Kirk DeHeer turns a threaded box

This demonstration covered three important issues:

1. Using epoxy inlays to allow applying threads to woods that are not dense enough and fine-grained enough to support fine threads naturally;

2. Preparing thread chasing tools to function properly;

3. Techniques for chasing threads in turning a threaded box.

Each of these issues will be treated separately in these notes.

Epoxy Inlays

Only a few woods are naturally dense enough and fine grained enough to receive threads successfully. These include: boxwood, blackwood, pink ivory, white ivory, lignum vitae, osage orange, mountain mahogany, some ebonies, cocobolo, and sometimes bocote. For other woods threading can be made possible by creating an epoxy inlay in the wood so the threads can be cut in the epoxy rather than the wood itself.

Kirk demonstrated this approach in a blank of box elder burl. He turned the blank into a cylinder and cut dovetail tenons in both ends of the blank. Following the normal standard, he parted the blank into two parts of approximately 1/3, 2/3 proportions retaining the shorter (lid) section in the four-jaw chuck. Kirk used a chuck with shark jaws to hold the work a bit further from the headstock. That section was faced off smoothly with a gouge. He then cut a groove into the face of the blank using a thin (3/32 in.) specially ground pen-turning parting tool. The outside of the groove was approximately ¼ inch from the outer edge of the cylindrical blank. The groove was cut 3/8 inch deep and about two tool widths (3/16-1/4 in.) wide. The walls of the groove must be parallel to the cylinder wall. This groove was to receive the epoxy inlay.

The longer (bottom) section of the blank was then mounted in the chuck and faced off. A groove identical in diameter to the one completed in the lid section was then cut using the same procedure as before. This groove was formed to align perfectly with the groove in the lid section. Kirk did this by using a scoring divider to transfer the outside diameter of the lid groove onto the face of the base section and then cut the groove inside of that diameter so both the inside and outside circumferences of the two grooves were concentric with one another.

With both sections removed from the lathe, Kirk used masking tape around the circumference of the lid section to create a form or dam above the grooved face of the lid section. This would allow over- filling the groove to create an exposed accent layer of epoxy between the two sections when finished. The tape may be skipped if you don’t want a contrasting center band.

A slow setting epoxy must be used. Kirk has used successfully both the 20 minute Stick Fast epoxy sold by Craft Supplies and the T-88 System 3 epoxy sold by Treeline. Dyes can be added to the epoxy to provide color contrast with the natural wood. These dyes can be obtained in a variety of colors. Dyes containing alcohol must be avoided, but epoxy based dyes and powdered dyes work well. Kirk has used the Inlace Translucent dyes and even powdered fishing lure dyes obtained from Cabelas. In this demonstration Kirk used a black Inlace dye. An appropriate amount of part-A epoxy was added to a mixing cup and dye was stirred into the epoxy until the desired color intensity was achieved. An equal amount of part-B epoxy was then stirred into the dyed mix. It is important to mix a batch large enough to complete the job at hand because it is almost impossible to mix a second supplemental batch and achieve a perfect color match. When thoroughly mixed the epoxy was poured into the grooves in the two prepared sections. The groove in the base section was filled just proud of the face of the piece but the groove of the lid section was generously overfilled within the tape dam so an ample accent layer was possible. The liquid mix will naturally contain some air bubbles, but the slow-setting epoxy allows time for the air bubbles to float out of the grooves so they are not problematic. The filled blanks were then set aside to harden completely.

Preparing Thread Chasing Tools

New chasing tools generally need some slight modifications to prepare them for use. A set of tools generally consists of a female thread tool for cutting threads on an inside surface and a male thread tool for cutting threads on an outside surface. Two other tools that can be useful are a relief-groove cutting scraper and an armrest. The relief-groove scraper must be custom ground, but the long point of a skew chisel can be used in lieu of this specialized tool. Chasing tools are available from both Sorby and Hamlet. The cutting teeth on the Hamlet tools are milled vertically and those on the Sorby tools are milled on a spiral. Kirk favors the Hamlet tools but many prominent turners prefer the Sorby. These tools are available in four thread pitch sizes, 20 threads per inch (TPI), 18 TPI, 16 TPI and 10 TPI. The 20 TPI tools are easier to use since they allow a slower advance of the tool across the work. However, some woods that will not accept the fine 20 TPI threads will accept the larger threads.

The female tool generally is manufactured from ¼ inch thick steel with a square tip with the cutting teeth offset to the left side of the tool shaft. The broad, square tip is difficult to use in tight areas and might be prone to bottoming out on the back corner. Kirk recommends regrinding the back edge of the tip so that the end of the tool tapers toward the tip and is radiused on the back edge. He also grinds the top face of the tool to reduce the thickness and to create a slight negative rake. The tool is sharpened by lightly laying that top surface flat against a fine-grit grinding wheel. He also softens very slightly the bottom edge of the cutting teeth.

The male tool is also made from ¼ inch thick steel with a square tip. The cutting teeth are milled right in the square tip. Kirk prepares this tool by simply grinding the upper face of the cutting tip to a negative rake by laying the upper face of the tip flat against a fine-grit wheel. This also results in reducing the thickness of the tool at the cutting tip. Subsequent sharpening is done by lightly repeating that grind on the upper face of the cutting end. Kirk also slightly softens the bottom edge of the cutting teeth as with the female tool. The left-most tooth on the tool is the lead tooth. It is important that this be a well formed and complete tooth. If it is marred or only a partial tooth it will be necessary to grind the left side of the tool back to reveal a full tooth.

Since the chasing tools must slide smoothly and easily across the tool rest it is important that the tool rest be perfectly smooth and free of any dents, scratches, or irregularities that might cause the tools to drag or hang up. The tool rest can be dressed on a belt sander or with a long file then polished with a fine abrasive. Allan Batty waxes his tool rest with a candle. The Tools, as with all tools, should have the shaft corners softened and the blades polished. 600 grit sand paper is great for this job.

Turning a Threaded Box

In this demonstration Kirk used an African blackwood blank so he could cut the thread directly into the wood rather than using an epoxy inlay. He also suggested that a 2×2×12 block of blackwood costing $26 can provide material for several threading practices.

The square blank was placed in a chuck using shark jaws. These jaws provide a secure mount and also hold the work a bit further from the headstock than standard dovetail jaws. The blank was turned to a cylinder and dovetail tenons were cut in both ends. A pencil mark was drawn along the axis of the cylinder to provide a grain alignment reference. The general dimensions of the box were laid out using the 1/3, 2/3 principle. Enough waste wood was left at the base of the longer section to allow forming a final drive chuck for finishing the bottom of the box. The blank was parted off leaving the lid (upper) section in the chuck. That section was faced off and hollowed out to the desired depth using a 3/8 spindle gouge and employing the usual hollowing back cut. The wall of the hollowed surface was refined using a box end scraper. This tool was made from a ½ inch square-end scraper reground with a straight taper along the left edge of the tip and a slight radius to the end of the tip. This allows scraping straight down the wall and then drawing across the bottom of the cut to smooth the end. The interior wall was thus scraped to be exactly parallel with the lathe bed to form a true cylinder inside. Parallelism was verified by laying a new and unsharpened pencil against the inner wall and checking the alignment of that pencil with the ways of the lathe bed. If an epoxy inlay had been used, the hollowing would have come out into just under half the thickness of the inlay. The long tip of a skew was used to cut a relief groove at the end of the section to be threaded. The groove prevents bottoming out the thread chasing tool which would quickly destroy the threads. The outer lip of the inner wall was then rounded over slightly. It is important that the entry point for the threading tool be nicely rounded, never a square corner or chamfer. If interior sanding is needed it should be completed before threads are cut.

The lathe speed for thread cutting is generally around 350 rpm. An easy way to set the proper speed is to remove the chuck from the spindle and turn the lathe on at low speed. Continue reducing the speed until you can follow a rotating thread on the spindle with your eye. If you cannot follow an individual thread your speed is too fast. If the speed is too slow the quality of the thread will suffer, but if too fast you wont be able to advance the thread chaser fast enough to establish a thread or withdraw the chaser before it bottoms out.

The tool rest should be placed at an angle to the lathe bed with the end furthest from the head stock farther away from the lathe bed than the inner end. This is to provide sufficient clearance to allow manipulating the female threading tool. If an arm rest tool is being used, it will lie on the tool rest and the chasing tool will rest upon the arm rest. The arm rest is then used to control the cutting pressure of the chasing tool against the wood. This is a very gentle pressure and can be applied by gripping the arm rest shaft between the thumb and forefinger of the left hand just outside the tool rest. Cutting pressure can then be applied by merely pinching the fingers against the tool rest while using the right hand to advance and retract the chasing tools from the wood.

The threading cut is begun at an angle to the wall being cut. The lead tooth does not contact the wood. Instead the third or fourth tooth is engaged lightly with a negative rake against the rounded edge of the wall and the tool wiped smoothly against the wood for a short distance. This cut must be made each time at or slightly above the center line. The tool is backed away from the cutting surface by pulling the handle of the chaser toward you slightly. This light cut is repeated several times until a thread becomes defined. Once the thread is defined the chasing tool will engage the thread and draw itself along the thread path. It is important to back off each cut before the end of the chasing tool bottoms out. If it stalls into a stationary position, all the threads will be destroyed. If you find it difficult to withdraw the tool from the cut in time to avoid bottoming, reduce the lathe speed or cut a longer relief groove. As successive cuts deepen the defined thread, the angle of entry is reduced toward being parallel with the wall being threaded so that when the thread is finished it will be truly cylindrical. When the thread is established, the lead tooth should be engaged to provide good cleanup cuts. The quality of the threads can be improved by applying a little lemon oil wax to the wood for the final cuts. The threads are at full depth when they become pointed. If light curly shavings are coming off the chasing tool it is cutting properly. If powder is coming off, the tool should be sharpened. With the threading completed apply lemon oil wax to the interior and remove the lid from the chuck.

Mount the base section in the chuck. Face off the end and create a spigot as you would for a friction lid box. With a skew chisel create a very narrow chamfer on the end of the spigot to provide a reference to determine the proper diameter for the spigot. Continue reducing the diameter of this narrow chamfer until it just enters the inner diameter of the lid threads. This defines the bottom diameter of the male threads to be cut in the spigot. Reduce the diameter of the remainder of the spigot to just more (1/16 in.?) than the defined chamfer diameter. Round over the chamfered end to provide a well rounded contact point for the chasing tool as before. Cut a relief groove at the base of the spigot using the point of a skew chisel.

Begin cutting the male threads with the outside thread chaser. The chaser will rest directly on the tools rest. No arm rest tool is needed. The technique is quite similar to that used for the female threads. The first passes will be made with the chaser contacting the wood on the rounded edge at the third or fourth tooth with the cutting edge in a negative rake position and angled slightly as if cutting on a taper. With a light touch, the chasing tool is wiped smoothly across the wood for a short distance and withdrawn. Repeat this cut until a thread is defined. As the thread develops, gradually reduce the angle of the chasing tool until it is perpendicular to the cutting surface. When threads are established, test the fit of the lid. If the fit seems tapered, continue deepening the threads at the thick end. If the overall fit is too tight, use a skew to scrape the tops of the threads down a bit then re-chase them to depth once more. Lemon oil wax can be used to lubricate the threads and improve the quality of the cuts. Continue this testing and cutting procedure to eliminate all binds and tapers until you achieve complete closure of the lid on the base. With the threads completed and the lid mounted on the base make a few gouge or skew cuts to true up the exterior surface of the box and to contour the top of the lid. Don’t worry about adjusting the grain alignment until after hollowing the base section since the hollowing and sanding might affect the fit.

Remove the lid and hollow the inside of the base to the desired depth using the hollowing back cut with the 3/8 inch spindle gouge and box scraper. If an epoxy inlays were to be used, this hollowing could come out almost to the inner edge of the inlay, but a little wood should be left inside to support the inlay. Sand the interior and finish with lemon oil wax. Use the skew to cut a narrow groove at the edge of the shoulder of the spigot to define the desired wall thickness for the finished box. With the lid threaded onto the base, check grain alignment. Misalignment will be corrected by using the skew to scrape tiny amounts of wood from the face of the shoulder at the base of the threaded spigot to allow the lid to tighten another fraction of a turn to bring the grain into alignment. Be patient with this fitting. You must sneak up on the proper alignment without going past it. A point tool is helpful to clean up the corner of the shoulder and spigot. It might be necessary to widen the relief groove at the base of the threads slightly to fine tune the fit.

Use a depth gage to measure the interior depth of the base then mark the desired external bottom with a parting too. Remount the lid and use the gouge and/or skew to cut the box exterior to finished contour and wall thickness. When the reference groove disappears you will know you are at the desired wall thickness. Cut a small groove at the edge of either the lid or the base to disguise the joint between them. In this demonstration, Kirk chose to generate a gentle hour-glass configuration with a rounded lid decorated with a very small bead around the top. Sand the completed exterior and finish with lacquer sealer. Friction dry then apply paste wax with steel wool to even out the lacquer and establish the level of sheen desired.

Part off the box with a thin parting tool. Create a threaded drive chuck with the residual wood remaining in the scroll chuck. This is done by following the same procedure used to hollow and thread the lid except that the threads can be less refined. Mount the base section in the threaded drive chuck and turn and decorate the bottom as desired. Finish the bottom with lacquer sealer and paste wax as before.

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