An Easy Technique for Pre-assembling Casing
(Photos by Kirk Grodske)
I don’t do a lot of trim work. But when I do, it’s often stain-grade or pre-painted, and the miters have to be perfect! I mostly hang doors, so I rarely have a full-size work table set up—usually, I just have a door bench. Most of the casing I work with is small. At first, I thought it was too small to survive a Clam Clamp, but I’ve learned better (more on that later!). So I came up with a method for pre-assembling casing that will guarantee tight miters and won’t require a large table. For this method, you won’t need to use staples, or biscuits—just glue. But the pre-assembled frames will be so strong that you can carry them in one hand.
I first made several sets of pre-assembly jigs, all of which are perfectly square and have a thin strip bordering two edges. I start by clamping the legs to two of the jigs with A-clamps. A single sawhorse is all I need for this technique.
|The back of each jig is covered with a non-slip material so the jigs won’t slide off my saw horse (I use Solid Grip Liner, Contact brand.).|
|Next, I glue up the miters with Titebond.|
|On wood moldings, I nail the corners with a brad, but on MDF moldings, I let the glue do all the work–MDF casing splits unless you nail it with a 23ga pin.|
|The assembly jigs do all the work. They keep the pieces square, and the A-Clamps help hold the casing flat, so the miters come out perfectly. I made several sets of these assembly tables.|
With multiple assembly jigs, I can leave a frame to dry while assembling another one. As soon as I finish a frame, I pick it up off the sawhorse, carrying it by the assembly jigs. It only takes a few clamps to stay busy.
One word of warning: If you’re like me, and you use these pre-assembly jigs frequently, you’ll want to wax them or coat them with some type of release agent so the casing doesn’t end up glued to the jigs.
The glue sets in about ten minutes.
|By that time, you can carry the frame in one hand and even shake it—the miters won’t open!|
The Clam Clamp Method
Since I first took the pictures you saw above, I was able to figure out an even better way to pre-assemble casing, especially small casing. Now I use a Clam Clamp!
To use Clam Clamps (see Gary’s tool review) on thin casing, I rabbet the assembly tables.
|This technique works much better with Clam Clamps, but the process is the same: Always clamp the casing to the assembly tables first. I position the casing so that it hangs over the rabbet just a bit.|
Once the Clam Clamps are on, the assembly tables make it much easier to move the frame. You can’t get this kind of clamping pressure from spring clamps, and when you use Clam Clamps, the joints are so durable, firing a brad nail through the miter becomes a waste of effort.
(This article originally appeared on GaryMKatz.com.)
I did the wood blocks and it just makes life easy.
Not sure I understood the rabbeting. Can you explain where the casing sits with respect to the rabbet and how that centers the clamp on the casing? Thanks
The Cam Clamp is thicker than normal thin casing.
The pins that engage to trim, I assume, are centered (roughly 3/8″ from one edge if 3/4″ thick.)
The rabbet in the board allows the Cam Clamp pins to offset for the thinner casing- which is usually about 3/8″ thick.
The casing should just overhang the rabbet slightly. The Cam Clamp will square up the assembly. Hope that helps.
Al, great idea! The best ones are always simple. I will have to give this one a try.
Thanks for sharing, Al.
For MDF casing, I have found that using 2P10 and hand pressure enables the elimination of the clamps, and still provides a joint that will stay together well.
Cool article with great ideas. I’ll have to try this technique on a window job I have going on this week. I have 5 windows to replace along with both interior and exterior trim. Looks like this will speed things up and leave me with nice tight joints.
Still don’t understand how you rabbet the corner boards. Can you provide a picture of the corner board w/out the casing?
Man do I wish I had some sets of these 25 yrs ago when I was trimming out houses – I tried all kinds of temp clamps for mitered corners but none of them worked easily enough.
Hopefully you’re still looking at the questions – I have one.
I kept looking for a close-up or a diagram of those corner jigs – the text says they have a raised edge on two edges, but some of the photos look like the raised “fence”/edge is only on one ? Am I seeing things? I envisioned a perfectly square piece of 3/4″ (?) plywood, with maybe a full 1″ x 3/8″ or similar piece of hardwood (?) fastened to each edge, so it stands proud a full 1/4″ and acts like a fence. Maybe hold them back a little from the actual corner 1/2” or so to allow for gunk/whatever in the very corner. Then put a piece of no-slip liner stuff on the back. Is that how they’re made? If not, what am I getting wrong (or right)?
As for release agent – why couldn’t you laminate the ‘tables’ with some scrap HP Laminate, or even use Melamine to begin with ? I was thinking a saw kerf right at the miter would also be helpful with squeeze-out or anything else preventing the two pieces from lining up perfectly.
Lastly – what’s the source of the non-Collins spring clamp? I can’t find them anywhere. Thanks! JLS
Yes, your description of how the jig is made is right on the money.
The spring clamps I think were bought at Rockler years ago.
I built this jig specifically for the assembly of the pre finished casing you see on top of the article and I could not be happier with the way the installation turned out.
The client bought a pre hung, pre finished door with casing and if it wasn’t installed right the first time around there wasn’t going to be a second try.