Model Locomotive Frames

Harry Coventry
Model Locomotive Frames

by H. J. Coventry

The Live Steamer, July-August 1950



The following procedure of making frames is offered with the hope that it will help to eliminate some difficulties of obtaining accuracy of wheel alignment and side rod fits, troubles which can frequently be traced to the frames. Bar steel is the best material to use in considering the disadvantages of other metals. Choice of using either CRS flat, or HRS flat. If the former is used, one precaution must be taken to allow for a certain amount of curvature of the edge opposite to the cut out, which is caused by the relief of the skin tension on one side. This will not occur with the HRS, but the edges may not be as flat or smooth as the former, so have to be corrected with the file before marking out.

Having two pieces of steel and the detail drawing, paint the surface of one side of each bar with one of the commercial marking out fluids, then scribe a line all the way along to represent the center-line of axles, also the cylinder centerline. Next, mark off the vertical center line of cylinder, allowing a little at end for the final finish. From this line, carefully mark the distance AB to the first axle (see sketch), then from the same point, mark off distance AC. If there are to be three or more axles, always mark off from point A, get the dimension from drawing by adding up the separate quantities. In this manner any error however slight will not accumulate. The aid of a watchmaker's loupe will hold dimensions to within plus or minus 0.005 inch. Having the main center lines, carefully center punch lightly each intersection with a sharp center punch.

Set a pair of dividers to half the pedestal opening and strike arcs at each. Scribe the openings square to top edge, lines to touch the arcs. It is well to mark out all holes or other lines given on detail print, before cutting away any part. Select a hole at each end and drill thru. Now clamp the two bars together and drill thru the second bar. Put a rivet thru the holes. Drill thru both bars with a row of say 1/8 holes at top of pedestals, and large drills where openings are made between pedestals. Allow 1/16 inch for finish for pedestal jaws and saw to the small holes, a jewelers saw will sever the bridges between holes. The larger openings may also be sawn out.

With the rough cutting completed, the first face to finish is the one indicated as E on the sketch. File this close to the line, testing frequently with the square from top edge of bar and also thru the opening. Finish the surface with a smooth file, finally "draw file". Now we need two pieces of CRS same dimension of pedestal opening. Square bar is preferred but a flat 1/2 inch thick will serve. This steel is true to two thousandths as manufactured and forms handy gage. Cut the pieces long enough to span the frames when erected. Now file the edge F until the CRS steel fits nice and snug, it should not be loose but require hand pressure to slide it down, and bear evenly over the surface.

Proceed with the next pedestal face E1, but this time set a pair of outside calipers to dimension equal to axle centers minus one pedestal opening. The loupe is helpful in setting the calipers. Test with the square in both directions as before and file until the calipers just slide over evenly. The face F1 is produced by using the gage of CRS. All the pedestals required, are done in the same way, calipers being retained at the same setting. This method insures evenly spaced and more accurate pedestals than merely working to the lines. The rest of the finishing may be done to outline, which care to produce square edges. Check over all holes drilled, as called for by print. Knock out the two holding rivets and clean off burrs with a fine file for both frames. You should now have the satisfaction of trying the two pieces of CRS steel gently pressed into the pedestals, one front and one rear, and find them square with frames and parallel to each other.

Later when the side rods are to be made, it will be necessary to have the centers exactly the same as centers of pedestals. This can be easily done with the simple jig (shown L on sketch). To make this, a piece of CRS flat, say about 1-1/4 inch by 3/8 inch and long enough to span two pedestals. Next take a short piece of CRS round, of same diameter as the pedestal opening, which will be found to fit as snugly as the E F blocks. Turn a shoulder K and drill thru with a small drill of 3/16 inch or 1/4 inch. The K diameter should be a wringing fit in a reamed hole in bar L. Now place the bushing G with the bar in a pedestal opening and clamp to frame. Make a ring H, of same steel as G, the hole to be reamed same diameter as K. Drop the ring in the other pedestal and clamp it to the bar L. Drill thru with K diameter. You will now have the bar L with two holes exactly same centers as the axles.

Any size shoulder bushings can be made later on, to suit the holes in the side rods.

If these few precautions are taken, little trouble will be had with "binding rods" due to inaccuracies of frame work or side rods, assuming of course that the crank pins are properly set at same angle throughout. But that is another story.

Shims
Steven Bible posted the following on Facebook:


 * Here is a CAD rendering showing my idea of the center driver journal-to-frame 3/64 inch shims. The frame is the waterjet cut. I'll fasten them to the frame with #4 flat head screws. Shims will go on both sides of the frame. This will limit lateral movement to 1/32" as the print suggests.