Bright EN24T steel vs Black EN24T for cutting gears

[JasonBModerator @jasonb]

Adam, read my reply again, it is teeth ON THE CUTTER not the gear

[Adam HarrisParticipant @adamharris13683]

Aha that make more sense!! Thanks for the correction. Well the cutter comes from RDG and expect to receive it tomorrow so will know then, but probably as you say 12!

Edited By Adam Harris on 10/10/2019 12:00:33

[Anonymous]

There are 36 teeth on the gear being cut.

Just noted that the involute cutter is around 65mm diameter. My spindle speed of 60rpm was based on a 3-1/4" diameter cutter.

Andrew

[Adam HarrisParticipant @adamharris13683]

so 12 x 60 x 0.004 = 2.9 inches/minute a much more sedate rate of feed!

Edited By Adam Harris on 10/10/2019 12:02:31

[Adam HarrisParticipant @adamharris13683]

Andrew thanks I'll adjust the RPM and thus feed accordingly when I get the actual cutter dimensions .

Edited By Adam Harris on 10/10/2019 12:12:39

[Anonymous]

Posted by Adam Harris on 10/10/2019 12:04:06:

Andrew may I come back to you with the actual cutter dimensions when I get it then?

That's fine – Andrew

[not done it yetParticipant @notdoneityet]

Posted by JasonB on 10/10/2019 11:02:32:

NDIY, commertcial gear cutting is unlikely to be done with involute cutters.

So, JB, are (almost) all involute cutters sold to hobbyists? Maybe most gears are cut by hobbing in the commercial sector these days, but I wasn’t thinking of David Brown Gears in this context – more like the commercial machine shops rather than high production manufacturers – like automotive gearbox suppliers for instance.

Adam,

Do compare your machine to that of Andrew’s. The feed rate may be restricted by your machine… I try to cut gears with my machine in horizontal mode rather than vertical, for instance, simply because the cutter has support at both ends of the arbor.

[JasonBModerator @jasonb]

Ah I was thinking along the lines of HPC, Duval etc. I suppose a jobbing shop would use Involute cutters for a one off, would really depend on what equipment they have as it may be cheaper to sub it out.

As for hobbing a modern machine will drive the blank independantly by electronic means no need for gear trains so no need to run a particular way to get started unlike free hobbing and they seem to use climb cutting too. Though you probably won't find many bodging lodges with something like this.

[Neil WyattModerator @neilwyatt]

Oil mist lubrication?

[Adam HarrisParticipant @adamharris13683]

Fantastic video Jason!

[JasonBModerator @jasonb]

Possibly turned off for the video, the two blue pipes clipped to those fan spray outlets probably supply flood coolant in normal use and there is what looks like a coolant pipe at the far end of the hob

[Anonymous]

Incidentally I make the OD of a 36 tooth 10DP gear to be 96.52mm.

Andrew

[Michael GilliganParticipant @michaelgilligan61133]

Posted by JasonB on 10/10/2019 13:24:01:

.

[…] something like this.

.

Wow … Do they have a Desktop version of that machine ?

MichaelG.

[Adam HarrisParticipant @adamharris13683]

Yes Andrew 3.8" – not quite sure how 98mm slipped in but probably because i understand Black is only made to 2% tolerance so I was pondering about 100mm diameter, is worst case 98mm size enough to turn down to a perfect 96.52 and I have decided possibly not so bought some 105mm Black instead

[Adam HarrisParticipant @adamharris13683]

Hi Andrew , cutter is 10T & 70mm diameter (made by HBM incidentally) so therefore feed 2.8 inches/minute (given my cutter circumference is 0.849 of yours)?

[Adam HarrisParticipant @adamharris13683]

at 70 rpm

[Anonymous]

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 HarrisParticipant @adamharris13683]

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…

[JasonBModerator @jasonb]

Are you sure your chart is not in m/min rather than rpm

[Anonymous]

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 HarrisParticipant @adamharris13683]

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

[Anonymous]

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 HarrisParticipant @adamharris13683]

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?

[SillyOldDufferModerator @sillyoldduffer]

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 HarrisParticipant @adamharris13683]

"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

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