Fly cutting puzzle WD40?

[Paul BarterParticipant @paulbarter66156]

[Paul BarterParticipant @paulbarter66156]

Hello all

I ask more experienced machinists to throw some light on why when fly cutting a piece of aluminium, albeit in a vertical slide on my ML4, using a honed sharp tool steel cutter at the poor old girls top speed. I found a pronounced burr on the "exit" sides when using WD40 as a cutting fluid, yet with the same set up and depth of cut /manual feed speed (slow) with no cutting fluid, razor edges all round with no apparent burr. In both operations the surface finish was good with similar faint regular machining marks. This activity was prompted by Ades workshop and his nice new mill, but only to satisfy my curiosity, I will not be tendering for any defence contracts.

Thanks in advance to anyone who has any ideas as to what is causing this apparent contradiction.

stay well all

Paul Barter

Edited By Paul Barter on 10/04/2020 15:47:37

[BazyleParticipant @bazyle]

Aluminium is notorious for welding to the tool tip. Even if not welded it is 'sticky' and grips the tool. So at the last thou before the end of the cut the last spec of swarf is held onto the tool and gets pulled right off. But if you apply lubrication, paraffin is often used as far cheaper than WD40 of which it is a constituent, the swarf is just being pushed/cut by the tool and as aluminium is rather ductile the last little bit bends away rather than being cut.

[Neil WyattModerator @neilwyatt]

Good answer Bazyle. That one was stretching my grey cells!

Neil

[Paul BarterParticipant @paulbarter66156]

Thanks Bazyle, for your clear explanation of this behaviour. I am most grateful once more for the willingness of forum members to share their hard won learning.

best regards Paul Barter

[HopperParticipant @hopper]

Posted by Paul Barter on 10/04/2020 15:45:13:

 

…, using a honed sharp tool steel cutter at the poor old girls top speed.

Depending on the diameter of your fly cutter but it sounds like the rpm is too high, even for aluminium. Try slowing it down a bit.

Edited By Hopper on 10/04/2020 23:37:04

[HopperParticipant @hopper]

Or you could try scribbling on the workpiece with a felt pen. Seriously. Check this out: LINK

[Clive FosterParticipant @clivefoster55965]

Bottom line is that there is a balance between the force needed to cut the metal and the force needed to bend it. As that last thin shaving isn't strong enough to stand up and be cut it is pushed over the edge instead.

Even if you don't have a proper built up edge on the cutter by the time you get to the end of the workpiece the cutter will have accumulated enough tiny bits of aluminium swarf to effectively make it blunter and slightly less sharp. This means a little more force is needed to make the cut so that last little bit is more likely to be bent over.

I use (expensive) Rocol UltraCut synthetic cutting oil / coolant at 40:1 dilution which is noticeably better than suds or WD40 at lubricating the cutter and reducing adhesion to lessen the effect. Horrible stuff in some ways as the narrow jet from my modified Bjur SprayMist system still gets everywhere, despite being turned right down to barely visible. Periodic deep cleaning is essential to get it off chucks and collets to restore grip. Far too expensive to use in a flood system! In the modified Bjur I'm getting approaching 5 years per litre which is livable with.

Well worth taking some trouble to devise a machining sequence that has all the edge cuts going into the metal rather than away from it. Which obviously avoids the problem. But its not always possible and even if it theoretically can be done it may well be far too much trouble. Especially when also coping with the limitations of a vertical slide relative to a proper mill. Even if the full monty is not possible its still worth arranging things so the "out of the metal" cut is on a concealed edge or one that will have more work done to it. Making a neat job of cleaning the burr off can be surprisingly difficult. A casual swipe of the de-burring tool, as I'm too often wont to do, usually isn't up to scratch.

Clive

Edited By Clive Foster on 12/04/2020 11:11:43

[andrew lynerParticipant @andrewlyner71257]

A learning experience:

I am just coming to terms with using my flycutter and, to me, it seems to be the Milling equivalent to parting on a Lathe. It's scary to watch unless you get it right. When it works ok, you cannot imagine what the problem was, ever.

My first results were more or less just banging and rattling as the tool hit the side of the work and was because I started trying to make the tool like a turning tool. It is now 'obvious' that the tool slopes diagonally down from the holder but I failed to realise just how much. It is very hard to think in the 3D that's necessary for understanding cutting tools and I failed to realise that the two cutting edges for a turning tool are not the same two cutting edges as on a fly cutter. (The two operate nearly at right angles to each other). The more I persevered with this mistake, the worse it got. I now realise that the basic relief slope of the fly cutting tool is already provided by the holder. I was putting a 'heel' on the tool which was destroying this relief and giving me a very long cutting (hammering) edge.

My mistaken ideas were not helped at all by all the videos I watched because the guy shows a diagram. He then uses the grinding wheel to produce a beautiful face but then, 'abracadabra' he moves it and rotates it in the air and I'm totally lost. Which face has gone where? Despite having grown up as a 'formal' learner of theory using Maths, I have now joined the kinaesthetic school (learning by doing).

The shape of a milling cutter and the way it functions somehow makes more sense to me (now) as you can eyeball the tool against an actual flat surface better than the curved surface you have when you're turning on a lathe. As I increased the bottom relief, the banging eased up until it turned into more of a 'swish' of a scythe through grass. Sudden grooves and stalling (only a toy mill) seem to be things of the past now and I am working on 'that curve' to spread out the grooves from a pointy cutter.

The patterns that the flycutter (and all the other cutters) are quite disturbing as the illusion is that the finger smooth surface has deep grooves running along it. I guess that is largely down to a record of tiny vibrations – like a vinyl disc.

On the subject of raggedy edges, I was looking at this YouTube link and, although it is mainly about plastics, the principle seemed sound (and he demonstrated hi theory works – for him). The rake on the outer face of the tool seems to be very relevant for materials that are soft and weak. He says that a negative rake allows the tool to get right to the edge of the piece without being pushed out, rather than cut properly, and then the last bit is levered out by the wedge and falls off as an ugly edge.

[Martin ConnellyParticipant @martinconnelly55370]

You can always use a sacrificial scrap on the side that will burr over and leave a nice edge on the actual workpiece.

What diameter was the tip running at and what is top speed for the machine? As Hopper said it may be too high a speed for the diameter you were running at. This is steel but it shows how low a fly cutter can be run at to be at the correct speed for the material. The HSS was ground but not honed.

Martin C

[Ady1Participant @ady1]

Faster is not always better when fly cutting

Fly cutting a big bore has the same issues, speed and feed can make a big difference

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