Wednesday, December 8, 2010
We left off on the crankshafts with all the lathe operations being complete and all 4 parts being ready for the 4 axis mill work.
The next operation is to do the offset machining needed for the connecting rod journals. There are two techniques available: eccentrically mount the crankshaft in a special chuck on the connecting rod axis and turn the journal or mount the crankshaft on the main journals and eccentrically turn the conrod journal. The first option is easier to program but more difficult to fixture, is very unbalanced during machining, and requires part repositioning for each conrod throw. The 2nd option is more complicated from a programming perspective but is easier to fixture, is balanced, and needs no part repositioning. I chose the 2nd option as once the upfront programming work is done making multiples is easy and repeatable.
Due to the low cutting forces due to the aluminum test part material and a desire to get a sample part machined I opted to leave the outboard 4th axis support off. Machine time these days is limited and getting a sample part quickly was of prime importance.
The mill setup is as shown:
The first operations are simple pocketing to remove the majority of the material in the most efficient machining technique. Pocketing was done at increments of 90 degrees and ended up with squarish journals.
Once this stage was complete I moved on to the 2nd program which created the fully round journals. The video of this move is subtle but cool. The machine is performing a G3 arc in the YZ plane (G19) while the 4th axis is doing a rotary move from 0 to 360 degrees. The result is an eccentric true cylindrical surface with none of the approximation concerns usually associated with surface machining type toolpaths.
And this is a short movie of both journals.
This part is being sent to a grinding house for finishing of the journals so the absolute tolerance and surface finish are not as crucial as long as there is enough stock left to clean up during grinding. After consulting with the grinder I am leaving .010" of stock on the main journals and .020" on the connecting rod journals as grinding allowance.
Now that I have a sample crankshaft we can test assemble a short block with the major rotating components: crank, conrods, pistons, and also trans and oil pump/drive components. Once we reach that stage there will be lots of pictures!
That's all for now.
Tuesday, December 7, 2010
The part started with a piece of 1" thick 304 stainless steel plate that was waterjet to a rough profile by North Eastern Water Jet. Andre of NEWJ is always very helpful in suggesting material they already have in stock to shorten lead time and reduce cost. The cost of the waterjet cut blank from NEWJ was less than a comparable piece of stock from the local supply house and saved me from having to hog out the majority of the cylinder bore.
The machining of the bores and screw/dowel holes was uneventful. The part is now nearly finished. The last issue is that the mill finish on the top of the material is not fine enough to clamp on the head surface without marring the aluminum casting. To fix this the part needs to be surface ground to a 8 microinch finish. I'll bring the part to a local vendor, Garden State Precision, who has the equipment and expertise necessary to do the job properly. I'll drop it off in a day or 2 and since this is not a rush pick it up in a week or 2.
While I am at GSP they will also do another small grinding job, thinning the crescent-shaped crankshaft thrust bearings.
The crank bearings are from a ZX10R. The journal bearings are to be used unmodified but the thrust bearing is slightly too thick to fit into my crank design. Instead of having a full-on special made, which is expensive and time consuming, I will have these stock bearings ground from the backside to the appropriate thickness. Inexpensive and just as effective. I'll have them grind matched sets in .0005 increments to allow me to dial in the desired crankshaft end float.
Coming up is the crankshaft rod journal machining. Hopefully in a few days but you know how it goes!
Until next time,