Supercharged Sherline Spindle - Part I, Sheaves

5 Bears Home Homebrew CNC bench mill

OK Back to work on keeping the website even a little fresh. I am perhaps 4 or 5 installments behind; the CNC mill is far beyond this page, and I have a folder full of photos to document! On my home page, I hinted at a "supercharged" Sherline spindle... rather than 2,600 RPM, I wanted 8,000+ and a bit more power as well. This is the first part of how I managed this, using a surplus motor and some imaginative mounting.

From left to right we have the stock Sherline motor, the spindle itself (in this case one of their ER-16 varieties), and the surplus motor which will be used. The motor is a Dayton "universal" (AC/DC) motor which will lend itself well to speed control with a modest controller such as a Dart or similar SCR 6 Amp system.

The problem I face is manyfold - first, I need new sheaves (pullies) which will reduce the 20,000 RPM output of the motor into something the spindle will digest. Next, I'll need to mount the motor and have some way to adjust the pulley tension, as these speeds can do odd things with pullies and belts without correct tension!

The stock Sherline Motor sheave. The belt is a rather smallish composite belt. When I first ordered the Sherline spindle head assembly, I ordered an extra belt, so I had two on hand. I decided to machine the sheaves so that it would use two belts simultaneously for strength.

My first task was to determine the geometry of the belt. The included angle, iirc (don't quote me here) was 30 degrees. I used a caliper to determine the factory groove depth and width.

Without a lot of fiddly top-slide manipulation, the best way to cut these pullies is to use a form tool, and an accurate sheave demands an accurate tool. Here, I have mounted a 1/4" HSS bit into a graver sharpener. This device is primarily used to sharpen what are known as gravers in the metal-engraving trade. It will hold any square-shanked tool at precise and consistent angles for application to the sharpening stone.

An old fellow with 25 years of experience might be able to grind this form bit freehand, but I cannot, so I cheat!

In use, the feet of the graver sharpener maintain a sliding contact with any flat surface. I am using a diamond stone to correctly sharpen the side flanks of the tool. Like most form tools, this one has no top rake. Adding top rake would destroy the correct geometry... it also must be positioned exactly at center height to induce the correct form shape into the aluminum.
The motor sheave is first. The grooves are roughed to nearly their full depth with a normal parting tool.

With the form tool at center height, it is plunged straight in to full belt depth, the jogged right and left to open up the roughed slot to the correct dimensions.

With a dead sharp tool and a slow spindle feed, it is very easy to generate the correct form, and the swarf comes off in amazingly thin but wide ribbons of material.