A Grizzly Adaptation

A machinists' lathe manufactured by Grizzly Tools of Bellingham, Washington has been adapted to create the ornamental turned work you can view in the Gallery. All of the machine work to create the apparatus for ornamental turning was machined on this lathe and a drill press.

Essential for the process of ornamental turning is an index plate and cutting frame. The index plate was made from a cast iron 8" lathe chuck backing plate mounted on a dividing head and drilled on a drill press with six concentric circles of evenly spaced holes of the numbers 84, 96, 108, 120, 144 and 192. When screwed on the lathe spindle and held in position by a spring-loaded detent, this device allows for the equal division of the circumference of a workpiece. Many divisions from 2 to 192 are possible.

The cutting frame is simply a small axle mounted in ball bearings in a yoke mounted on the lathe toolpost. The axle is broached across its center with a ¼" square hole to hold a standard ¼" high-speed steel lathe bit held in place by set screws. Power is brought to the axle via a Foredom-type flexible shaft drive controlled by an electronic variable speed controller. The cutting frame may be rotated through 90° so that the cutter can be adjusted to cut from a horizontal plane to a vertical plane. The bits may be ground to various shapes to effect different cuts on the workpiece.

The overhead drive used on this setup is provided by a Foredom-type flexible shaft drive which sockets into the end of the cutter frame spindle. Traditional ornamental lathes use an overhead power transmission to the cutting frame by means of a system of pulleys and drive belts. I decided to try the approach of using the flexible shaft drive and it works well as a simpler alternative. The speed of the cutter is maintained and adjusted by an electronic variable speed control. Drill frame work is done with the handpiece of the flexible drive mounted into a jig on the tool holder. Small router bits serve as the cutters and drill bits can be mounted in the chuck of the handpiece to hold various sized drill bits to drill holes for mounting stones, inlay of silver wire, etc.

The eccentric cutting frame allows shallow circles to be cut on the workpiece with adjustment for the radius of the circle and its displacement from the axis of the lathe. By indexing and changing the radius and eccentricity of the cutter, complex geometric patterns can be cut as illustrated in the History section. A better eccentric cutter has been devised by Al Schwarz which is circular and allows for balancing so that it is smoother in action. Such an improvement is on the list of continuing refinements to be made to the Grizzly.

So far, all of the possible ornamentation that can be created by the Grizzly lathe is done with the lathe spindle in a fixed postion, held in position by the index plate and detent. The various cutters move in and out controlled by the lead screws of the tool holder sliderest and make a cut on the fixed workpiece. The index is then moved to a new position and another cut effected. Another type of ornament can be effected, however, when both the workpiece and toolholder move in a synchronized motion.

The Grizzly lathe, being an engineering or machinists' lathe, is capable of cutting screw threads of various pitches. These different screw threads are determined by adjusting a built-in set of gears called the quick-change box. With this adjustment, 44 different screw threads can be cut. With the coarsest thread being one of 4 threads per inch, or ¼" lead, the problem was to extend this lead so that a screw thread of, say, one thread per six inches was possible. This would allow for slow, gradual spiral patterns to be cut on an ornamental turning workpiece. So a new gear train was devised on the outboard end of the Grizzly with a 96-tooth gear being mounted on the spindle meshing into a 24-tooth gear on the idler shaft, thence to a 144-tooth gear on the idler shaft meshing into a 24-tooth gear on the lead screw. What this accomplished was to place the spindle in a new relationship to the lead screw which was derived from the new gear relationships. With a 4:1 ratio from spindle to idler and a 6:1 ratio from the idler to lead screw, the lead screw now was in a 4:1 x 6:1 = 24:1 ratio to the spindle. So with the quick-change gear box set for the coarsest screw thread of ¼" lead, now the toolholder moved 6" for every rotation of the spindle. These new gears were cut from 0.5" aluminum stock on the Grizzly lathe using the ornamental turning index plate and horizontal cutting frame.

It was found that the torque needed to hand crank the lathe with this setup was too great to insure a smooth motion to the cutter. So a 30-rpm DC gearmotor with an electronic speed controller was installed to drive the lead screw by means of a universal joint connection. Now with this setup, it was possible to cut 40 spirals, with leads ranging from 0.2" to 6", upon the workpiece.

In conjunction with the spiral apparatus, a device called by Holtzapffel the reciprocator was constructed on the Grizzly lathe. This device allows sine waves to be cut upon the workpiece. It consists of one arm fitted and locked tight to the spindle and another mounted to the idler shaft upon an eccentric hole within a hub. The arm from the spindle is drilled with a series of holes to allow for adjustment and the outer end of the arm from the idler shaft has a screw which is attached to one of the holes in the arm from the spindle. This attachment allows for rotation and the motion imparted from the eccentric hole on the idler shaft causes the idler shaft arm to crank and so move the spindle arm back and forth in a regular period. This motion causes the workpiece to engage in a recurring back and forth partial rotation. So as the toolholder is moved along the bed of the lathe by the lead screw, the workpiece is rotating backwards and forwards by a regular amount and regular sine waves are cut by the powered cutter. The size of these waves depends upon the diameter of the workpiece, the eccentricity of the hole in the hub on the idler shaft, the adjustable hole setting on the reciprocator arm, and the setting of the quick-change gear box. Combining possible hub eccentricities with the adjustable hole settings on the reciprocator arm gives a possible 27 different amplitudes or heights of wave; and by varying the settings of the quick-change gear box, 40 different lengths of waves are possible.

In order to cut more than one pattern around the workpiece, either of spiral or reciprocated design, a new means must be used to index the lathe. This requires a spiral chuck, which has a ratchet and pawl, or worm gear to allow for changing the position of the workpiece in relation to the spindle. This chuck was machined on the Grizzly also. A cabinet was built under the lathe to contain the new spiral/reciprocator apparatus when not in use. The Grizzly lathe has proved to be an excellent tool to adapt to ornamental turning, serving as both the ornamental lathe in use and the machine tool to create new apparatus as needed. In time, more apparatus will be created on the Grizzly to extend its ornamental turning capabilities.

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World-Wide Web presentation Copyright © 1996, James E. Harris. All Rights Reserved.
Last revised March 12, 1996.