Need advice on surface finish

[FatFreddie]

Hi,

I've converted a Clarkes Mill / Lathe combo with the intention of using that to build a gantry style mill for cutting wood / aluminium.

http://www.mycncuk.com/forums/showth...9-The-sows-ear

I can get reasonable accuracy from it but the surface finish on the vertical plane is poor...



I'm assuming the poor finish is due to flex in the milling part of the machine not helped by a rather convoluted drive arrangement. The spindle bearings at least seem quite good!

Before I spend more money on it, is my assumption correct?

I think I could just about work with it to build the gantry mill (probably converting to ballscrews to improve the X/Y accuracy) but could I expect a better surface finish from that? I'm planning to make it from 3"x3" 1/4" thick box section and thick plate with a working area of no more than a metre square (probably 750mm) and using 20mm or 25mm supported rail for the slides and direct drive 16x5 ballscrews for positioning. I'm pretty sure I can make the frame rigid enough but how much flex will the supported rail add? I could go to profile rail but that's doubling the price but if it's necessary to get a good finish...

Alternatively I could look for a second hand "proper" milling machine to convert but I'd prefer a gantry style machine for the extra room (one project I'm thinking of is making a guitar).

Basically I suppose what I'm asking is whether this sort of surface finish is common and how rigid does a machine have to be to minimise / eliminate it - my own experience is restricted to this mill which does it with even a very fine cut and a Bridgeport that had to be sorely provoked to create a poor finish so I don't have much of a reference frame for the middle ground.

Sorry for the ramble!

Cheers,
Mark