Rotor balance weight system

1. A rotor having:

rotor body having:

a flange with a circumferential array of discontiguous apertures; and

a surface spaced apart from the flange; and

one or more rotor balance weight assemblies, each having:

a weight having:

a passageway having a first end and a second end;

an internal thread along the passageway; and

a boss at the first end of the passageway, the boss in a respective associated one of the apertures; and

a fastener having:

a shank having a first end and a second end and an external thread engaged to the passageway internal thread;

an engagement feature at the shank first end for engagement by a tool to turn the fastener; and

a head at the second end, the head contacting the surface.

2. The rotor of claim 1 wherein:

the circumferential array of apertures is 20-100 apertures.

3. The rotor of claim 1 wherein:

the flange is an axial flange.

4. A rotor of claim 1 comprising:

a rotor body having:

a flange with a circumferential array of discontiguous apertures; and

a surface spaced apart from the flange; and

one or more rotor balance weight assemblies, each having:

a weight having:

a passageway having a first end and a second end;

an internal thread along the passageway; and

a boss at the first end of the passageway, the boss in a respective associated one of the apertures; and

a fastener having:

a shank having a first end and a second end and an external thread engaged to the passageway internal thread;

an engagement feature at the shank first end for engagement by a tool to turn the fastener; and

a head at the second end, the head contacting the surface,

wherein:

said surface has a plurality of recesses, each aligned with a respective associated one of the apertures; and

the fastener head is in the recess associated with the associated one of the apertures.

5. The rotor of claim 4 wherein:

each said recess is blind.

6. The rotor of claim 4 wherein:

each said recess is doubly concave.

7. The rotor of claim 1 wherein:

each said head lacks engagement features for tool engagement.

8. The rotor of claim 1 wherein:

each said head is doubly convex.

9. The rotor of claim 1 wherein:

each said engagement feature is a socket.

10. The rotor of claim 9 wherein:

the socket is a hex socket.

11. The rotor of claim 1 wherein:

each said weight consists essentially of stainless steel or nickel alloy.

12. The rotor of claim 1 wherein:

each said boss protrudes from a singly convex surface portion of the weight.

13. The rotor of claim 1 wherein:

the one or more rotor balance weight assemblies are fewer in number than the circumferential array of apertures.

14. The rotor of claim 1 wherein:

each shank first end is subflush to a rim of the respective associated boss.

15. The rotor of claim 1 further comprising:

a plurality of stages of blades.

16. The rotor of claim 1 wherein:

each fastener shank is in compression between the surface and the associated weight.

17. A gas turbine engine comprising the rotor of claim 1 .

18. A method for assembling the rotor of claim 1 , the method comprising for each of the one or more rotor balance weight assemblies:

with the fastener in a relatively retracted condition contacting the fastener head with the surface;

engaging a tool to the engagement feature; and

using the tool to turn the fastener to extend the fastener and drive the boss into the respective associated aperture.

19. The method of claim 18 further comprising:

spinning the rotor and determining balance of the rotor and wherein the one or more rotor balance weight assemblies improve balance of the rotor.