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Milling around with bits

Discussing the merits and uses of different types of mill bits

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Andrew Johnston17/04/2021 11:52:38
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A few more notes:

There's no fundamental reason not to use high flute count cutters on aluminium. The advice to use 2, or possibly 3, flute cutters arises from the fact that aluminium alloys are easy to machine and so feedrates can be much larger than those used for steel. So there is more swarf. Using fewer flutes helps prevent jamming of the swarf in the flutes. For instance the manufacturer of my cutters recommends, for a 6mm cutter, 3800rpm and 210mm/min in steel cutting full width and 0.5D deep. But for aluminium the recommendations are 7600rpm and 440mm/min, cutting full width but also 1D deep, so roughly four times the removal rate. Interestingly the feed per tooth is only slightly higher for aluminium. This is what happens when aluminium swarf clogs flutes:

clogged cutter.jpg

No idea why SoD says don't use suds on aluminium. On the manual mill I machine aluminium dry, or with the odd squirt of WD40. The WD40 doesn't lubricate, or do much cooling, but it does help prevent swarf from gumming up the cutter. On the CNC mill I use suds flood coolant, mainly to wash away the swarf. Recutting swarf is a quick way to kill a cutter. I've never used paraffin on anything.

You don't need long flutes to cut deep pockets. It's unlikely you'll be doing a full depth pass, and if you do the cutter will deflect. A cutter with short flutes and large stickout will be stiffer than a cutter with long flutes and the same stickout. So less tool deflection. This pocket is 48mm deep and was machined with a 2.5mm stepdown and standard length flutes:

cylinder_deep_pocketing.jpg

I start with selecting a width and depth of cut and then select an appropriate chipload. Then a spindle speed appropriate to the cutter and material, and that then allows the feedrate to be calculated. The critical parameter is chipload, in mm/tooth, rather than feedrate. It's counterintuitive but increasing chipload is often a cure for chatter. When I first started using my horizontal mill I was cautious. Result, the whole machine shook, which was impressive given it weighs 3500lbs. Double the feedrate, and hence chipload, and it cut perfectly with no chatter or vibration.

Andrew

Stuart Munro 117/04/2021 12:33:43
76 forum posts

Andrew, hyper clarity! Your comments make sense to me so I must have taken something in from this blog.

I think you have identified two misunderstandings on my part.

1/ Your picture of swarf is a familiar sight. I had thought it was due to the aluminium alloy melting at the edge and sticking to the bit, so thought slowing the SFM/RPM and feed rate would help. But clearly reducing the swarf with say WD40 will also ease the problem.

2/ I had read that two flutes was the way to go with aluminium but did not align this knowledge with the speeds that large CNC milling go at. So for my speeds 4 flutes should be no harder to use than 2 flutes. My Sherline has a maximum RPM of 2800 so I'm never going to go too fast unless my bits diameter exceeds the capacity of the mill! Worth understanding but I'll probably stick with 2 flutes for price and to be able to use a 'slot drill' when needed.

As to the chatter issue, I know the theory about increasing feed rate or reducing SFM but it is challenging to a novice. I've only been machining during lockdown so have never had an experienced machinist work with me. I do believe that one session with someone telling me 'you can feed a bit faster' will give me the confidence but that may still be a few months off. Happily, there is a professional engineer with his own shop in the family who I'm sure will help when we can do so!

Regards

Stuart

Tony Pratt 117/04/2021 12:59:33
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Don't want to get in a 'pi****g' contest but I have done quite a few jobs in Aluminium where only a long series end mill will do the job to drawing requirements & of course you get more deflection the longer the cutter, that's where the skill comes into the equation.

Tony

Edited By Tony Pratt 1 on 17/04/2021 13:08:30

Stuart Munro 117/04/2021 16:54:23
76 forum posts

Tony,

Sounds like your skill level is a different order to mine, as an accountant I could deflect numbers anyway you wish (but of course didn't) but deflecting a cutter sounds beyond me.

Stuart

Tony Pratt 117/04/2021 18:05:56
1503 forum posts
7 photos
Posted by Stuart Munro 1 on 17/04/2021 16:54:23:

Tony,

Sounds like your skill level is a different order to mine, as an accountant I could deflect numbers anyway you wish (but of course didn't) but deflecting a cutter sounds beyond me.

Stuart

yes, sorry for being too technical, all will become clear as you progress in your machining journey, as a general rule say for milling use the shortest sharp cutter you can & only use long series cutters if you have to. Loads of online advice will come your way but yes get hands on experience with a decent machinist if you can. Where are you located?

Tony

Andrew Johnston17/04/2021 20:36:48
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6009 forum posts
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Posted by Tony Pratt 1 on 17/04/2021 12:59:33:

Don't want to get in a 'pi****g' contest...........................

Probably wise, as those types of contest usually end up with someone getting wet shoes. smile

Andrew

Dave Halford17/04/2021 20:45:34
1439 forum posts
12 photos
Posted by Andrew Johnston on 17/04/2021 20:36:48:
Posted by Tony Pratt 1 on 17/04/2021 12:59:33:

Don't want to get in a 'pi****g' contest...........................

Probably wise, as those types of contest usually end up with someone getting wet shoes. smile

Andrew

But only the bloke next to you.angel

Stuart Munro 118/04/2021 07:21:15
76 forum posts

......... but yes get hands on experience with a decent machinist if you can. Where are you located?

Tony

Tony,

Essex - some good sources of 'hands on' advice once covid restrictions allow. Till then fun learning.

Stuart

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