Work Question - concentricity of holes

[Tris]

A long shot possibly, but has anyone on here got any experience machining concentric holes (20mm/50mm) in a large SG iron casting using an angled head on a maching centre

I'm trying to get a feel for what concentricty can be acheived reliably

Cheers


Tris

[timwilky]

I would have thought it would be down to the quality of your machining centre and tooling. SG iron should cut freely enough, maybe reduce feed rates for increased hardness depending on the make up of the iron

It is a very long time since I did any machining and pre dates modern machining centres etc, but I would have thought 20 and 50mm holes would not present a problem.

[AndyBrad]

we generally keep holes that size to within 0.001"

[Tris]

I would have thought it would be down to the quality of your machining centre and tooling. SG iron should cut freely enough, maybe reduce feed rates for increased hardness depending on the make up of the iron

It is a very long time since I did any machining and pre dates modern machining centres etc, but I would have thought 20 and 50mm holes would not present a problem.

The drawing asks for a concentricity of 50 micron between the through hole and the counterbore. Given that the casting is a meter tall and about 800mm in diameter with these concentric holes drilled and bored through the sides, this a difficult challenge.
The machine was "sold as being capable" but I'm begining to suspect that it isn't - and maybe no other machine is

we generally keep holes that size to within 0.001"

Andy is that the concentricity of one hole to the other or the tolerance on the diameter. If the first then you're the man I need to talk to ;)

Cheers

[timwilky]

Seeing as you are in Lincolnshire, I would guess you could get some "informal" advice from the local turbine manufacturing community on precision machining, are there any local institute of mech/prod engineers discussion groups etc?

[embee]

It's going to depend a bit on what sort of geometrical tolerancing they are calling for on the bores themselves (roundness, diameter, straightness, parallelism, surface finish etc). When you're down in the 50m range these are going to eat up a lot of the tolerance you have to play with. Is that positional tolerance of the average centres or TIR, each relative to a datum or to each other?

The other fundamental question is how is it going to be measured/inspected?

From my experience of cylinder block tolerancing (V12 etc) I'd reckon you're likely to be approaching the capability of the machine. In a casting the size you're describing the deflections of the fixture and even the temperature is going to be significant.