Segments not aligning after cut...

[andrew zukewycz]

247861
i made some nice segmented bowls as xmas gifts... however, i just cant figure out how to get the segment halves to lign up better so i dont have to sand so much...

currently i do the following steps;

1-cut segments
2-glue up (2 halves each with 3 pieces) for a 6 piece level
3-sand each half on belt sander so each half is true
4-glue each half together
5-plane it flat
6-glue all levels together and to base
7-turn round

my math is - 360 devided by 6 pieces = 60...then devide that by 2 and you get 15 degrees...

so i cut 15 degrees for each side... but each time i glue up each half, i end up with greater than half a circle... do i just need to use some scrap pieces to dial my saw in to the exact angle, or is my math or technique wrong...

im just trying to save the time it will take to figure it out by asking for some operating experience.

[andrew zukewycz]

also, if it makes a difference i use a chop saw for the cuts.

[Richard Coers]

chop saw is not accurate enough. You can make a jig and use a stationary disc sander to touch them up, or a sled on a table saw. A lot of segment guys glue up two halves and then trim those. You then hold the halves on a jig and run that through the table saw. Then glue those two halves together to get a full circle.

[Curt Fuller]

Andrew, are you cutting the segments from long strips of wood? And what are you cutting them with? When I cut segments I first rip my wood into long strips with the width and height as consistent as possible. I now have a planer so I run them through the planer to get an even consistency in both width and height. Then I cut the segments on a miter (chop) saw by cutting one, clamping a stop block to the back guide of the saw for consistent segment length, and then just turning the wood over for each cut. In Malcolm Tibbetts book he says he prefers a miter saw but many use a table saw with sled. To get as close a fit as possible, the saw needs to be set at exactly 30 degrees for 6 segments. My saw has a stop at 30 but it's just enough off that I get segments that are a hair too big of an angle. So I don't use the stop. I made a mark of my own to show exactly where I want it and then tighten it down there. But even then I still have to sand the two halves just a little to get them to fit with a good joint. One of the things that also makes it hard to get the segments glued just right is too much glue. Because it's so hard to clamp them I rely on finger pressure to hold the segments. Too much glue and they squirm around so much it's hard to keep them aligned. And too much glue also holds them slighty apart. I'm really not a segmenter in the true sense of it but those are a couple things that help me. Malcolm Tibbett's book 'The Art of Segmented Turning' is an excellent resource for learning to cut and glue and everything else about the art of segmenting.

[John Beaver]

Chop saw is fine. (50/50 on which way is better)

I think the best bet is to use some scrap wood and dial your saw in. Just make 1/2 a circle, tape the segments together and set them against a straight edge, then adjust the saw accordingly. Might take 2 or 3 tries, but well worth the time.

[John Keeton]

I don't do segmentation, but your bowls look nice! And, this is somewhat off topic, but I was just curious as to any concerns you may have about wood movement given the large, non-floating, bottom. It would seem that might be an issue over time.

[Steve Rost]

Andrew, You say "greater than half circle" my brain is a little slow this morning but I don't know exactly what you mean. But to go on, is the math formula you are using calculating the length of the arc, not the segment length. Length of the arc will be longer. This will give you a circle larger than you expected. Your saw is accurate enough but you need to dial it in with scraps. You may have done too much sanding and cut less than a 180 half. Lots of variables....

[Michelle Rich]

.My best guess is your cuts are not precise enough. You must get your segments cuts perfect, and each half 180. when you are sanding your halves to get them to glue up, you are sanding the pieces to an oval. Take your time here and set your saw. Don't think sanding in halves will work, and do a lousy seg cutting job.
When I divide 360by 6 I get 60 then divide by 2 I get 30!!! If you are cutting at 15 you are making a 12 segment row.
Get one of the new digital protractors and set your saw up. I never had one until recently & made many many segmetations. Today i check with the protractor..how easy today it is for segmenters.
John is correct about the bottoms, but not at the size you are working with..you are ok as you are

[Thom Sturgill]

I just built the table saw sled shown in the current 'Woodturning Design' and after a little tweaking produced a 12 piece ring with no visible gaps when held together with rubber bands. I would modify the build instructions slightly if I were to build another, but it is a good design.

In addition to Malcom's book mentioned above he also has some video available. Curt Theobald has an a video (poor sound quality, but excellent info) that details a jig for a 12" disk sander to allow you to correct each piece of a ring before glue up. consistency in both length and angle is critical. Even very small errors in angle add up when you have large numbers of them, for example 24 cut angles in a 12 piece ring. So half a degree error would show up as 12 degrees total or 6 degrees on each half circle. If the total correction is large enough to be visible you may end up with a segment that is visibly shorter than the others, so make sure that you tale some off both ends to pread the correction. Hope that makes sense....

[Bill Bulloch]

If you sand in halves you will never have a perfect ring; the ring will be oval and will never align perfectly. For this reason, I always take the time to make sure that I am cutting my segments at the right, precise angle before gluing. I use the Incra 1000HD Miter Gauge and take the time in making adjustments until I get it right. I do not sand each segment. I dry fit each ring before gluing and check the alignment. Occasionately, there will be a misalignment caused by some mishap while cutting. If the gap is very small, and the ring is not part of a feature ring, I will glue in halves, otherwise I recut the ring. So, whatever method you use to cut yours segments take the time to make sure your alignment is exact -- why proceed with a project that you know you will not be satisfied with when completed?

[Steve Rost]

BINGO** I think Michelle found the source of Andrews original problem *math* Gotta make all your cuts accurate and repeatable. Early this year I built sleds for my table saw and I make 20 segment rings with no gaps on the first try. I assemble my rings now in one step, no half rings. When you go to feature rings a miter saw works.

[Montgomery Scott]

Andrew, You say "greater than half circle" my brain is a little slow this morning but I don't know exactly what you mean. But to go on, is the math formula you are using calculating the length of the arc, not the segment length. Length of the arc will be longer. This will give you a circle larger than you expected. Your saw is accurate enough but you need to dial it in with scraps. You may have done too much sanding and cut less than a 180 half. Lots of variables....

The opposite is true. The segment length is longer than the arc length.

The equation for arc length is: D*pi/n
The equation for segment length is: D*tan (pi/n)

[David C. Roseman]

The opposite is true. The segment length is longer than the arc length.

The equation for arc length is: D*pi/n
The equation for segment length is: D*tan (pi/n)

Hmm. Montgomery, I'm unburdened by any talent for math, but I do know that the shortest distance between two points is a straight line. So given two points on a circle, the arc length of the included angle must always be greater than its segment length. I think that's what Steve means. Am I missing something?

Andrew, beautiful work on the bowls. Haven't tried segmented turning yet, but I've done flat work for many years, and made hundreds of miter cuts for furniture and cabinetry. A carefully made sled on a table saw is great, but for the few simple cuts that you're making (they're 30, not 15 degree), I actually prefer a tuned-up chop saw. Agree with other posts that your issue is probably just a matter of being slightly off in the segment length or angle, maybe exacerbated by too much glue. As long as the saw is set precisely to 30 degrees by test cuts on scrap, and stop blocks are used for repeatability, this should work fine. Sanding to fit shouldn't be necessary, and for me seems to introduce more rather than less imprecision.

David

[Montgomery Scott]

Yes, you are missing a critical point. The edge of the segment represents the tangent point of the radius. We're not measuring the distance between two points on the curve but extending the cut angle past the curve to the edge of the segment. I am not using the mathematical definition of segment but the turners definition; the distance along the length of the piece from corner to corner.

[Steve Rost]

Montgomery, help.... now I am confused. I cannot see how the segment length is longer than the arc between the same two points...

[Montgomery Scott]

It is not. Your polygon is drawn so that the circle is inside the polygon and the tangent points on the circle touch the mid-point of each segment of the polygon.

http://marleyturned.com/Shop_Utilities.html Check the formula I provided against the link you are familiar with. It is the same formula he uses.

[Steve Rost]

Montgomery, I found Marleys site several months ago. The calcualtor he provides is Dead-On-Accurate.. I have built rings up to 16 inch diameter and down to 2inch. Thanks

[Montgomery Scott]

Yes, it is dead on accurate as it uses the formula I provided.