Cylinder Bases. Lathe or milling machine?
I read an expert treatise on making a double expansion steam engine, and I imagine that the comments applies to triples also. One aspect emphasised the importance of accuracy in making the cylinder bases. The parallelism of the surfaces, the concentricity of the piston rod hole and the other circular elements, and the thickness. The usual method for making these items is to turn them in a lathe with a 4 jaw chuck, then to reverse the item in the 4 jaw to turn the other face. It is possible, but very fiddly and time consuming, and relies on expertise, patience, good eyesight, and a good lathe. All of which are in short supply around here. A triple expansion steam engine requires 3 of these base plates, and while there are some common dimensions, the cylinder bores are all different. Many of the screw holes are common to the 3 plates. The thicknesses are all the same. To shorten this rather boring epistle, I decided to have a go at making the base plates on the CNC mill. Given my previous muck ups, broken bits, crashes, this was a courageous decision, as evidenced by having to bin the first effort. But the next 3 all seemed to work OK. First I studied the plans and noted the common elements, then I made a jig, with holes drilled at the common positions.
To see a video of the CNC mill cutting the external profile click on the link below http://youtu.be/m0d5yuX96Uc
BTW. In a previous post I mentioned a 1 mm inaccuracy in a CNC milled part. It happened again when I milled the first base plate, which ended up exactly 2mm smaller than programmed, and had to be re-made. This time I discovered the cause of the inaccuracy…. I had used an 8mm milling cutter, but had forgotten to tell the CNC computer that I had changed from using the 6mm cutter. The CNC machine did not notice the change, and cut the part exactly as instructed, very accurately, 2mm smaller than intended. CNC machines are incredibly clever, but very very dumb. They do exactly as instructed, even if the instruction is wrong.