Crankshaft Toolbit

[Bob_W]

I was curious about how others may turn features like the connecting rod throw on a crankshaft. For the model engines I build, this feature could be fairly narrow relative to the amount of tool extension required to reach the finished diameter. The crank I am turning now requires the tool to extend approx 1.3" beyond the tool holder due to the 2" stroke (1" throw).

I have included a picture of the first tool I made for a smaller crank. Instead of using left and right hand tools that would be narrow, I made this tool bit from a 1/4" square bit and ground the cutting edge as shown. The tool is similar to a parting tool in function but has several ground notches to eliminate chatter. By traversing left and right 1/16" while turning I can create the finished surface 5/16" wide using this tool. The notches have to be spaced so that the cutting edge cleans up the adjacent material remaining from the notches.

To ensure a cylindrical surface, I grind the cutting edge of the tool in the lathe as follows: After approx shape is ground by hand, the tool is mounted in the lathe with the sides of the tool bit square to the lathe axis. Then a small grinding wheel is placed in the lathe chuck. With the tool bit slightly above center, to create relief, the front cutting edge is ground parallel to the axis by traversing the tool back and forth across the wheel. Then the tool is lowered to the correct height. Now the crank can be placed in the lathe between centers and turned. This larger tool has no side deflection while trueing the side surfaces and no tool changes required.

I have used this successfully several times. The current tool I just finished using is a larger 5/16" tool (no picture) and only has two cutting points at the corners with front and side relief. It extended approx 1.3" and was used to turn a .625 diameter bearing. Only two points are required since the traverse distance is large enough to clean up with only two points. Within a few thousandths of final diameter I checked the left and right side diameters and stoned the tool in place to finish within two tenths diameter left to right side.

How do you guys do these?

Bob W

[Al_Messer]

Sounds good to me---I'll have to try it, Last crankshaft I did was for a Stuart V-10 and it was a PAIN! I managed to ruin the drop forging that came in the kit--twisted it up like a pretzel. Second one was turned from the solid, very slowly and very, very carefully!!

[Bob_W]

Oooh Bummer. Haven't seen too much on other ways to turn deep sections between shoulders. How heavy of a cut were you taking? I typically take relatively light cuts on these because they are not traversed for a long distance. I think I was taking about .005 deep cut I tried single point type tools but they deflected laterally too easily since their thickness needed to be less than half the width of the cut. How were you turning yours.

Anyone else with some suggestions. Got to better ways to do this than what I had posted earlier.

Bob W

[JeffinWI]

Bob, I've only ever made a couple in crankshafts. Probably a little larger, journals 1.5"Ø and about 2" wide. We used a 1/4" wide "T" type cutoff tool, with a radius on each corner. Turned from a solid, with the tool hangin' out about 2.5". Had to make the bearings also. Two pieces of brass? silver soldered together, turned/bored complete, then heat to separate the halves. Fun stuff!

[UnkaJesse]

Bob, I use essentially the same type tool design as you do, but my tool is narrower so there is only one wide notch in the center. Has perked werfectly for me many times.

Unka Jesse [img]/ubb/images/graemlins/laugh.gif"%20alt="[/img]

[Al_Messer]

Bob, I used a bit like in the picture, except mine did not have the serrations. It was just a wee bit narrower than the length of the crankpin. I was working on an Atlas 6 x 18 lathe and was using hand feed, got a little careless and gave it too much feed on one rev and bingo!! Steel pretzel!! Then I started over with a nice piece of 1018 steel rod and carefully marked and drilled the centers and went from there. The swarf would come rolling off looking like a very thin steel ribbon as wide as the tool. You know, that's the last crankpin I've turned and this was back in the early 70's. Nerves just couldn't take it, I guess.

[R_Johnson]

Could'nt you make up a crank by using pressed fit's. This is done on some snow mobile, and motorcycle engines. You could use ground steel pin' for the rod throw's. You could make up a steel cylinder that would fit the crank boss's to hold alinement during pressing

[Bob_W]

The attached picture is not the best, but it shows the completed crankshaft and tool bit. This particular bit has only the two left and right side cutting edges. For a size reference the tool bit is 5/16 inch square and the shaft diameters are 5/8". The center was milled out then turned to final dimensions. The tool shown was hanging out unsupported approx 1 3/8". Worked pretty good.

Some people do use pressed assemblies and if I made anything larger than this engine I would probably use that technique,

Bob W