Quarter Scale Merlin V-12

[ozzie46]

WOW!! It's a shame you have to hide it in the crankcase.


Ron

 

[Scott_M]

Outstanding !
Working with SprutCam can sometimes be maddening, especially with such a complex operation, my hat is off to you for getting it to do what YOU want !
It is looking really good Terry.

Scott

 

[Davewild]

You sir are a master, beautiful

 

[Charles Lamont]

Most impressive. I think a sharp HSS tool would get you closer than 1 to 2 thou.

 

[bigrigbri]

Very skillful indeed. Well done.

 

[mayhugh1]

With the crankshaft fully roughed-in the 'easy' part is over, and now it's all about discovering and dealing with all the tiny details that affect its final accuracy. As I get closer to the final finishing operations there's less and less stock available to protect me from my mistakes.
The rear of the crankshaft on the full-scale Merlin is internally splined for a rear driveshaft. Rather than risk the crankshaft with a difficult splining operation, the quarter scale model uses a separate internally splined coupler that's Loctite'd into a half inch bore in the rear of the crankshaft. The drawings show this coupler having a circular array of mounting holes, but the crankshaft drawing doesn't show a corresponding array of tapped holes. The recommended Loctite can probably handle the torque requirements of the magnetos and maybe even the supercharger, but I'm not sure it will be adequate for the starting torque requirements of an electric starter. So, I decided to drill/tap an array of holes in the rear end of the crankshaft for six 3-48 mounting screws. These holes should have been drilled much earlier in the workflow, but I've had my head down in the crankshaft drawing which I had been assuming was complete. I wasn't happy with the setup that I had to use at this late date to support the crankshaft for drilling, but it was adequate for the forces needed to drill the tiny holes.
I left excess stock on all the crankshaft webs, and now it must be removed before machining the counterbores at the ends of the bearings. These counterbores will eventually be plugged in order to seal up the pressurized oil passages between the main and crankpin journals. Since I can't trial fit the crankshaft into the crankcase, I built partial CAD models of my crankcase and the crankshaft workpiece in its current state using measurements of the actual bearing locations. This allowed me to overlay the two and determine the best way to distribute the removal of the excess web material. The center bearing establishes the forward/aft position of the crankshaft in the crankcase, and so I provided a .0015" clearance for it. I followed the drawing for the remaining six bearings which calls for .040" clearances. I used a radially mounted right-hand boring bar to finish the left sides of the webs. Rather than invert it and run my lathe in reverse as I did previously, I made a left-hand tool to finish the right sides. It started out as a boring bar for a triangular insert, but due to a measurement error I ended up using a trigon insert, instead. I also ground a form tool to machine a radius on the neck of the crankshaft just behind the front flange.
Now the real fun begins...
I had .070" excess material on the main bearings, and so I decided to experiment some more with my bifurcated cutter. Since I wasn't 100% happy even with my new insert, I honed its front edge on a diamond hone and added more rake to the insides of the Britnell notch. I think its edge is now as keen as anything I could have done using HSS. My results using the improved insert was a slightly smoother cutting action and an improved d.o.c. resolution to less than .001" while cutting near the supported ends of the crankshaft. This depth control will be important later when I try to finish all the bearings to a common diameter.
I still had some serious workpiece deflection while turning the three center bearings, however. Initially, I wasn't aware of it because the edge of the tool was hidden deep inside the webs and under a thick layer of the moly-based cutting oil I've been using. The symptom that alarmed me was a ridiculously high TIR of two to three thousandths on a freshly turned bearing. The deflection was created by the compromised rigidity of the workpiece which is a result of the now fully machined webs particularly around the offset crankpins. The real problem for a crankshaft is that this deflection can't be compensated by taking a heavier cut. The offset crankpins cause the workpiece's reaction to the cutting tool to be a function of the angle of rotation of the workpiece. This can be measured using a dial indicator on an adjacent bearing while cutting. The actual amount of deflection continuously changes as the workpiece rotated. At high rpm the symptom would have been a squeal as the cutter chattered against the workpiece. At my 50 rpm turning speed, though, it showed up as a non-circular turned bearing. Because of the limited space between the deep bearing webs it's very difficult to get accurate measurements, but I'm reasonably certain that I measured a couple thousandths difference between two almost orthogonal measurements taken on the diameter of the center bearing. This closely correspond to the unreasonable TIRs I measured just after turning the bearing. The bearing breathes instead of wobbling as it spins, and it's hard to notice a problem with the naked eye.
Some quick and dirty things I did to understand the problem included wood shims forced between the crankpin webs as well as a length of close fitting drill rod inserted through the hole in the center of the crankshaft. I was able to significantly affect the results but not cure the problem since I'm sure even the oak was reacting to the high cutting forces.
In any event, I completed the web machining, and this produced identical final finished spaces between the webs. By this time I had removed .020" of my .070" safety margin from the main bearings.
After re-turning the main bearings with the wood shims in place, I removed the workpiece from the lathe and then re-installed it to check the repeatability of the TIR measurements. I found the runout to be similar for all the bearings, but it had nearly doubled to an unbelievably value of .010". This result led me to believe I may have yet another problem with my headstock fixture and, in particular, the newly machined buttons. I suspected the center-drilled holes weren't deep enough and that my setscrews that bear against the spigot flats might be pushing the workpiece around slightly. I made new buttons with deeper center-drilled holes, and I re-turned the bearings with the wood shims in place and then repeated the test. Again, I measured almost the same ridiculously high run-out, and now I have only .030" safety stock remaining.
After spending a couple days doing various experiments but no additional cutting I concluded the issue related to the fixture was probably due to my method of tightening the setscrews. When I previously checked the consistency of the fixture I wasn't paying close attention to any particular tightening sequence. I had been just tightening the screws against the flats, but my workpiece at that time was a heavy chunk of rigid steel. Now the workpiece is much more flexible, and the setscrews need to be iteratively tightened so the workpiece is allowed to find its center with little or no net stress.
To check out these theories I machined a set of aluminum cervical collars which fit between the crankpin webs. These are not jammed into place but were lapped for a close sliding fit. I think it's important to not fixture the workpiece under any stress that can affect dimensions because after machining and when the part is removed from the fixture these dimensions will change. I Ty-Wrapped these collars in place, and for good measure I also inserted a length of close-fitting drill rod through the previously drilled hole through the center of the workpiece. (I could really use a steady rest behind the center of the crankshaft, but my lathe carriage is in the way, and there is no room for a follow rest.) I carefully tightened the setscrews to allow the workpiece to find its own position on the headstock center, and then I turned just enough material off the center bearing to clean up its TIR. I then removed the workpiece from the lathe so I could re-fixture it and re-check the TIR. Thankfully it now repeated.
And so, to answer Peter's earlier question about whether stiffening spacers are needed while turning a crankshaft, my answer is a resounding 'yes'. And thanks to Charles who warned me about being too quick to finish the most important surfaces on the part.
It's now obvious to me that there is a major advantage, besides surface finish, to grinding crankshaft journals rather than turning them. The grinding forces against the workpiece will be much less than a cutter's turning forces, and the resulting deflection even on a 10-1/2" long crankshaft would be nearly insignificant.
Now, I plan to finish up the last few semi-finishing operations before returning to the bearings. Hopefully, I've paid my dues and the problems are understood well enough to successfully finish up. - Terry

 

[gbritnell]

Terry,
This is a wonderful presentation of the amount of work and fixturing it sometimes takes to make a part. By the time a project is finished you have a whole box full of plates, pins bushings and what-have-you parts that were required to complete a part.
I don't know how stress proof this 1144 material is. By that I mean when I made the crank for my inline 6 cylinder engine I left the shaping of the counterweights till last. That way I could use hose clamps in conjunction with bushings to clamp the round counterweights together for added stiffness. I then made up a fixture with support blocks mimicking the main journals to clamp the crank into to remove the remaining stock from the counterweights. When I got finished I had a total runout of .0012 on the whole crank. It's not like you're going to make 2 of these to try out different approaches but sometimes a different process works a little better. I'm sure grinding is the best way to go but that involves another whole process and heaven knows we have enough specialized tools laying around.
http://www.modelenginemaker.com/index.php/topic,2295.0.html
gbritnell

 

[makila]

Hi Terry,

That crankshaft is a work of art, as mentioned on an earlier post, too good to hide in the engine!
Great presentation as always, you should write a book!

Steve

 

[mayhugh1]

I have only blind access to the journals that need to be counterbored since their ends are blocked by the workpiece spigots as well one or more adjacent journals. My counterboring tool will have to pass through the bores of the adjacent journals in order to reach the ones to be machined. What's required is a toolholder with a removable insert that can be assembled and disassembled while the tool is between bearings. It also has to be long enough to pass through the entire crankshaft from either end in order to reach all 23 journal ends.
The tool I made was a single-insert manual piloted counterbore. I had some tiny triangular carbide inserts on hand that seemed to be perfect for the job. A simple toolholder was milled from a piece of drill rod that was .004" under the reamed diameter of the bearing bores. I pressed a bronze pilot bearing onto the nose of the tool that was just a thousandth under the journal hole diameter. I relaxed the clearance to the tool's main body so it could be easily rotated while passing through the entire crankshaft on any of its four axes.
In use, the tool is inserted through the crankshaft until the pilot reaches the journal to be counterbored. The insert is then screwed into position on the tool and coated with cutting oil. A tap handle attached to the rear of the tool is then smoothly rotated while applying light pressure until the depth gage bottoms out. Since the insert's cutting edge has a tiny radius, it's easy to overpower the tool and end up with a spiral groove in the wall instead of a nice smooth finish. I made this mistake a couple times but was able to repair the finish, with some difficulty, by adding several additional passes. The insert must, of course, be removed before the tool can be withdrawn.
Two counterboring depths were used: .060" for the main journals and .090" for the crankpin journals. The difference between the two is because a portion of the crankpins' counterbores will be removed later when the windage chamfers are turned. The counterbores are only .030" over the bore diameters, and each was checked with a shop-made gage to make sure they all ended up with a smooth, common diameter. Although the process took several hours, the tool worked surprisingly well. The next time, though, I'll use an insert with a larger cutting radius.
With counterboring completed the webs could be finish machined. The last operation was an 11 degree windage chamfer turned on the counterweights and crankpin throws. I desperately tried to use the cross slide on my Enco lathe to cut these chamfers, but I ran into multiple interference issues between the carriage and tailstock and the narrow cutting window I had to work with. Reluctantly, I moved the workpiece over to my 9x20 lathe where I compiled g-code to cut the chamfers. The workpiece was simply held in the lathe's 3-jaw with tailstock support since, for this operation, there's really no great center precision required. The support collars couldn't be used, but I was able to install the central support rod. Although the program was fairly simple, the approach and retract moves for the cutter had to be carefully considered. There was barely enough room between the webs even for the tool that I had to radically modify, and so my pucker factor was off the charts the first several times the program was run. For simplicity and ease of testing I created a small program to cut only a single chamfer. This meant the cutter had to be re-referenced and the program run twelve times giving me ample opportunity to set up a crash. Once I had turned the six left side chamfers, I reversed the workpiece in the lathe and re-ran it six more times to turn the right side chamfers.
The next step will be to drill the oil passages between the main bearings and the crankpin throws. I may have a major fixture coming up just to drill these six holes. - Terry

 

[petertha]

Your tool is ingenious! Is the insert just flush mounted or does it sit within a pocket/edge that's not quite visible?

 

[DICKEYBIRD]

Absolutely mind-boggling. Having recently done a bit of Mach3 close-quarters CNC lathe work, I can truly understand what the pucker factor must've been when you pressed Cycle Start with so much time & effort invested in that crankshaft. Your mental clarity & focus is astounding.

 

[mayhugh1]

Peter,
There's no pocket, but the insert is a snug fit inside the slot cut into the tool.
Terry

 

[mayhugh1]

The Merlin's crankshaft oiling system is a little different from those I've encountered on other engines. Instead of a pressurized oil channel daisy-chained through the main and crankpin bearings, all seven of the Merlin's main bearings are parallel plumbed to an external oil pressure regulator. Each main bearing, except for the rearmost, provides oil to an adjacent crankpin bearing through an internal drilled passage that connects the two. So, in addition to drilling seven entry holes in the main journals, six 60 degree oil holes must also be drilled through the crankpin journals to connect them to the main journals.
Although I've never intentionally rotated the head of my Bridgeport clone off vertical, it can be set at the required angle and save me the construction of yet another single-use fixture. I set up an indexing head with its tailstock support under the spindle, indicated everything true, and then chucked up a test piece of drill rod to practice on. I tried to use a 60 degree carbide v-bit to spot the holes since it would have produced a nice transition between the angled holes and the journals' surfaces, but I chipped it on the first practice spot. I eventually used the cutting tip of a HSS center-drill to spot the holes. This worked OK, but it didn't produce quite as nice of a result. Because the angle-drilled holes pass through the bores of the crankpin journals, the journals had to be plugged with a filler rod so the drill could properly re-enter the crankshaft on the other side of the bore. For nearly all the journals I used the body of the counterboring tool I made earlier as the filler.
The diameter of these holes wasn't specified, and so I drilled them .087" which was the diameter of the Guhring drill bit I used on the crankshafts of my two radials. I really like these bits because unlike my 300 piece import set these drills are straight, cut smoothly, and handle deep holes really well. The main journals were drilled with a conventional .070" diameter twist drill.
After the drilling was completed, I de-burred the edges and polished all the machined surfaces with a medium grit Scotchbrite pad. I'm not as good with a file as George, and so I elected to not try filing fillets around the counterweights.
The crankshaft was then moved back into its fixture on the lathe so the journals could be brought to their final diameters. I really wasn't looking forward to these next finishing operations because of all the TIR issues I ran into during the roughing steps. Since this was my first one-piece crankshaft, I was worried that I didn't yet understand all the problems that had been causing the runout, and now they might show up with the finish line in sight.
I started the finishing operations with the crankpin journals which still had some .025" excess stock left on them. I used the support collars on every crankpin except for the one being turned, and this time I was especially careful about sequentially tightening the setscrews in the headstock fixture. I managed to turn all the crankpin journals to the same diameter within a half thousandth except for number three which came in a full thousandth under. All but the two middle journals had beautiful surface finishes straight off the bifurcated cutter.
The final TIR of the 1&6 crank journals measured .0005". These are the journals which are nearest the supported ends of the crankshaft. The final TIR of the 2&5 journals measured .0007". The final TIR of the 2&3 journals, the poorest supported center pair, measured .0007" and .001". The runout of the lathe spindle measured at about a tenth, and the TIR of the live center in the tailstock measured three tenths. I suspected the crankpin run-outs in excess of a half thousandth, or so, were most likely due to circularity errors left behind by the rotational flex created by the cumulative slop in my six support collars. And, as expected, the higher numbers were measured on the two center crankpins.
I tackled the main journals next. After re-installing the workpiece in the headstock fixture with all the crankpin support collars in place, I was relieved to find that the central bearing TIR measurement closely matched the one made several days ago using the support collars. After a couple passes through all seven journals with the bifurcated cutter, they were all at the same average diameter +/-.00025". I say 'average diameter' because I still had significant runout in the center bearings. Six of the TIRs ranged from .0005" to .001", but the center journal ended up with .0018" no matter what I did. Maybe my sliding fits to the support collars could have been a bit tighter. I was eventually able to verify that the bearing TIRs were definitely due to circularity errors after I found my long lost narrow body micrometer at the bottom of one of my toolboxes. A multi-lobe TIR pattern was also obvious in the oscillating needle movement on the dial indicator as the crankshaft was rotated through a full revolution. The main journal diameters ended up .015" below the values called out on the drawing, but this was OK since I had been planning to make the bearing sleeves somewhat thicker than specified.
While taking .001"-.002" diameter passes I found that my thick moly-base cutting oil was a detriment even though it seemed to perform well on deeper cuts. I found WD-40 worked best while taking very light cuts.
After turning, I polished all the journals with 400g paper to remove the minor machining scratches and then followed this with 600g. I doubt that I polished off even a tenth, and so I wasn't concerned with changing the profiles of the journals.
The steps remaining are to remove the spigots and complete the machining on both ends of the crankshaft. It looks like I'm definitely going to need a steady rest for these operations, and my best candidate seems to be a modification to the one on my 9x20 lathe. The stock support arms are too wide to fit between the webs, and so it looks like I will need to make a set of new ones.
Six weeks ago I would have considered a runout of .0018" to be a failure and almost enough reason to start over. After my experience with this project, though, it's not clear what I could do differently to improve the results. I think the only way to get better numbers is to grind the journals so the flex on a very flexible workpiece is reduced. - Terry

 

[stevehuckss396]

Holy Freakin Smokes!

 

[gbritnell]

Terry,
I think what we fail to realize when working on these small parts is just that, they are small parts. The tolerances that we are trying to achieve using 'shop tools' is generally extremely accurate, even if they are out by .0012. The complexity of this crankshaft makes the task even that much harder. I would venture to say that there is no other process, pressing or silver soldering, or material that could have given you the results that you have achieved.
I have mentioned in other threads and postings that there are extremely talented machinists and miniature builders out there but they aren't contributors to forums such as these and with that being said the ability to learn from their experiences is lost.
I personally am delighted and impressed with your work and documentation. Even garnering one or two bits of knowledge from your postings is a great learning experience for me.
There's no doubt that this will be a magnificent engine when finished.
gbritnell

 

[ozzie46]

("I personally am delighted and impressed with your work and documentation. Even garnering one or two bits of knowledge from your postings is a great learning experience for me.
There's no doubt that this will be a magnificent engine when finished.
gbritnell")

I fully agree. Well said George.
I know personally that I couldn't come anywhere near to what you have accomplished. Keep up the good work.

Ron

 

[RiekieRhino]

All I can say is awesome work.

 

[jschoenly]

Really fun to watch this build and learn!

 

[mayhugh1]

Thanks everyone for your kind comments...
.
While waiting on some special ball bearings that I need to modify my lathe's steady rest, I started thinking about the shell bearings that I'll soon need to install the crankshaft. Although I have some experience with replacing these bearings on full-size engines, I've never actually made any from scratch. I searched the forums to see how other model builders are making them, and I was especially interested in the two-piece soldered-blank technique. After thinking about my journal TIRs and wondering what I was going to use for a diameter to bore them, it occurred to me that I needed a shell that would easily adjust to the effective diameter of my non-circular journals after they had been allowed to rotate for a while. Instead of all the soldering, turning, boring, and unsoldering I wondered if I could press-form the bearing halves from a soft metal.
I thought I'd perform a quick experiment. When I was much younger I collected coins, and like a few others at the time I purchased a number of 'collectible' silver bars at a premium price back when the price of silver was less than $6/oz. Since the silver in these bars is very pure I figured it should be very malleable and perform well as bearing material. I rolled one of my bars into a flat sheet having roughly the thickness needed, and then I cut off a narrow strip to make a test bearing shell. I annealed the silver using the same torch technique used earlier to anneal the aluminum castings. Using just my hands and a wood dowel that closely matched the diameter of my crankshaft's main journals I pressed the silver strip into one of the finished bearing caps. Sure enough, it conformed perfectly to the bearing cap with no spring-back.
Unless someone talks me out of it, I think I'm going to try to make my Merlin bearings this way. My plan is to machine a simple two-piece die that I can use to press-form the bearing halves. Since the main bearings require an internal oil groove I'll try to include that feature in the die. I believe that some of the older racing engines used silver bearings, but I don't know why it isn't common practice today. - Terry

 

[gbritnell]

Terry,
That's what Ron Colonna uses for his Offy engine bearings. He bought sheet silver material from a jewelry supply house.
gbritnell