cut bowl angles

Re: cut bowl angles *LINK*

Mark Kauder. Phenix City, AL

>Jack,

I have some experience making segmented bowls. However, I have never used a radial arm saw to do it (I use a sled on a table saw). Segmented bowls can be made with Bandsaws, TS, CMS, etc.

Regardless of the method, most a segmented turners will tell you that there are no factory settings on these tools or their miter gauges that come out of the box with the setting accurate enough to make a 12, 18, or 24 sided circle without leaving gap somewhere.

What you have to do, is to make a test ring using the stock setting, and then adjust from there. Keep testing until you have it perfect, and then lock it down. This is easier to do with a sled on a TS, because most of us are not willing to dedicate a tool to just that operation, and instead would have to do test rings every time they wanted to do a bowl.

What I would suggest with a radial arm saw, is once you find the proper angle, cut a 2" thick piece of hardwood with that setting, to use as a setup jig for the next time you make a segmented bowl.

Feel free to ask more questions here or email me directly.

Mark


Mark Kauder's Web Page

Re: cut bowl angles

Lan Brady

>When gluing up the rings, I glue 1/2 at a time. Then sand the 2 halves flat and glue them together. The slightly smaller segments are hardly noticable.

Re: cut bowl angles

Al Crandall

>Precise angles are obtained in the tooling trades by use of a sine bar to set the angles. I have never tried segmented turning, but I have set a lot of sine bar angles. I wonder why a sine bar carefully made from a good dry hardwood wouldn't give you the needed precise angles. The "jo block" stack up for each angle could be made from wood also and carefully tweeked by sanding or adding paper or cellophane shims to get the required angle. For that matter, I bought a "real" set of 81 new jo blocks on ebay for around $62 if memory serves. They are dimensioned in one ten-thousandths of an inch increments and are accurate to less than +/- fifty millionths and should be close enough for wood working. We spend more than $62 on one gouge. And jo blocks will never wear out.

Re: cut bowl angles

Keith Tompkins

>Hi Al, could you post photos and a description? sounds great.

Re: sine bar 101. Hang on, it's long!

Al Crandall

>Kieth, I don't speak html, but a google search for "how set up sine bar" produced this hit:

http://claymore.engineer.gvsu.edu/~jackh/eod/manufact/manufact-130.html

A search for jo block or gage block might produce an image and some more information.

Let the treatise begin.

The sine bar must have two rolls of "exactly" the same diameter and a known distance between them. The greater the distance, the greater will be the accuracy. For segment cutting I would imagine 12" or 15" between centers of the rolls would work out great. The top surface of the bar must be "exactly" parallel with the centers of the rolls. For woodworking purposes I would bore two holes in a straight piece of wood with the centers parallel to an edge, and the distance between the holes held as accurately as anything you have ever done in woodworking. Find or make two hardwood dowels to fit into these holes as tightly as possible. On, the table saw rip enough of the closest edge off to expose a small portion of the holes. The dowels are going to be held in place eventually by cross drilled and countersunk screw holes, because you are going to rip again enough off the edge to expose just slightly more than 1/2 of the dowel. Now is the time to determine, to whatever degree of accuracy your shop is capable of, the exact distance between the hole centers. If you were shooting for 12" c-c and got 12 1/64", that is ok. It will just make the math a little different when figuring the jo block stack up. By the way, the term "jo block" derives from the name of the first person to come up with a set of standard precision set up blocks for tooling use, and his last name was Johannsen or something similar. Anyway, mark indelibly on the bar the length of its c-c distance. Cross drill and countersink the dowels and holes for the mounting screws. Remove the dowels and rip the bar again so that less than 1/2 of each hole is left. Install the dowels and screws for the last time. When the bar is set on a flat surface like your lathe bed, it should sit firmly on the entire edge of both dowels with no gaps or rocking. The last thing to verify is that while it is sitting on the rolls, the top surface is parallel in two axes to the plane it is resting on. If all this checks are ok, congratulations. You have a sine bar. To increase utility, you might have to cut a chamfer, on the ends, which comes close to the half holes so that when the bar is tilted the end does not touch and cause the dowel to loose contact with the registering surface.

The sine function in trig is the ratio between the hypotenuse and the side opposite the angle in question. If we know the length of the hypotenuse, then the length of the side opposite is the value found in a trig table for the given angle. The trig tables are formulated with the value of the hypotenuse always at "1" unit. So if we want to set the bar at an angle of 30 degrees, we look in the trig tables for sin 30 and find a value of .5. Any right triangle with a 30* angle will have a side exactly 1/2 as long as the hypotenuse. Likewise, for a 45* angle, we find a value of .70711, and this would be the length of the side providing the hypotenuse were 1 unit in length. But our hypotenuse (the center to center distance of the bar) isn't 1 unit long. We made it 12", or 15", or 12 1/64" which we will round off to 12.016. So all we have to do to find the proper stack up to place under one of the dowels is to multiply the sine value from the trig tables by the length you marked on your bar.

In use, the sine bar would be placed on its side on your cross cut sled with one of the dowels (in the toolroom it would be called a roll) firmly against the fence of the sled, and the other roll would be pulled tightly against the stack up to trap the stack up against the fence. At this time I would suggest some means of mechanical clamping to insure that the bar doesn't move and change the angle you worked so hard to accomplish. If your fence isn't dead square to the blade your angle will not be what you calculated. If this happens, you have two choices: improve the accuracy of the sled fence, or fudge the stack up to get the angle you want. If you choose the latter, the fudge factor will be a constant that you will need to add to the stack up each time you calculate a new angle. I would prefer a dead square fence myself. A sine bar can set an angle to degrees, minutes, and seconds, accurate to a portion of a second with repeatability day after day. Probably more than you wanted to know, but you asked.

Re: cut bowl angles

John Lucas

>Mark

I found the thickness of the wedge is also critical. Just getting the angle right is only half the story. If even one wedge is slightly longer than the others the circle won't be complete. I use a variety of homemade stops on my table saw and miter saw to get the cuts the same size.

I use a block of wood with a bolt threaded into it. This way I can fine tune the distance if necessary.

When I make my 80 segmented rings for the mirror inlays this still isn't accurate enough so I glue them together in sets and check them at 45, 90 and 180 degrees and true them up if necessary with a sanding block bolted to the end of my workbench. This is set exactly 90 degrees to the bench. I still have to be carefull no to round over the edge but it works pretty well.

Re: sine bar 101. Hang on, it's long!

Keith Tompkins

>cool, thanks Al!

Gage Blocks 101

Al Crandall

>I did a search like I suggested for gage blocks and was surprised at how complicated they made it seem to get the blocks to "wring" together. In a shop environment, just take the two blocks and pass the surfaces which will be mated over the skin on the inside of your wrists. This will clean them and oil them sufficiently to allow them to stick together. Most people (you can substitute the word aprentices here) are amazed at the power which holds gage blocks together when properly wrung to each other. Start with the two blocks contacting at just the corners of the lapped surfaces and slide them onto each other while giving them a slight twisting motion. Slide them about half past alignment and then slide them backward into position. They will be amazingly "locked" together. Take them apart by twisting and sliding in a reverse sequence. The blocks will act as one piece of steel when they are stacked and you won't have to worry about multiple block stack ups falling apart.