I have done hundreds maybe thousands of ejector pins. So I will try and shed a little light on how I do it.
I build Investment Casting molds (no U in my molds), so the mold base is aluminum. I always drill and ream for ejector pins "after" all the cavity work is done.
9 times out of 10 I will drill from the back side of the mold base. I clamp the ejector plate to the back side of the base and drill, ream and counterbore all in one setup. If you have a base that is very thick or you are drilling for small ejector pins, a wandering drill might be a concern. Then you might want to drill and ream from the cavity side and then I might open up the hole a bit from the back side.
After all my holes are done, I will assemble the mold. With the ejector plate in place, I will depth mic through the ejector holes and establish all ejector pin lengths.
With my pins and dimensions in hand I head over to the surface grinder. I have an ejector pin fixture for the next steps. First rough length is established with a cutoff disk. Then I grind the tops of the pins to the correct lengths.
Sometimes pins are located in a non-flat or contoured area of the cavity. In this situation I will "time" the pin by cutting a "D" shape on the head. These pins must not rotate. In the ejector plate, instead of a counterbore I will machine a matching "D" shape. I assemble the base with pins in place and machine the top of the pin to match the contour of the cavity.
I try to establish the lengths of the pins with "metal safe" in mind. If I make a mistake, I want them to be too long and "not" too short. Although I try to establish the lengths dead on, my customers would rather have the pin impressions on the "part" to be slightly recessed rather than raised. The tolerance that I try to follow for the length of the pin is: - .000 + .001 (inches of course)
Hope this helps.
Scott  |