Experimenting with summing in a query.

Have a 2 simple tables with part number and qty as fields

ie table 1 part1.....qty1
tablw 2 part 2......qty2

the I set up a make new table query using the two tables with a link between
part1 and part2, and a column that sums the 2 qty together.

Now all is ok as long as I have identical part numbers in both tables, it
returns the values correctly and sums them up. But if I enter a unique part
number in one table and not the other it will not return that part number as
part of the query result.

How can I get the query to return all part numbers even if they only appear
in on table.

I'm using this as a test to then set up a query that will search across a
number of tables and return say all the bolts found even if they do not
appear in all the tables and total up all the same part numbers.

Any help appreciated

SR