What is the Hoffmann Dovetail System?

What is the Hoffman SystemVermont Hardwoods is an ardent fan of the Hoffmann Dovetail joining system. Engineered and manufactured in Germany, while it’s “just a router” the way it’s put together is ingenious and ideal for assembling picture frames, especially ones that are narrow and/or tall.

Benefits of Hoffman Dovetail System

Not everyone can justify having one of their machines, but you don’t need one to take advantage of this option. We offer the service on all mitered chops and it allows us to ship ready-to-assemble frames. You can easily assemble the frames yourself instead of paying for a pre-assembled frame and the additional shipping charges usually associated with them.

How the Hoffman Dovetail System Works

Triangular slots are routed into the ends on the back of each rail which form a butterfly shape, or dovetail, when joined. Depending on the moulding profile, 1−4 slots are made at each joint. To join the frame, the edges are glued and a butterfly-shaped plastic piece, called a key, is inserted into each slot. The insertion of the keys provides the correct clamping pressure for the glue, and the end result is a tight, perfectly aligned joint with no markings on the face or sides.

Keys

The keys are made of a high strength plastic and made to very high tolerances to ensure a perfect fit in a properly routed slot. There are a total of 5 key cross sections, from the miniscule W0 up to the building-grade W4. We use the W0, W1 and W2 and have separate machines dedicated to each size.

  • W0 is used for profiles with a wall thickness of 3/8” or less.
  • W1 is used for all other finish moulding profiles
  • W2 is used for stretcher and strainer

The keys are available in several lengths, and in a few of our profiles we “stack” them to reach the desired length. For instance, our W1 machine can route to over 90mm (3-1/2”) high, and since the longest W1 key is 60mm we use a 40mm and 50mm or 30mm and 60mm key to maximize the strength of the joint.

Benefits

  • Plastic keys retain their shape and size no matter what the humidity, keeping your frame strong and properly aligned
  • Clamping, pressing, nailing, and screwing is eliminated − that saves time, money, and aggravation
  • The net cost of a ready-to-assemble mitered chop with dovetail keys is considerably less than a pre-assembled frame

Assembly

Follow the simple steps below to put together your ready-to-assemble mitered chop with dovetail keys.

  1. Lay the four pieces upside down on a flat workspace in the shape of the frame
  2. Apply glue to the mitered edges of two opposing pieces
  3. Gently drive a plastic dovetail key into each pre-routed slot at the joint
  4. Wipe any excess glue from the joint before it dries
  5. Repeat steps 2−4 for the remaining joints
  6. Fitting can proceed immediately − the key will hold the joint while the glue dries

Tips on assembly

The Joining Surface

One of the most important parts of getting a good joint is having smooth, hard and flat surface on which to insert the keys. Many people want to cushion the face of the frame as they are afraid of damaging the finish. While that’s a valid concern, as long as it is smooth and flat, the energy from the hammer blows will be widely distributed and there is little chance of damage.
The reason for it needing to be hard is that it will ensure the face of the two pieces will remain even with each other, provided a near “closed corner” look.

An ideal material for the joining surface is “phenolic sheeting”, a synthetic material made with resin and generally either cloth or paper. It’s available from some industrial supply houses and specialty suppliers. There are a myriad of options but we make it simple for you, as we source it and offer it in standard 12” x 12” pieces [insert link to item in our store] (you can use a piece of any ½” thick material under the other corners to keep the frame level as you’re joining it) as well as custom sizes per request.

Inserting Keys

Being that most of our mouldings are made from very hard wood and the tolerances on the slots and keys are very close, it can sometimes be a challenge to get the keys properly seated. The W0 keys are very small and bend easily if you’re not very careful to strike them perfectly straight on. In stacked key situations the friction of the combined length can make it very difficult to get the second key all the way in. Here are couple things you can do to ease your job.

  • WD-40 or similar lubricant. The best way to use it is spray some in a little cup or the lid of a small jar and use a small paint brush to apply it to the ridged edges of the key. This does not weaken the joint at all, as the key-to-wood surface is not a glued joint.
  • On stacked keys, insert the first one as far as you can, then use a T-handle allen wrench, thin screwdriver or similar to drive it all the way to the bottom of the slot. Then insert the second key.

Hoffman System - Dovetail Wedges 2 Hoffman System - Dovetail Wedges

 

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