Milling: creating a smooth end finish in a slot

[Rob Meades]

The machine purchased was specifically stiffened for cutting brass; at least, that's what they told me, there was an extra stiffening bar, out of sight up-top in the picture of the machine above, for this purpose. I have a feeling that the mess you see on that last picture is because the tool had degraded by that point and hence was rubbing. If the machine were too sloppy, I'd have thought the same mess would appear early on as late on (see right-hand side of middle picture versus left-hand side)? Maybe a better question would be: could lack of machine rigidity somehow cause damage to the cutting edges of the bit?

It is worth noting that I'm cutting a very long continuous slot here as this is a maze that I'm cutting.

Plunge rate is 0.3 mm/sec. Not sure about spindle run-out: how would I go about checking it?

[cropwell]

Rob,
Are you using anything to clear chips, cool the tool and possibly lubricate it ? A good blast of air and a squirt of WD40 seems to be the recommendation on other forums. You need to get the chips out of the way to stop them reattaching. Two flute carbide is also a better cutter for brass.

Cheers,

Rob

[Rob Meades]

No, I'm picking up a compressor tomorrow and hope to use Boyan Silyavski's guidance to make myself an air blower. Maybe I'm just asking too much, making such a long slot-cut without cooling/blowing? I moved away from carbide to HSS because I could run at a lower speed (and hence reduce the heating effect) but it doesn't seem to have been a sufficient reduction. I think I'm going to try again with the same settings but reducing the feed rate right down, to 2 mm/second. I'll keep the vacuum handy and give it a suck every so often. I'll also order some more bits, 2 flute this time, and a lot more brass. Gonna get this right eventually...

Thanks Rob,

Rob.

[Rob Meades]

To conclude this thread, I've now managed to cut the brass maze successfully. The secret was, as you probably expect, cooling. With a 1 mm internal diameter copper tube backing 2 bar of air pressure (no water, too messy for my loft) from a Jun Air compressor, cutting for 10 hours caused no damage to the bit whatsoever and the finish in brass is as good as that in perspex. I only applied the cooling during the looong cut around the side of the maze so as not to overheat the compressor but that was enough. Happy milling Christmas everyone!

[Robin Hewitt]

Now you need to rework your G-code so it pierces on a slope rather than a plunge. Thus allowing you to use a conventional end mill, rather than a slot drill, and slick up that maze floor

[Rob Meades]

Say what with the who now!? There are always wheels within wheels with this hobby. How do I pierce at a slope!?

[Robin Hewitt]

Your slot drill is bound to chew up the metal directly below it's centre, it is just something they do. A milling cutter has no cut towards the middle so it can leave a fine finish below. They all have their tricks.

However a milling cutter can only plunge vertically so far before it's non-cutting middle bottoms out, so you have to move sideways as you move downwards to remove that middle.

Well, either that or restrict your Z travel to what it can manage