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The Cambridge Turning Trials

Introductory Post - EN3B

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Andrew Johnston29/05/2011 11:18:17
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Here are the first of my results on surface finish versus turning speed and coolant. In this first post I will reiterate my results for EN3B, so that all the results are in one thread. I'll put the different materials in their own posts, so that we don't end up with very long posts. I haven't posted pictures of my latest tests as I don't think my photography is good enough for the pictures to add anything. All the materials used were sourced from professional metal stockists, so they should be the grade they claim to be; although harsh experience has shown that the same nominal grade from different suppliers can vary widely.
 
Disclaimer: I don't claim that these results are definitive, there are too many variables for that! However, they may be useful as a starting point and to dispel some myths.
 
Unless otherwise stated the common factors for all tests are:
 
Lathe: Harrison M300
Coolant: Biokool14 soluble oil, from Hallett Oils
Depth of cut: 40 thou, 80 thou off the diameter
Feed: 4 thou per rev
Tip: Korloy CCMT positive rake insert with 0.4mm radius, grade N3120
 
For completeness, here are my results using EN3B, a steel that I have always found to be a bit of pig (or should that be badger) on which to get a decent finish:
 
Material: 1.375" diameter steel EN3B

The four trials were:
 
Trial 1: 260rpm (94fpm), no coolant
Trial 2: 260rpm (94fpm), coolant
Trial 3: 1200rpm (432fpm), coolant
Trial 4: 1200rpm (432fpm), no coolant
 
General appearance:
 
Trial 1: Surface is rough and inconsistent axially, if wiped with a paper towel it picks up fibres
Trial 2: Looks and feels slightly smoother and is more even axially than 1
Trial 3: Smoother and more shiny than 1 & 2, but is inconsistent axially
Trial 4: Surface feels smoother than 3, is very uniform axially and shiny
 
Under a magnifying glass:
 
Trial 1: Surface is dull, torn and uneven
Trial 2: Surface is dull and torn, but not as uneven as 1
Trial 3: Surface is slightly torn, but is also slightly burnished
Trial 4: Surface is much more highly burnished than 3
 
Swarf:
 
Trial 1: Came off in small, cold, uneven pieces, generally a few mm long
Trial 2: Came off in small, cold, uneven pieces, generally a few mm long
Trial 3: Came off in long cold curls
Trial 4: Came off in pretty dark blue long curls; longest piece was over a metre

Measurement of Surface Roughness (Ra), taken axially at three points around the periphery (all in micrometres):
 
Trial 1: 6.01 6.75 6.30 Average=6.35
Trial 2: 4.97 4.52 4.16 Average=4.55
Trial 3: 2.72 2.62 3.05 Average=2.80
Trial 4: 2.12 2.45 2.76 Average=2.44
 
So, for me, using carbide at high speed without coolant, gives the best finish and appearance.
 
As an additonal test I tried a fine cut, 2 thou off the diameter, at 1200rpm and 4 thou per rev, no coolant. Measured difference before and after was 2.1 thou, and the finish was good, but slightly uneven axially. So, while it is perfectly possible to take fine cuts on EN3B it is not necessarily a good thing to sneak up on dimension from a surface finish point of view.
 
Regards,
 
Andrew
 
Note: I'm not measuring to a tenth of a thou (!) just interpolating the readings on the 1"-2" micrometer to get the resolution.
Andrew Johnston29/05/2011 11:59:12
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Here are the turning trial results for stainless steel.
 
Material: Stainless steel 1.25" diameter, grade 303 free machining
 
Trial 1: 180rpm (59fpm), no coolant
Trial 2: 180rpm (59fpm), coolant
Trial 3: 800rpm (262fpm), coolant
Trial 4: 800rpm (262fpm), no coolant
 
General Appearance:
Trial 1: Surface is rough and torn, picks up fibres when wiped with a paper towel
Trial 2: Smooth finish and uniform
Trial 3: Smooth finish and uniform
Trial 4: Smooth finish, but slightly less uniform that 3

Under a magnifying glass:
 
Trial 1: Surface is dull, torn and uneven
Trial 2: Surface is smooth, little or no tearing
Trial 3: Surface is smooth, no tearing
Trial 4: Surface is smooth, no tearing, but slightly uneven axially
 
Swarf:
 
Trial 1: Long tight coils, several feet long
Trial 2: Short coils, about 1"
Trial 3: Short pieces, not coiled
Trial 4: Short pieces, not coiled, light yellow colour
 
Surface roughness (Ra µm):
 
Trial 1: 5.07 3.52 3.84 Average=4.14
Trial 2: 1.83 1.80 1.72 Average=1.78
Trial 3: 1.97 1.86 2.12 Average=1.98
Trial 4: 2.06 2.06 2.30 Average=2.14

Small depth of cut test 2 thou off diameter, 800rpm, 4 thou per rev, with coolant:
 
Difference before and after, 2.1 thou, excellent finish, as good as the best of four trials above.

Conclusion: Contrary to popular opinion stainless steel is easy-peasy, provided you don't turn slowly without coolant! Best is slow speed with coolant, although high speed with coolant isn't far behind. It is possible to take fine cuts and still retain a good finish. Personally I'll probably stick to high speed, for the time saving, and take a slighly worse finish.
 
Caveat: I don't know what the error on my surface roughness gauge is, but I'm guessing +/-10%? Remember that a difference of 0.2 in the readings is 200nm, about 8 millionths of an inch, even a gnats wotsit is huge in comparison.
 
Regards,
 
Andrew
Andrew Johnston29/05/2011 16:14:02
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Here are the turning trial results for brass. I've never seen it recommended to turn brass with coolant, and have never done so, so I didn't include coolant in this trial.
 
Material: Brass 1.375" diameter, grade CZ121
 
Trial 1: 800rpm (288fpm), no coolant
Trial 2: 1700rpm (612fpm), no coolant
 
General Appearance:
 
Trial 1: Uneven surface, axially inconsistent
Trial 2: Even surface

Under a magnifying glass:
 
Trial 1: Regular fine radial pattern, repeating consistent with the size of the chips
Trial 2: Smooth surface, no discernable radial pattern
 
Swarf:
 
In all cases the swarf came off as a stream of fine particles, and got everywhere!

Surface Roughness (Ra µm):
 
Trial 1: 3.56 3.66 3.55 Average=3.59
Trial 2: 2.61 2.41 2.41 Average=2.48

Fine cut test, details as before, at 1700rpm, 4 thou feed, and no coolant.
 
Difference before and after 1.9 thou, slightly uneven finish, not as good as trial 2.
 
Conclusion: As expected brass is pretty straightforward; the finish does seem to be better at higher speeds. A fine depth of cut does degrade the finish.
 
Regards,
 
Andrew
Andrew Johnston29/05/2011 17:58:49
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More results, this time for EN1A.
 
Material: EN1A, 1.25" diameter, free cutting, but not leaded
 
Trial 1: 370rpm (121fpm), no coolant
Trial 2: 370rpm (121fpm), coolant
Trial 3: 1200rpm (393fpm), coolant
Trial 4: 1200rpm (393fpm), no coolant
 
General Appearance:
 
Trial 1: Uneven finish, torn, picks up fibres when wiped with a paper towel
Trial 2: Smooth and even finish
Trial 3: Smooth and even finish
Trial 4: Smooth and even finish

Under a magnifying glass:
 
Trial 1: Surface uneven and torn
Trial 2: Surface looks uniform and smooth
Trial 3: Surface looks uniform and smooth, some burnishing
Trial 4: Surface looks uniform and smooth, surface is burnished
 
Swarf:
 
In all cases the swarf came off in small (few millimetre) pieces.
 
Surface Roughness (Ra µm);
 
Trial 1: 5.85 5.35 4.87 Average=5.36
Trial 2: 2.86 2.83 2.65 Average=2.78
Trial 3: 2.46 2.47 2.36 Average=2.43
Trial 4: 2.24 1.94 2.15 Average=2.11
 
Fine cut trial, 2 thou off the diameter, 4 thou/rev feed, 1200rpm, no coolant. Difference before and after, 2.0 thou, slightly uneven finish axially.
 
Conclusion: Not surprisingly very similar results to EN3B, but a better finish overall, as one might expect, as it is sold as free-cutting.
 
Regards,
 
Andrew
Andrew Johnston29/05/2011 20:41:29
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Final set of results, for aluminium alloy. The recommended turning speed for HSS is 500fpm, for carbide it is an eye-watering 1800fpm. My lathe doesn't go that fast! The 500fpm equates to about 1700rpm, and the lathe is pretty noisy at that speed. I'm not going to annoy the neighbours on a Bank holiday Sunday by running at the top speed of 2500rpm. So the tests for aluminium are at one speed only, with and without coolant.
 
Material: Aluminium alloy, 30mm diameter, grade 6082T6 - tempered and artificially age hardened, ie, precipitation hardening
 
I used a different tip here; a high positive rake and polished tip from Greenwood Tools, advertised as specifically for light alloys. One of the main problems in machining light alloys is BUE, built up edges. The material softens and forms an edge on the tip, which mars the finish, before it breaks away and another edge forms, and so on. I have never suffered from this using these tips, and I normally turn light alloy dry, so they do seem to work. These tips have a radius of 0.2mm, so I halved the feedrate to 2 thou/rev for these tests.
 
Trial 1: 1700rpm (526fpm), coolant
Trial 2: 1700rpm (526fpm), no coolant
 
General Appearance:
 
Trial 1: smooth and even finish
Trial 2: smooth and even finish, but not as bright as 1 - I think this was caused by the swarf bunching up, getting trapped, and rubbing on the workpiece

Under a magnifying glass:
 
Trial 1: Surface smooth and even
Trial 2: Surface smooth and even, but with a faint helical pattern in places; caused by trapped swarf?

Swarf:
 
Trial 1: Long tight curls
Trial 2: Some tight curls, some messy balls of swarf, all mixed up

Surface Roughness (Ra µm):
 
Trial 1: 1.12 1.12 1.16 Average=1.13
Trial 2: 1.21 1.16 1.16 Average=1.18

 
Fine cut, 2 thou on diameter, 1700rpm, 2 thou/rev, with coolant. Difference on diameter before and after was 2.0 thou. Surface finish looked excellent, so I measured it as:
 
1.12 1.11 1.14 Average=1.12
 
Conclusions: No real surprise, aluminium alloy turns beautifully at high speeds and gives a good finish. Coolant doesn't really affect the finish, but it might affect the swarf production, which in turn may affect the finish due to trapped swarf. However, I expect I'll stick to cutting dry, as the speeds involved tend to result in washing the workshop with coolant.
 
Regards,
 
Andrew
Andrew Johnston29/05/2011 21:32:35
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Here are the final set of test results (stand up the boy who said hooray). It was suggested to me a while ago, on this forum, that high positive rake polished tips could be used to get a good finish on materials when using fine cuts (I'm embarrassed to say that I cannot recall who made the suggestion).
 
All tests were done with the afore mentioned tip from Greenwood Tools taking 2 thou off the diameter and at a feedrate of 2 thou/rev. All the materials are as listed in the previous posts. After each test I made a subjective assessment of the finish and measured the Ra values. Here are the results:
 
Brass CZ121 1700rpm, no coolant
 
Excellent surface finish, completely uniform
 
Ra values: 1.15 1.27 1.32 Average=1.25
 
EN3B 1200rpm, no coolant
 
Uneven finish axially

Ra values: 1.81 1.92 1.95 Average=1.89
 
EN1A 1200rpm, no coolant
 
Smooth finish, but slightly uneven axially
 
Ra values: 1.70 1.67 1.75 Average=1.71
 
Stainless Steel 303 800rpm, no coolant (I forgot to turn it on!)
 
Excellent finish, completely uniform
 
Ra values: 1.61 1.86 1.72 Average=1.73
 
Aluminium Alloy 6082T6 1700rpm, coolant
 
Excellent finish, completely uniform
 
Ra values: 1.12 1.11 1.14 Average=1.12

So, the polished tips do give an excellent finish, but not apparently on low carbon steels, at least not visually. It may be that the finer finishes are partly due to the slower feedrate.
 
Ho hum, there's a point when experiments have to stop and the real work begins. So tomorrow in the workshop I'm actually going to make parts for my traction engines.
 
Regards,
 
Andrew
nick morrison30/05/2011 21:23:54
23 forum posts

Congratulations, this is very interesting and useful, please extend and continue the
 
'Cambridge trials'. regards Nick
Nicholas Farr30/05/2011 22:11:24
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1913 forum posts
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Hi Andrew, very interesting, and a very good presentation of your results, very easy to follow the individual trials.
 
Regards Nick
 
 
Andrew Johnston31/05/2011 22:54:33
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4724 forum posts
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Thanks, I'm glad the results have been of some use. They have certainly made me think! At some point I expect I'll try the Sumitomo tips from Arc, as they seem to be reasonably priced and come recommended.
 
I would also hope to add the results of extra tests as interest and time permits. The next test will be on EN8, as this is pertinent to another thread.
 
Regards,
 
Andrew
mgj31/05/2011 23:00:39
1008 forum posts
14 photos
Yes thanks. It would seem that there is a lot of variation!
chris stephens31/05/2011 23:32:03
1045 forum posts
1 photos
Hi Guys,
I am sure that the tips that Arc sell are marvellous things but they are not necessarily the best for "our" use. If you want a really good tip for a very fine finishing cut, wel,l up to a mm or so, T1200A cermet is the material to go for. The only chap that I know sells to these to the hobby market is the guy who advertises in the comics. I have not bought from him, but I did from his predecessor. I believe that he still supplies instructions for there best use, follow them closely if you don't want to chip the tip!
To sum up, Sumitomo make hundreds of tips but not all of them are for us. Don't be disappointed and buy the right ones first time, as first impressions tend to last!
 
http://www.carterprecisiontools.co.uk/index.html , is offered for information but I have not had any dealings with them and am not connected in any way.
christephens
Hugh Gilhespie01/06/2011 09:10:31
130 forum posts
45 photos
Hi, Just to say that I have dealt with Carter Precision Tools several times and it is a superb service. Highly recommended.
 
Regards, Hugh
HasBean01/06/2011 09:18:30
135 forum posts
30 photos
Gents,
I have used the predecessor to Sumitomo T1200A (T12A) and found it gave a superb finish but, as mentioned, care is required as they did chip easily.
I now use Mitsubishi NX2525 which is also a cermet which appears to be just that little bit tougher, recommended to me by, and available from, Greenwood Tools.
 
Regards,
 
Paul
Andrew Johnston04/06/2011 21:24:03
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4724 forum posts
532 photos
It's what you've been waiting for; the turning trial results for EN8! Not surprisingly in many ways it was similar to EN1A and EN3B, but in other ways was sufficiently different to be interesting.
 
First, turning trials as before, same insert from Korloy, 40 thou depth of cut, 4 thou/rev feed.
 
Material: EN8, 1.625" diameter
 
Trial1: 260rpm (111fpm), no coolant
Trial2: 260rpm (111fpm), coolant
Trial3: 800rpm (340fpm), coolant
Trial4: 800rpm (340fpm), no coolant

General Appearance:
 
Trial1: Uniform finish but slightly rough, picks up fibres from a paper towel
Trial2: Uniform finish, feels smoother than 1
Trial3: Uniform finish, feels smooth, but picks up fibres from a paper towel
Trial4: Uniform finish, feels smooth

Under a Magnifying Glass:
 
Trial1: Surface is slightly torn
Trial2: Dull but not torn
Trial3: Dull but not apparently torn
Trial4: Uniform, not torn, slightly shiny
 
Swarf:
 
In all cases the swarf came off as a short (5mm) tightly curled helix; at higher speeds it was hotter!

Surface Roughness (Ra µm):
 
Trial1: 5.73 5.36 5.90 Average=5.66
Trial2: 3.95 4.02 3.52 Average=3.83
Trial3: 3.06 3.36 3.06 Average=3.16
Trial4: 3.27 3.20 2.66 Average=3.04
 
So similar to previous steels, but with two major differences. The turned surface was always very uniform axially and radially, even when slightly torn. Secondly the higher speeds didn't give as great an improvement as might be expected from previous results.
 
So, do another trial, same conditions as trial 4, except at 1200rpm (511pm). Like night and day! The finish is now almost a mirror finish, easily the best of the low carbon steels seen so far. The surface is smooth, but with slight radial grooves under the magnifying glass.
 
Surface Roughness (Ra µm)
 
1.55 1.76 1.62 Average=1.64
 
I'm gobsmacked at the difference in finish between 800rpm and 1200rpm.
 
Finally the fine depth of cut trial, insert from Greenwood Tools as before, 1200rpm, 2 thou/rev feed and no coolant.
 
The finish is smooth and uniform but not as good as for the deeper depth of cut. Under the magnifying glass the surface is smooth and uniform.
 
Surface Roughness (Ra µm)
 
1.92 1.96 1.86 Average=1.91

So there we have it, similar to EN1A and EN3B, but different too.
 
Regards,
 
Andrew
Andrew Johnston26/06/2011 12:31:08
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Here are some more tests, this time using the Mitsubishi NX2525 tips from Greenwood Tools, with a 0.2mm corner radius. These tests are specifically looking at the finish obtained with small depth of cut. Speeds for each material were those found to give the best finish in the above tests. Common for each test are the following:
 
After a 5 thou depth of cut to ensure concentricity of the part, a test cut of 1 thou depth, 2thou off the diameter. In all cases feed was 2 thou/rev.
 
EN3B: 1200rpm, no coolant
 
Surface finish is inconsistent axially and has a rough feel; looked better after the initial clean-up pass
 
Ra Measurements (µm):
 
2.95 3.32 3.55 Average=3.27
 
EN1A: 1200rpm, no coolant
 
Same as EN3B, surface finish is inconsistent axially; looked better after the initial clean-up pass
 
Ra Measurements (µm):
 
3.15 3.06 3.15 Average=3.12
 
Stainless Steel 303: 800rpm, with coolant
 
Very uniform finish with no tearing visible
 
Ra Measurements (µm):
 
0.95 1.05 1.12 Average=1.04
 
Brass CZ121: 1700rpm, no coolant
 
Slightly uneven finish axially
 
Ra Measurements (µm):
 
3.12 2.97 2.86 Average=2.98
 
Aluminium 6082T6: 1700rpm, no coolant
 
Excellent finish, but some slight axial variation, possibly caused by swarf getting trapped
 
Ra Measurements (µm):
 
0.80 1.21 1.01 Average=1.00
 
EN8: 1200rpm, no coolant
 
Slightly uneven finish, not torn but a series of axial grooves visible under the magnifying glass
 
Ra Measurements (µm):
 
2.70 2.72 2.91 Average=2.78
 
For convenience the Ra finish obtained with a polished positive rake insert above are repeated here:
 
EN3B: 1.89
EN1A: 1.71
Stainless Steel 303: 1.73
Brass CZ121: 1.25
Aluminium 6082T6: 1.12

Discussion: These inserts gave a very good finish on stainless steel, the best seen so far, and marginally better on aluminium, although I expect this to be within the error bounds of the measurement. However, the results for low carbon steels, and surprisingly for brass, were worse than those for a polished insert. All in all a bit of a mixed bag. At some point I will buy some Sumitomo inserts from Carter Precision Tools, but I don't find the website easy to understand, so it'll have to wait until I have more time.
 
One thing I am learning from these tests, is that I cannot get a good finish on low carbon steels without using a decent depth of cut. I just need to learn to be confident in my micrometer readings and cross-slide dial, and don't pussyfoot around with small depths of cut; or just cheat and use the cylindrical grinder.
 
When time allows the next tests will be on annealed silver steel.
 
Regards,
 
Andrew
 
Steve Withnell03/07/2011 17:59:48
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794 forum posts
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EN16T Turning trials from Blackpool
 
Carbide tip 3/4 inch EN16T round bar under test
 
1500 rpm no coolant
 
Following advice went hard at it.
 
Blue turnings stuck to arm and burnt same - must wear long sleeves
Sparks off workpiece - not seen that before
Lathe (5 x 22) not happy at 0.5mm cut
Mirror finish on workpiece
 
Conclusion
 
EN16T should only be used by real men with real lathes and tools, anything associated with rice or pasta won't make the grade.
 
EN8 will have to do in my 'shop...
 
Regards
 
 
Steve
 
Nicholas Farr03/07/2011 18:57:21
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1913 forum posts
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Hi Steve, according to the Model Engineers' Workshop Data Book, that was issued as a collection of cards in the early days, EN16 and EN16M are considered as being poor at ease of machining, it dosen't list EN16T so I don't know if it is any different.
 
Regards Nick.
KWIL03/07/2011 19:04:14
3106 forum posts
56 photos
EN16T is, as it says Tempered.
Steve Withnell03/07/2011 19:32:20
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794 forum posts
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The piece I have must have been EN16BT - "Bad Tempered"
 
Seriously, I did get an excellent finish, but the skill to deliver that finish "at size" is probably beyond me. You can't creep up on this stuff. It also tests the rigidity of the machine too.
 
Steve
Andrew Johnston03/07/2011 22:27:04
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4724 forum posts
532 photos
Hi Steve,
 
Thanks for posting the results for EN16T. Oeeeer, I like rice and pasta, so I'd bettter steer clear! In my defence I can at least say that I don't like, or eat, quiche.
 
It's interesting that your results seem to reinforce my experiences. In order to get a decent finish on low and medium carbon steels you need to run fast, with a decent depth of cut.
 
I always wear long sleeves when turning for just that reason; my normal fleece has loads of metal embedded in it where the swarf has melted the material. I have seen sparks coming off steel when turning, but only on hot rolled or black steel. I assume that the sparks are actually the mill scale coming off red hot, rather than the swarf itself. Can anybody confirm this?
 
Who wants to try EN24?
 
Next time I order from my steel supplier I might get a few lengths of some of the more unusual steels, just to see how they perform.
 
Regards,
 
Andrew
 
On edit: Steve, you're ideally placed to answer a machining question; what feeds and speeds should we use when turning Blackpool rock?

Edited By Andrew Johnston on 03/07/2011 22:29:14

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