Tools - Angle Plate

Milling an Angle Plate by the Trigonometric Method

In this post I demonstrate how to create an angle plate (also called an angle block) with the angle accurately milled to any desired angle. Although I am sure this method has been around for a long time, I first became aware of it by reading this post. In brief, the method is to drill two accurately located holes in a rectangular plate, and by inserting rods into the holes, suspend the plate from the top of the mill vise at the desired angle; the exposed edge of the plate can then be milled "flat" to create a finished plate with the desired angle. Angle plates are typically used to set up work in a mill vise when the work needs to be held at a specific and accurate angle.

In the example shown here, a plate is create

d with an angle of 29.5° for use in setting up the compound slide on a lathe for screw cutting. We begin with the trigonometric formula for converting the desired angle into a set of X,Y coordinates (see figure to the left). Using this formula we can calculate the distance Y for an angle of 29.5° and a length of X. You can do this with any calculator that supports trigonometric functions, but I created a chart and spreadsheet so that I could have a printed reference handy for general use. Using the chart I created, I find that for a length X of 1", the value of Y is 0.566".

I now set up a 0.25" thick aluminum plate (which I previously squared on the mill). For the purposes of illustrating this technique, I scribed an angle of approximately 29.5° on the plate using a protractor. I then center drill at the lower right of the plate, and establish the coordinates of this location as X,Y = 0,0. Next, the drill is moved 1.000" to the left, and 0.566" up (X,Y = 1.000,0.566) and a second hole is center drilled.

Next, the two holes are through-drilled using a #14 drill.

Finally, each hole is finished with a 3/16" reamer.

The next step is not typically done for a general purpose angle plate. However, for the special purpose angle plate I am creating here, I need to add a "ledge" or step to the plate so that the plate can be held aligned with the end of the lathe cross slide while the compound slide is adjusted to the desired angle. For this purpose, the plate is flipped over and approximately 0.80" of the plate is milled away, starting 0.10" inches from the edge.

Next, the section of plate to be milled at an angle is rough cut (using a hacksaw) to remove the bulk of the material.

Two pieces of 3/16" drill rod (approximately 2" in length) were previously prepared by cutting to length with a hacksaw. Both ends of each rod were then slightly chamfered by chucking in the lathe (the pieces were wrapped with paper before chucking to prevent gouging or scratching), filing lightly, and then finishing with emery cloth.

Finally, the rods are inserted in the plate, and the plate is mounted in the mill vise as shown, taking care to make sure that each rod is held firmly flat against the top of the vise. The exposed edge is then milled "flat" to complete the finished angle.An additional step particular to my use was to bevel the top edge (using a 90° point angle end mill) to ensure that the angled edge would lie flat against the compound slide.

The photo to the left shows the completed angle plate in use on the cross slide (I used a Dremel tool to scribe the angle on the plate for future reference). You can see the beveled edge of the angle plate next to the compound slide; a slightly protruding screw head on the compound slide shows why the bevel was needed (my compound slide was modified with a cam lock by a previous owner).Some closing comments:

• I have not given overall plate dimensions for this build, as they are not critical. The most critical concerns for getting an accurate angle is the positions of the holes with respect to the edge of the plate, and a smooth fit for the rods in the holes.
• I do not believe the rod diameter is critical, except that the rod should not be too large in relation to the size of the plate, or so small that it might flex. It seems to me that a rod diameter of 3/16" to 1/4" is about right, although I have no theoretical basis for saying this. I suspect that a rod size as small as 1/8" would work, but as I do not have a reamer this small, I have not tried this.
• It is important that the two rods used have identical diameters, as consistent as possible. For this reason, drill rod is a very good choice. Care should be taken in preparing the rods to make sure they are not gouged, scratched, bent, or deformed in any way.
• The use of aluminum for angle plates is not ideal in that aluminum is a soft metal which can be easily dented or scratched with heavy or careless use. However, for the kind of light use expected in a home shop, I feel that aluminum is an acceptable choice as it is easily machined and more than hard enough for the home shop.

Although steel angle plates can be purchased at relatively low cost, I often prefer to make my own tooling. As a "newbie" home machinist, I find that making my own tooling provides valuable learning experiences. In addition, there is a lot of satisfaction is using tools I have made for myself.