Armstrong RML Chassis Girders

by John


chassis R rear obl


Having made the decision to try to mill the girders from solid steel bar, I bought some 50x16mm bar and cut it into 400mm lengths.


Then milled it to 46.4 x 14mm, then used carbide end mills to form the girder profile.  This process produced a large amount of hot, sharp chips, and took 2 full day sessions in the workshop.  Each evening I spent about 30 minutes pulling bits of swarf from the soles of my boots with pliers.

And I discovered the limits of my milling machine.  The 5hp spindle motor never hesitated.  Nor did the axis AC servos.  I did manage to chip the cutting edges of  a 12mm carbide end mill when it dropped onto the milling table.  And I blunted another one.  Not sure how that happened.  Maybe hit a hard bit in the steel.   No, the limit of the machine was the ability of holding the end mills in the ER40 collet chuck.  If I pushed the depth of cut or the feed rate too hard, the cutter would start to move in the chuck.  I managed to ruin one work piece in discovering that fact.


There is virtually no distortion resulting from the milling.  The apparent bend in the photo is photographic distortion.


The Vertex milling vices are within 0.02mm for height.  I picked up the second vice cheaply on Ebay a couple of years ago, with this exact purpose in mind. (milling longish workpieces)


3 made.  One to go.  Plus the bottom 6º shape and the 4º ends.   Each 4.5mm deep pocket takes about 25 minutes, at 300mm/min feed rate, 1.5mm depth of cut, 2700 rpm.

I should be able to finish the girder shapes tomorrow.


These milling marks are visible but very not finger tip palpable.  Any suggestions for a good method of improving the finish?

Then to mark out the rivet positions, and insert about 100 rivets into each girder.  In the model these will mostly be decorative.  In the original they held the components of the girder together.  Luckily for me, a fellow member of our model engineering society is a very experienced riveter, having worked in aircraft manufacturing, and he has offered to spend a session teaching me some basics.  In the original cannons, the rivets are superbly neat, regular, and obsessively carefully laid out.  I will try to do likewise.