Precise Miters

We all know that producing tight, precisely mitered corners is not easy to achieve. Here are a few tricks that we have discovered that help us set up our machinery, make our cuts precise, and maintain good miters.

1. Saw or chopper? When cutting hardwoods, we recommend using a saw over a chopper.
   • Saw advantage: when properly equipped and maintained, a saw will provide cleaner miters and have a higher success rate than a chopper. Saw disadvantage: sawdust
   • Chopper advantage: less mess and dust. Chopper disadvantage: can have difficulty cutting cleanly through hardwoods, as they can “crush” or “tear” the wood fibers.
2. Use the correct, properly sharpened saw blades. Cutting mixed hardwoods with a 10″ miter saw, we recommend 80 tooth, double-sided veneer, carbide blades. We use a negative hook angle (approx. -5°) alternate top bevel (ATB) blade with the thickest plates possible. Call us for brand names and recommendations. For choppers, we recommend honing your blades when you get them back from your sharpening service. Honing means putting a smooth, very sharp “finish edge” on a blade. When this is done to chopper-style blades, not only do they last longer in between sharpenings, but they have a much longer life before they need to be replaced. Honed blades also produce a much higher quality cut. It easy and cheap to set up and teach yourself to hone chopper blades. (See below)
3. Keep the moulding from moving during the cut. Sometimes the blade on your saw or chopper will either push or pull on the moulding while the cut is in progress. For the best mitres, ensure that your piece does not move at all during the cut. There are many ways to do this, and the proper method depends on the shape and size of the moulding. For most profiles, a hand-actuated squeeze clamp provides plenty of force and secures the piece quickly using only one hand. When clamping, be sure that the clamping pressure doesn’t cause the piece to lift away from the table or tilt away from the fence. Using a spacer in the rabbet that is thicker than the rabbet depth (so it sits proud of the lip) is a good way to steady the piece and keep it from tilting. Top clamps are also helpful in certain situations, especially on mouldings with wide, flat faces. The concern with top clamps is that they can tilt the piece away from the fence, so make sure the piece is snug to the fence and table before making your cut. Top clamps are usually pneumatic and take some effort and knowhow to install, but are a good tool to have at your disposal.  
4. Make sure your corners are exactly 90 degrees. Usually we just measure the right and left 45-degree cuts separately. If this is all we do to check our corner accuracy, it is not the most reliable method. Very small yet significant deviations from 45 degrees are difficult to measure. These inaccuracies amplify by the fourth corner, when putting the frame together. An effective cross check is to measure an assembled scrap corner to see if is 90 degrees. Make sure that the scrap moulding used is straight and otherwise true. With a machinist’s-type square held on the outside of the corner, you can easily see if the two corner angles combine to form an exact 90 degrees. Adjust your right or left angles as necessary until your test corner is a true 90 degrees. Provided your moulding is reasonably straight and your opposite leg lengths are exactly the same, the entire frame should fit together with no gaps. We cut a test frame from scrap moulding on each of our mitre saws at the beginning of each week to test for square and tight corners. Using a wide, flat moulding, and a harder wood such as maple, is a good way to ensure that your saw and assembly setup is ready to handle the most difficult mouldings. 

Honing chopper blades:

Acquire a 12″ square piece of ¼” thick plate glass (have your glass company polish the edges so it is safe to handle) and glue a sheet of 600 grit wet or dry sandpaper to its surface. The sandpaper must be glued down smoothly with no wrinkles (3M 77 Adhesive works well.) The glass and sandpaper are essentially a large, very flat sharpening stone. Use this to hone your chopper blade as follows:

1. Oil the surface with light machine oil. One thin line of oil is sufficient.
2. Remove a single chopper blade (leave the other blade on the chopper to help realign the first and vice versa) and place the beveled edge on the surface of the “stone.” The cutting edge should face away from you.
3. Push the blade away from you, into the edge, using even strokes of moderate pressure. Be careful not to rock or tip the blade while pushing. Four to six strokes should be enough to polish the edge of the blade. Be careful handling the honed blade, it will be razor sharp!

Once you are set up, the honing process for both blades should take only 5-10 minutes from start to finish. Hone chopper blades when you first get them back from your grinding service, then at the first sign of dullness, indicated by the extra force required to make your cuts -this is usually after cutting 80 to 100 frames. The sandpaper will last for several honing cycles. Just replace it when it is obviously too dull to sharpen any more. The simple practice of honing will save you money in increased blade life and better quality cuts. Better quality cuts save moulding, assembly time, and frustration.

JOINING HARDWOOD FRAMES

Assemble frames using a corner vice with glue and nails (pre-drilled) or with dovetail-type inserts. We recommend Hoffman or similar joining systems when possible because it is the most foolproof way to join hardwood frames. In general, we do not recommend using v-nails in hardwoods because they can cause splits and cracked, especially in maple or with tall mouldings where v-nails are stacked. 

For more information on how to assemble dovetail-routed frames, please review our Dovetail-routed Frames guidelines within our How-to Tips section.