Effective Training Inc. Message Board

Hi,

Is there a difference in inspection methods or tolerance zones for a hole toleranced as perpendicular to a datum or true position to a datum, if they have the same tolerances & same datum references.

A little more information. I have two holes in line that a pin goes into. The holes are different sizes and are dautms -C- & -D-. There are three holes in a radial pattern around the center of the round part. The pins that go in to the holes have gears mounted on them and all three gears mesh to a gear in the center of the pattern, not part of this part. Datum -A- is the face of the mounting flange of the part perpendicular to the holes. Datum -B- is the pilot dia. of the mounting flange. My tcf callout would be a composite callout for aixs C-D of true position dia. 0.05 to A and B & true position dia. 0.1 to A. Our manufacture is recommending true position on the larger hole to -A- & -B-, and a seperate tcf for the axis C-D to be perdendicular -A-. The holes are dimensioned with a basic dim for the dia. of the pattern and two basic angle dims for the radial location.

I hope I having given enough infomation and thanks for any help you can give.

The problem a lot of people have is that they try to get the cart before the horse. You first have to place six INDIVIDUAL holes into the overall component, three on each side. The overall component can have functional datums assigned. You have done that by identifying two features as datum A (planar) and datum B (axis). If the "clock position" of the 3-hole pattern (one pattern on each side of part) is important then you'll need to establish an orientation datum (tertiary) to constrain rotation about datum axis B. If that constraint is not necessary then only two datums will suffice (A and B). I have always said that (in most cases) we don't design datums... we design features that may become datums for downstream features to reference from. First you have to put some holes (6) in a big honkin part! Then, you can choose one or more of those existing holes to act as datum references to control other features. Take a look at my suggestions on the attached marked copy of your pdf file. You'll see that I locate the 3-holes on each side of the part to a common datum framework (A, B, T). I then select the individual hole on one side to act as a datum reference for the location of the opposing hole (coaxiality).

The previously proposed two single-segment TP control shown in the center of your file called "Design" is an incorrect usage. If the upper FCF constrains the orientation to datum A to Ø0.05 the lower FCF tolerance becomes incoherent. The lower FCF can only further constrain the upper control (be less that 0.05 tol in lower FCF).

My thoughts, anyway!