Woodturner's Resource
Woodturner's Resource  
  Featured Artist    Websites   Support Wr
Tutorials, Projects & Tips   Event Calendar   Tool and Book Store
  Home Page Forum HelpSearch Map TPT Resources LoginRegister
 
Page Index Toggle Pages: 1
Send Topic Print
Seg Bowl bottoms (Read 241 times)
 
Bruce Kamp
Active Member
***
Offline


WR Rocks!

Posts: 199

Seg Bowl bottoms
Dec 25th, 2017 at 12:06pm
 
What do most of you do with the bottom of a segmented bowl? I have had varied luck with getting a good enough cut from the wedgie sled to have the points meet. It seems the larger the diameter the more difficult it is. I have been resorting to using plugs. Problem is I then have end grain in the center of the bottom.
I have used solid pieces  and a floating base but do not care for the look.
Thanks
Back to top
  
 
IP Logged
 
Register To Remove Ads
Ed Weber
WR Global Moderator
WR Patron
*****
Online



Posts: 4,931

Wilton, California, USA
Wilton
California
USA

Gender: male

JET 1642
Grizzly G0584
Re: Seg Bowl bottoms
Reply #1 - Dec 25th, 2017 at 12:17pm
 
To make a base from segments and have them meet "perfectly" (no gaps of any kind) you must construct it in stages.
You'll need to glue up sub-assemblies, not the entire thing all at once. For a 12 segment piece, I would glue up 4 assemblies of 3 segments. Each 3 segment assembly will be a total of 90 degrees. Everyone should be able to get a pretty good 90 degree angle using a square or something in their shop. What this method does, is to allow you to sand both sides of 90 degree segment/assembly to refine the point so that it's exactly even. Then glue up two halves, this way you have 2 half circles that you can sand and perfectly flatten (if needed).  Take the two halves, align the center points and glue to completion.

Regardless the number of segments in a given ring, building pairs or small sub assemblies first and refining them, will result in a much better (tighter fitting) center point.
Back to top
  
 
IP Logged
 
Ray Stubbs
Full Member
**
Offline


WR Rocks!

Posts: 67

Columbia, Mo., USA
Columbia, Mo.
USA

Gender: male
Re: Seg Bowl bottoms
Reply #2 - Jan 3rd, 2018 at 5:59pm
 
I'm confused about how you have end grain with the plug, unless you are cutting the plug from a piece of dowel. If you draw a circle on a board and saw on a band saw, how does this give you end grain with the plug?
Back to top
  
 
IP Logged
 
Louie Powell
WR Addict
*****
Offline



Posts: 671

Saratoga Springs, New York, USA
Saratoga Springs
New York
USA

Gender: male

PSI 12" Turncrafter Commander
Re: Seg Bowl bottoms
Reply #3 - Jan 8th, 2018 at 11:01am
 
Ray Stubbs wrote on Jan 3rd, 2018 at 5:59pm:
I'm confused about how you have end grain with the plug, unless you are cutting the plug from a piece of dowel. If you draw a circle on a board and saw on a band saw, how does this give you end grain with the plug?


Very true.  However, cutting a 'plug' from a board means that it will be side grain that will expand/contract across the grain.  If the plug is then glued tightly into the bottom of the segmented bowl, expansion could split the sides, while contraction could split the bottom.  If it's small enough, the amount of expansion/contraction probably won't matter, but if it's larger, it could be a problem.

The solution is to cut a mortise around the bottom of the bowl, and then turn the plug with a tenon around the entire circumference.  Add a spot of glue at the end of the grain, but allow the 'plug' to float in the mortise just like the end of a breadboard.  A bit tricky to assemble, but completely eliminates the expansion/contraction concern.
Back to top
  

Louie
 
IP Logged
 
Ed Weber
WR Global Moderator
WR Patron
*****
Online



Posts: 4,931

Wilton, California, USA
Wilton
California
USA

Gender: male

JET 1642
Grizzly G0584
Re: Seg Bowl bottoms
Reply #4 - Jan 8th, 2018 at 11:51am
 
Louie Powell wrote on Jan 8th, 2018 at 11:01am:
The solution is to cut a mortise around the bottom of the bowl, and then turn the plug with a tenon around the entire circumference.  Add a spot of glue at the end of the grain, but allow the 'plug' to float in the mortise just like the end of a breadboard.


The OP wrote

Bruce Kamp wrote on Dec 25th, 2017 at 12:06pm:
I have used solid pieces  and a floating base but do not care for the look.
Thanks


IMO
A "plug" in the center of a segmented bowl bottom can be end-grain (a dowel) or side-grain (cut from a board). In either case wood movement is "possible.
With an end-grain plug, tangential shrinkage could play a factor.
In a side or face-grain plug, Radial shrinkage could play a factor
Radial shrinkage is typically less than tangential shrinkage.
Unless a piece is moved from any extremely low ambient humidity environment to an extremely high one, wood movement on a "small" plug is negligible.
In general terms,
Any plug that is less than 1/4 the diameter of a segmented base will most likely not effect the bowl in terms of cracking. Assuming the joint is correctly made, the plug could conceivably contract (shrink) enough to have a crack in itself but is highly unlikely.

Wood species selection always plays a part when building a segmented piece, where you place it and it's orientation can certainly have a impact on the piece. These days, modern wood glues have all but eliminated joints separating, so now any weak point will show up in the wood itself. ( wood cracks not the joint ) This usually only occurs in pieces of wood that are much larger than would typically be used in segmenting.

The larger the piece of wood, the more potential energy can be imposed during movement, the more likelihood of cracking itself or surrounding pieces.


Back to top
  
 
IP Logged
 
George Stratton
Full Member
**
Offline


WR Rocks!

Posts: 87

Gender: male

Old Delta 12" 46-701 in great condition. Had it about 20+ years and turned 1 part.
Re: Seg Bowl bottoms
Reply #5 - Jan 9th, 2018 at 5:08pm
 
I have only turned one larger segmented bowl, around 10" dia. and in the process of glueing up another from 6 different types of wood. The first one was all cherry. I draw up what I'm making on Auto-Cad on the computer in a sectional view, so I can se the sizes of each layer as far as diameters and angles to band saw the rings to. These were then glued together except for the Bottom. On the first bowl I made the bottom from 4 90 deg. pieces which made it very easy to get the center to match. The rest of the bowl was 30 deg. segments. I mounter the bottom with screws to a face plate, turned a dovetail recess on what becomes the bottom surface as well as the surface itself and rough turned the OD. Then I glued it to the rest of the bowl and turned the entire thing using the dovetail on my chuck. I'm doing the same thing on the next bowl but hopefully with 30 deg. segments on the bottom. My questions are, do others attach the bottom last? Is it a dumb way to do it? Forgot to mention I finished the inside bottom before glueing it on too. Still leary about trying to finish the inside bottom afterwards.
Thanks much, Geo
Back to top
  
 
IP Logged
 
Ed Weber
WR Global Moderator
WR Patron
*****
Online



Posts: 4,931

Wilton, California, USA
Wilton
California
USA

Gender: male

JET 1642
Grizzly G0584
Re: Seg Bowl bottoms
Reply #6 - Jan 10th, 2018 at 9:24am
 
While there always exceptions that are usually due to access limitations (neck size) pieces are typically constructed as you described.
Start with a good solid base and build upon that. This makes it easy to turn and refine a few rings at a time as you progress, making it unnecessary to return to the bottom once turned.
Back to top
  
 
IP Logged
 
Page Index Toggle Pages: 1
Send Topic Print