LumberJocks

Turning #1: Segmented Bowl Process

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Blog entry by OSU55 posted 04-07-2015 09:38 PM 3534 reads 5 times favorited 6 comments Add to Favorites Watch
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This tutorial is about the process I use to create a segmented bowl, or about any segmented turning item. After researching many methods and trying many of them, I’ve been able to put together a fairly simple and robust process that uses as few specialty tools or jigs as I could. Where possible I used existing jigs and tools that I already had and others may also have.

#1 – Design

Once you have the approximate size, shape, and feature designs of the end item to build, the number of ring layers and segments need to be determined. There are several approaches to this, but I like to use the software program Segmented Project Planner (SPP – I did a review here). I find it allows me to change the design all I want very quickly (compared to manual methods). Ring layer thickness and the number of segments are variable, and the program generates a cut list with all pertinent data to know material thickness, width, and segment length. I will mention that if you want rings less than ½” tall, glue the thinner material to the material for an adjacent ring prior to cutting segments (allow for a little trimming in the material width for post glue clean up). Trying to glue segments less than ½” is just too much of a pita.

Think through what your process will be at each step and make sure your equipment can handle the size (if you plan to use a 12” disc sander to flatten ½ ring ends, then the largest ring is limited to ~12”). If it’s a long/tall item, such as a vase or lamp, it probably needs to be made into upper and lower halves, inside turning completed, then assembled prior to OD turning. You also need to determine how the bowl will be held to the lathe spindle and accommodate the design and process for it. I used a ¼” deep tenon cut into the base layer of the bowl in this tutorial.

#2 – Material Prep

SPP provides a cut list by layer for the material. I find that my planer leaves an adequate top and bottom surface and I don’t need to plane or sand them, but that’s going to be an individualized decision based on planer knife condition. Those surfaces will be worked again after the rings have been glued up. The sides of the material do need to be straight so they register properly against a fence when cutting segments, but surface finish is irrelevant – it will all be turned off on the lathe. I usually run a hand plane down each side of the table saw ripped surface to check straigntness. Perpendicularity to the top and bottom surfaces is not critical. I generally cut the stock a few inches longer than the cut list value to allow for a safe distance from the table saw blade, and add a bit to the width to allow for any clean up needed.

#3 – Segment Cutting

There are plenty of places to find details and equations to determine angles, lengths, etc. online. I use the SPP info. I’m a big TS sled user and not much of a miter gage user. For the 1st segmented bowl I did, I thought I would give the miter a try, and then build a sled if needed. I didn’t need to. In the picture below is the Bosch 4100 OEM miter with a shop made fence, with the angle being set with a giant protractor. This gets me very close, and I’ll make tiny adjustments if needed as I cut layers. I never get it perfect, but so close that an adjustment throws it out the other way. I did come across a “wedgie” that someone makes that is for a double fence sled. I’m sure it works, and it would take out the step of straightening ½ rings (covered later), but I’m not running a production shop. In the overall scheme of a project it isn’t a great time saver.
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The picture below shows setting cut length on the saw. I take the SPP generated segment length and multiply it by the Cosine of the segment angle and add the saw kerf in, then measure with calipers. The “stop” in the picture is a thin rip guide with the bearing removed (I already had the guide). The adjustable “fence” can be snugged in place and the miter guide will still move in the slot. A piece of wood with double stick tape works as well, just put a mark on the table inline with the blade to measure from.
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Picture below shows everything set to cut a segment. After each cut, the material is flipped upside down, resulting in angled cuts on each end like a pie. This is using the “economy” method. There is another method for grain matching that I won’t cover here – it can be researched online.
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I find I do not need to sand the ends of the segments. With a sharp blade and the blade set perpendicular, the ends are flat and “square” enough for gluing. I just knock off any tear out on the segment ends so it doesn’t end up in a glue joint. After the 1st layer, lay the rings out and butt them together to check the cut angle. It will rarely be perfect and doesn’t have to be. I will usually go with ~0.020” gap between ½ rings without angle adjustment. I find I start chasing the gap from inside to outside trying to get closer, and it doesn’t really matter unless you are making 48 or 96 segment rings

#4 – Segment & Ring Glue up

I glue up ½ rings first, then the whole. Reference the pic below. This method ensures tight joints, and accumulates angle error at the ½ ring gaps, which is addressed in the next step. The small amount trimmed from the segments at the end of each ½ ring is not noticeable – don’t tell the admirer of your project and they will never know.

I lay out the segments, 12 in this case, with a ½” dowel to separate the ½’s. Use hardwood dowels – softwood can collapse, unevenly, giving a poor glue up. I apply glue to each segment end for the entire ring at one time. I go around the ring twice, in order, to apply glue. It is end grain and will absorb more glue. The ½ ring ends are left dry. I then assemble the ring with dowels in place using stainless hose clamps. I use a cordless drill and set the clutch on 4, not a real tight clamp. Many use rubber bands or inner tubes. Have a soft blow hammer or a mallet ready to tap/hammer segments in to final position. Segments need to be level on top/bottom and the outer corners aligned. Sometimes I have to let off clamp pressure to get things lined up, which is why I like the hose clamps. All of the hose clamps get a good coating of wax to prevent glue from sticking. I use Titebond III, and any quality wood glue will work. I like the longer open time. I use freezer paper, with plastic on one side, under the rings to keep the glue from sticking and to make clean up easy – throw it away instead of scraping glue off the bench. Glue will get everywhere, and that’s ok. It will all be machined off the piece.
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After a couple of hours the glue has set enough to make the ½ ring ends parallel. Many use a disc sander, which I don’t have. I have used a shooting board and plane, but easiest method I have found is to use a sled on the table saw. I already had the sled in the pic below, a Charles Neil design taper leg sled, available on his website. Just line the ends up, clamp, and cut.
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The two ½’s are then clamped in hose clamps again to make a whole ring. The pic below shows a full ring, except for the center. If your base ring will be segmented like this one, DO NOT try to get the points to come out perfectly. It is an exercise in futility. This ring has about a ¼” hole left in the center, which will be drilled out to a ½” for my clamping fixture. You can see how it is not perfectly round. The hole is plugged prior to turning. I don’t have a picture of it, but on one side of this base ring I cut a ¼” deep rabbet on the lathe, creating a tenon, to grip in a chuck.
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. #5 – Flattening Rings
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The top and bottom surfaces need to have the glue cleaned off and need to be flat enough to make a good glue joint (you decide how flat that is, everyone has their own opinion). I try for ~0.10” on each surface determined by a straight edge across the ring. There are several methods, such as disc sanders and sanding discs on the lathe. I use a couple of methods depending on ring wall thickness.

For full rings like above, or rings with wall thickness more than ~1-1/2”, I use a planer. First though, using flat jaws, I will flatten one side of the ring on the lathe so it won’t rock on the sled. I am not good enough to get them flat enough for gluing directly off the lathe so I will plane the lathe turned side also. I’ve used sandpaper on the lathe, but since I’m going to plane anyway, it’s cleaner and just as fast to plane them. For thinner rings, I can get them flat enough on the lathe using scrapers and sanding.

Planing does create a little chip out when exiting a segment with grain parallel to the blades, but 1) it’s minimal, 2) I locate the ring with a joint perpendicular to the planer blades, so the grain is at a slight angle. I use a sled made of coated particle board for shelving. I use double sided turning tape to hold the rings down. About 4 x 1” long x ¾” wide pieces of type have been enough to hold the rings. I use 4×4” long “rails” (pieces of wood), 2 at the front and 2 at the back, overlapping a ring by ~1”, to set the planer cutter head entering before and leaving after the rings. These are also held by turner’s tape. I will plane multiple rings at one time that are within `1/16” thickness, but they need to overlap one another or snipe will put a dip in the rings. Sometimes I’ll use “rails between rings. I allow the rings to dry overnight before flattening.

EDIT: I have improved my ability to flatten wide rings on the lathe. I use a 1” flat edged scraper with a very slight radius and a very flat board ~1-1/2” wide x ~ 14” for sanding. The scraper is used 1st to get all the glue off and as flat of a surface as I can. I then use the sanding stick with a standard 9×11 sheet of sandpaper, 60-80 grit, and fold the paper around the front and sides of the stick and just hold the paper on by hand as the flat surface of the stick is pressed to the surface. Lathe speed 600 rpm, the lowest mine will go. I use a raking light placed under the ring and a steel ruler to check flatness.

#6 – Bowl Glue Up

You want the least amount of run out of the bowl after glue up. There are several methods used for aligning the rings concentrically, one being to use the lathe and a cone, as well as several approaches to shop made presses. I didn’t want the lathe tied up for gluing, and wanted something effective but cheap. After a few iterations, here is what I came up with.

I used a couple of 1’ square pieces of the coated shelving material for the press, with a ½” hole in the center. A 12” piece of ½” allthread rod is secured in the center hole of one piece with nuts and washers, making it perpendicular to the board. The pic below shows the press with an 8 layer glue up. On top the small square pieces of wood are just spacers so the nut doesn’t have to be ran down all the threads. The bowl is positioned upside down when built up on the press, one layer at a time. The small pieces of wood located on the bowl rim and sitting on the bottom board of the press are to hold the 1st ring in position as the other layers are added. They are held in place with turner’s tape.
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Below is a pic of the cones used to radially align the layers for concentricity. These are made from ¾” MDF. For each cone, I marked and rough cut two discs of each size with a jigsaw, then glued them together. I then used a circle jig on the bandsaw, with the table tilted 45°, to cut each cone to size. Each cone was then mounted on the lathe, and the cone cleaned up, sanded, and well sealed with shellac to harden the surface and resist absorbing liquid. A coat of wax helps them slide and prevent glue sticking to them. A ½” hole was drilled in the center of each to locate on the allthread shaft in the press, and a larger hole drilled halfway through the disc to clear the nut holding the shaft to the lower board of the press when locating the 1st layer.
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The pic below shows the press with a cone in place. You can just see the upper rim of the cone sticking out under the upper board of the press. I mark each layer with the outline of the mating layers to know where glue needs to be applied, and then 2 segments, 180° apart, are marked in the middle. Each layer is offset a ½ segment, aligning the ½ way segment marks with a glue seam, to give a “brick layering” construction. This provides a lot of structural integrity to the shape. I apply glue to both surfaces of the joint, lay the ring layer on the press, put the cone on, the upper board, spacers, washer, nut, then turn the layer being glued back and forth to spread glue and start seating the layer. I use levels set perpendicular to each other on top of the upper press board to make sure the cone is level and properly centering the ring. It’s an iterative process as I tighten the nut on top. Once the nut is snugged up, I let it sit for at least 5 minutes, then move on to the next layer. By the time the next layer is added and being moved around, the previous layer has been in contact for over 10 minutes, has taken a good set, and doesn’t move. You can stop at any level and wait for a later time, but put the press together and apply pressure to all the layers assembled to that point before leaving it. After all the layers are assembled, I snug the press down well and leave it at least an hour. I do like to let the glued up assembly sit out of the press unrestrained for 8-10 hours minimum (overnight) to let the glue dry and stresses to relieve.
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. #6 – Turning The Bowl

Here is the glue up mounted on the lathe ready for turning. I didn’t have much vibration at all from this glue up. It was well within 1/8” run out, not bad for 8 layers about 6” tall and 13-1/2” in diameter. As you can see, I don’t worry about excess glue. It all gets turned off in the process, so I don’t waste any time concerning myself with it. I highly recommend wearing a face shield during the roughing process. The glue chips can hurt when hit in the face with them. While bowl gouges can be used for roughing, with the interrupted cuts and the dried glue, I prefer carbide inserts (reviewed here and here). Scrapers can also be used.
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Here are pics of the finished turning, and the finished bowl. I use a combination of bowl gouges and scrapers once the glue is gone, and power sanding as needed. The wood is Walnut and Soft Maple. The bowl was dyed with Transtint dye in Target EM4000 stain base, and finished with a light coat of thinned oil based poly on the lathe.
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6 comments so far

View lew's profile

lew

11340 posts in 3219 days


#1 posted 04-07-2015 11:40 PM

Super write-up, Thanks! And thanks for the link to the software, too.

-- Lew- Time traveler. Purveyor of the Universe's finest custom rolling pins.

View TheWeiss's profile

TheWeiss

50 posts in 700 days


#2 posted 08-12-2015 08:30 PM

Indeed. I enjoyed this write up quite a bit as well. Much thanks and great work!

View SteveGaskins's profile

SteveGaskins

647 posts in 2051 days


#3 posted 08-30-2015 07:42 PM

Exceptional tutorial. Well stated. Added to favorites.

-- Steve, South Carolina, http://www.finewoodworkingofsc.com

View Graybiki's profile

Graybiki

1 post in 383 days


#4 posted 11-20-2015 02:50 AM

Great tutorial. To your comment about the “wedgies”. I have used them and they are fantastic. I built the sled and the stop and it works perfectly. Even my first test ring came out perfect. It was hard to believe. The secret is that the second fence forms an angle that is complementary to the first fence angle to get the total angle of the wedge. It is amazing.

View Grumpy's profile

Grumpy

21564 posts in 3315 days


#5 posted 02-17-2016 10:40 PM

Great blog OSU55 & excellent result.

-- Grumpy - "Always look on the bright side of life"- Monty Python

View Ken90712's profile

Ken90712

16955 posts in 2653 days


#6 posted 02-27-2016 03:29 AM

This was great , thx for taking the time to teach us guys new to turning. Loved reading this.

-- Ken, "Everyday above ground is a good day!"

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