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Manual machining 4340 steel.

Manual machining 4340 steel.

Manual machining 4340 steel.

I’m working on a major ) for) me project. It’s an 18 cylinder model radial engine. This is very similar to a 1/5 scale size P&W R 2800. These have been done before by others so the design is good. The machine work is very taxing as it’s small and lots of parts that fit together.the issue is the crankshaft. This is built up of several pieces press fit together as most small engines are.

The counter weights are flat fully machined plates with the master rod journal being the main piece but there is a center bearing. Now the plates may be machined flat but days later when getting ready Th assemble they hav warped. It’s a lot of work to machine these and numerous setups. I can’t imagine thes being made in the 1940’s , all the respect in the world to the people that made this motor what it was. The first one I made looked nice but was a dead failure. Al four counterweights warped like pretzels. The second go around I caught by closely checking on my surface plate. But the first time I pressed the rod shaft in the plates both bowed out. There is only about .002. Press fit on the parts. The finish is pretty good for being machined, not ground. The rod pin had nice radiused lead in so it did not gall up.but even severa days later the secon counter weight is no longer flat. They are at room temp about 70 deg. f. They dit nor get very warm during machining. I used a very sharp edge carbide cutter to finish cut the plates. The stock looked like hot rolled. I cut about equal stock off each dice. It was about 1/8” . There was no porosity and the cut was good with good finish. Even the bores were quite smooth. I don’t have a giant mill or lathe but both are near new. I try to get the best cutters for the job but I can’t pull the nice insert tools like bigger machines. I have a good friend that could surface grind the plates but I think something goes wrong in the press fits. I have a pretty solid hyd. Press and I carefully get things lined up. I’m thinking there is too much interference in the fit. Others have made thes motors from the same drawings and some have had this problem. Other small model motors have pressed together crankshafts. Weed eaters are a good example. Maybe they are just looser construction. The pin is 1.5000 inches. Small engines often have bent it out of line cranks. There are guys that make a living straightening them.

RE: Manual machining 4340 steel.

Exactly how big/thin are the plates you're making?

It's possible they have significant internal stress, or that the press fit itself is changing the internal stress field and causing your problem.

If they are thin enough, you may be able to cut them from shim stock or precision sheet - your base material would be more expensive, but would give you very flat base stock with no internal stresses; you'd have to cut the perimeter profile but the flat surfaces would require no attention at all, which may make up the difference as far as cost.

RE: Manual machining 4340 steel.

What is the rod diameter, and how wide is the plate? 0.002" interference sounds pretty tight, and would take some fairly high push to get the joint to slip together. Could you make the joint looser then braze it, or do the loctite and pin method?

RE: Manual machining 4340 steel.

Cool the pin, heat the counterweights, less force to press the parts, less likely to bend during insertion.

If the counterweights still warp, reduce the amount of interference.

Best regards - Al

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