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Mini Cove Tool as a 4 page pdf
This article describes how to make a Mini-Coving tool. I’ve found it incredibly easy to make, easy to use and very versatile. The Mini-Coving tool cuts as a shear scraper, so it leaves a surface that needs little sanding. It will work on both spindle and face-plate turning. It will cut coves that are impossibly small for a gouge (my favorite is 1/16” wide). The only limitation is size; it only works well in small sizes. For any size that can be cut with a gouge it’s more efficient to do so, even if you have to practice a bit first. You can make a tool with the same profile in large size, but it is then more useful as a refining shear scraper than as a coving tool.
The Mini-Coving tool is simply round stock sharpened at a 45° angle. I suggest you use hardened HSS Drill Blanks for stock rather than drill rod, as ordinary tool steel dulls very quickly when used as a shear scraper. You’ll probably have to order HSS Drill Blanks from an Industrial Supplier, but don’t be tempted to use the shank end of a HSS Drill Bit you already have, as the shank ends of drills aren’t adequately hardened. I’ve found that 1/16”, 3/32” and 1/8” size best suit my work.
A ¼” Drill Blank (which is about the maximum size for a useful coving tool) is large enough to mount directly in the handle. For smaller coving tools the primary problem to overcome is vibration; a 1/16” shaft long enough to be useful at ordinary tool rest offsets would be more of a chatter tool than a coving tool. The fix is easy though; just mount a short length of the HSS Drill Blank in a larger rod.
Start making the tool by turning a short (about 6”) handle with a 1” deep 3/8” diameter mounting hole. Next find a 3” length of 3/8 rod. You can use drill rod or even the smooth shank end of a bolt. I used brass rod for the tool in the pictures because I had some and it’s easier to drill.
Drill a hole in the end of the rod about ½” deep the diameter of your HSS Drill Blank. A drill press, if you have one, is the easiest way to drill the hole. Use a center punch to give the drill a place to start and clamp the rod in place. Don’t try to hand-hold the rod as that’s not safe and even if you don’t hurt yourself the hole would probably end up oversize.
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[Fig1: Drilling a hole the size of the Drill Blank in the 3/8” Rod using a drill press. I’m clamping the rod in a cross-vise. Clamp the rod with however you like, but for safety and accuracy, please clamp it.] |
If you don’t have a drill press you can drill the hole on your lathe. The only problem is holding both the rod and the drill bit, as most of us don’t have two drill chucks. A collet chuck will hold the rod if you have one. Most Four Jaw Chucks will hold the rod if the top jaws are removed. Use the point of a skew or the corner of a parting tool to give the drill a place to start and select a slow speed.
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Fig2: Drilling a hole the size of the Drill Blank in the 3/8” Rod using a lathe. I’m using my Stronghold chuck with the top jaws removed to hold the rod. Make a starting dimple for the drill bit, and turn the speed of your lathe down.] |
Once the rod is drilled mount it in the handle. Then temporarily insert the HSS Drill Blank in the rod and mark how long you want it to be. Remove the HSS Drill Blank and cut it to length. You can cut the Drill Blank to length with a cut-off wheel mounted in a side grinder. Instead I used the corner of my bench grinding wheel to notch the blank and snapped it by tapping with a hammer. Once you’ve cut the blank to length, glue it into the rod with super-glue. Put a drop of super glue on a disposable surface such as making tape, and roll the rod in the drop of glue. Then insert the Blank. If the fit is tight enough to compress the air in the hole you may need to clamp the Blank in place until the glue sets. Save the remaining part of the HSS Drill Blank. When the tool has been ground down past usability you can remove the used Blank either by twisting it out or by heating it first, and then glue in a new piece.
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[Fig3: Marking the Drill Blank for Cutting. I’ve temporarily inserted the Drill Blank in the Rod and marked how long I want it to be with a marker. The longer you make it the more you’ll be able to sharpen it, but the more prone it will be to vibration. 3/8” to ½” seems a reasonable compromise.] |
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[Fig4: The Drill Blank after being notched. I rolled the Drill Blank on the corner of my Bench Grinder stone. Sure, I could have cut the Drill Blank with a cut off saw, but my angle grinder had something else mounted at the time.] |
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[Fig5: Ready to Snap the Drill Blank. I’ve wrapped masking tape around the notch and clamped the Blank in a vise. A hammer tap will snap the Blank at the notch. The tape will keep metal pieces from flying around, but wear eye protection anyway.] |
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[Fig6: Clamping the HSS Drill Blank in the rod. I’ve rolled the end of the Blank in CA glue and inserted it in the Rod. To keep a piston effect from pushing out the Blank I’ve clamped it while the glue cures.] |
The simplest way to sharpen the Mini-Cove on a Bench Grinder is to put the butt of the handle in sliding V-Block and adjust it so that you grind a 45° angle. The only trouble with this approach is that the tool has to be kept very sharp to work well, and the hollow of the grind will be oriented to that the tool can’t be effectively honed with a slip stone. Constantly returning the tool to the grinder will quickly shorten the Drill Blank past usability.
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[Fig7: Sharpening the Mini-Cove Tool with a sliding V-Block on a bench grinder. This is the simplest way, but doesn’t allow for as many sharpenings.] |
The tool can be sharpened on the bench grinder in a hone friendly fashion, but it requires making a simple jig. If you’ve read any of my previous articles, perhaps you know of my love affair with jigs. The jig is simple to make. Cut a V-groove in a small block of wood that is thick enough to keep the tool handle from fouling on the grinder platform. Cut the block at a 45° angle to a length that allows the Drill Blank to protrude slightly. Then take a small strip of wood that can be fastened across the V-Notch to clamp the tool in place. I was feeling fancy and drill and tapped for some ¼-20 thumbscrews. Wood screws would work as well as long as you keep a screwdriver handy. Adjust the platform rest so that the tool points directly at the center of the wheel and sharpen, using the 45° face of the jig as a guide.
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[Fig8: Sharpening the Mini-cove Tool with a jig on a the bench grinder tool rest. Sharpening this way puts the hollow of the grind across the tool, which allows you to hone it with a slip stone.] |
It’s easy to sharpen the Mini-Coving tool on a Tormek. It’s so small that the slow grinding speed of the Tormek doesn’t seem over-long. You can even do it without using Tormek jigs. Just make a couple of modifications to the tool handle. First, when making the handle, cut a notch with a ½” round file or cut a V-Notch on a band saw across the handle just back of the ferule. When inserting the rod in the handle, add a little super glue or polyurethane glue so that the rod will stay oriented the same way in relationship to the notch, otherwise it may twist slightly in use. To sharpen the tool, simple place the notch on the tool rest and adjust the rest so that the tool is sharpened at a 45° angle.
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[Fig9: Sharpening the Mini-Cove Tool on a Tormek. I’ve cut a notch in the handle that rides on the Tormek tool guide.] |
Using the tool to cut coves on spindle work is very simple. Just orient the tool so that the bevel is UP and points towards the center of the spindle and push in. On a sturdy spindle you can even cut quickly. On thin or long spindles you will have to work to prevent vibration. Start by cutting gently, perhaps backing up the spindle with your fingers. If you still have vibration you’ll need to suppress it with a back-steady. I’ve had very good results using a strip of UHMW (Ultra-High Molecular Weight) Polyethylene that has a V-Notch cut in it. A small strip of wood with a waxed v-Notch would also work, although you’ll have to be more careful about scorching the work.
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[Fig10: Cutting a Cove on a spindle. This spindle is substantial enough that I can cut aggressively at first as witnessed by the stream of shavings coming off the tool] |
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[Fig11: Cutting with a Back Steady on a slender spindle. If vibration is a problem (you’ll hear it) back up the spindle. I’m using a notched UHMW strip to avoid burning my fingers. The UHMW won’t burn the wood and can be easily shaped with woodworking tools.] |
You can simply plunge the tool in to the depth you want, and the tool will cut a perfectly symmetrical, if slightly elliptical cove. You can enlarge the cove by recutting on each side. You can even under-cut the sides if you wish. If you keep the tool sharp you should find that the cove requires little sanding. To avoid rounding over the edges of the cove I usually skip sanding the interior of the coves until I get to 220 grit or so.
If you try making a mini-cove tool that is ¼” or larger you’ll find that it tends to cut very slowly and have serious vibration problems. Shear scraping with a large clearance angle requires a fair amount of pressure to cut. The total force required is proportional to the width of the tool engaged in cutting which is why bigger tools don’t work as well. You can partially remedy this by pre-cutting the recess with a more efficient tool such as a parting tool. You’ll have to experiment to learn the depth you can precut.
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[Fig12: Detail of two 1/16” coves cut in an Ornament Icicle.] |
Use the Mini-Cove tool the same way on faceplate work. Orient the tool so that the bevel is up, point it to the center of the work and push in. Again, the shear cutting action keeps sanding to a minimum, although as usual, faceplate work takes more sanding than spindle work. You can keep chatter to a minimum by cutting gently and as early as possible before cutting supporting material away.
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[Fig13: Detail of a 1/8” cove cut on the rim of a bowl.] |
I think you’ll find that the Mini-Cove tool is an easy to use tool that will increase your design possibilities. It is especially effective on small scale spindle work such as pens, lace bobbins, and ornament icicles. I’ve also found that it can add interesting details to faceplate work such as bowl or vessel rims. Being such an easy to tool to make and sharpen there’s little to keep you from giving it a try. Let me know how it goes.
HSS Drill Blanks and UHMW Polyethylene can be obtained from any industrial supplier, such as MSC, 1-80-645-7270, www.MSCDirect.com:
1/16” HSS Drill Blank, MSC# 01100049 $.49
3/32” HSS Drill Blank, MSC# 01100064 $1.00
1/8” HSS Drill Blank, MSC# 01100080 $1.00
3/8”x3/4”x5’ UHMW Polyethylene, MSC# 52433448 $5.47
The author spends entirely too much of his basement time making and modifying tools and jigs. He welcomes comments, questions and suggestions via email at David@DavidReedSmith.com.
