Introduction: Angle Divider for Perfect Miters

About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first to…

In 1903 Stanley Tools patented their No. 30 Angle Divider. It accurately divides an inside corner angle* for transferring the correct miter angle to your saw. This is especially useful for installing baseboard molding where corners in homes are seldom an exact 90 degrees.

The photo shows my facsimile for a Stanley No. 30 Angle Divider. Original Stanley angle dividers can be found on eBay and range in price from around $40 to $200, depending on condition and how much original packaging is included. This Instructable will show how you can make your own facsimile of a Stanley No. 30.

For the sake of full disclosure, various versions of an angle divider are available at Amazon and other places for less than $20 up to nearly $100, depending on the design.One company now offers a copy of the original Stanley No. 30 for $65, but made from stainless steel sheet. And, Harbor Freight has a plastic version that will do the job for around $5. It all depends on how committed you are to making your own.


  • 3/4 inch hardwood
  • 1/8 x 3/4 inch aluminum bar
  • 10-32 flat heamachine screws and nylon locking nuts
  • 1/4 inch carriage bolt, wing nut, and flat washer


  • Table saw
  • "C" clamps
  • 3/16 inch drill
  • Drill guide or drill press
  • Vise-Grip Pliers
  • Countersink
  • File
  • Sandpaper
  • Pliers
  • Screwdriver

*In step 10 of this Instructable I will mention how to use this angle divider to get the proper angle to cut on an outside corner.

Step 1: Cut Hardwood

I ripped two pieces of straight and dry hardwood (oak) to make two pieces 1/2 x 3/4 x 10 inches. Rip a third piece 1/4 x 3/4 x 10 inches. Cut two pieces from this and set the rest aside. One piece will be 4 inches long. The other will be 1 inch long.

Lightly sand saw marks smooth, even if they do not fully disappear.

Step 2: Glue

Glue the pieces as shown so there is a slot 1/4 inch wide. This slot will allow a 1/4 inch machine screw to slide smoothly, but without looseness. It may be necessary to use a few gentle strokes with a file or rasp on both sides of the slot if the screw sticks. Clean and sand where there is dried glue or uneven edges.

Step 3: Drill

(I made a mistake and initially drilled on the wrong end. Follow the text rather than the photo for guidance.)

On the end of the wooden assembly where the 4 inch piece was glued between the larger pieces measure in from the end 1/2 inch and drill a hole through the wooden assembly 3/16 inch in diameter. Try to make this hole centered on the slot as accurately as possible. The ideal is for the angle divider to give equal readings between either arm and the body. The precise location of this hole has an effect on whether readings between the arms and the body of the angle divider are the same or vary slightly. (Even if they vary slightly, there is a way to work around that problem in use, and it will be described in the step 12. Since building this one, I have built two more. The inclusion of an adjustment for the pivot pin can be helpful if you are having problems with unequal angles. [I can say I checked the angle divider shown here. The pivot hole is accurately placed and the tool gives the same reading without an adjustment feature.])

Step 4: Shape the End

As I mentioned in the last step, I actually made a mistake and drilled my 3/16 inch hole on the wrong end, but this photo still illustrates how I narrowed the end near the pivot screw. After I had sawn the corners away, I used a sander to round the end a little.

Step 5: Cut Aluminum, Then Drill

See the text boxes in the first photo for help with the size and placement of the holes shown, as well as the length of each piece of aluminum. Be sure to follow those specific dimensions fairly closely. (I actually made an earlier version of this from steel. It worked well. I took dimensions from it for this version.)

See the second photo. I clamped each pair of pieces together with a Vise-Grip pliers and drilled holes through both pieces at the same time to insure precisely spaced holes on each pair of aluminum pieces. Each set of holes may be a little to one side of the exact location I wanted, but drilling each set of holes at the same time keeps the spacing between them the same on each pair of aluminum pieces.

Step 6: Assembly

I used a countersink in an electric hand drill to remove burrs around the holes I drilled. I also sanded the surfaces of the aluminum bar to remove marks and make its appearance more pleasing. And, I filed the corners to round them. Corners also need to be rounded so they do not extend beyond the working edges of the longer aluminum pieces.

Place a 10-32 machine screw 1 1/2 inches long through the hole in the wooden assembly. Place the end holes in the longer aluminum pieces over this screw. Add a nylon locking nut. Tighten, but allow the arms to move smoothly. Still, tightening as much as you dare removes whatever looseness or play there is in the screw connections and yield more accuracy in the tool. (Do not worry if there is a little play between the holes you drilled and the screws. In use you will take the play out in the same direction by pushing the tool into the corner and by pushing against the tool again when reading the divided angle.)

Step 7: Countersink

Countersink the hole in the bar that slides over the wooden assembly. (The view is from the back of the angle divider. The second photo may be helpful for understanding the why and the how of which pieces to countersink.) Add a flat head 10-32 machine screw 3/4 inch long and a nylon locking nut to fasten one of the shorter aluminum pieces to the longer aluminum piece. See the orientation of the pieces in Step 5 if you need to refresh how the pieces are to be arranged. Pay special attention to the placement of the holes. The angle divider will not work correctly if any pieces are turned over or turned end for end. Tighten, but not so tight that the pieces do not move smoothly and easily.

Step 8: Countersink Again

Drill another countersink, but make it in the shorter piece of aluminum bar that will slide directly over the wooden assembly. Add a flat head 10-32 machine screw and a nylon locking nut. Tighten, but not too tight.

Step 9: Insert the Carriage Bolt and Use

The 1/4 inch carriage bolt goes through the slot in the wooden assembly and through both 1/4 inch holes in the shorter pieces of aluminum. Add a washer and a wing nut. (If the square portion of the bolt head is too large for the slot, file or grind on it until it fits smoothly.)

To use--

Push the angle divider into a corner and spread the two longer aluminum pieces until they meet the sides of the corner as well as possible. Be careful to hold the angle divider as flat and level as possible. Tighten the wingnut so nothing moves while transferring the angle. The sides of the wooden assembly and of one of the aluminum wings define the angle that bisects the angle of the corner.

Step 10: Capturing and Transferring the Angle

Use a sliding bevel finder (also known as a T bevel) to measure the angle from one leg of the angle divider to one side of the wooden assembly. To check accuracy, flip the angle divider over and see if your results are the same.*

Gently move the settings on your electric miter saw until one leg of the bevel finder is against the fence and the other is parallel to the blade's cut. I have found it helpful to set the power miter saw as best I can, and then check the bevel finder against the angle divider again to make certain I did not bump the bevel finder while setting the saw. If the bevel finder still fits the angle divider exactly, I know all is good. If the bevel finder no longer fits the angle divider, I know I need to check the angle on the saw before I make a cut. (In step 13 I added a way to set angle measurements directly from the angle divider without the use of a T-bevel. This can work with either a table saw or a power miter saw. That way makes use faster and makes accuracy more certain)

This is a link to original instructions for the Stanley No. 30.

Some of the Stanley Angle Dividers came with a mahogany handle or body between the two wings (arms). Later that changed to nickel plated steel. The steel version also had a crossbar at the bottom end in the final version that allowed the angle divider to be used as a try square when the wings were folded in.

For an outside corner with this angle divider, record the actual angle of the corner with the bevel finder and lock it securely. Then fit the angle divider to the bevel finder without jarring the bevel finder's setting. Loosen the bevel finder and use it as before to record one-half of the angle of the corner from the angle divider. The second photo shows a miter I cut with the angle divider on an outside corner that is not an exact 90 degrees. (It is not yet glued or nailed, which is why I am holding it.)

Finishing touches--

I could stain and varnish the wood. I could also trim and smooth the ends of the machine screws.

*See step 13 and the text related to photos 5, 6, and 7. Taking the angle reading for the miter cut from one side of the angle divider may result in a slight inaccuracy. But, do take the angle reading from one side. Cut one piece of the finished corner from that reading. Then take an angle reading from the other side of the angle divider and cut the other member of the corner from it. The finished miter will be perfect because any inaccuracy in one cut will be complimented by a slight opposite inaccuracy in the other cut.

Step 11: Addendum - Accuracy

The photo shows another version of the angle divider I made after giving the one shown in earlier photos to my son-in-law for his birthday. I wanted to attempt a welded version, but ran into problems.* I did, however, discover something about countering inaccuracies in the tool.

If everything in the angle divider is as accurate as hoped, you should be able to push the arms against a straightedge and the angle between the body and the straightedge is exactly 90 degrees on both sides of the body. (This means the angle divider can easily be used as a try square. Just push its arms against something straight firmly enough to remove any play and tighten the adjusting nut.)

The next step explains why the steel version tool shown was just a bit inaccurate and what I did to fix that.

*Square tubing looks like it is straight and true, but usually is not. After I had the body welded together, I discovered I needed to true the sides. The process I used is the same I used in the Instructable at this link. The other problem was that I could not get a precise enough placement of the center pivot axle. Welding always causes shrinkage at the weld, and that makes things move. See the next step for my solution.

Step 12: Center Pivot Point (adjustable)

See the text boxes in the first photo. I added a new adjustable center pivot axle. In the photo you can see what remains of the original pivot axle welded between the two pieces making up the body. In the welding process it moved and came to be offside by thousandths of an inch. When checking the miter angle from both sides, there was a slight difference. It was very slight and probably would scarcely be noticed in a miter on stock only 3/4 inch or less in thickness.

The second image shows an exploded view of the adjustable center pivot axle mount I made from 3/16 inch rod and 1/8 x 1/2 steel bar. The red lines indicate the location of welds. The holes are for #8 machine screws. The holes are oversized so some adjustment is possible.

The third photo shows the underside with the adjusting screws. I threaded holes in the 1/2 x 1/2 square tubing.for the #8 screws.

The fourth image shows an alternate method of providing an adjustment for the pivot that does not involve welding, but does use a wooden body for the angle divider. The hole in the body for the 10 x 32 pivot screw is oblong side-to-side. The wooden body has been rabetted for a piece of 1/8 x 3/4 inch steel bar. Drill a hole in the steel that aligns with the oblong hole in the body. Tap it for 10 x 32 threads. Set the adjustment of the pivot as in the next paragraph.

To set the adjustment, spread the arms against a straightedge as shown in the fifth photo. With the #8 screws not quite tight (steel version from 1/2 square tubing, loosen the #10 x 32 pivot screw on the wooden body version), press the square against the straightedge and press the angle divider body against the square. Carefully tighten the adjustment screws. Check the other side of the body for squareness, too.

Step 13: Simplify

In my experience imprecise handling of the bevel finder during transfer of the miter angle easily introduced error problems that resulted in a poorer fitting miter. (It is especially easy for the position of the bevel finder to twist a little so the arm is a little higher or lower than its body and not really flat. This results in a different reading that produces a slight variation in the angle between the blade and the body.) It is possible to eliminate the use of the bevel finder by setting the saw directly from the angle divider.

See the first photo. Capture the corner angle as before.

See the second photo. Slide a piece of scrap wood with parallel edges lightly against the side of the blade.

See the third photo. While holding the scrap of wood so it does not move, slide the body of the angle divider against the edge of the scrap wood. Keep the angle divider and the scrap of wood from moving. Adjust the saw's miter gauge to fit the arm of the angle divider. (If you are using a power miter saw [See the fourth photo. The angle divider shown in the fourth photo is another version I made.], fit the body of the angle divider to the fence of the saw. Swing the motor until the blade is parallel to the arm of the angle divider.)

See the fifth photo. Cut one of the pieces of wood to be mitered. (If the angle divider is precise and aligned as in step 11, both pieces forming the corner may be cut from the same side of the saw blade. If not, see the next photo.)

See the sixth photo. If the angle divider is not absolutely precise (i.e. The angle read on one side of the body is slightly different from that read on the other side of the body.), set the miter gauge to make the second cut from the opposite side of the blade. Each cut may vary a tiny amount, but the errors will compliment each other and the finished corner will be exactly what was "read" by the angle divider in the first photo.

See the seventh photo. This particular corner is greater than 90 degrees, but the miter is precise and tight. Both pieces were cut from the same side of the blade, too.

I made this welded version of the Stanley No. 30 Angle Divider to keep after giving an earlier steel version to one of our daughters and giving the oak and aluminum version to our son-in-law. I did not expect to use it much, if at all. But, after discovering I can set my table saw directly from the angle divider, I will use it frequently for nicely mitered corners of all kinds.

If you would like to make the version of this Angle Finder shown in the fourth photo, I did a separate Instructable on it. I am making those as gifts for friends. They are more work than the basic version in the first part of this Instructable, but are very accurate the first time without fail.