Supercharged Sherline Spindle - Part II, Sheaves

5 Bears Home Homebrew CNC bench mill

In the previous installment, we have been machining the motor sheave for the supercharged Sherline spindle system.

The first full form cut is the toughest, as the final dimensions are best acheived with a trial and error fitting of the belt. Obviously I have a bit of additional turning to go on the flanks of the V-groove.

Once the first V-groove was satisfactory, the right and left jogging amounts are noted, and the second V-groove was completed exactly in the same manner. Excess cutting on one groove or another would make the belt in that groove sit much deeper, and result in a sloppy fit. Remember, we are trying to use two belts simultaneously. The grooves must match or one belt will be tighter than the other.

With the motor sheave complete, except for the dual set screws, the spindle sheave is started.

I measured the no-load speed of the motor (STUPID!) and calculated a 2:1 reduction of the 20,000 RPM motor to 10,000 RPM spindle. Yes, I did not account for load. Obviously the completed setup didn't come close to 10,000 RPM at the spindle.

The Spindle sheave is bored to correct dimension.

Again, the V-grooves are cut in a manner similar to the motor sheave. Groove to groove dimension is 0.400"
I used a slug of aluminum which was rather short. To remove the excess, I had to swap the slug and part off the remaining stock. The chuck jaws are on a hollow boring in the sheave, and are tightened from the inside-out.
The finished spindle sheave is displayed, with the previously mentioned boring exposed. This boring is also present on the stock Sherline sheave and is used to clear the spindle nut at the top of the spindle itself.

Another view. The rims of the V-grooves are radiused gently with a file while still in the lathe to keep the raw grooves from chewing up the belt.

Both the motor and spindle sheaves were drilled and tapped for 2 ea. 8-32 set screws

A final view of the finished product. A test of the system showed all A-OK except the additional load of the spindle took my 10,000 RPM spindle to 7,500, and that without any cutting loads... that's why I said STUPID! earlier. I may have to turn another motor sheave to get the spindle back up to where I want it.

Deceptively easy, these sheaves, but they took a bit of work to do right.