MTM 2 NotesUnit 69Page 1 of 4

Unit 69: Machining Compound-Angular Surfaces

Big Idea: A compound angle surface can be machined by placing the piece on an angle plate, rotating it with respect to the tilt axis, then tilting the plate up until the surface to be machined is perpendicular to the axis of the cutting tool. When the compound angle surface is specified by angles on the orthographic drawings, then calculating the angles of rotation and tilt are done using these formulas:

Type of Given Dimensions / Angle of Rotation / Angle of Tilt
Angles / /

Big Skill: You should be able to calculate the angles of rotation and tilt for a compound angle surface given angle dimensions on the faces of the piece to be drilled.

Let’s say you have to machine a compound angle surface on a block of material as specified in the orthographic views below. Note that angle A will always be on the front face, and angle B will always be on the end face, and that they are measured from the horizontal (and not the vertical like for compound angle holes).

The way to figure out the angles of rotation and tilt is to think of the surface as cutting across all three faces of a rectangular solid defined by the angles on the orthographic drawing. The trick is to assume that the height of the solid is H = 1”.

  1. What is the angle of rotation R for the compound angle surface above?
  1. What is a formula for the angle of rotation R in terms of A and B?

  1. What is the angle of tilt T for the compound angle hole above?
  1. What is a formula for the angle of tilt T in terms of A and B?
  1. Calculate R and T when A = 25 15 and B = 22 37.