calipers or a beam compass. You make a small indentation in the center of the work with a very small center drill and then use the caliper or beam compass to mark the circle you need. Been doing that for years and it works everytime.