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This article was published in the February 2013 (#41) edition of Woodturning Design.
Foam Cone Sander
A while back I wrote an article on a Foam Ball Sander (Woodturning Design Spring 2008). I thought it was terrific to use--very stable in use, consistent scratch pattern, and capable of being used very aggressively without shedding its abrasive and leaving a smear of velcro stickum across the wood. But alas, it was quite sub-optimal when it came to preparing or changing the abrasive. Life involves compromise. The Foam Cone Sander is a different compromise. It's stable in use, but you won't be tempted to hold the drill with one hand as you can with the foam ball, and it wouldn't work well for the interior of a hollow vessel. It gives pretty much the same consistent scratch pattern, without swirls, and can be used aggressively without shedding abrasive due to overheating. It's reversible to make it easier to get into some areas. You have to pay a bit more attention to presentation, but it's WAY easier to prepare and change the abrasive.
I think a consistent, swirl free sanding pattern is more efficient. Also I tend to use a power sander for the first grit or two and then switch to hand held abrasive when all you're doing is removing the scratches left by the prior abrasive. Except for perhaps the very center of a bowl the scratch pattern is straight and similar to hand held abrasive, as shown in Fig01.
First the article will discuss preparing and turning the cone blank. Then the cone will be covered in foam, tape, and velcro. The abrasive will be prepared and applied, and then the using the foam cone sander in various bowl situations will be discussed.
Making the Cone
Begin the blank by bandsawing a disc, 2-3/4" in diameter, out of wood about 1-1/8" thick as in Fig02. The next step is to drill and tap the center of the disc for 3/8x16 threads. Fig03 shows a mini-tap guide, on top, which enables you to easily tap threads perpendicular to the blank because of its spring loaded point that follows the tap, and two slightly different styles of taps. The tap in the middle has a shank that is the diameter of the outer threads. This style limits the depth you can tap. In this case, it will tap deeply enough. If you made the cone of thicker wood it would have trouble sanding more concave surfaces (or you would have to use a lot more foam layers). The bottom tap, a cheaper one with a reduced shank, could tap more deeply.
Mount the blank on your lathe using a 4-jawed chuck (or faceplate, etc). Mount a combined drill and countersink or spotting drill in your tailstock mounted drill chuck as in Fig04 and drill a small dimple to aid in getting the tap drill started straight. Then mount a 5/16” drill bit in the chuck as in Fig05 and drill through the blank.
Now mount the mini-tap guide in your drill chuck. Place the nose of the tap in the drilled hole and bring up the tailstock so that the point of the mini-tap guide engages the dimple in the back of the tap (or the tap handle if you're using one). Advance the tailstock ram to compress the spring of the mini-tap guide. Lock the lathe spindle and turn the tap so that it cuts the threads. The spring loaded mini-tap guide will follow the tap in (at least for the critical first half inch), keeping it aligned with the axis of the lathe and preventing skewed threads.
Now prepare a drive stud for the cone. Use either 2-1/4” of 3/8 x 16 all-thread or cut the head off of a 3/8 x 2-1/2” bolt. Thread on two nuts and lock them against one another about 1/2" from one end as in Fig07. Then remove the blank from the chuck and thread the short end of the stud into the cone blank.
Mount your drill chuck in the headstock, and then mount the cone blank, via the drive stud, in the drill chuck. Bring up your tailstock so that you won't have worry about the drill chuck vibrating loose as in Fig08. Using a small bowl gouge, reduce the diameter of the blank to 2-1/2" as in Fig09.
Then use the bowl gouge to taper the nose of the blank so that the shape becomes a truncated cone. A good general purpose slope to aim for is 45°, or measured as in Fig10, 135 degrees. This angle works fairly well both inside and outside of bowls unless they have very concave surfaces. Another tactic would be to use a pair of cone sanders, one flatter, maybe 155 degrees for bowl insides and one steeper, perhaps 125 degrees, for the outside.
Fig10: Checking that the Cone is 135°.
Covering the Cone
The wooden cone needs to be covered with 3 layers of 2mm craft foam to allow it to conform to the bowl surface and sand efficiently. It is possible to calculate the exact size and shape of foam needed (it will be on my website if you're into the math) but it's easier to start bigger than necessary and trim, as craft foam isn't very expensive (unless you splurge on the new sparkly foam). Start by making a template out of poster board or cereal box cardboard. Begin by making a 5" circle, then a smaller 1-1/2” circle with the same center. Draw two radial lines starting at the center at right angles to another. The result is three quarters of a ring as sown in Fig 11. Use the template to trace out 3 of the foam shapes on the craft foam using a fine lined sharpie or pencil. Now cut out the foam pieces.
Lay the foam pieces out on newspaper or another disposable surface and spray one side of the foam pieces with 3M 77 spray adhesive as in Fig12. Wait for the glue to set up following the directions on the can. Then apply the first piece of foam to the cone blank as in Fig13. The foam will overhang top and bottom and overlap. First trim away the overlap with a knife or scissors. Then trim the overhang at top and bottom as in Fig14. Repeat the foam application for the two additional layers, being sure to put the overlap in a different place for each layer. The result is shown in Fig15.
The foam is somewhat fragile—if you had to change the Velcro it would likely take some foam with it so protect the foam with a layer of duct tape. I've not discovered a terribly efficient way to cover a cone with duct tape...so just get it done, a couple of layers worth, and trim away any overlap as in Fig16. Now apply the hook layer of “Industrial Strength sticky back” Velcro. Again, there is no magic way, just apply and trim. The result is shown in Fig17.
Using the Foam Cone Sander
The first step in using the Foam Cone Sander is to size and attach abrasive. Use 5" discs. Unperforated discs would be best, but perforated discs from Home Improvement big box stores will work fine. Lay a disc abrasive side down on a work surface and find the center. Draw two radial lines at right angles from one another. Then measure the side of your cone. Add 1/4" to that (this lets a little abrasive fold over the nose if you need use just the nose for bowl bottoms). Subtract the total from 2-1/2" and draw a circle with that radius as in Fig. 18. Cut the abrasive on the lines as in Fig19.
Now apply the abrasive to the foam cone, trying to match at the widest diameter, and overlap it on the side as in Fig20. Trim the overlap as in Fig21, leaving about 1/4" of overlap. You can now use the trimmed abrasive as a template for any future ones.
Mount the foam cone with abrasive in a power drill. You can use any drill you like, but if you have an air compressor, I recommend using air powered drills. An environment of sawdust and abrasive is not good for electric drills that suck in outside air to cool themselves (I ruined a really nice close quarter drill this way). Air drills, however, use the clean air from your compressor for cooling and will be none the worse for sanding duty provided you remember to oil them occasionally. You can mount the cone with the large end close to the drill for most uses as in Fig22, or unscrew the mounting stud and screw it in the other end to mount the cone with the large end out as in Fig 23 for hard to reach areas. Reverse the overlap when you do this.
To maximize sanding efficiency when using the Foam Cone Sander ensure that the bowl surface and the abrasive surface are moving in opposite directions at the point of contact. Depending on your lathe and drill you can reverse either if necessary. Also make sure the overlap is trailing. Try to do most of the work with the side of the cone, not the rim or nose. The following figures show several applications in sanding a bowl.
Fig24 shows sanding the foot of a bowl in close to the chuck. The cone is in the reversed position, and a right-angled drill is being used.
Fig 25 and Fig26 show sanding the inside of a bowl. The cone is in the regular position and the side of the cone is in contact with the work. The foam gives enough to allow the abrasive to adapt to the curve and make contact over most of its side.
Fig27 shows sanding the foot of a bowl. In this case, little but the rim is making contact and some swirling is present in the pattern. In Fig 28, which shows sanding on the outside of the bowl, the whole side of the bowl is again making contact and the abrasive pattern is straight.
Tools and Materials
3/8" x 16 tap
2-3/4"D x 1-1/8" wood disc
2-1/4” of 3/8 x 16 all-thread or 2-1/2” 3/8” bolt
2 each 3/8" x 16 nuts
2mm craft foam
3M Super 77 or other spray adhesive
Industrial Strength Sticky Back Velco
5" hook and loop abrasive discs
David Reed Smith lives to turn and tinker in Hampstead, Maryland. Regular readers may be surprised (or relieved) there was no blue tape visible in this article, but be assured there was some behind the scenes when the bowl was reversed. This article, along with perhaps a supplementary video will be available at www.DavidReedSmith.com along with 60 or so other articles. He welcomes comments and questions at David@DavidReedSmith.com.