Sphere-Cuttingjig
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Shopbuilt Sphere-Cutting Jig Eugene Lee ’ve been turning for a few years object. I wanted to spend more time (Above, l to r) Juniper spherical hollow form with and have honed my skills so that exploring surface enhancements a tilted band, 4¾" (12 cm)—band produced by I I can turn a sphere-shaped object, rather than turning the sphere itself, multiaxis turning. Walnut, 4¼" (11 cm); Avocado, either as a solid or hollow form. They so I decided to build a sphere-cut- with John Jordan-inspired carving, 4½". Spheres are always sphere-shaped rather than ting jig. were turned with the jig described. near perfect because when I rotated The jig’s cutter has to be mounted them in my hands, I felt some bulges at the lathe’s center height, on an arm and depressions—they were not as that pivots around a point directly spherical as I wanted. under the intersection of the lathe designed a jig that can be built with When turning a spherical object axis and the equator of the sphere- hardwood, phenolic-faced plywood, without the use of a jig, I am to-be. Swinging the arm through a and metal hardware. mindful that at any time I can cut semicircle sweeps the cutter across too much and go inside the target the surface of the sphere. There are a Make the jig radius. When that happens, I either number of metal sphere-cutting jigs There are two parts to this jig (Photo recut the sphere to a smaller radius on the market (see page 46), but I have 1): a platform mounted on the lathe or leave it alone as a sphere-shaped no experience with machining, so I ways, and a pivoting turret that 42 American Woodturner October 2012 Copyright 2012, 2016, American Association of Woodturners. All rights reserved. 42_SphereCuttingJig.indd 42 9/11/12 10:43 AM FEATURE carries the tool. The turret pivots (73 mm) thick. The bottom view of not right on, you will get a football on a vertical pin embedded in the the platform (Photo 3) shows ¾" × shape. I fastened the top of the plat- platform directly below the lathe ⅜" (20 mm × 10 mm) T-track dadoed form to the sides with #8 sheet-metal axis. For tooling I am using the Easy into a guide bar of ¾" hardwood. For screws and washers in oversized Wood Tool’s Ci1 Easy Rougher and my lathe, the guide bar fastens 2¾" counterbored holes to permit a little the Ci0 Easy Finisher. For flatness, (70 mm) from the back edge of the play for final adjustment. The dis- stability, and low friction I made the plywood. Photo 3 includes two blocks tance from the top of the pivot to platform and turret base using ¾" of wood with ¼" (6 mm) T-bolts and a the lathe axis determines the biggest (20 mm) phenolic-faced plywood, plastic knob for locking the platform sphere, in this case a radius about for which you can find suppliers on onto the lathe. 4¼" (110 mm). the Internet. Photo 4 shows the underside of To locate the hole for the pivot, The jig dimensions are not critical the platform top plate with the ¾" place the top onto the platform so and can be tailored to your own lathe plywood block that carries the pivot, that the tip of a revolving center is and tools. The parts described here a 2½" × ¼" (63 mm × 6 mm) bolt with above the middle of the platform. apply to my JET 16-42 lathe. Photo 2 washer. A ¼"-20 threaded steel insert Use a square to mark the platform shows only the platform, a flat box secures the bolt to the platform directly under the tip. Use Forstner that slides along the lathe bed with about 1" (25 mm) protruding bits to counterbore the hole for a and houses the pivot. My platform from the top as the pivot. The pivot steel threaded insert, then complete measures 10" (25 cm) wide, 12⅜" needs to be directly under the lathe the hole with a ¼" bit. Run the bolt (31 cm) front-to-back, and 2⅞" axis in order to turn a sphere. If it is into the threaded insert to help 1 2 3 Sphere-cutting jig consists of a base The jig platform is made of ¾" phenolic-faced Underside of jig platform shows the platform mounted on the lathe ways with a plywood, which is flat, stable, and smooth. T-track, blocks, and knob that lock it onto pivot directly under the lathe axis. Pivoting Note the bolt as pivot. the lathe ways. the turret assembly and its carbide-tipped tool cuts the surface of the sphere. 4 5 6 The pivot is threaded through an internal The pivot mounts directly under the lathe axis. The jig turret consists of a housing for the plywood plate to keep it in place. cutter (parts A and B), a spacer block (part C), and a mounting block (part D) that connects the turret to the bottom plate (part E). woodturner.org 43 Copyright 2012, 2016, American Association of Woodturners. All rights reserved. 42_SphereCuttingJig.indd 43 9/11/12 10:43 AM dado about ⅜" deep into the bottom plate, part E, and glue the connecting spline into it, then carefully fit part D onto the spline and screw the assem- bly together. With the completed turret in place, fit a ¼" nut onto the pivot and tighten it to barely touch the plywood. The sphere-cutting jig is complete. 7 8 To use this jig, first turn a cylinder Brass connector bolts and cross dowels, You may need to notch the turret’s that is longer than the diameter of the parts F and G, hold the turret assembly bottom plate so it can clear the headstock. sphere. Center the jig on the cylinder together. The bottom block, part D, fits over a hardwood spline glued onto the and start cutting from side to side bottom plate E. The nuts and bolts at right while you move the cutter toward the hold parts A, B, and C together. desired radius. Videos are available for viewing online with instructions on how to use a similar jig, so I will limit my discussion to a few advisories. you screw it into the hard plywood not critical, so long as the cutter is When turning larger spheres, I have without any misalignment. Recheck secure and ends up exactly on the found it best to rough out the shape with a square to make sure the pin is lathe axis. I used seasoned hard- using a bowl gouge before bringing directly under the tip of the revolv- wood blocks measuring 4" (100 the jig into play. I start with the Easy ing center (Photo 5). Clamp the top mm) long by 15⁄16" (30 mm) wide, for Rougher and, as I get close to the down, drill pilot holes, and screw a total height of 5⅛" (130 mm), which final shape, I switch over to the Easy the top onto the platform. for safety’s sake I shaped and cut from Finisher. This speeds up the process The turret’s bottom plate, part E in a larger board. and puts less wear on the carbide tips. Photo 6, has several holes for the pivot, Photo 7 also shows the ¼" brass to cut spheres of various diameters connector bolts and cross dowels, Cup centers without the cutter overhanging too parts F and G, that hold the blocks The last step in turning a solid far. To reduce wear on the holes in the assembly together. Cut a ¾" dado sphere is to part off the ends and bottom plate, I epoxied ¾" long by ¼" about ⅜" deep into the bottom mount the workpiece between cup steel spacers into it. Since the nominal of part D for attaching it to the centers to turn away the stubs. I ¾" phenolic plywood is actually a bottom plate with a ¾" square use shopmade cup centers like the little thin, I had to grind the spacers spline and screws. Make part A ones in “Spherical Thinking” by to match. last and install the ¼" × 20 steel Frederick Hill (AW, vol 25, no 4). threaded insert for the locking However, instead of fitting the cup Adjust the cutter height knob, part H in the photos. Fasten center over the Oneway-type revolv- The greatest challenge in the design part A atop part B using #6 sheet- ing center, I screw it onto the cen- of this jig was how to adjust cutter metal screws and washers. With the ter’s threads by epoxying a ¾" × 10 height without using metal parts. I turret assembled, trim the length to nylon insert lock nut into the wood ended up with the four-block turret 4" and set it aside. (Photo 9). The tip of the revolving shown in Photos 6 and 7. From the The bottom plate, part E, is 10" center juts out a fraction of an inch, top, block A and knob H clamp the (25 cm) long by 6" (15 cm) wide so if you epoxy a couple of washers cutter in place. Block B has a ½" × with a series of five holes 1" apart and a coin to the lock nut, it will ½" (13 mm) dado sized to house the for the pivot. The turret mounts completely cover up the threaded cutter. Block C is a shim that can be 1" from the last hole. This enables end of the revolving center. swapped out for height adjustments.