Bright EN24T steel vs Black EN24T for cutting gears

[Adam HarrisParticipant @adamharris13683]

I need to cut a spur gear using an involute cutter and Black EN24T seems to be more easily available than Bright in the right large size of bar. What is the difference in the ease of machining experience between the two, and is there a difference in performance of finished product re- shock and shearing and wear resistance? Thanks

[Adam HarrisParticipant @adamharris13683]

[David George 1Participant @davidgeorge1]

The material of both bright and black steel are the same the hardness is the T which us the same in both products.

David

[vintage engineerParticipant @vintageengineer]

Make sure the black bar has enough material to machine to size.

[Simon Williams 3Participant @simonwilliams3]

As alluded to above, I assume it doesn't need saying that you don't want to let your nice sharp involute cutter anywhere near that nasty black mill scale. It'll knock the edge off the cutter quicker than you can say pickle.

There, I've gone and said it now…

[Adam HarrisParticipant @adamharris13683]

So how much will I need to turn it down in size in order to remove the nasty black mill scale and leave some nice machineable shiny stuff to get my involute cutter into?

[Adam HarrisParticipant @adamharris13683]

And why is it that EN24T Black 130mm diameter bar is more expensive than EN24T Bright 130mm diameter bar if Black is less nice to machine?

[JasonBModerator @jasonb]

You can pickle the bar to get rid of the scale, 1hr in brick cleaner will see it all drop off.

Less processing of teh black bar makes it cheaper.

[Anonymous]

Given both are EN24T it doesn't matter if they're cold drawn or hot rolled. In theory the cold drawn will have internal stresses. But on the assumption that the material is hardened and tempered after drawing that won't be the case.

The question of mill scale isn't really an issue. One, when cutting a spur gear the cutter normally approaches the OD from within the material. So it never has to cut the scale. Two, hot rolled is unlikely to be to size, or even round, so the OD will need to be machined first anyway.

Andrew

[David NobleParticipant @davidnoble71990]

I believe that cold rolled steel has a greater yield and tensile strength due to the manufacturing process.

David

[not done it yetParticipant @notdoneityet]

Andrew (or others), is gear cutting always carried out by conventional milling – even on large commercial machines?

Clearly, climb milling is not usual for our smaller, less rigid mills – even a bump, and not table movement, might upset the more flimsy rotary tables used by some.

What about hobbing, while on the topic. I suppose this metod of gear cutting has to be done conventionally, just to get started?

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Adam Harris on 08/10/2019 22:46:19:

And why is it that EN24T Black 130mm diameter bar is more expensive than EN24T Bright 130mm diameter bar if Black is less nice to machine?

.

To which Jason replied: “Less processing of teh black bar makes it cheaper.”

… Which leaves me confused, nay kerfuddled !!

MichaelG.

[Adam HarrisParticipant @adamharris13683]

So I am expecting to have the mill scale disappear in a pile of chips as I turn down the 100mm Black EN24T bar to 98mm (at about 80 rpm with carbide insert?), but when I then attack it with the HSS involute cutter these 10DP teeth are pretty large requiring a cutting depth of about 5mm – with a 1.5kw motor on a horizontal milling arbor, how many passes of the HSS involute cutter should I be planning on? I read somewhere that the feed of HSS to EN24T should be moderately aggressive ….given a cutter diameter of for example 65mm, what speed , rate of feed , and depth of cut should i be aiming for? I would like to get all 36 teeth cut with one new cutter without having to resharpen it.

[JasonBModerator @jasonb]

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

Michael, Bright would need more rolling/drawing or may be supplied as peeled both processes would add to the cost.

I have generally found that Black is cheaper than Bright whenever I have bought steel, Don't know where Adam is getting his from but as an Example 300mm of black 105mm dia for £45 vs. 300mm of bright 100mm dia for £83 would seem to confirm that, even their 130mm black is cheaper than 100mm bright. What do you normally pay for it?

[Michael GilliganParticipant @michaelgilligan61133]

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

[…]

Michael, Bright would need more rolling/drawing or may be supplied as peeled both processes would add to the cost.

I have generally found that Black is cheaper than Bright whenever I have bought steel, Don't know where Adam is getting his from but as an Example 300mm of black 105mm dia for £45 vs. 300mm of bright 100mm dia for £83 would seem to confirm that, even their 130mm black is cheaper than 100mm bright. What do you normally pay for it?

.

I don’t buy it, Jason … so I can’t answer that

The reason for my confusion was that Adam asked why ‘black’ was more expensive, and you explained why it was cheaper.

MichaelG.

[Anonymous]

Posted by David Noble on 10/10/2019 08:42:33:

I believe that cold rolled steel has a greater yield and tensile strength due to the manufacturing process

Very possibly, but not in this case. Commercial suppliers quote the same physical properties for cold drawn and hot rolled EN24T. Presumably this is because the hardening and tempering is done after drawing/rolling. So any properties imparted by the drawing process are reset during heat treatment.

Andrew

[Adam HarrisParticipant @adamharris13683]

I was looking at M-Machine prices – Bright EN24T 130mm @ £10.32 / 25mm, Black EN24T 130mm @ £11.14 / 25mm. Maybe it is an aberration to do with their particular stock levels

[Anonymous]

Posted by not done it yet on 10/10/2019 09:17:22:

Andrew (or others), is gear cutting always carried out by conventional milling – even on large commercial machines?

I've always cut my gears using conventional milling. Bit ironic really as my horizontal mill is the only one with a backlash compensator; and it's the only one I haven't used for climb milling. Something to do with if all goes pear-shaped the mill isn't going to stall, it'll just plough on and cause damage.

Gear shaping, as used commercially, is by definition a sort of conventional process.

Looking at videos of commercial hobbing some climb mill, but most don't. So it probably depends upon the manufacturer and/or user.

Andrew

[Adam HarrisParticipant @adamharris13683]

So Andrew what speed, rate of feed and depth of cut, do you recommend in my situation ?

[Pete RimmerParticipant @peterimmer30576]

My small hobber cuts conventionally, no facility for climbing.

[Anonymous]

Must be getting old, I thought I made my 6DP traction engine crankshaft gears from EN8, but looking back I actually made them from EN24T. Gears shown here, before the splines were machined:

If I recall correctly I had to turn at fairly high speeds to get the nice finish with insert tooling; probably 800 or 1200rpm. Depth of cut and feeds were nothing special.

I cut the gears in one pass; anything else is a waste of time and just increases wear on the cutter. Spindle speed would have been around 60rpm. I can't remember what the feedrate was, but I normally aim for around 4 thou per tooth when horizontal milling. Anything much less and the mill shakes. Gear cutting shown here; coolant not essential but it does help cool things down:

The cutter was secondhand, bought from the US via Ebay. A new cutter should cut a complete gear without a problem, although it may depend upon the source of said cutter.

Andrew

[Adam HarrisParticipant @adamharris13683]

Andrew many thanks – so 60 rpm, full depth "one pass" cut, but rate of table feed (I don't understand what you mean by 4 thou per tooth)?

Edited By Adam Harris on 10/10/2019 11:44:06

[JasonBModerator @jasonb]

Multiply RPM x number of teeth on the cutter x 0.004" and that will give distance per minute feed.

[Adam HarrisParticipant @adamharris13683]

Aha thank you Jason. 36T x 60RPM  x 0.004" = 8.6 inches/minute

Edited By Adam Harris on 10/10/2019 11:50:34

[JasonBModerator @jasonb]

That seems quite a lot of teeth

I would have expected about 12, have you got a picture or link as teeth would be very thin

 

Edited By JasonB on 10/10/2019 11:54:39

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