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Old 08-08-2009, 10:06 PM   #1
gbritnell
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Default A multi-cylinder crankshaft machining tutorial

Gentlemen, the following will be a tutorial on machining a multi-cylinder crankshaft. In this case it's the crankshaft for my Holt tractor engine. I feel when making this type of crankshaft the most important factor is the machineablility of the steel. Although everyone has their favorite material, partly based on the equipment you have available, I now use 1144. I have made cranks from 1018, 12L14 and cast iron. I guess if a person had the right equipment 4140 would make a nice part. 1018 steel which is a common CRS is available as a flat rolled stock which expedites machining because you don't have to take a piece of round bar and machine it down to a rectangular shape. The problem with this material is that with cold rolling it builds up internal stress and when machined it likes to warp. Sometimes when you get the bulk of the material chopped off it stablilizes to some degree. 1144, commonly refered to as 'stressproof' is great stuff to work with. It has a higher machineability rating than 1018 ( 83% vs. 78%) and one of it's most endearing features is that it doesn't warp, or at least it's negligible. The bad thing is they don't make flat stock so you have to cut a piece of round down if that's your starting point. Most everyone here knows the virtues of 12L14 (leaded free machining steel) with a machineablility rating of 193%. I have made a couple of cranks from this steel with no adverse affects. The tensile strength ranges from 78,000 for 12L14 to 115,00 for 1144. What that all means to us as modelers I can't say. Our cranks are generally small and don't have great torsional loads put on them so the tensile strength is probably not as important as how well you can machine it. I don't know of anyone who has run their engine for a long period of time and then torn the engine down to check and see if the crank throws are still in the proper radial postion as when it was machined. That's not saying that nobody has, I just haven't heard anyone say they have. OK, enough with all this and on to the build. I started out by making a piece of rectangular stock to suit my requirements. I layed out the shapes on the stock and then started cutting. I first chuck the stock up close in my 4 jaw chuck, indicating it close and center drilling each end. After this I extend the stock far enough to cut one mainshaft. I leave a little material on it for the final setup. I then turn it around and do the other end. After this I mount the stock between centers and finish the main journals to size. I might add at this point that the procedures I use for this crank could be somewhat different depending on what size crank you're making and how ridgid it is. The first couple of pictures show the machining of the mains. I didn't take pictures of the center drilling as my explanation should suffice.






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Old 08-08-2009, 10:10 PM   #2
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Default Re: A multi-cylinder crankshaft machining tutorial

After the mains are finished I take the crank over to the mill and cut the material out of the first web. I only do one at a time to keep as much strength in the stock as possible.





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Old 08-08-2009, 10:22 PM   #3
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Default Re: A multi-cylinder crankshaft machining tutorial

Now it's back to the lathe to turn the first throw journal. I use high speed tooling on my cranks. That's not to say that carbide can't be used, I just like high speed for this type of work. I can put a much sharper edge on it and hone it if it gets a little dull. The tool I use for the throws is like and extended cutoff tool made long enough to get to the bottom of my cut. It has clearance on both sides so I don't scuff the sides of the throws. The width is already finished on the mill. I only need to make it round and to size. When you start out the cut will be intermittent but not as bad as trying to cut all of the stock out on the lathe. The nose shape is what's refered to as 'bifurcated'. It has a notch gound into it so that you're only cutting with the 2 small outboard edges. The total width of the nose is a little less than 1/2 the width of the throw. This allows you to move the tool back and forth to clean up the whole journal. There is also a small radius on the corner. This gives the crank strength at the junction point. When first setting up the tool mount your dial indicator somewhere and indicate across the 2 points on the tool. This is very important to get a parallel cut on the journal.





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Old 08-08-2009, 10:44 PM   #4
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Default Re: A multi-cylinder crankshaft machining tutorial

I'm going to interject this step to clarify the mounting of the shaft for turning the throws. As you can see by the pictures I have made up 2 blocks, one for the chuck end and one for the tailstock end. These blocks need to be dimensionally the same as they will be used not just for clamping but setup. The tailstock end has one finished hole, reamed or bored, and one center drilled hole. I try to make the center drilled hole as large as possible for as much support as I can get. For cranks like the ones on the Stuart engines I split the block and put a cross threaded hole in it to clamp onto the crank. On these blocks I drilled and tapped from the side parallel to the through hole but on the side oppostite the center drilled hole. This will be for clamping the shaft into the block. The chuck side block has 2 finished holes in it, one the diameter of the shaft and the other whatever you choose. It just needs to be in line and smooth for indicating. This block also has 2 tapped holes in the bottom for clamping. These were 3/8-16 holes. I also make up a brass shoe with the radius of my shaft on one end to clamp against the shaft. Once my block is clamped into the 4 jaw chuck and indicated it stays there. When I have to cut the inside mains I remove the chuck with the block in it and replace it with my collet chuck. This keeps the accuracy of my setup for the subsequent throws. You might say that this takes quite a bit of time, changing chucks and all, but if you removed the block and had to reindicate it every time it would take just as long or longer. I am fortunate enough to have several chucking devices available.




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Old 08-08-2009, 10:46 PM   #5
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Default Re: A multi-cylinder crankshaft machining tutorial

Gbritnell, once again, EXCELLENT - Thank you .
Edit: Can I ask: in your 2n'd last post - photo 2 - How did you fasten the main shaft in the "block" you use for turning off-center ? - Thanks- you answered it in the third post
Sincerely, Arnold
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Old 08-08-2009, 10:56 PM   #6
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Default Re: A multi-cylinder crankshaft machining tutorial

You will notice in these steps that I cut the first throw up against the tailstock end and then switch to the chuck end. The reason for this is I try to cut as close to the support block as possible. The next thing you might ask is "why don't you slide the shaft all the way into the tailstock block?" I can't, it hits the live center. I don't have one of those long skinny center attachments. After the first throw is cut the crank goes back to the mill to rough out the next main. This process is repeated all the way down the crank. I made up some snug fitting brass blocks and super glued them into the finished throws. This provides longitudal support when the live center is pushed into the tailstock block.








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Old 08-08-2009, 11:13 PM   #7
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Default Re: A multi-cylinder crankshaft machining tutorial

I slowly work my way down the crank, one main, one throw, one main, one throw, until I get finished. WARNING!!! Always make sure your tool is clear before starting the lathe. Turn it over by hand at least 2 revolutions to check everything, then start. Another thing I forgot to mention. You know very early on I said the clamping blocks needed to be dimensionally the same, here's why. When you clamp the crank into the chuck block it first needs to be parallel to that block. For that I use a small dial indicator mounted in a block. I slide it along the clamping block and rotate the crank till it's parallel, then clamp. Now I slide the the tailstock block on and snug up the clamping screw just enough so I can still turn the crank. I Then take a steel parallel 1/2x3/4 (factory made) and clamp it against the sides of both blocks with small machinists clamps, then tighten the clamp screw and remove the parallel. The worst that can happen is the throws might be a minute amount out of parallel on the axis. For model work I don't think you'll ever notice it. We now have a finished crank, at least to this point.







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Old 08-08-2009, 11:16 PM   #8
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Default Re: A multi-cylinder crankshaft machining tutorial

I should have thrown in this picture with the last one but it's hard keeping all this stuff straight without a script. LOL
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Old 08-08-2009, 11:23 PM   #9
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Default Re: A multi-cylinder crankshaft machining tutorial

This crank has radiused edges on the throws and mains. Not just a radius concentric with each. I therefore had to cut them afterwards. The method I used was to make a layout in AutoCad of what I needed and then sined and cosined (stepped) the radii off with a ball mill. I might also say that they are two different sized radii so two separate step off charts were needed.


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Old 08-08-2009, 11:29 PM   #10
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Default Re: A multi-cylinder crankshaft machining tutorial

Now I had to join the radii with a straight cut. I calculated how much I would need under the throw to get the required angle and set up an adjustable parallel. This is where you start getting butterflies. I sure hope something doesn't come loose!! It would only take me seven hours to get back to this point. You'll see I used the clamping blocks from the lathe to hold the crank secure while doing this. In this view you can see the tapped holes in the bottom for the set screws.











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