Questions about expansion & contraction - Woodworking Talk - Woodworkers Forum
 
Thread Tools Display Modes
post #1 of 10 Old 08-29-2012, 05:22 PM Thread Starter
Junior Member
 
Join Date: Feb 2010
Location: East Tennessee
Posts: 27
View ETBee's Photo Album My Photos
Questions about expansion & contraction

I am building a table that will be outdoors. It's made of cypress. The top will be attached to aprons using figure 8 clips to allow it to expand and contract as needed. There is also a lower shelf that will be attached the same way, but expansion of the shelf will be limited by the table legs on each corner. The shelf will be 22 inches wide, made of edge-joined cypress boards. I need to leave gaps around the legs but I am not sure how much expansion to expect. Any idea how big the gaps should be?

As part of the same table I am building a circular pedestal, also from cypress. I plan to start with a 16-sided polygon made by gluing 3.75 inch wide boards together, then rounding it. The end result will be a circular pedestal about 4 or 5 inches tall. Again my concern is about expansion. If I construct the pedestal with the grain running up and down, won't it expand/contract across the grain and tend to tear itself apart? If I orient the grain side to side the glue joints will be on end grain, which doesn't seem like a good idea.
ETBee is offline  
Sponsored Links
Advertisement
 
post #2 of 10 Old 08-29-2012, 11:17 PM
Senior Member
 
Join Date: Sep 2011
Posts: 25,995
View Steve Neul's Photo Album My Photos
I would probably only leave about a 1/16" gap around the legs. Your shelf is likely to expand some while it is new however it is more likely to shrink a great deal next summer especially if it sits in the sun. I think if you start out with too big of a gap around the legs now when the top shrinks it may be more than you want later.
Steve Neul is offline  
post #3 of 10 Old 08-29-2012, 11:26 PM
Senior Member
 
BernieL's Avatar
 
Join Date: Oct 2011
Location: Greenville NH
Posts: 1,399
View BernieL's Photo Album My Photos
It being summertime, your wood is about as wide as it will ever get, so you need to be concerned about shrinkage. I am presently building a country hutch and I'm making it fairly snug because it will shrink in the house during our NH winter heating season. It sounds like you already know that wood moves across its' width and not length. I hope this info sets your mind at ease. We all learn as we go and I'm still learning at the door step of my golden years.

Its' never hot or cold in New Hampshire... its' always seasonal.
BernieL is offline  
Sponsored Links
Advertisement
 
post #4 of 10 Old 08-30-2012, 03:19 AM
Senior Member
 
Join Date: Sep 2011
Posts: 576
View Midlandbob's Photo Album My Photos
Outdoor is not a big problem. The humidity and moisture content is more stable. Be sure the wood has been outdoors for a month or two. If its house dry it will expand the first time it's out. Quarter cut wood expands almost 1/2 as flat or tangentially cut boards.
The best reference is still UNDERSTANDING WOOD by Bruce Hoadly. It has the calculations, discussion and charts with the expansion of the commonly used woods.
Bob
Midlandbob is offline  
post #5 of 10 Old 08-30-2012, 07:23 AM
Senior Member
 
bob sacamano's Avatar
 
Join Date: Jan 2012
Location: la mirada ca
Posts: 566
View bob sacamano's Photo Album My Photos
since it will be outdoors im curious what type weather it will be exposed to and what kind of finish are you going to use ?

build it right or not at all
bob sacamano is offline  
post #6 of 10 Old 08-30-2012, 08:08 AM
Old School
 
cabinetman's Avatar
 
Join Date: Jul 2007
Location: So. Florida
Posts: 24,027
View cabinetman's Photo Album My Photos
A sketch of the design would answer the question about the shelf and the gap needed if any. Just allowing a space could be a bad idea as if the wood is high in moisture, it will be be shrinking to a gap much larger than what is allowed. If it's not enough, it may get too tight.

The point here is that the design and layout of the shelf would determine how to handle the movement issue. IOW what are the legs like, and their size and shape? Does the shelf span between the legs in both directions? And, how is the shelf planned to be fitted to the legs?

As for the pedestal, I would join at the end grain. At least it's the lesser of two evils. If there is an expansion/contraction happening, all that is likely to change is the height. If joined at the long grain edges, it may break the joint, or buckle one or more of the sections.





.
cabinetman is offline  
post #7 of 10 Old 08-30-2012, 08:45 PM Thread Starter
Junior Member
 
Join Date: Feb 2010
Location: East Tennessee
Posts: 27
View ETBee's Photo Album My Photos
Better than a sketch, here's a pic of the actual joint and the gap I have at the moment. The shelf is 22 inches wide and all four corners look just like this. That's about 1/8 of an inch, so the total gap for exapnsion is 1/4 inch before it starts pressing on the legs. As others have commented, the cypress might be at its most "expanded" state right now, so I may be looking more at contraction making the gap wider, which is OK.

I am planning to finish with several coats of spar varnish.

ETBee is offline  
post #8 of 10 Old 08-30-2012, 08:50 PM
Old School
 
cabinetman's Avatar
 
Join Date: Jul 2007
Location: So. Florida
Posts: 24,027
View cabinetman's Photo Album My Photos
Quote:
Originally Posted by ETBee View Post
Better than a sketch, here's a pic of the actual joint and the gap I have at the moment. The shelf is 22 inches wide and all four corners look just like this. That's about 1/8 of an inch, so the total gap for exapnsion is 1/4 inch before it starts pressing on the legs. As others have commented, the cypress might be at its most "expanded" state right now, so I may be looking more at contraction making the gap wider, which is OK.

I am planning to finish with several coats of spar varnish.

How is the shelf attached to the legs?





.
cabinetman is offline  
post #9 of 10 Old 08-31-2012, 06:40 AM Thread Starter
Junior Member
 
Join Date: Feb 2010
Location: East Tennessee
Posts: 27
View ETBee's Photo Album My Photos
The shelf is not attached directly to the legs. The legs support aprons that are attached via mortise and tenons. The shelf sits on the aprons, attached via figure 8 clips so it can move. My concern is the position of the legs represents the maximum expansion possible, so I want to leave a sufficient gap around them
ETBee is offline  
post #10 of 10 Old 08-31-2012, 08:31 AM
Senior Member
 
Join Date: Sep 2011
Posts: 25,995
View Steve Neul's Photo Album My Photos
I would just continue with the way you have it built. Be sure when you varnish it that you varnish all the areas that don't show.
Steve Neul is offline  
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Woodworking Talk - Woodworkers Forum forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in










Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Similar Threads
Thread Thread Starter Forum Replies Last Post
sanding/varnishing pine floor - expansion/swelling questions johnee Wood Finishing 7 08-09-2012 07:38 PM
Expansion Pebble41mike General Woodworking Discussion 4 12-28-2011 06:46 PM
Shiplap Shrinkage, Expansion, Contraction FinallyGreen Timber Framing 2 05-24-2011 05:44 PM
Wood Expansion/Contraction windstorm General Woodworking Discussion 3 11-08-2009 08:50 PM
expansion / contraction ecw Joinery 3 08-07-2009 08:50 AM

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

 
For the best viewing experience please update your browser to Google Chrome