I am trying to make a "bowl from a board", where concentric rings are cut from a board with an angled blade, then "reverse-stacked" to make the rough shape of a bowl, then finally turned on a lathe. One technique involves cutting the board in half, cutting out "half-rings", gluing back the cut half-rings back into full rings, then stacking the rings into the bowl blank by gluing up the stacked rings.
Here are some pre-pictures of this half ring setup:
I have seen (and built) a band saw circle cutting jig that require the workpiece to have a dowel/pin of some sort attached to/inserted in the center so that the workpiece can pivot around its center while the blade automatically makes a nice circular cut. However, in the case of half-rings, half of the "center" of the workpiece seems to be missing, so there is no way to insert/attach a pin. I saw one method that glued an auxillary piece (the pine in the pictures) of the same thickness onto the edge of the workpiece so that the center "reappeared". Then the circle cutting operation is performed, and the auxillary piece removed (how? as now since you have a semicircle you will not have a parallel edge to run against a table saw fence, or a perpendular edge to hold against a miter gauge to sand off the piece, etc.)
Postponing that problem until later, I went ahead and epoxied a 1/2" dowel onto the "center" of my workpiece+glued auxillary piece. I cut a ring, and it worked perfectly! Then I cut a second ring, and the dowel broke off (and since I was holding it against the table, I didn't realize what had happened until I ruined the workpiece, grrr).
Finally, to my question .... is there a trick that I am missing to this method? To anyone who uses it - how do you attach the dowel/pin to the half-workpieces? I had not recessed the dowel, as I didn't want to damage the workpiece by drilling holes in it, but maybe this is required and then you have to remove enough material to remove the hole later in the process?
It's funny you say that - I tried this first on a scroll saw, but found it to be EXTREMEMLY hard to (freehand) keep the cut even within a 1/4" of the line, while the bandsaw (with the circle cutting jig) cuts a perfect half ring. There was also the problem of getting the scroll blade through the workpiece, which required drilling a hole that requires a reducion in the final thickness of the turned rings. Were you suggesting cutting freehand on a scroll saw? Or using the same type of pivot jig? If the later, my question about how to mount the pin stands
Cut it freehand on the scroll saw. Yes, it takes some practice. If you had trouble keeping to the line I bet your blade tension was too low or your feed rate too high. Use a drill bit that just fits the saw blade. Smaller than 1/16" I have done just a few of these with a scroll saw but others have done a lot of them that way. Pretty wood you have there. Will make a handsome bowl.
I have a crazy technique that seems to work for me. For the center pin, I use a pop rivet - 1/8" works. I drill a very small hole that fits the pin of the pop rivet and a 1/8" hole for the rivet itself in the jig. You can cut off the pin a little above the board if it gets in your way. But the pivoting hole is much smaller and as long as you don't put too much side pressure on it, it works quite well. I use the same method for turning on a faceplate. Always keeps my turnings centered, even if I pull them off the plate and re-attach. If the pin does bend, just pull it out and put another pop rivet in the jig.
Ddockstader - that sounds reasonable. Do you glue on an auxiliary piece to give you the center at which to drill the hole? How do you cut it off before gluing the middle half rings (the bowl base) back together?
i'm a little confused. If you are going to turn it on a lathe, why is it so important to stay to the line. I agree with Jim Finn. Tighten the sucker (blade) up. You should be able to keep reasonably close to the line.
Randy - every bit you miss the line by you double it to get the amount of wall thickness you have lost (as it affects 2 rings). It is not a huge problem when the rings are as thick as I've shown here, but if I want to move to a steeper angle preserving thickness is very important.
That is a great video, thanks. I had been screwing my slider into the base each time, but the toggle clamps are a much better idea. However, I didn't hear him say anything about the pivot point of his jig. It seems to have the dowel under the surface of the hole only, and then a HALF dowel above the surface. I wonder how he cut that? I also wonder how he is attaching his workpiece to the small piece of wood with the dowel+half dowel - could it just be double sided tape?? How do you do it?
You are right, the dowel goes through the base and that is the pivot. To cut the dowel exactly on center, I drew a line along the length of a piece somewhat longer than the "carrier" would end up being. I didn't want to be playing with short pieces on my table saw. I drilled a hole through that board centered on the line. I glued a piece of dowel into the hole, making it proud of the surface of the board by slightly less the thickness of my base. My base is 3/4 and the dowel is about 5/8. Then, I ripped the board along the line, cutting the dowel down the middle, too. Finally, I cut the board to length, with the center of the dowel at the center of the final length of the board.
I made two of these. That way, I can cut one half ring, then do the other half using the second carrier, without moving the slider. That way, they must be the same diameter.
As you guessed, I use double sided tape to hold the pieces to the carriers.
Perfect - I am excited to try it this weekend! (I've screwed this up enough times that this is my time to get it right ).
What is the diameter of the dowel that you used? I had been using 1/2".
Also, just to make sure I understand the geometry correctly, when you rip the board+dowel in half, it will still be attached by a partial circlular cross-section piece of the dowel, right? That is, you'd have to move the fence and cut again, a few times, until you've removed all of the material of the half-dowel before the off-cut will actually separate and leave you with the finished jig, right? And you also have to make sure the height of the blade is set to pretty much exactly the height of the board or you'll end up with less than the full thickness attached with the tape? (If these questions are not clear as just text, I'll take some pictures as I'm doing this to illustrate what I mean. I think pictures are a good idea anyway because I looked for quite a long time and was unable to find exactly how to do this. Whenever you find a hole in the knowledge of the internet, you must fill it! ).
You're right and my explanation left a lot t be desired. Let me try this again.
You glue a dowel into a board, through a hole that is centered on the line you draw along the length of the board. You set the height of your table saw blade to cut just through the board. it can be a bit higher, but you want to leave a good length of dowel in tact. Then, you need to cut through the part of the dowel that is holding the two pieces of the board together. You can do that by nibbling away the board, use a flush cut hand saw, or flip the piece onto the side perpendicular to the cut you just made and cut the board and the dowel.
As to dowel size, I don't believe there is any magic. I used 11/16 because that's what I had.
I think that I'm attaching a couple of pics of the final carrier. The face that you see is the one that the double sided tape sticks to.
Ah, yes, a flush trim saw would work too (and seems safer). Thanks for the pictures and the clarification. Now all that's left is to trust my double sided tape! Do you have a favorite style/brand that you use for this? I've successfully used some double sided duct tape (Shurtape Hold Strong) for holding down router templates, etc., but it seems like there it would be a lot easier for something to slip with this small of a tape surface.
David: I use the turner's double sided tape that Lee Valley sells. Their HQ store in only 5 minutes from me, so it's an easy sell. I'm sure that there are other brands/types that would work. The cutting does not put a lot of stress on the joint.
I came up with a much easier method when I wanted to make half-bowls using the scroll saw. I believe the same concept can be used on the band saw.
I cut a square notch in the middle of a long edge of the stock rectangle and screwed in an eye-screw until it's center was even with the edge. The pivot was a machine screw, inserted in a nut that was epoxied into a sliding base on a line even with the front edge of the blade. The sliding base slides perpendicular to the tilt axis of the saw's table. The pivot nut is always in line with the front edge of the blade, regardless of the radius set by the sliding base. (The screw/nut diameter is matched to fit with the eyescrew hole. The screw part of the eyescrew needs to be long enough to remain secure during rotation of the workpiece around the pivot screw. )
The last 2 pics of this project show the jig and the cuttings made with it: http://lumberjocks.com/projects/65803
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