Milling: creating a smooth end finish in a slot

[Robin Hewitt]

Brass should not be a problem, don't understand why you are having trouble.
Maybe we need a close up of the cut edge, is it square, have you got vibration marks, is it smearing at the top?
Maybe a picture of the machine that is doing the cutting?

[Rob Meades]

Thanks for the swift response and offer of help, here's some more detail. First, the machine (High-Z S-400/T):



Then, the material overall:



Then the cut; left is early on when some clear-outs are being cut, middle is at the start of the slot-cut (where the right-hand side of the picture is the earliest, the left-hand side is several minutes later) and finally right is the slot-cut just before the bit snapped:



For good measure, here's a video of the slot-cut phase, taken shortly before the right-hand side of the middle picture was cut.

[Jonathan]

From the third photo it looks like you have issues with rigidity. Your machine has unsupported rails on all three axes, so this is shouldn't come as a surprise. That shouldn't mean you can't make this part though, just makes it more of a challenge.

Is the tool plunging too fast? Ideally it shouldn't much at all.

Have you checked the runout of the spindle? With such a small tool this may have an effect.

[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!