how do you machine a crankshaft or cam

[skipper]

Hi All,

I been looking at a few model engine sites and I am curious to know how the crankshafts and cam shafts are made. Are they made on a mill or on a lathe? How do you make the offset journals, do you offset the center line of a lathe? Some of those model engines are amazing.

Thanks,
rene

[ALLtra Mach]

They use a crank grinder, shaft spins on center and a grinding wheel moves in and out with the profile of lobe

[holly]

to machine a crank shaft, this can be done on a lathe by off setting the crank shaft in a 4Jaw chuck and the tail stock untill the big end journal runs true to the centre line of the headstock.The journal can be machined with a parting-off tool which will give square ends.

To machine a cam, which I take you mean the cam used for operating the vave block operating ports on steam engines, then this is can be done by use of a three jaw chuck and placing a packing piece under one jaw by the amount of throw required.

hope that this helps,

[CarbideBob]

Making a crankshaft,
I've seen lathes with the crank offset and counterbalanced so the pin turns in the center. Lathes with the toolholder following a cam so the holder moves around in a circle while the crank spins on center. Horizontal type milling cutters that interpolate the pin while the crank rotates (or the crank is mounted off center while the cutter is stationary). Internal mills where the crank goes inside the cutter which interpolates the pin. VMCs with the crank on a 4th axis whittled out with an end mill. Oh yea - turnbroaching is popular on mains. You name it and it's probably been done.
Finishing is grinding. This requires a specialty machine. Same thing as roughing though. Either offset the crank, or move the work head with a cam. Newer cnc machines interpolate the wheel in and out as the crank spins on centers (or interpolate the workhead while the wheel is stationary). Alot depends on how big the crank is and how many miles you want it to live for (and of course how many you are going to make) .
I'm sure NC Cams will post something on camshafts.
Bob

[Andre' B]

You missed one Bob.
I seen one single cyclinder model engine where the crank was made with a hacksaw and a file, and it looked good.

[Mcgyver]

rene, the fun of model engineering is pressing our simple home shop equipment into making tricky bits like model cranks, cams etc. How this is done in the model engineer's home shop is I'm sure a very different from someone building stuff for cars or industry.

there are a few tried and true techniques for cranks. the ones I'm familiar with are, from the solid, turned from a fabrication, and built up. Built up means the components are turned and then pressed or otherwise assembled together, usually without additional machining. Fabricated means the rough shape is approximated by brazing/silver soldering together pieces and then turning. From solid is just that, the crank is turned out of a solid bar. the latter is my preference, size permitting.

For the last two, you have to devise a way to hold the work piece so that each axis of the work (pins, journals etc) can be held along the axis of the lathe. This is most commonly done between centres. you drill centre holes in the end of the stock (or have clamp on pieces withe holes drilled) such that you can alternately mount the work on different sets of centres, each setting putting one of the work's axis in line with the lathe axis.

needless to say for multi throw cranks this is not trivial; the work ends up long and skinny with lots of flex and you must get left and right hand tools into so tight places. you must pay attention to the sequence of cuts so that rigidity is maximized (start with the pins, and use spaces in the gap, then the journals)

In industry, these cranks would be out of say chrome moly and then hardened and ground - this isn't usually done in the home shop as the grinding equipment is not available and the engine's duty cycle and loads don't require it


A lot of people make dedicated cam machining equipment, basically a way to hold and index the cam shaft while a cutter head or grinding wheel follows a pattern. index the shaft position, cut the cam, index to the next cam, cut etc. i made one once in the mill using an index head. By working out the math such that for each movement of the index head, I calculated a Z axis number - produced the profile through hundreds of facets. It came out beautifully after hardening and polishing. also, there is a great example of a home made cam machine on this site somewhere, searching will find it.

cranks and cams are a couple of the more complex ops in model engineering (although nothing here is that complex if broken down enough) so I'm not doing much justice covering them in a couple of paragraphs, but hopeful that gives you the idea.

[Wiseco]

Here is a video of an integrex making a crankshaft.
http://fr.youtube.com/watch?v=E1Fj1j8Sg1g

[NC Cams]

In a recent issue of Home Shop Machinist, there was a well documented "how to" article on "how to make a cam grinder". The method of designing the cam profile leaves a lot to be desired as far as dynamics goes but, open and close a valve it will do.

Simply offsetting the center of a circular object in a 3 or 4 jaw won't get you a viable cam profile nore anything even close - perhaps an ellipse or a rhombus but not an IC engine cam profile.

The next inevitable question, "how do you design a cam profile?" is not answerable in a message board posting.

Do a search for "cam design" via the search engine - in a prior thread, a number of sources where info can be found re: the subject were discussed in the thread. It is not a trivial subject nor one for those who might be mathematically challenged. A foundation/background in dynamics helps as well...