dynamotive
Plastic
- Joined
- Jan 20, 2007
- Location
- San Diego
I joined this forum to ask questions about machining a crankshaft for my quarter-scale Merlin engine. You can see the engine at www.quarterscalemerlin.com. I first started the topics 'fast stock removal from steel', and 'forging' to simply ask questions about processes, and there's some good answers in those. Here's a picture of an investment-cast 4130 steel crankshaft blank, one of four I had done recently.
It's 10.3" long. It looks nice, but it's unusable because there are voids in the casting, and it's just a bit too long. This batch was actually the third time I've tried this. In previous attempts, the casting places picked their favorite exotic alloy, and the results were not machinable or straightenable by me, plus they were incredibly warped. I added the extra bridges between the crank webs to reduce warpage, and it worked pretty well. But, of course, if the previous ones had been straightenable, it wouldn't have mattered. All part of the learning and flogging around process.
The casting place warned me that this shape might not cast well, and they were right. Years ago, I machined a crank for the prototype out of a 2.5" diameter 4140 rod, and had it ground at a precision grinding place. It came out fine, but ever since, I have been most anxious to find a way to simplify the process of producing more crankshafts. That's why the effort and expense on the cast steel crankshafts. Many people will undoubtedly suggest exotic ways of doing this, so let me state right up front that I do not want to invest large amounts of money in any part of this project. No large quantities, and no expensive processes or tooling. Ok?
Somebody suggested a cast iron crankshaft. That is not something that I have considered before, because I think of cast iron as being brittle and weak. I know that many cars use cast iron crankshafts, but racing cars rarely do, and the real Merlin certainly didn't. However, this model engine actually is not that highly stressed, at least the way I envision it. The real Merlin turned a maximum of about 3200 rpm, and in order to make the scale version as realistic-sounding as possible, it should be kept in that range. According to some supercharger experts I consulted, this engine should be capable of producing about 16 hp at 3600 rpm, which should be enough to power a 1/4-scale Mustang or Spitfire at scale speeds. An engine this size running at 3600 rom should be relatively unstressed, so maybe a cast iron crankshaft would work fine, but I still feel very reluctant about going that way. What are the chances that I could get a cast iron crankshaft blank that is reasonably sure to not have any dangerous defects in it?
Incidentally, I just got a book called 'Vees for Victory' which describes the Allison V-1710 in great detail. In the early versions of that engine, the crankshaft did not have any counterweights. It was designed to run at 2400 rpm, and at that speed, the bearing loads due to imbalance were acceptable. Weight is obviously very crucial for any aircraft engine, and leaving the counterweights off saves weight. When the design speed was increased, counterweights were added, but only to reduce the bearing loads to acceptable levels, never to try to achieve complete balance. Again, it's that weight thing that's hanging over them. The Merlin's counterweights do not aim for complete balance either.
Getting back to the investment-cast steel crank, I think that if I give up trying to cast a near-finish shape, and make it look more like what a forging would like, in other words very rounded, and only an approximation of the finish shape, and with no counterweights, that it would cast fine. The problem is finding places that are willing to try it for small quantities. I think the place that did the last ones doesn't want to hear from me any more.
The other option brought up was machining a flat blank and twisting it to get the correct throw orientation. The mains are pretty narrow - would that work? Would that be something I could do with a torch and some sort of jig?
It's 10.3" long. It looks nice, but it's unusable because there are voids in the casting, and it's just a bit too long. This batch was actually the third time I've tried this. In previous attempts, the casting places picked their favorite exotic alloy, and the results were not machinable or straightenable by me, plus they were incredibly warped. I added the extra bridges between the crank webs to reduce warpage, and it worked pretty well. But, of course, if the previous ones had been straightenable, it wouldn't have mattered. All part of the learning and flogging around process.
The casting place warned me that this shape might not cast well, and they were right. Years ago, I machined a crank for the prototype out of a 2.5" diameter 4140 rod, and had it ground at a precision grinding place. It came out fine, but ever since, I have been most anxious to find a way to simplify the process of producing more crankshafts. That's why the effort and expense on the cast steel crankshafts. Many people will undoubtedly suggest exotic ways of doing this, so let me state right up front that I do not want to invest large amounts of money in any part of this project. No large quantities, and no expensive processes or tooling. Ok?
Somebody suggested a cast iron crankshaft. That is not something that I have considered before, because I think of cast iron as being brittle and weak. I know that many cars use cast iron crankshafts, but racing cars rarely do, and the real Merlin certainly didn't. However, this model engine actually is not that highly stressed, at least the way I envision it. The real Merlin turned a maximum of about 3200 rpm, and in order to make the scale version as realistic-sounding as possible, it should be kept in that range. According to some supercharger experts I consulted, this engine should be capable of producing about 16 hp at 3600 rpm, which should be enough to power a 1/4-scale Mustang or Spitfire at scale speeds. An engine this size running at 3600 rom should be relatively unstressed, so maybe a cast iron crankshaft would work fine, but I still feel very reluctant about going that way. What are the chances that I could get a cast iron crankshaft blank that is reasonably sure to not have any dangerous defects in it?
Incidentally, I just got a book called 'Vees for Victory' which describes the Allison V-1710 in great detail. In the early versions of that engine, the crankshaft did not have any counterweights. It was designed to run at 2400 rpm, and at that speed, the bearing loads due to imbalance were acceptable. Weight is obviously very crucial for any aircraft engine, and leaving the counterweights off saves weight. When the design speed was increased, counterweights were added, but only to reduce the bearing loads to acceptable levels, never to try to achieve complete balance. Again, it's that weight thing that's hanging over them. The Merlin's counterweights do not aim for complete balance either.
Getting back to the investment-cast steel crank, I think that if I give up trying to cast a near-finish shape, and make it look more like what a forging would like, in other words very rounded, and only an approximation of the finish shape, and with no counterweights, that it would cast fine. The problem is finding places that are willing to try it for small quantities. I think the place that did the last ones doesn't want to hear from me any more.
The other option brought up was machining a flat blank and twisting it to get the correct throw orientation. The mains are pretty narrow - would that work? Would that be something I could do with a torch and some sort of jig?
