By continuing to use this site, you agree to our use of cookies. Find out more

Member postings for Andrew Johnston

Here is a list of all the postings Andrew Johnston has made in our forums. Click on a thread name to jump to the thread.

<< First < Prev 331 332 333 334 335 336 337 338 339 340 341 Next > Last >>
Thread: Machining what am I doing wrong?
29/05/2011 20:48:40
Hi Steve,
 
What type of lathe? Can you describe the differences in finish? For instance is the surface texture different over a small length, indicating that the cutting process is different, or is it inconsistent over larger lengths, indicating that the cutting process is flipping between different regimes?
 
It might be something to do with the headstock bearings being loaded differently?
 
Regards,
 
Andrew
Thread: The Cambridge Turning Trials
29/05/2011 20:41:29
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
29/05/2011 17:58:49
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
29/05/2011 16:14:02
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
Thread: What's gone wrong with the badgers bum
29/05/2011 12:16:38
I can't see a link, long or otherwise, anywhere in the thread?
 
Regards,
 
Andrew
Thread: Machining what am I doing wrong?
29/05/2011 12:12:14
Posted by Ian S C on 29/05/2011 11:55:53:
Another thing that most of our lathes lack is HP. To get the speed, feed, and depth of cut often takes considerable power, a good way to finish off a carbide tip is to dig in and stall the lathe, or slip the belton a belt head lathe. Ian S C
 
I agree that a certain amount of horsepower is needed to achieve the higher speeds and feeds, but it is interesting to note that at the higher speeds and feeds the power needed to remove a given volume of metal per unit time is lower than at low speeds and feeds.
 
Regards,
 
Andrew
Thread: The Cambridge Turning Trials
29/05/2011 11:59:12
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
29/05/2011 11:18:17
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.
Thread: Badger's Bum Trials
29/05/2011 10:49:45
A small point; I've found that the finger nail is an excellent comparative tool for surface roughness. It is possible to tell the difference between surfaces that vary by rather less than 1µm
Ra.
 
Regards,
 
Andrew
28/05/2011 22:16:25
We could say that the formatting is badgered; must be using the wrong speeds and feeds.
 
Regards,
 
Andrew
28/05/2011 21:00:27
Oh, b****r, and I've just spent an evening doing more trials on different materials, only to be beaten to publication!
 
Mind you the evening hasn't been entirely wasted, as the CNC mill has been whirring away in the background making another prototype bevel gear pinion for my traction engine.

Regards,

Andrew
Thread: Machining what am I doing wrong?
26/05/2011 19:09:09
I suspect that EN1A is more forgiving than EN3 as regards finish versus speed, certainly within my limited experience. We'll find out when I do some tests this weekend.
 
Contrary to popular opinion I don't recall having a problem taking small depth of cut with carbide tools in various materials, including stainless, albeit free machining 303. I'll add a shallow depth of cut test to my trials at the weekend.
 
Regards,
 
Andrew
25/05/2011 23:42:10
Hi mgj,
 
Errr, I'm not looking for an argument!
 
Here are a few random notes:
 
I have very little experience of using HSS tooling on the lathe, mainly specials, screw cutting and form tools, so I don't really have any comparative experience. I've never used a tangential holder.
 
The trial turning was done using a Burnerd collet chuck. I've no idea about the spindle bearings, I assume that they are the original Gamet bearings as fitted by Harrison.
 
I've no doubt that different brands of tip produce varying results but, like most of us, I don't have the luxury of doing comparative trials. So far the Korloy ones seem pretty good and very resistant to impact damage, and believe me I've done some pretty heavy interrupted cutting. In general I buy recognised makes of tip from professional tool suppliers. It's a problem faced by the professionals too, judging by some of the posts on the practicalmachinist forum.
 
In theory most of the heat from the cutting process should be in the chip, which is why it is blue. However, you are right that the workpiece does get warm. I've never measured the temperature, but I've always been able to hold it for at least a few seconds. That means less than 100°C, probably about 80°C? So from 20°C that's a 60°C rise. The linear thermal coefficient of expansion for carbon steel is about 11x10-6. So for a 2" workpiece that's an increase of about 1.32 thou on the diameter. If dimensions are critical (less than a thou) then I'll go and make a cup of tea or do check of the drawing etc while the workpiece cools down.
 
I deliberately chose EN3B because I find it, and hot rolled steel, a pig to machine while getting a good finish.
 
I rather suspect that my conclusions would not apply to other materials. The weather over the Bank holiday weekend isn't looking great, so I probably won't be flying. If that proves to be the case I might find time to do so tests on other materials. I have in mind free machining EN1A, aluminium 6082, brass CZ121 and stainless steel grade 303.
 
If I do so I'll post in a new thread, rather than hijack this one again.
 
Regards,
 
Andrew
25/05/2011 21:52:49
Posted by mgj on 23/05/2011 18:33:13:
I beg to differ about carbide and feeds and speeds. Use it all the tipped tools at exactly the same speeds that I use with HSS - works perfectly, with several different makes of tips in both positive and negative rake configurations. For those who don't believe, go try it.(Yes I know carbide will go a lot faster, but its nonsense to suggest it HAS to - especially on a lathe where coolant is being used, for the very good reason that the coolant can go everywhere which is messy and uncomfortable)
 
You are using coolant - yes?(At which point a lot of people say they cut dry. Good luck to them, a dose of coolant produces a finer finish and tools last longer. My definition of a good finish is =/-.002" on diameter, and a mirror shine.
 
(Selective quote to save space)
 
Hmmmmm, time for an experiment. I did a series of four trials. Common features were:
 
Lathe: Harrison M300, running from a true three phase supply
Material: 1.375" diameter steel EN3B
Depth of Cut: 40 thou, ie, 80 thou off the diameter
Feed: 4 thou per rev
Coolant: Biokool14 from Hallett Oils, or whatever they're called these days
Tool Tip: Korloy - CCMT postive rake insert 0.4mm nose radius, grade NC3120, general purpose turning of steel
 
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
 
Pictures of the four turned surfaces are in one of my photo albums, although my photography is not really good enough to be able to discern the differences.
 
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. Given what I understand about the cutting process that makes sense to me. It is interesting to note that at slow speeds coolant helps, at higher speeds it doesn't. I can think of reasons why that might be, but have no definitive proof.
 
Disclaimer: The above doesn't mean mgj is wrong, just that it doesn't work for me!
 
Regards,
 
Andrew

Edited By Andrew Johnston on 25/05/2011 21:53:48

Thread: Tooling questions for a KX1
22/05/2011 22:52:13
Blimey, 1.5m by 3mm, you'll need a lot of horsepower for that!
 
However, to answer the question, 250rpm for a 3" or 4" slitting saw will be fine in aluminium. Whether the mill has the power to drive it is another matter. When I've cut heatsink fins on a CNC mill I used a small slot drill rather than a slitting saw.
 
I use an ER20 collet chuck on a bigger CNC mill (a Tormach) and have never felt the need to go bigger. CNC tends to involve small cutters running fast, rather than large cutters running slow. The ER20 collet chuck also works fine on my manual Bridgeport.
 
I've got no direct experience of the KX1, or the ARC rotary tables, so I'll leave that question for those who are more knowledgeable.
 
Regards,
 
Andrew
Thread: That's better!
22/05/2011 22:34:09
Swarf - I just take mine to the nearest council tip and recycle it with all the other metal. Nobody has ever complained, in fact the helpful council man pointed out that they had a separate skip for aluminium scrap.
 
Regards,
 
Andrew
Thread: New technology in Model Engineers Workshop
20/05/2011 14:26:23
Posted by Keith on 20/05/2011 13:31:41:

There are a number of contributors to this forum who believe that if they are not controlling their machine tools themselves then there is no skill in it.
 
I do hope that said contributors (if they exist) do not stoop so low as to use power feeds; definitely not cricket.
 
Regards,
 
Andrew
Thread: What's Changed?
18/05/2011 17:13:10
Likewise it happened to me a couple of months ago, but I've had no repeat performance since. I'm using Firefox 3.6, since the upgrade to 4 fell over while downloading, which doesn't exactly give you confidence in the upgrade!
 
Regards,
 
Andrew
Thread: What Collet type, 5C or ER25
16/05/2011 08:29:42
Hi Kai,
 
That does seem quite cheap for a centering indicator. It depends upon where it is made, you may find it instructive to do a search for co-axial indicators on:
 
 
If were to buy a coaxial centering indicator I would look at Blake from Long Island Indicator or, closer to home, the Centro from Haimer in Germany. In practical terms I find I can get centres close enough for my purposes using a length of precision ground steel in a collet and a cigarette paper. But each to his own.
 
Off topic: Can I assume that you are Austria? I see that Bernardo have a factory in Linz; lovely city, it's just unfortunate that both times I have visited we have been cursed with travel nightmares.
 
Regards,
 
Andrew
15/05/2011 23:24:06
An advantage of 5C collets for work holding in the lathe might be that you can get them in square and hex variants.
 
Regards,
 
Andrew
<< First < Prev 331 332 333 334 335 336 337 338 339 340 341 Next > Last >>
Magazine Locator

Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!

Find Model Engineer & Model Engineers' Workshop

Sign up to our Newsletter

Sign up to our newsletter and get a free digital issue.

You can unsubscribe at anytime. View our privacy policy at www.mortons.co.uk/privacy

Latest Forum Posts
Support Our Partners
cowells
Sarik
MERIDIENNE EXHIBITIONS LTD
Shopping Partners
Subscription Offer

Latest "For Sale" Ads
Latest "Wanted" Ads
Get In Touch!

Do you want to contact the Model Engineer and Model Engineers' Workshop team?

You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.

Click THIS LINK for full contact details.

For subscription issues please see THIS LINK.

Digital Back Issues

Social Media online

'Like' us on Facebook
Follow us on Facebook

Follow us on Twitter
 Twitter Logo

Pin us on Pinterest

 

Donate

donate