Making a multi-cylinder crankshaft

[gbritnell]

I read Terry Mayhugh's post about making the crankshaft for his 289 v-8 engine. While I think it would be more than strong enough given the testing he performed on it I wanted to present my way of making one piece, multi-cylinder crankshafts.
Over the years I have made many crankshafts for engines ranging from single throw to 2, 3, 4, 6 and 8. I have made them from fabrications to one piece units. Crankshafts are undoubtedly one of the biggest pains in model making.
I thought I had posted a thread on building my flathead V-8 engine but in going back through my postings they ended in 2020. ???

As for metal I use what we have in the States classified as Stressproof steel. 1144 grade. With the dimensions finalized I first start by center drilling the stock on both ends then rough turning the front and rear of the crank leaving the front .020 oversize. The rear is left much larger in diameter so that flats can be milled for indexing the throws and still leave stock for the flywheel flange.
Picture #1 the stock rough turned.
Picture #2 the flats being cut on the flywheel end.
Picture #3 the main journals being cut to width but here again leaving .02 finish stock.
Picture #4 the crank rotated and multiple cuts made to remove the main bearing stock.

 

[gbritnell]

The next 2 pictures show the bifurcated tool and indicating is parallel with the lathe axis. The tool has to be thin enough to overlap cuts when moving from side to side inside the journal pockets. My tools are ground from H.S.S,. lath blanks. The purpose of the bifurcation is to help reduce chatter when cutting,. Even though I'm cutting steel the tool had 0 top rake. Just like a cutoff tool.
Picture 3 shows the crank blank with all the main journals turned but still with .005 remaining. The blank is then put in the mill vise and one of the flats corresponding to one of the throws is indicated flat and the first throw is roughed out in a similar manner as the main journals. The throws are done one at a time staring from the front end of the crank and working toward the rear. This way you always have more stock on the chucking end for support.

 

[gbritnell]

Along the way two fixture blocks need to be made. One for the chuck end and one for the tailstock end. Both blocks need to be the same width dimensions. I'll explain later. The chuck end block is bored to allow the flywheel end of the crank to be inserted. This is the end that is larger with the flats milled onto it. A hole is drilled and tapped for a large set screw, which in my case was a 5/16-18. The set screw is ground flat on the bottom.
The chuck end block is mounted in the 4 jaw chuck and indicated for the correct offset. In the picture you see the brass bushing which holds a suitable ground rod for indicating. In my case it was a 1/2" end mill. I could have used a hardened dowel but they are usually a few tenths oversize so reaming the hole won't allow them to be used.
Picture #1 set screw in block
Picture #2,3,4 indicating the block offset. I used the micrometer calibration rod to be able to clear the indicator from hitting the chuck. Once in position the block remains in the chuck.

 

[gbritnell]

The next series of pictures show the tailstock end of the crank blank. This fixture block is as I mentioned earlier the same width and height dimensions as the chuck block. It is bored to accept the rough turned end of the crankshaft. A second hole is drilled and countersunk to accept the live center. This needs to be large enough to provide adequate support. The block is then drilled and tapped for a lock screw. If you go back in the pictures to where the crank is in the vise you will see that the front end of the crankshaft also has 4 flats corresponding to the rear. The reason the blocks need to be the same size is so when inserting the crank blank and tightening the set screws you can put a straight edge across the block so make sure that the crank is in good alignment. The screws are snugged and the crank twisted into alignment then the screws are tightened. The chuck end block won't move in the 4 jaw chuck but you can twist the tailstock block enough for perfect alignment.
Pictures #1,2,3 show the blank and fixture blocks.
Picture #4 shows the first throw turned and polished.

 

[gbritnell]

After the first throw is finished the crank blank is removed from the chuck fixture and put back in the mill. The second throw is roughed as the first and the blank is then put back in the lathe. This operation is repeated for each throw.
The next series of pictures show the crank with the throws and main journals cut to size and polished. What isn't shown is once the journals are finished the crank is then put back into the lathe between centers and the mains are cut and polished to their finished size. This allows for truing up any distortion in the crank. I have found that using the 1144 steel on a crank this size I get about .003 deflection. To help support the crank when finishing the mains I make a split steel bushing which slips over the crank. I then use hose clamps to tighten the bushing

 

[gbritnell]

The crank will then need to have the counterweights cut down. For this another fixture is made. This plate has blocks corresponding to the mains and the caps can be tightened to hold the crank securely for cutting. The fixture will also be used latter for drilling the oil passages. An adjustable parallel was set and used to make sure the crank was indexed parallel with the main and the throw prior to cutting. The outer ends of the webs are cut at an angle to reduce the mass as the throw.

 

[gbritnell]

The next operation was to set up the mill to drill the oil passages between the mains and throws. I have an angle mill table and it was set up and indicated to the proper angle. The fixture was then mounted and indicated square. The crankshaft was then mounted in the fixture and clamped in place. An edge finder was used to find the center line of the crank.

 

[gbritnell]

The crank was center drilled deep enough to allow the drill to start accurately on the angle. The holes were first drilled with a standard length drill then followed with a long drill. The drill was pecked many times to insure the chips were cleared. The crank was rotated to each position and drilled. It was then taken out and turned end for end to do the other journals.

 

[gbritnell]

The final operation was to chamfer and radius all the corners and edges. The polish everything.
It's a big job but completely doable if you take your time.

 

[petertha]

George, thanks for putting these pictures & description together. The process makes more sense to me now that I see them strung together. I had a similar idea for end fixtures but I like your version more!

- did the bifurcated tool last the whole cutting operation or did you have to re-dress a few times?

- I didn't see pics of any clam-shell like lapping tools for the journals/throws but the end result looks great. Did you turn it within 0.000X" using the same cutting tool & then finish with flat backed abrasive stick or something like that?

- the oil passages aren't really the topic theme but dumb question, how are they fed in this engine?

 

[gbritnell]

The oil pump feeds oil passes in the engine which feed the main bearings and then through the crank to th rod journals.
The tool held up for the entire job because it wasn't cutting much material. The journals weren't lapped, just cut and polished using fine emery on a flat piece of steel.

 

[ozzie46]

George, I remember following your Flat head V-8 build and the trouble you had getting it to run. Did thread just vanish into thin air?

Ron

 

[gbritnell]

Hi Ron,
I looked for it but it was gone along with the Vtwin and others????

 

[minh-thanh]

Hi @gbritnell .
The crankshaft is great .
Thanks for sharing !

 

[michelko]

Beautiful crank!!

 

[Mike Ginn]

George
Excellent write-up and pics. My question is- How did you get the surface finish and what did the cutting tool look like?
In my very limited experience of cranks I have always been plagued with chatter and the inability to have good support for the cutter with its long overhang. Is there a secret??? How did you do it?
Many thanks for sharing.
Mike

 

[gbritnell]

Hi Mike,
The tool is shown in the second series of pictures.

 

[petertha]

After the first throw is finished the crank blank is removed from the chuck fixture and put back in the mill. The second throw is roughed as the first and the blank is then put back in the lathe. This operation is repeated for each throw.

You obviously did this successive mill/turn iteration for a specific reason, I assume to minimize CS distortion as the machining progresses from throw to throw? But I'm having trouble visualizing the CS rigidity from 'springiness' standpoint as more & more of the material is removed say on the last couple throws. Versus roughing out all the throws beforehand on the mill & then turning down all throws in succession on the lathe (obviously re-aligning the TS fixture block). Sorry for clumsy wording, hope the question makes sense.

I have found that using the 1144 steel on a crank this size I get about .003 deflection. To help support the crank when finishing the mains I make a split steel bushing which slips over the crank. I then use hose clamps to tighten the bushing

I didn't see any pics showing temporary spacers between the webs as you progressively turn the throws. Is this bushing kind of like a split tube that fits over the now still fully circular webs segments & supports them that way?

 

[Mike Ginn]

Thanks George - I missed the picture. Following on from Petertha's post I find chatter a major problem with a single throw crank but with a multi crank like yours I don't understand how you manage to achieve the surface finish shown.
Any insights would be helpful
Thanks
Mike

 

[gbritnell]

Hi Peter,
You're exactly right! The crank is removed from the lathe fixture after each throw is cut to reduce the springiness. The chuck end (fixture end) always has more stock and therefore makes the blank more rigid than cutting all the throws at one time. Realigning the tailstock block only takes a minute. The crank is lightly clamped and the straight edge laid between the blocks then the fixture is tightened. I never found that I had to put spacer blocks between the already cut throws because this crank had enough 'beef' to it that unless the tailstock was over tightened it doesn't distort the crank. If a smaller crank was cut then spacer blocks would certainly help. The split bushing is for finishing the main journals. Yes it fits over the O.D. of the blank and is then held tight with hose clamps. There is always one on each side of the journal being cut.