Tutorial AA: Composite position, evaluated from CMM data

Books	Cogorno, Geometric Dimensioning and Tolerancing for Mechanical Design, ch. 12 (composite position) and ch. 13 (graphic analysis); Drake, Dimensioning and Tolerancing Handbook, ch. 5
Standards	ASME Y14.5 (composite position tolerancing)
You learn	Frame Style Composite Frame, PLTZF and FRTZF, the gauges of a composite frame, Analysis › Position Evaluation
Sizes	A plate 80 × 80 × 10 with four holes Ø5 +0.2/0 on a 40 mm square, 20 mm from datums B and C

A cover plate is bolted on with four screws, and the plate’s holes must line up with the tapped holes underneath. The pattern may sit 0.2 mm off where datums B and C put it. The holes must stay within 0.04 mm of each other, square to A, because the screws go into a matching pattern. One composite frame says both things.

The hole layout is that of Cogorno’s graphic analysis example (ch. 13), converted to millimetres. The measured data are ours.

Step by step

1. The plate, its datums and its holes

The plate is made by script. Its datums are A (underside), B (left) and C (front), and its holes are dimensioned 4X Ø5 +0.2/0, all on the four hole faces. The part’s standard is ASME Y14.5.

2. The composite frame

Select the four hole faces, then PMI › Feature Control Frame:

Position, Ø ticked, 0.2, MMC, datums A B C;
Frame Style Composite Frame, lower row 0.04, MMC, datum A.

The Feature Control Frame dialog with the lower row

The label is a two-row frame with one position symbol: position | Ø0.200 (M) | A | B | C / Ø0.040 (M) | A.

3. The inspection plan

PMI › Inspection Plan lists each row as its own characteristic:

PLTZF: Ø0.2 at MMC, up to Ø0.4 at LMC, which locates and orients the pattern to A|B|C;
FRTZF: Ø0.04 at MMC, up to Ø0.24 at LMC, which holds the spacing and the orientation to A.

The functional gauge has two gauges. The first has pins of Ø4.8 on A, B and C. The second has pins of Ø4.96 that float on A, with no stops for B and C.

4. The rule on the lower row’s datums

Set the frame’s LowerDatumReferences to B in the property view: the plan’s check reports an error. A composite lower row may only repeat the upper row’s datums, in their order. They orient the row and never locate it. Then set it back to A.

5. Position Evaluation

Select the frame, then Analysis › Position Evaluation. The table lists the true positions from B and from C, read from the model. Type the CMM’s results into the table:

Hole	From B	From C	Size
1	19.94	20.06	5.12
2	20.08	60.08	5.16
3	60.12	59.96	5.20
4	60.04	19.96	5.08

The result is ACCEPT.

Each hole’s bonus is its size less 5.0. With it, the pattern-locating zones are 63 % used.
The feature-relating zones may slide and turn on A. Fitted to the holes, they are 72 % used.

6. If the lower row could not turn

Double-click the evaluation, then choose Lower row zones › Translate only. This is what a lower row repeating A|B would allow. The same holes now use 103 % of the zones: REJECT. The datums a composite lower row repeats decide whether this part passes.

The replay

#	In the book (NX)	In FreeCAD NX-Style	Result	Notes
AA1	The cover plate: four holes 4X Ø5 +0.2/0 on a 40 mm square; datums A (underside), B (left), C (front); ASME Y14.5	modelled by script, the datums and the size made as in tutorial T	differs	datum feature A; datum feature B; datum feature C; 4X Ø5.000 +0.200/0
AA2	Feature Control Frame, Frame Style Composite: the pattern Ø0.2 (M) to A\|B\|C, the holes to each other Ø0.04 (M) oriented to A	4 face(s) selected + ribbon 'PMI › Annotation > Feature Control Frame'; Position, Ø ticked, 0.2, MMC, A B C; Frame Style Composite Frame, lower row 0.04, MMC, datum A; MB2 = OK	same	position \| Ø0.200 (M) \| A \| B \| C / Ø0.040 (M) \| A
AA3	Inspection Plan: one row per segment, one functional gauge per row	ribbon 'PMI › Inspection > Inspection Plan'	differs	PLTZF Ø0.2 at MMC up to Ø0.4 at LMC, gauge pins Ø4.8 on A\|B\|C; FRTZF Ø0.04 up to Ø0.24, gauge pins Ø4.96 that float on A (Cogorno ch. 13: a composite frame takes two gauges). NX lists the frame; the plan and the gauges are NXStyle’s
AA4	A composite lower row may only repeat the upper row’s datums, in order	the frame’s LowerDatumReferences set to B in the property view; Inspection Plan; set back to A	differs	the check reports B as an error: the lower row orients to the datums it repeats, starting with the primary (Cogorno ch. 12, Drake ch. 5)
AA5	Analysis > Position Evaluation: the measured holes against both rows	the frame selected + ribbon 'Analysis › Tolerance > Position Evaluation'; the CMM’s 12 values typed into the table; MB2 = OK	differs	ACCEPT: the pattern-locating zones (with each hole’s bonus) 63 % used; the feature-relating zones, moved and turned to fit, 72 % used. The paper gauge of Cogorno ch. 13 done numerically: a minimax fit. NX has no evaluation of measured data (that is a CMM program’s job)
AA6	The same holes if the lower row could not turn (as if it repeated A\|B)	the evaluation double-clicked, Lower row zones 'Translate only', MB2 = OK	differs	REJECT: 103 % of the feature-relating zones. The datums a composite lower row repeats decide how its zones may move, and so whether the part passes
AA7	The cover plate with its composite frame	render of the graphics window	same	6.8% of the view shows the part

In the book (NX)

In FreeCAD NX-Style

Result

Notes

AA1

The cover plate: four holes 4X Ø5 +0.2/0 on a 40 mm square; datums A (underside), B (left), C (front); ASME Y14.5

modelled by script, the datums and the size made as in tutorial T

differs

datum feature A; datum feature B; datum feature C; 4X Ø5.000 +0.200/0

AA2

Feature Control Frame, Frame Style Composite: the pattern Ø0.2 (M) to A|B|C, the holes to each other Ø0.04 (M) oriented to A

4 face(s) selected + ribbon 'PMI › Annotation > Feature Control Frame'; Position, Ø ticked, 0.2, MMC, A B C; Frame Style Composite Frame, lower row 0.04, MMC, datum A; MB2 = OK

same

position | Ø0.200 (M) | A | B | C / Ø0.040 (M) | A

AA3

Inspection Plan: one row per segment, one functional gauge per row

ribbon 'PMI › Inspection > Inspection Plan'

differs

PLTZF Ø0.2 at MMC up to Ø0.4 at LMC, gauge pins Ø4.8 on A|B|C; FRTZF Ø0.04 up to Ø0.24, gauge pins Ø4.96 that float on A (Cogorno ch. 13: a composite frame takes two gauges). NX lists the frame; the plan and the gauges are NXStyle’s

AA4

A composite lower row may only repeat the upper row’s datums, in order

the frame’s LowerDatumReferences set to B in the property view; Inspection Plan; set back to A

differs

the check reports B as an error: the lower row orients to the datums it repeats, starting with the primary (Cogorno ch. 12, Drake ch. 5)

AA5

Analysis > Position Evaluation: the measured holes against both rows

the frame selected + ribbon 'Analysis › Tolerance > Position Evaluation'; the CMM’s 12 values typed into the table; MB2 = OK

differs

ACCEPT: the pattern-locating zones (with each hole’s bonus) 63 % used; the feature-relating zones, moved and turned to fit, 72 % used. The paper gauge of Cogorno ch. 13 done numerically: a minimax fit. NX has no evaluation of measured data (that is a CMM program’s job)

AA6

The same holes if the lower row could not turn (as if it repeated A|B)

the evaluation double-clicked, Lower row zones 'Translate only', MB2 = OK

differs

REJECT: 103 % of the feature-relating zones. The datums a composite lower row repeats decide how its zones may move, and so whether the part passes

AA7

The cover plate with its composite frame

render of the graphics window

same

6.8% of the view shows the part

Result	Means
same	The book’s NX steps work as written.
differs	It works, but the FreeCAD way is different; the notes say how.
limit	The book’s way fails in FreeCAD itself; a workaround follows in the next steps.
missing	FreeCAD has no such function; a stand-in follows.
skipped	Not replayed; the notes give the reason.
FAILED	Did not work.

Result

Means

same

The book’s NX steps work as written.

differs

It works, but the FreeCAD way is different; the notes say how.

limit

The book’s way fails in FreeCAD itself; a workaround follows in the next steps.

missing

FreeCAD has no such function; a stand-in follows.

skipped

Not replayed; the notes give the reason.

FAILED

Did not work.

What differs from NX

NX’s Feature Control Frame has a composite frame style as well. NX-Style also checks the lower row’s datums.
NX states the requirement but does not evaluate measured data; CMM software does that. Position Evaluation is NX-Style’s.
STEP AP242 receives the two rows as two position tolerances, because OpenCASCADE cannot link them.