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Curved Panels - adding another dimension to woodworking. #2: Building "flat" curved cabinet doors - 101

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Blog entry by John8059 posted 02-22-2009 03:06 AM 6598 reads 0 times favorited 3 comments Add to Favorites Watch
« Part 1: Curved Panels #1 - Introduction and some uses for curved panels. Part 2 of Curved Panels - adding another dimension to woodworking. series Part 3: Building a better form for curved panels - 201 »

Note that this is Part 2 of a series and you may want to look at Part 1 to make some sense of it.

I don’t have any pictures of the form I built for the flower boxes or any construction pictures so I am skipping right to the 2nd project I introduced in Part 1. If I explain the flower box panels at the end of the series, it will be much easier to understand. Further, it was very similar to making the panels for the last project I’ll cover – the curved vanity doors.

The entertainment center I’ll discuss here was a commission a buddy of mine got. I typically made the working drawings for his projects and produce 3D renderings for him to present for approval. Since I had experience veneering and making curved panels, he asked me to help with those parts of the project also and I’m glad he did. It was challenging and I am proud of the results.

The design started as a picture the customer had cut out of a magazine. He put overall dimensions to it and we took it from there. I purposefully designed the cabinet so there was an abrupt transition from the curved sides to the flat front. I don’t think it detracted from the design of the piece and it made it a lot less complicated than building part-curved/part-flat doors. It was going to be difficult enough to get the doors square, snug to the case, and the gaps right. The alternative to blend the curve into the front of the cabinet required a much tighter radius and it just didn’t look right. Perhaps this is a good time for a picture showing the shape of the cabinet.

You can see that in addition to the curved doors, at either end there were smaller fixed close-out panels needed. The front edges of the close-out panels were to be cut parallel to the radius of the panels like the outside door edges; however, the rear edges had to be cut at a much different angle. Likewise, the inside edges of the doors had to be cut to match the 90 degree angles of the front doors. I learned the hard way on this project to plan ahead because it took me days to cut the doors and close-out panels out of the curved panels. Just think about how you would keep them plumb to the curvature of the door, square, and the correct size. More about that later.

Further, the toe kick also needed to be curved but at a smaller radius. Rather than make another form to make curved panels with a different radius, I essentially built two small forms using the same principles I used to make the large panel form. I combined them with the square base/toe-kick for the middle section of the cabinet and veneered it as one piece. Take a look at the top view of one end of the cabinet:

OK so the toe-kick isn’t shown exactly how it turned out but you get the idea.

Finally the good stuff. This was my first encounter with these materials so I had to learn what was what. Visit this link to see the selection of products I had to choose from http://www.qualityplywoodspec.com/ and/or check out specialty plywood stores in your area. I choose not to use the products that achieve their bending capabilities from kerfs, but have nothing negative to say about them.

I did pick from their selection of bending plywoods. I still don’t get the difference between what they call bending birch and bending Lauan, but I do get the barrel vs. column part – do you want it to bend along the 8’ axis or the 4’ axis? It is available in 3 thicknesses; 1/8”, ¼”, and 3/8”. I’ll warn you these are not the actual thicknesses. You will NOT get what you expect! I used one 3/8” sheet in the middle sandwiched by two 1/8” sheets on either side and ended up with a thickness of 0.78” instead 0.875”. I saw this coming and used what I did to get as close as I could to the thickness of the plywood used in the flat doors. This picture is stolen from the lectern project and is out of context here, but you can see the laminations (look below the special router base addition);

Did I mention that the panel gains strength from each layer of lamination? I hope you don’t need a citation and will just take my word for it. Believe me when I tell you that these panels are every bit as rigid as the flat plywood you buy. And just in case I forget to mention this later, they DO NOT flex when you take them off the form. They retain the shape of the form to the point that if you have defects in the form, you will see them in the panels. Trust me!

OK – I think Veneer Joe has touched the hearts of every woodworker who has considered vacuum bagging. If you don’t know where to find him, you better start looking. If you are going to attempt this, you will need some of his equipment. The adhesive I am going to recommend using requires 6-8 hours in the press so you better get that check valve, presssure regulator switch, and build a reservoir tank or you will surely burn out your pump. Joe has all the info you need so visit his site and download the instructions there. A little hint: If you just can’t hold a vacuum, be sure to check the check valve for debris. A minute piece of Teflon tape was stuck in the valve seat of mine and held me up for 2 days while I tried to seal perfectly good joints that were not causing the problem.

The glue I used is a DAP product that was recommended by a friend and readily available in a local hardware store. It was my first encounter with urea formaldehyde glue (I’m pretty sure they only have one). It worked great and the only thing I have to say about it is make sure you buy or make something to stir it with as it does not want to dissolve easily. Read the directions carefully and measure precisely. Use a scale if possible.

You will need a roller system to apply the adhesive because you won’t have a lot of time. I recommend applying the glue to both sides so for me that meant spreading it out over eight surfaces, stacking the panels, getting them into the bag, sealing the bag, and pulling a vacuum before the open time ran out. It is very doable but don’t get hung up trying to save a roller. And DON”T be cheap about the glue! Buy more than they recommend and mix plenty. You can’t possibly mix more in time if you run out!

You will need a vacuum bag comfortably larger than your form. Have the form ready and waiting inside the bag so when you get the panels glued up and stacked, you can slide them onto the form and close up the bag. You don’t have time to fart around! See Joe again. There you can buy a valve and the glue to make your own bag out of vinyl that you can buy in the fabric department at Wal-Mart. It is ridiculously cheap! Find the one you think is strong enough and spend a few extra pennies to buy the next thicker one. DON’T FORGET TO TEST THE BAG/VACUUM SYSTEM FOR LEAKS BEFORE YOU MIX ANY GLUE!

Now you can be sure that glue is going to run out of the seams and your panels can easily become a permanent part of your form if you don’t take the time to protect it. I am fortunate enough to have a large roll of real cellophane and I cover the form with it. Nothing water-based sticks to that stuff and I find it indispensable. You will have to get creative. I suppose a plastic garbage bag would work?

You aren’t going to get anywhere without a form. I am warning you the second time now that if you aren’t careful making the form, your mistakes will be reflected in every panel you produce on it. If it is wavy or twisted, your panels will be exactly just as wavy or twisted. And they WILL stay that way! Build it on a “flat” surface and take NO SHORTCUTS! More about this in Part 3.

You will have to make some calculations based on the actual thicknesses of the bending material and the panel radius you want. Here you need to include the gap between the door and the carcass and consider whether you are looking for the inside or outside radius. I admit that I made my form for this project backwards and regret it now, but we will continue to work with what I have here.

From that calculated radius, you need to deduct for the thickness of the bender boards you are going to use to construct the curved surface of the form. I used whatever I had left over that added up to about ¾” thick and it has worked fine for me every time. Using that radius, I constructed a plywood pattern from which I cut 12 identical ribs which would support the bender ply sheets. Below you can see the plywood pattern resting between the 2nd and 3rd ribs of the form. I rough-cut the ribs on the band saw and used a trim bit on the router to finish the job.

Notice it has an index on the left side so each rib could be aligned to the next so long as they were all attached to the edge of the base correctly and the base was square. Below you can see all the ribs, spaced 3” on center, ready for the bender ply sheets to be added.

I learned later that this next step is one that should not be skipped at any cost! I used the urea formaldehyde glue and the vacuum bag just like I would on the real curved panels and bonded the bender ply panels to the ribs of the form. I only used about 8 lbs of vacuum so as not to warp the panels by sucking them in-between the ribs. I repeat – I would not make the form like this again so there is no need to point that out. It will become clear in the next part of this series.

After the form was set up, I block sanded it using a curved block I made on the form with Elmer’s glue and some scrap bender ply. It didn’t take much sanding and I was ready to lay-up some panels. I hope I covered that part well enough above so I will skip to a picture of a panel in the bag. For what it was worth, I tried to get one edge of the panel straight and aligned with an edge of the form. It didn’t do much good as far as trimming the panel into a door later.

Well now you have had a crash course in curved panel building. I am purposely skipping the part about how I got the panels square and plumb because I wouldn’t go through that again for all the money in … well I won’t put you through how NOT to do it. You can see below that I eventually got the panels to fit perfectly with European-style gaps.

If you noticed that the 3D rendering above was covered in beautiful lacewood but the project ended up being curly maple, you’re correct. The people rejected the lacewood when they discovered that all of the fleck would not be the same size. No comment! Thank goodness I didn’t actually apply the lacewood veneer. And speaking of veneer, it was just another trip into the bag.

A final note here. I mentioned that panels made this way do not lose their shape (don’t spring back) when removed from the form; however, I strongly suggest that when possible you build your curved panels BEFORE you complete the design and/or shape any of the other elements of your project. I swear I am not telling you beacuase I make this mistake. My panels turned out incredibly close to plan, but I was able to make the curvature of the cabinet match the doors perfectly because I waited until the doors were made. Too bad no one will ever be inside the cabinet to see just how good. Well at least the customer could see before the granite top was installed.

Thanks for looking and watch for Part 3 – The Correct Way To Build A Form so it is easy to trim your panels. If all this is too much for you, Part 4 will be about making less complicated curved parts without a vacuum system.

-- Cuz



3 comments so far

View kiwi1969's profile

kiwi1969

609 posts in 2904 days


#1 posted 02-22-2009 06:46 AM

very informative, thanks a bunch for this

-- if the hand is not working it is not a pure hand

View douginaz's profile

douginaz

220 posts in 3465 days


#2 posted 02-22-2009 03:24 PM

Wow, Very nice project but even nicer tutorial. There is a ton of critical information and watchouts. I for one appreciate the time and trouble you went through to share this with us. Thank you. I have made curved pieces before but mostly small rails & stiles, nothing that required a vacuum. I hope to collect all of your posts on this to use a reference. Thank you again for taking some much valuable time and effort to enlighten us.
Later,
Doug in AZ.

-- If you need craft books - please visit our small business at http://www.wittywife.com

View cabinetmaster's profile

cabinetmaster

10874 posts in 3021 days


#3 posted 02-22-2009 06:02 PM

Dido Dougs remarks. I will keep this post as well. I need to get started on that vasuum system now.

-- Jerry--A man can never have enough tools or clamps

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