Frames

A couple of important decisions before we start ordering material ………

• What thickness is to be used?  I chose to use imperial size 1/8″ to avoid making mistakes with dimensions that would need to be altered if a metric size were adopted.
• What to buy? basic material, laser cut with holes spotted or laser cut without holes spotted. My enquiries found that laser cut frames were only available in 3mm thickness so that ruled out any laser cut frame anyway. Laser cutting does leave a hard edge which can be problematical so I decided to investigate water cutting.

Water cutting contractors are quite readily found but you do have to supply them with a DXF file of the part to be cut. What is a DXF file? well its a Computer aided design file that traces the contour of the part that is then used by the water cutting machine to control the profile cutting. How is one produced? By a computer aided design software package. If you have no access to one of these programes then water cutting is not an option.

Fortunately I do have a CAD (computer aided design) software suit so I was able to draw out the frame profile using the dimensions from the drawing and thus produce a DXF file. I chose not to include the holes as my mill and its DRO would be perfectly capable of drilling them accurately and it saved on the cost.

If you do use the water cutting option do check with the contractor what accuracy they will cut to as you may find they have different options on accuracy and hence cost. Also check if they work in metric or imperial (or both) as your DXF file if in imperial will have to be coverted by them to metric if that is the only machine option they have. My contractor only worked in metric but he did the conversion and sent me screen shots of the dimensioned profile  that resulted for me to check that the metric measurements tallied with my imperial dimensions. I’m glad to say they did. The accuracy of cutting was 0.2 mm or 7 thou in old money.

Finally my contractor sourced the 1/8″ material. It was 6″ x 1/8″ flat bar and I had also found two suppliers of the same in case I had to free issue the material.

In order to keep my costs as low as practical I had the main frames, rear frames, bogie frames and tender frames all done on the same order.

Within a couple of weeks of placing my order I had all delivered. Quite pleased it worked out OK and I got back the off cuts that were unused due to minimum lengths of bar having to be purchased.

A couple of things that were noticeable: there is a very slight chamfer due to the wter cutting operation nothing that needs anything to be done to it however and when the bar is cut the frames may have some bows due to the stress relief that occures when cutting large bits out as happens towards the frame rear. Again nothing serious that cannot be rolled out flat (ish).

It is worth noting that the left and right frame are not identical as there is a cut out at the rear of the frame that is in a different position. So my DXF file did not include these and I chose to cut them out on the mill seperately.

before doing any drilling a decision is needed on the type of main springs to be used as it affects the holes needed. There are three options, one being prototypical leaf springs and the other two being versions of coil springs. There are a number of opinions on live leaf spring design from the leaf spring being all metal to only a few leafes being metal with non metallic or phosphor bronze filler leafs. My personal preference is all metal springs as I have seen too many springs invert under load and use where non metallic filler leafs have been used. So my frames are drilled to accommodate the spring hangers for live leaf springs.

In order to drill the frames they are placed back to back and clamped together. I set the frames up on the mill table on a wood base to allow the drilling to penetrate the frame but not damage the mill table. The frames were clocked parallel on the table from the top edge of the frame. Now the mill table travel was insufficient to do all the drilling without moving the frames so at some stage in the drilling process a mark had to be made from which the moved frame could be lined up to and then clocked again to ensure they were parallel to the table. Using the DRO the holes were all drilled and reamed (where noted) according to the drawing dimensions. My process was to highlight the drawing dimension and hole as it was drilled to ensure that no hole was omitted.

A couple of holes were too large to be drilled and they were bored to finished size.

With all the holes drilled the frames were seperated and the piller drill set up to countersink the holes so marked. The countersing depth and diameter was checked from the screw head to be used and the drill press stop set accordingly. The frames were held by hand for countersinking as the drill naturally centred on the hole. Finally all holes were deburred both sides and those requiring to be tapped had their threads cut.

The main frames are not bent at the rear yet as the hornblocks have to be fitted from the frames and then machined with the frames back to back to ensure the axle box slots are accurately squared with each other. A process to be descibed later.

Frames as finished drilling with horn blocks fitted see later blog.

Having assembled my frames with a number of parts I found when examining the drawing for the cylinder blocks that the K exhaust casting requires holes in the frames around the rectangular cut outs that are not shown on the frame drawing. I will have to drill them in situe but anyone starting this now with this knowledge can transpose the holes from the drawing showing the K exhaust casting onto the frame drawing. Unfortunately the holes are not dimensioned so they will have to be scaled from the drawing.