Milling: creating a smooth end finish in a slot

[Boyan Silyavski]

VERY impressive! Especially your worked numbers (2.5 bar, 2 mm inside-diameter feed tubing, 0.8 mm inside-diameter nozzle). I'm working in my loft, so I have nothing like the space that you have, however there are dental compressor rigs that should work well (and quietly) available for around UKP150 on ebay so, using these numbers I should be able to make something similar.

Thanks!

No problem. There are many builds on the net, but when i started making mine i found that they or intentionally skipped explaining the fine details or were just unaware of them. basically almost useless information. So like always i had to start from scratch.

Here are some more details if you have not figured all from the video:

- the brass bar body ideally is >=8x8mm
- fitting holes translate seamlessly to inner bore hole.
- the 2 pneumatic fittings are m5 to 4mm OD tube
- the distance between them should be at least 2cm from center lines
- the way/bore hole/ that connects all is 2mm ID
- the brass tube is 2mmOD and 1mmID. I cramp it at the end with pliers, drill it 0.6-0.8mm, sand it flat, fine sand carefully the edge to chamfer without destroying the flat nozzle face Check correct flow with more output. This is the critical part that defines the blow pattern.
- brass tube is super glued and pushed fit at brass body. Dont overdo it as you will need to exchange tha tube sometimes for some jobs
- for hevy cutting up to 20-25mm cutter 1 tube is ok even blowing heavy chips from working surface, then better 2 tubes positioned from opposite sides
- hose is 4mm OD 2mmID PU hose from China , both air and liquid hose are the same
- quality water filter body will resist even thinners / mine is made in Italy/, a crappy Chinese one will explode if sth else than water, especially using alcohol if you see even one micro crack throw it away.
- 1x baby oil+0.1 x dish washer+ 1x water or kerosine / careful with the container, test it first without pressure and then fill it to the top, full container just breaks, does not explode/

[Rob Meades]

Well, I'm not having much success here. I don't have my compressor yet so I thought I'd try a HSS/8% cobalt end-mill (4 flute, 7 mm flute length, 6 mm diameter shaft) which requires a lower speed. With the tool sticking out 20 mm to avoid my clamps, I've taken the HSM Advisor recommendations and then backed them off, so 5 mm/second feed rate and 0.5 mm depth of cut, tool running at 14000 RPM as recommended. As a reminder I'm cutting a slot in [343-460 N/mm2 tensile strength] brass plate.

Result: tool snapped off in the middle of the first pass. It looks as though the tips have worn quite quickly and then the tool has become clogged with brass:



Here are my tool details, material details and the HSM advisor screen to match. Where am I going wrong?



FYI, until it snapped, it appeared to be making quite a reasonable cut:



Rob

[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!

[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