Replies

1. 
   RANGERP75R | Feb 23, 2003 07:41pm | #1

   Marv

      Just went through something similar where I had to build a large semi-round counter. Sit back and I have a feeling the cavalry is on the way. They came to my rescue in a hurry and it was a piece of cake.

   Thanks Dan, Richard, jdg, Tom and all the others that pitched in.

   Good luck with this endeavor...

   sarge..jt

2. 
   jackiechan | Feb 23, 2003 11:24pm | #2

   Marv,

   The cavalry has arrived.  (Thank you Sarge for the complement!) 

   There are a number of ways that can be used to create an arch top.  For this project, I would recommend using one of the following four techniques: bending plywood lamination, kerf bending, coopering, or flute and bead routing.

   Plywood lamination would be my number one choice, as it produces the smoothest curve with little to no additional milling needed. But to achieve the most stable arch possible, a vacuum press should be used to clamp the layers together over the form.  This can be a large set up, and an expensive option if you don't have a vacuum press.

   Kerf bending, flute and bead routing, and coopering are close second choices.  I would rank the set up and procedure from easiest to hardest in that order as well.  I'll give you the procedure for kerf bending.  (If you'd like the details on the other techniques just let me know.)

   For kerf bending, you will need: a lower grade piece of plywood (a cost saver right there!) a table saw, a few clamps, some epoxy that can be thickened (or auto body filler! - Bondo), and a sheet of veneer of your choice.  (Rockler carries many different species of flexible veneers, starting at $39 for a 36" x 96" sheet.)

   Kerf bending is exactly what it sounds like.  By making a series of parallel saw cuts, or kerfs, in a piece of wood, you have effectively made the piece thinner, thus easier to bend.  The closer the cuts are made to each other, the easier the wood will bend.  The procedure entails kerf bending the plywood substrate, reinforcing it with the filler or epoxy, and creating a smooth and visually appealing exterior with the veneer.

   On paper, draw the curve to full scale, and determine its radius.  The depth of each kerf should be enough to leave no more than 1/8" intact (usually the thickness of one layer of plywood.)  (Example: 3/4" thick wood is kerf cut to a depth of 5/8") The next step is to determine the spacing between kerfs.  Cut a scrap piece of the plywood you'll be using to 6" wide x (the arch's diameter long).  Measure the length of the arch's radius from one end, and make a single kerf cut across its width.  Clamp the wood to the top of your bench kerf side up, with the clamps only to one side of the cut.  Slowly raise the unclamped side until the kerf is almost closed.  At that point, measure the distance from the end of the wood to the bench - this is the maximum distance each kerf should be from each other.  (This may take some trial and error to achieve the optimum bend.)

   A bending form will need to be made to the exact dimension of your drawing.  This can be done with scrap.  (See the enclosed drawing for a fast and easy bending form.)  Make additional cuts in your test piece and bend it over the form to be sure the spacing between kerfs is correct.  (Make adjustments as necessary.)

   Here's where the fun begins.  Cut a piece of plywood to the length and width needed.  Reset the saw blade to the exact height of your test cut, and make all of the spacing between them also determined from the test piece.  Completely fill all of the kerfs with epoxy or auto body filler - I find the auto body filler is easier to spread and less noxious to use.  Bend the piece over the form, and clamp.  A piece of scrap clamped across the width of each end should be sufficient.  You will get some filler squeeze out, scrape away any excess, and fill in any gaps.  When the filler is completely dry, release the piece from the clamps and scrape away the excess. 

   Cover the inside and outside with the veneer and you're done!  (Sarge bookmarked the Knots thread where I gave the procedure for veneering.)

   Please feel free to send me any additional questions that you may have!

   Dan Kornfeld, Owner/President - Odyssey Wood Design, Inc.

1. 
   mcg6637 | Feb 24, 2003 12:02am | #3

   Thanks for the tips on building an arch.  The kerf bending technique is certainly something I'm familiar with though on a smaller scale.  But your suggestion about flute and bead rang a bell.  I've seen flute and bead router bits sold but only for 1/4" thick wood.  Do you happen to know if they can also be had for 3/4" wood.  Since I'm familiar with barrel stave constuction,(again on a smaller scale), this sounds like it might be another posibility.  Since I would use a moulding around the outside the top wouldn't show and a faceted surface on the inside might look interesting. (always interested in saving time and effort!)  Do you have any opinions about that?  Any suggestions you have on actual construction techniques would be appreciated.

   Again, thanks for the reply.

   Marv

1. 
   jackiechan | Feb 26, 2003 07:30am | #4

   Marv,

   The largest flute and bead router bits I've seen can accommodate wood stock up to 1/2".  When a 3/4" flute and bead are needed, a shaper is normally used.  If you would like to go with the routing method, I'd recommend using the 1/2" bits for 1/2" stock.  Glue up the arch at this thickness - once the glue has cured, lay a 1/8" sheet of bending plywood onto both the inside and outside of the curve.  Not only will this greatly reinforce the arch's strength, it will also give a beautifully smooth curve both inside and out!

   Coopering the arch is also a good alternative to the above method.  Regarding your question about leaving the facets visible inside the cabinet - it's really a matter of personal preference.  It would definitely be a visually unique structure, and would offer the admirerer an insight as to how the curve was achieved.  One thing to note, there will be glue squeeze out that will have to be scraped away once everything is dry.  This may effect the visual if it's not done equally all around.  If this is the route you choose, it is important that the arch is supported well on both ends (side to side), and front to back.  This will reinforce the strength of the arch.  An internal support, molding or otherwise, would not be out of the question.

   Always cut a few extra pieces for either method.  Also, use a slow setting glue - there will be many pieces to glue together, and you don't want to play beat the clock!

   Now to the fun part - actually building the arch!  No matter which method you choose - flute and bead routing or coopering, the key to a successful arch is the gluing form.  The form consists of a series of plywood cradles evenly spaced along the length of the arch (curve side down), that give space for clamping (see the drawing below).  (Think of a boat cradle in dry dock.)  Each cradle will use two clamps (Bessy Tradesmen or similar).  The pieces to be glued are laid into the cradles.  The clamps on either side drive the pressure down into the arch, thus offering consistent pressure throughout the curve.

   To build the form, use your full scale drawing to lay out the curve on pieces of plywood.  I use a double thickness of plywood for each arch to handle the clamping pressures.  The halves can be glued and screwed together, just be attentive to the screw's locations, as you'll be drilling holes for the clamps.  The form should be a few inches longer than the arch is wide, and deep enough to be able to handle large holes drilled into it without loosing strength.  The easiest way to achieve consistent curves is to cut the first one with a jigsaw or bandsaw, and then use a flush trim bit in a router to match up the rest.  The arches should be screwed to a piece of plywood to keep everything in alignment.  Line the inside surface of each cradle with packing tape to avoid the pieces sticking to them. 

   Please feel free to send me any additional questions that you may have.

   The graphic did not attach - see the next post down!

   Dan Kornfeld, Owner/President - Odyssey Wood Design, Inc.

   Edited 2/25/2003 11:40:49 PM ET by Jackie Chan

   Edited 2/25/2003 11:42:29 PM ET by Jackie Chan

   
2. 
   jackiechan | Feb 26, 2003 07:44am | #5

   Here is the graphic file mentioned in my previous post.

   Dan Kornfeld, Owner/President - Odyssey Wood Design, Inc.