Cutting long, tapered surfaces

This article shows a setup method for cutting long, tapered surfaces on a three-axis milling machine. The part to be made is 533.4 mm (21") long by 101.6 mm (4") wide by 50.8 mm (2") thick. It has a 0.3-degree taper in the length dimension and a 4.3-degree taper in the width dimension. There are also threaded holes perpendicular to each tapered surface.

I find it convenient when cutting a long part on a milling machine to hold the part with two milling vises. Figure 1 shows the finished part set on two vises. The surface parallel to the machine table with the threaded holes has the 0.3-degree taper with 13 5/16-18 holes perpendicular to the tapered surface.

The finished part is shown set on two vises (left). The finished part is displayed on the setup with the angle blocks used to set the 4.3-degree angle (right). Image courtesy of B. Taylor

So the question is how to accurately set the taper. What I do is make L-shaped angle blocks to fit between the vise bed and the part with the short leg of the L against the end of the part. One of those blocks is shown on a vise bed beside the part. The angle blocks are CNC-routed from 6.35 mm (0.25") thick aluminum plate. The taper amounts to 2.794 mm (0.11") in the 533.4 mm long part. The inside surfaces of the L are at right angles to each other and tapered at 0.3 degree from the outside surfaces. When the short leg of the L is against the end of the part, the correct angle is ensured. It gives visual assurance to an operator that the angle being cut is proper. Let a CNC tool make the angle, and the work of the machine operator is simplified.

Note the work stop against one of the angle blocks. This is a little more insurance that the setup is correct. You want a rigid, positive setup so the machine aggressively can cut the metal. That’s one way that the setup time pays off.

Figure 2 shows the finished part on the setup with the angle blocks used to set the 4.3-degree angle. Three angle blocks are used here. Because of the relation of the taper to the other surfaces, holding the part with vises is not appropriate. Notice the tab on the bottom of the angle block. That tab fits into the T-slots in the machine table. This speeds up setup since the y-axis position of an angle block is set by the tab being inserted into a T-slot. When the part is placed on the three angle blocks, the y-axis position is set and the x-axis surfaces of the part are parallel to the x-axis of the machine tool. The part is held by a T-slot clamp in the usual way.

Figure 3 shows the three different angle blocks used. This way of setting angles allows a more rigid setup than using a tilting table, tilting vise or sine plate. A more rigid setup means faster metal removal and better surface finish. I find that the time spent making the angle blocks makes this type of job easier than fiddling around with gauge blocks to set angles.

All in all, it’s a timesaver.