Filtering a subform by LinkChildFields and LinkMasterFields not operating --?

[plh]

I have a form which contains a SubForm. Opening the form to a specific record is
no problem. It opens with filter [IDTool]=64 (for example). I thought I was
applying the same filter to the SubForm when I originally set the
"LinkChildFields" and "LinkMasterFields" to IDTool. But in the drop down box in
the sub form I see all records. Note that the SubFrom has a Union table as its
source object. It has two foreign keys IDTool and IDMaterial. They are both
linked up with referential integrity enforced. The idea is to show all
IDMaterial but only one IDTool (64 in this example). It appears that I need
something else because the aforementioned set of "LinkChildFields" and
"LinkMasterFields" to IDTool does not seem to be having any effect.
What Am I doing wrong?
Thanx,
-plh