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Miter Clamps
Perfect Miters ...... Fast

An Article
"Perfect Miters Every Time"
by Jim Chestnut

Article First Published in "Fine Homebuilding Magazine"
issue #164, July 2004

Edited by Jim Chestnut for web site publication.

All pictures are courtesy of "Fine Homebuilding Magazine", though most were not used in the orignial publication. Any inaccuracies, omissions, advertising, or inappropriate verbiage that may appear in THIS article did NOT appear in "Fine Homebuilding" and are solely the result of my own ineptitude and/or megalomania, greed, perversity, or any other of a wide range of diverse character defects.

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Gluing Set Up

I use a full 4x 8 sheet of plywood covered with formica for a glue-up table, though any sheet goods will work such as melamine. Glue up is messy, especially when I'm doing it, and dried yellow glue pops off formica easily with a taping knife. My preferred glues are Wilsonart yellow glue, which is made by Lock Weld, and Titebond Yellow glue with the red cap. All the titebond II I've tried is too runny and it ruins all my clothes.

The 4x8 foot width will handle most door and window casings. The wider assemblies, such as wide closets and french doors etc., are clamped on the floor directly in front of their destinations, and immediately nailed to the jambs with the clamps still on them, for their own protection as much as to free up floor space. Our jobs are filled with clumsy oafs tromping on trim, like it's a sporting event or something. Fortunately, they don't wear their golf spikes.

Under the business end of the table, where it hardly ever gets knocked over, I keep an old pot half filled with hot water and bristling with toothbrushes, on a cheap electric hot plate. This combination cuts glue squeezout amazingly fast, and an air gun removes the evidence while drying the casing - again, fast. We have been using this process on both paint and stain grade work for years without problems for the finishers - though for stain or clear finish, we repeat the process with very clean water after the first blowoff.

To be a worthwhile method for me, any new method of trimming had to be faster than the way I had been doing it but without sacrificing quality. The following is an advertisement and if I get any guff about it, I'll fill the site full of blinking signs. When I first started preassembling casings, the only clamp up to both was the Hartford Clamp, but they rusted easily, leaving black stains where wood, clamps and glue met, were difficult to operate unless the corners were cantilivered off the work bench or floor, and they tended to slide the mitered pieces out of alignment. If the man doing installations wasn't paying attention to the proximity of adjacent walls, he would sometimes nail casings home only to have to chisel the clamp off later because the release lever would hit the adjacent wall and not release the clamp. The pins were not removable or replaceable either. So I now manufacture and sell my my own clamps, called Clam Clamps, designed specifically for clamping mitered casings. They don't rust, and you'll never have to chisel one off a casing. End of advertisement.

Man, you finished that beer already!



We cut a bunch of thin shims from zero to an eighth or so in 3 or 4 inches and leave them on the glue-up table. Then we butter up all the cut edges, slots and biscuits, shove the joints together and align the details. I use a lot of glue and try to avoid sliding the miters against each other, because this tends to remove too much glue and can result in a starved joint. The thin shims are used to adjust the face alignment slightly when a head or leg has a slight twist, or the glue-up table itself is torqued.

When it is good, we hold it in position with one hand, slide a clamp into position with the other and clamp it home. The Wilsonart glue seems to tack up and cure even faster than Titebond - which is a good thing in this application. The clamped casing unit is cleaned of glue then leaned up against a wall in a way that the joints are not stressed.

Using poplar at 7% moisture content, and working in temperatures above 70 degrees, the clamps can safely be removed in 3 to 5 minutes, so long as the casings are not badly manhandled.

When pressed by either scheduling, lack of space or, more likely, an immediate need of eye candy for either homeowner or architect, we use the following technique. We clamp and lean 4 casing units against the wall. Then we remove the first set of clamps, clamp and lean another, remove the next set of clamps in order and so on, until we have 8 , 12 or 16 sets done. Then we install those with the clamps still on them, nailing into the jamb only. When finished with those four we come back and do the same with the ones from which we'd removed the clamps earlier - progressing from the first ones clamped to the last. Then we take the clamps off the first batch installed, and start all over again.

But both James Chanbers and I much prefer to just clamp and lean all day long till we have a huge tunnel of casings leading to a blank wall, and let someone else nail them up the next day.

On doors where the rock is proud of the jamb, we cut back the rock such that 3/4" or so will be left under the outside edge of the casing. This is most quickly accomplished with a battery powered skill saw fitted with a cheap, carbide tipped blade of small diameter to keep the foot speed down. Good eye protection is a must. The carbide tips do not last long on the drywall screws, and when all are gone the tipless blade will do just fine. There is actually less possibility of damage done, less dust and less cleanup (of the area) required with this method than with the traditional hammer mashing technique. This job is reserved for the guy who screwed up worst yesterday; that decision rendered, of course, by you and I, and not subject to discussion.

Quick Grip clamps are hooked around the jamb at the inside corners and the casing drawn in to touch the jamb on those with the rock proud. A 3 ½ “ casing can usually be drawn in 5/16ths without cracking the joint (if done as described in the above paragraph), providing the casing has been allowed to sit over night in temperatures above 70 degrees. This will depend to some extent on the casing profile at the inside edge.

We run a tiny bead of glue along the outside edge of the jambs before shooting the casings on - small enough that we rarely get any squeezeout at the reveal. This adds enormous strength and rigidity to the jambs. Though this effect is tremendously diminished with pre-primed parts, we still do it nonetheless.

The outside edges of the casings are nailed off by the man installing the baseboard. That way, the miter joints have had more time to set up, and the base man already has long nails in his gun. Also, he has become the sole person responsible for the door to jamb reveal at the bottoms of doors, and is reminded of that fact before starting.

Baseboard wedged too tightly between corners and door casing by quality-consious, though less experienced help is the primary cause of shrinking door reveals down bottom. This shift of responsibility to him forces an awareness of this potential problem.


System Components

The story pole method of measuring, and the registered stop method of marking lengths is far more complicated to describe than to do. I could have set up and knocked down everything at least 4 times by now - if you started at the beginning of this article.

Each component of this system also stands independent of the rest. For instance, you can forget the story pole, and measure traditionally as already mentioned. Or you can leave out the step above and drag casing and saws all over the house cut them in place and glue them up on the floor. Or you can cut and fit, tack them to the jamb, and then clamp them, with or without biscuits.


Miter Joint Strength

But no matter how you do it, you just can't deal with such a problematic joint as a mitered casing any better than with clamps, glue and biscuits. The biscuits add a lot of surface area to the inside corners of the joint, and that surface area has glue strength in shear, as opposed to tension.


Titebond has general guidelines for suggested clamping pressure ranges that should result in optimum strength glue joints. 100-150 psi for softwoods, 125-175psi for harder woods, and 200 and up for woods like hard maple. A 3 1/2" by 3/4" flat casing has a surface area along its miter joint of 3.7 square inches. Assuming a clamping pressure of 125psi for poplar, you're talking a recommended total clamping pressure of 464 pounds for the joint. But Titebond stresses that a very thin uniform glue line is the objective, not enormous pressures, so you can probably get by with less.

My mud bucket fitting technique required the face or backplaning of cocked casings for a "good fit" - or more precisely, "a good cosmetic fit". There is simply no way to block plane mitered cuts so they mate precisely over their entire surface areas every time - unless you make a career of trimming just one house. And without continuous contact, glue does not function well - particularly when in tension.

And nails simply do not put pressure on miter joints. You would need a seven inch long nail to go through a 3 1/2 inch casing where it would do any good - at the inside corner where they always open up. So clamping is the only real solution - but of course we have know that for hundreds of years. But now there are clamps out there which will do the job FAST.

Now that wasn't so bad was it? Only one an a half little commercials.


Happy Trimming,

Jim Chestnut

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