Hello, I really need this drawing very much. I love it. Do you have any plans to sell it?I came across two online references to help me with machining the Merlin crankshaft. The first is a thread by a Belgium builder 'Zapjack' who fabricated this exact part with some 200 hours of work over a period of two months nearly three years ago. It's located at
He first published his build on a French forum and then cross-posted its highlights on HMEM in 2012. The original non-English forum where he posted his realtime build as well as an additional two year's work on his Merlin is located at:
Unfortunately, his posts faded away in 2014 after completing the crankshaft, prop shaft, and cylinder liners as well as the crankcase and some of the cylinder block machining.
The second reference is George Brittnel's crankshaft tutorial inside his V-8 flathead build thread starting at:
Since I have some limited four axis CNC capability, my hope is to combine the information in the two threads and take advantage of my Tormach's fourth axis. I don't if my particular CAM software can be convinced to continuously machine the offset throws from billet, but it's worth several days of experimenting to see just what it can do. Hopefully, I can at least come up with g-code for some of the tedious roughing.
Work started on the crankshaft by sawing off a 10-1/2" length of 2-3/4" diameter 1144 steel. I've not used this particular alloy before, but it comes highly recommended for crankshafts by George. I bought a piece long enough for two parts just in case my learning curve takes an ugly turn. I purchased the metal from an online supplier who advertises it as 1144 Stressproof or 'equivalent'. The 'equivalent' sounded ominous, but their price was nearly half that of the other online supplier that I've used used in the past for material not available in my scrap collection. Since Stressproof is a brand name, I'm not sure it's legal to use it to advertise a generic equivalent.
Anyway, after facing and center drilling one end, I turned the o.d. down to 2-1/2" over as much of the length as I could before flipping it around, facing and center-drilling the opposite end and then turning the rest of the o.d. After cutting through the black outside layer I was relieved to find the material turns pretty similarly to mild steel. The chips resemble those from free machining steel, and the surface finish is similar. An amazing thing I noticed was the material's consistent o.d.. The run-out at the end of the 10.5" long un-machined round was only .002" after being chucked in my lathe's 3-jaw without tailstock support. The material I purchased was their low-end cold-roll, but it is also available as precision ground and polished.
After studying the crankshaft drawing I realized just how complex this part is. The webs are not identical, and there are many machining features associated with them. Another wrinkle is that each bearing and crank pin is bored-through in order to reduce weight. In addition, both ends of each of these bores must be counterbored for end plugs since internal oil passages supply pressurized oil from the mains to the crank pins. The workpiece I'm starting with weighs 18 pounds, and the weight of the finished part will be only 1-1/2 pounds. A lot of metal has to be removed from some very difficult to reach locations.
The first and probably most important decision to make is how the workpiece will be held for offset turning. George's offset end blocks looked good to me as they positively grip both ends of the heavy eccentrically rotating load. When I tried to adapt his technique to my crank I realized the four-sided headstock block he used for his 90 degree throws would not work with my crank and its 120 degree throws. I looked at using a hexagonal end block but I wasn't happy with two of the four jaws gripping on the corners of the block. A 12-sided polygon would work, but it wouldn't have long enough sides to handle the crank's 1-1/2" stroke in my 4-jaw.
Zapjack center-drilled the ends of his workpiece for center-turning on each of the three offset axes. I don't have much experience with center-turning, but supporting the weight of this workpiece between two centers concerned me. None of Zapjack's photos showed his headstock drive, but I can't imagine it was merely a conventional drive dog.
I decided to both center-drill and mill reference flats on both ends of the workpiece. Currently my plan is to use the center-spots to locate the workpiece between centers while finish turning the crankshaft. However, I will also add a head support block similar to George's to secure the crankshaft to my lathe's faceplate. The tailstock end will just be supported in an offset center-drilled spot by either a live or dead center. Most of the material will be initially roughed out on the mill and probably with the workpiece held horizontally in a vise. If I run into problems and have to come up with a plan B, at least I'll still have the flats and center-drill references to work with.
A first pair of reference flats was milled into each end of the workpiece while it was held horizontally in a vise. The workpiece was then stood vertically in the mill and clamped against an indicated reference plate using a ground block between the flat and the plate. I was relieved that this rather dicey set-up was actually able to hold the workpiece truly vertical and was rigid enough to mill the additional flats. Zapjack actually removed the table from his mill so he could perform a similar operation. The 120 degree center-drills were then drilled, and the remaining two flats were milled on the perimeter. Both ends of the workpiece were similarly machined but an additional nine holes were added to the front-end. These will eventually be tapped and used to secure a driveshaft to the front of the finished crankshaft.
Because of its complexity and the need to modify its dimensions to fit my 'short' crankcase, I modeled the crankshaft in SolidWorks so I could better understand what I will be up against. This crankshaft looks like a part that can be easily ruined by lapse of attention. It also looks like it will be the most complex part I've ever attempted to machine. It wasn't too long ago, when I was intimidated by what now looks like a pretty simple crankshaft in my 18 cylinder radial. - Terry
View attachment 77211
View attachment 77212
View attachment 77213
View attachment 77214
View attachment 77215
View attachment 77216
View attachment 77217
He's the person you communicated with in post 843 above.Randy？Excuse me, who is it