Blog Post:

The Crankshaft Pulley

07/03/2007

You hear a tick, tick, tick sound in your engine compartment of your highly modified 1977 Corvette. Is it the lifters? In this case, after sophisticated diagnosis using an automotive stethoscope, we determined that the sound did not emanate from the engine internals - in fact it seemed to be coming from the front of the engine. Behold the problem. It's amazing that these pulleys didn't spin off the crankshaft.

Mastercam CAD Pulley

So - let's make a better pulley. And let's use lots of technology, because we can. The goal is to replace the two stamped steel pulleys with one billet aluminum pulley - not purchased from Summit Racing. Since the car will never have air conditioning, the A/C sheave was eliminated. Here is the CAD model created using techniques from Jill's Ph.D. research. She used non-contact reverse engineering tools, as well as advanced design optimization techniques. (Remember, we are using lots of technology…)

Now that we have the design, it's time to have some fun and actually make it!

We started with an 8" diameter by 4" long billet of T6061 aluminum…

Shopmaster

…and a ShopMaster BridgeMill "3-in-1" CNC machine.

The task proved to be quite a challenge, as:

The maximum diameter that the chuck can grip is 4 ½"

The milling and turning spindle motors are only 1 ½ hp.

The issues were overcome with some creative tool path programming in Mastercam. First a lot of material was removed using milling toolpaths.

As the OD of the billet exceeded the swing over the saddle, the first turning cuts were done using a reversed boring bar so the saddle didn't have to pass under the part.

The OD and ID were then roughed and finished. We discovered that our pulley design was basically a bell from an acoustic point-of-view and controlling chatter was a problem, especially during grooving. With only a 1 ½ hp motor on the spindle, taking heavy cuts was not an option. A custom ground grooving tool proved to work much better than the Iscar cut-grip tool we initially tried. Here are the chatter marks from the Iscar grooving tool.

Here is the part with the OD finish cut with the custom insert and quickly polished.

And the completed part…

Test fit - bolted to the balancer of the new 409 C.I. small block Chevy motor along with the mating pulleys.

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