abridged
Quote Originally Posted by mklotz View Post
An array of orthogonal T-slots may be a better alternative than the traditional array of drilled or tapped holes. The slots are easier to clean and, used with slotted strap clamps, can offer even greater flexibility in clamp placement than holes can.

The plate should be fitted with a removable block that can be attached to the plate bottom. Size said block so it fits in both the mill vise and the bench vise. Not having to remove the mill vise in order to use the plate will save time. Holding the plate in the bench vise while attaching parts is often easier than doing it under the mill head. The attached block need not be square; for instance, an hexagonal block allows for more clamping angles.

Removable fences on the sides of the plate can be helpful in managing part arrangements while clamping. Their removability is important so that once the parts are clamped the fences won't interfere with the mill tooling.

If you are going to employ a hole array, consider letting it design itself. Scribe what you think would be a good array on the plate. Later, when you need a hole, try to drill it on one of the points where the lines intersect. If through holes are drilled at the tapdrill size of the standardized thread you've decided upon for your clamping, the holes can be tapped at a later date if so required.
Agreed 100%, Mr. Klotz. Slots are the go-to in flexibility, for all the reasons holes are not. The only advantage of holes are in being seemingly easier to make.
Hole or tee-slot spacing should base on slot length of clamp tooling. The failures of strap clamping often due to misapplied ratio between toe and heel to fastener.