Adjustable Drill Jig
Adjustable Drill Jig
by R. G. Neff
The Live Steamer, September-October 1950
In building "O" Gauge live steamers it is difficult to scale down some of the parts and maintain correct proportions so they will work properly. This is especially true of Baker valve gear, as very small errors in the length of the various rods and levers will throw off the action and the engine will not run smoothly at all settings of cut-off. Of course, the holes should be jig drilled, but the problem is in making accurate jigs. This job is too small to work out by the usual surface palte methods. You can come fairly close by using magnifying glass, a 100th scale and needle scriber and after the holes are drilled you can insert pins and check them, but you may have to make several tries, which takes a lot of time.
Becoming tired of the above procedure, I devised an adjustable drill jig with which the holes in the Baker gear parts can be located with less than 0.001 inch error.
Construction is shown plainly in the sketch. Brass or steel may be used for the body of the jig. The bushings are made of 1/8 inch drill rod, drilled in the lathe, hardened and pressed into the body. I use 1/16 inch as a convenient size for the holes and pins. This size is small enough to redrill for tap and clearance holes. As drills always make an oversize hole, drill the bushings first with #54 or #55 drill and redrill with 1/16 inch which will make them exact size and the pins won't wobble. As these small drills sometimes wander, after the bushings are in place insert pins and check to see they are square with body and parallel. If not, bend or twist the ends to get alignment. Then file the side you will use as the bottom to remove high spots. Hold the jig on a dead flat surface while adjusting. To drill two holes a given distance apart, set the mikes to that distance plus 0.0625. Insert pins in the holes in the jig and set them a little wider than the opening of the mikes. Put a light tension on the clamp screw and turn in the adjusting screw and turn in the adjusting screw until you get a feel across the pins. Set the clamp screw tight, recheck and you are set to go. Mark and drill the hole at one end of the work, first the smaller drill and then 1/16 inch. Place the jig on the work with a pin thru one bushing and the hole you have just drilled, and drill 1/16 inch thru the other bushing.
This will space the holes accurately. The sketch is the exact size of the jig as I made it. Overall size is not critical but for "O" Gauge it should be adjustable from 3/16 inch or 7/32 inch to about 5/32 inch. I believe such a jig would be helpful in 1/2 inch or even 3/4 inch scale.
Models with this jig build Baker valve gear operate five and six minutes per ounce of water (this is at a normal road speed pulling average load) and is a 25% increase in efficiency over previous models without use of the jig.