Need advice on surface finish

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...

Attachment 2115

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

The finish may not just be a case of rigidity, it can also be related to transmitted vibration and damping. Even the most rigid structure will transmit vibrations... see this article on fignoggle.com on their trials and tribulations with surface finish on a RF31 clone...

That looks like you are using the side flutes of the cutter. Maybe a cutter with only two or three flutes.

When using the side flutes, it will try to work like a screw thread against the side of the job, running up and down the material. So any up/down movement of either your quill or vice will give those results. Try locking up the quill and tighten up the gibs on the cross and topslide.

To obtain really superior results, get yourself a good quality end mill with as many flutes as possible, and razor sharp, the ones I use have at least 6 flutes. Run at high speed, conventional milling only, no climb milling, slow feed and fine cuts, plus of course, plenty of lube. If you get it right, the surface should be like a mirror.

Bogs

It's called "chatter". A sort of sympathetic vibration and you've got it bad.

The easiest way to get chatter is big overhangs, reduce your overhangs.

Apart from that you can either increase the rigidity to damp it out or change the frequency either by adding flutes, as bogstandard suggests or by changing the RPM.

A radical spindle speed change before the finishing cut might help but chatter does tend to echo through on to the next cut.

With that much chatter, I agree, cutting uphill is probably a no-no.

Robin

Thanks everyone for the suggestions, I've been trying different feeds and speeds and more flutes as suggested and I have improved things by running 4 flutes at minimum speed (about 300 RPM I think) - feed doesn't seem to make too much difference except that it has to be fairly slow at that speed to avoid machining marks. Locking the quill doesn't make too much difference but I assume the resultant cutting force is downwards which is against the feed mechanism so it's fairly rigid in that plane.

I wouldn't even consider climb milling with this rig :-)

Looking at the milling head, the part of the casting that connects the main body of the machine to the spindle is just two thin vertical plates - I might try and bolt some cross members in to stiffen it.

I think I can make some improvements with work holding as well though the vice is good and solid.

I'll also try and source some 6 flute cutters - any recommendations?

Thanks again,
Mark