The Gearbox Case / Cover I


Construction begins with the gear case (155) and the case front (157). You must have the correct bearings on hand to begin machining. Because I am going to use a vertical mill, I did not follow Mike's exact procedure. Whenever you deviate from Mike's operations sheets, be sure your setups will produce an engine with critical surfaces straight, concentric, parallel, etc. In other words, know how the parts relate to one another, and where it is critical that bored holes be in line (intermediate shaft) and surfaces be parallel to one another (rear of case, and cover)

Becase the case and the cover are essentially built together, I have combined them into what will probably be 3 or 4 pages. We start with just the Case.

  A slug of 6061-T6 aluminum was chucked and faced so that the ends of the bar are truly parallel. This was done by swapping end for end and taking facing skim cuts. I left the blank a bit longer than needed. The outside diameter was turned to print.

I then drilled entirely through the slug with a 1/2" bit. Leaving the slug in place (after carefully ensuring that it ran true, to within .0002" or so), I bored the center, rear bearing hole to a depth of 5.5mm. To do this, you must really sneak up on the last few tenths. When you get close, a piece of 600 grit wet/dry paper may be used to loosen the fit slightly.

  It is a matter of moments with even 600 grit paper to go from uninstallable, to too loose, so again, use care. Here, the correctly-sized bearing is used as a guage to check the progress. As Mike says, aim for a close sliding fit.
The viton groove was cut next with an appropriately ground boring tool. Any time you bore in such a fashion, you will raise a ridge of metal on the rim of the groove, so after the groove is cut, you must deburr the rims. You may be tempted to use abrasive paper, but it is better to use the tip of a round needle file. Rotate the chuck by hand, and allow the file to slightly radius the rims of the viton ring groove.
The case blank was swapped end for end, placing the bearing seat next to the chuck. If you are using a normal 3-jaw chuck, you cannot do this! The slug must run true, and a normal 3 jaw will not permit this end for end swap without running eccentric, often by as much as .010". I am using a six-jaw Buck Adjust-Tru chuck which permits centering of stock to perfection.

If you elect to perform such a swap, use a 4-jaw chuck so that you may realign the case so as to again run true. Also note that I did not cut the 63 degree bevel in the rear of the Gear Case... this is going to be the last thing I do on this part. The reason? Having no bevel on the rear will permit me to chuck the case with much greater accuracy.

The blank was now faced to the correct length of 36mm. Note pictured here is the front of the case, with the original 0.500" hole still showing. That is where I began to bore.

I started with a small boring tool, boring to a depth of 27mm. Note that with major boring operations like this, you do not want to bore each pass to the finished depth. Leave 2mm or so at the bottom of the bore for subsequent accurate cleanup when the bore is opened to the correct diameter.

Note too the stiff and heavy boring bar for the grunt work, which uses a ground 1/4" HSS bit. Much more effective at hogging than a finer boring tool.

Bored now to 63.5mm, the bottom was cleaned up to the correct 29mm depth with a finish boring tool. The undercuts were turned next using the same heavy boring bar in the above picture, except the bit had the necessary radius ground into it. These undercuts and tapers were tedious... only fine cuts could be made.
The Gear case turned and bored. The next step will be to fashion the gearbox front, and integrate the two!

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