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Bright EN24T steel vs Black EN24T for cutting gears

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Andrew Johnston12/10/2019 10:30:23
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It depends upon what is available but as a starting point 70rpm and 2.8"/min feed is good. I'd be cautious and go to the next lower available spindle speed and feed.

I might have been over-ambitious suggesting turning 4" diameter at 1200rpm. I've just turned some 1" EN24T to make some bolts. At 1200rpm the finish was ok, but not perfectly smooth, so i could have gone a bit faster. May be 600rpm would be better for 4" diameter.

Andrew

Adam Harris13/10/2019 23:19:24
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Thanks Andrew - that turning is INCREDIBLY fast! According to my charts for 1" with carbide inserts that is almost at maximum speed for Hard Brass at 1200 - my chart range for Medium Carbon steel with inserts is 350-500 for 1". I was thinking for 4" around 100-120, but I take your point and will experiment at higher speeds but 600 is at Aluminium speed...

JasonB14/10/2019 06:49:50
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Are you sure your chart is not in m/min rather than rpm

Andrew Johnston14/10/2019 08:52:25
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For the US equivalent material 4340 with a coated carbide tool and tempered for a Brinell hardness of 275 to 325 Machinery's Handbook gives a turning speed of 400 feet per minute. For 1" diameter work that equates to 1528rpm. Experience indicates that the medium carbon alloy steels need high surface speeds when using carbide tooling to prevent tearing of the surface.

Andrew

Adam Harris14/10/2019 20:31:21
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Well my chart does give for 400 ft/min material in 1" diameter exactly 1528rpm using HSS tools (actually it gives 1986 rpm for Coated tools). My chart (and I cannot remember where on earth I found it on the internet) offers that a general category of material has a wide "range" of speed in ft/min, depending presumably on its alloy variations. It gives Aluminium at 400-1000 ft/min, but I see that L.H. Sparey gives Aluminium a rate of 300 ft/min. My chart gives Stainless 60-90 ft/min and I see that Sparey gives it 50 ft/min and David Clark's Teach In article in MEW 216 (corrected by Neil Wyatt) gives "Carbon Steels, Stainless Steel and Alloy Steels" all at 50 ft/min . My chart gives High Carbon steel a range of 40-70 and Low Carbon steel a range of 80-140 and Stainless a range of 60-90. Perhaps, embarassingly, my chart is all nonsense, or fit only for large industrial machines, but until now it is fairly academic because I use it as a broad indication, since the best speed (and feed) for me has been one found by trial and error that produces nice chip formation and partly because I am rarely sure of the spec of material I am cutting! I do try to follow the principle that one should start erring on the slow side. However I have always believed that harder material should be turned much slower than softer material, hence my surprise that this hard EN24T stuff should be turned as fast as Aluminium. Have I been doing it all wrong, or is the major discrepancy the difference between Coated tools and Carbide Inserts (for which I have no data and actually hardly any experience)? I know inserts can be used faster than Coated cutters, but how much faster, and is the faster speed an option or a necessity? And if my chart's  ft/min ranges for materials are no good, can you point me to a chart that you recommend as giving better results?

Edited By Adam Harris on 14/10/2019 20:34:01

Edited By Adam Harris on 14/10/2019 20:47:35

Andrew Johnston14/10/2019 21:21:21
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EN24T isn't particularly hard. It's around 250-300 Brinell, which is mid 20s to 30 on the Rockwell C scale. While that's harder than soft steels it's well below cutting tools and things like springs.

A lot of the early amateur cutting data was intended for lightweight lathes and basic tooling. Carbide inserts weren't even a dream. Unfortunately starting slow doesn't always work, some materials just need to be run fast to get a good finish.

Andrew

Adam Harris14/10/2019 21:29:58
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Thanks Andrew, so are you saying that my chart of speeds is acceptable for HSS and Coated tools but if using carbide inserts I should go at as much as 4 x those speeds? Ie Stainless I should be looking at 240-360 ft/min with inserts, and aluminium I should be looking at 1600 - 4000 ft/min with inserts?

SillyOldDuffer14/10/2019 21:50:22
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Posted by Adam Harris on 14/10/2019 20:31:21:

... Perhaps, embarassingly, my chart is all nonsense, or fit only for large industrial machines, but until now it is fairly academic because I use it as a broad indication, since the best speed (and feed) for me has been one found by trial and error that produces nice chip formation and partly because I am rarely sure of the spec of material I am cutting! I do try to follow the principle that one should start erring on the slow side. However I have always believed that harder material should be turned much slower than softer material, hence my surprise that this hard EN24T stuff should be turned as fast as Aluminium. Have I been doing it all wrong, or is the major discrepancy the difference between Coated tools and Carbide Inserts (for which I have no data and actually hardly any experience)? I know inserts can be used faster than Coated cutters, but how much faster, and is the faster speed an option or a necessity?

Edited By Adam Harris on 14/10/2019 20:34:01

'partly because I am rarely sure of the spec of material I am cutting!' Yup that'll do it. Because you don't know what's being cut the best speed and feed rate can only be found by experimentation. Knowing the family a metal belongs to helps a bit, but not accurately.

A metal's machineability is determined by its combination of ductility, tensile strength, hardness and grain-size. The internal structure is also important and this is often altered by heat-treatment. Whether the same metal is hardened, tempered or annealed can effect cutting radically. Many metals and alloys don't machine at all well under any circumstances; they're chosen to suit some other process like stamping, casting, welding, or extrusion. They can be very hard, very tough, and very gritty. Or easily torn and sticky. Yuk.

Lubrication and cooling is important with some materials.

The other factor is the machine: production cutting rates assume heavy powerful machinery, not Hobby Lathes.

As a generalisattion Carbide works best at 5 to 10x the speed of HSS, and it gets better finish with deeper cuts and faster feed-rates. Most amateur equipment can't achieve this, fortunately it's not essential. Carbide, especially the sharper inserts, work well at slower speeds, But as they like it rough, very often the answer to a disappointing cut is to push carbide harder, the exact opposite of HSS where slowing down usually helps.

I find it easier not to prat about with feet per minute, calculators and tables. Dividing 10000 by the diameter of the job in mm gives the approximate speed in rpm for HSS and mild steel. Slow down by half for Cast Iron, increase x1.5 for brass, 2x for Aluminium. Double speeds for carbide, more if the machine will take it. The crudity of the cakculation is a strong reminder this is only a rule of thumb, not to be taken too seriously, but it's a good way to start. If acut doesn't go well, experiment.

As a complete begineer I wasted a lot of time trying to cut unknown scrap. Poor and inconsistent results confused me. Unless you know it machines reasonably well, my advice is to avoid scrap and buy metal intended to be machined. It's worth it.

Dave

Adam Harris14/10/2019 22:06:38
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"But as they like it rough, very often the answer to a disappointing cut is to push carbide harder, the exact opposite of HSS where slowing down usually helps" - thanks Dave, that is useful to know. And I will try carbide Inserts at 4x "Coated" speeds and experiment increasing from there. 

Edited By Adam Harris on 14/10/2019 22:21:33

Nigel Graham 230/10/2019 20:28:59
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A thread I'm looking at because I'm wondering whether to cut my own gears or modify stock ones for my project, since Edwardian trade magazine review photos are not exactly accurate works drawings so something that respects 1908 practice is near enough. (The originals were "machine-cut steel" though - it says so in the contemporary publicity material!)

Not Done It Yet _

Not sure where the "climb milling" comes from here, unless as being not sure of what Andrew Johnston's earlier comment meant about the cut being from "inside " the material.

Assuming horizontal milling, or the vertical-mill equivalent using a gear-cutter on a stub mandrel, for down-cutting the teeth approach the side of the blank and cut outwards or up (!) towards its circumference.

Look at Andrew Johnston's photo. If we assume the cutter is revolving anticlockwise as we look onto the machine: for down-cutting the table would therefore move leftwards (away from the camera). So each tooth, below the spindle axis so moving rightwards, would enter the blank's face remote from us and emerge up through its circumference.

If the cutter revolved clockwise for the same left-going table-travel, clearly that would be climb-milling, with the active teeth and table moving in the same direction. As you say, not for the relatively light-weight machines and accessories most of us possess.

I would be surprised if industrial gear-cutting by horizontal profile-milling rather than generating by gear-shaper or hob, would normally have been done in climbing mode, except perhaps for finishing cuts, but if someone knows otherwise...

not done it yet31/10/2019 05:47:40
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Thanks, but I do know the difference between ‘conventional’ and ‘climb’ milling. My query was a simple one. I most certainly use conventional milling when cutting gears with involute cutters on my machine. Not yet tried hobbing gears...

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