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5th August 2014, 09:43 AM #1SENIOR MEMBER
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Lathe fluff cuts in steel - what tool?
I have been building a few parts on my lathe. I recently had a 10.03mm shaft around 100mm long that needed to be reduced in size by 0.03mm. I used a DCMT 070402 (NX2525 grade) insert running around 640rpm with a slow feed. Instead of cutting it rubbed in part. The end result was half OK (somewhat nice finish - but not uniform in size), but left me wanting for a better solution. I assume that the 04 nose radius was a bit much? I am thinking of trying out a DCMT cutter with an 02 radius to see if that helps.
About half way through the cut it started screeching like a pig. Not sure why that would happen. Possibly deflection and rubbing due to lack of DOC.
What do you generally use for really light fluff cuts in steel?
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5th August 2014 09:43 AM # ADSGoogle Adsense Advertisement
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5th August 2014, 10:39 AM #2
Hi Anthony,
For cuts that small you could try the high positive rake ally tips, but the finish will probably be crap. I would either use HSS or just use emery.
I have taken some small cuts with CCMT cermet on the C10 but not as small as .015.
Ew1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.
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5th August 2014, 10:41 AM #3future machinist
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You not going to have much luck taking a .03 cut with carbide. By there nature they are blunt and push material away relying on heat generation to make the job soft. You need a nice Sharp HSS cutter.
BETTER TO HAVE TOOLS YOU DON'T NEED THAN TO NEED TOOLS YOU DON'T HAVE
Andre
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5th August 2014, 10:58 AM #4SENIOR MEMBER
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G/day variant22, never used one myself but I think a shear tool might be the answer.
shed
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5th August 2014, 12:07 PM #5SENIOR MEMBER
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Not necessary, the main advantage with that tool it they're easier to sharpen, otherwise much ado about nothing in my book.
It's not always the case that carbide is "blunt", but they tend not to be as sharp as HSS. If you need to use inserts, then I would recommend an insert designed for aluminium, as it will be high rake. Choose as small a nose radius as you can, as the general rule of thumb is the minimum depth of cut on a carbide tool is 1/2 the nose radius, below that and the tool will tend to be deflected. It's quite easy to further refine inserts by spending just a few minutes by tipping them up on edge (out of the holder), and honing them with a diamond hone. You can also do this when they get worn. In this case however, use it to sharpen the carbide tip even further to allow finer depths of cut. However be careful with that insert, as carbide is brittle and that insert will be fragile if you bring it to a needle sharp point.
Personally when I want to do this type of thing (well I use a file to be honest ) but when wanting an extremely fine finish and just "fluff" coming off, I have a very well rounded HSS tool I've ground with significant back rake. It just fluffs steel off. But 0.03? I would be going emery on the back of a file for that!
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5th August 2014, 12:12 PM #6Senior Member
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G'day Variant,
I'd have to agree with both Ueee and Shed,
I've managed to do very fine cuts (in steel) using the high positive aluminium inserts and get a really nice finish.
A 0.4 nose radius is more suitable to deep cuts and this is possibly why you are struggling although I would have thought a DCMT 070402 would have a 0.2 nose radius?
From the link in your post it looks like your tips might work for precision finishing in stainless or cast but not steel, they are also described as milling inserts but I have no idea if this makes them less suitable for use in turning or not?
To be honest though, if you are only working with mild or free cutting stock, just go with a HSS shear tool as suggested by shed.
Carbide is nice if you fluke upon the correct grade of insert and speaking for myself I can assure you that finding the right insert was nothing more than a fluke. Selection of the correct grade of carbide seems to be a bit of a black art and is very dependant on the stock you are working with.
As hobbyists we work with all sorts of stock from scrap yard mystery metal to known grades which makes insert selection a crap shoot at best and can get expensive and frustrating. For this reason it is always handy to have some HSS handy so you can grind your own tool when carbide insanity sets in.
Of course failing all this, a warding file and good old emery cloth work well too.
Cheers,
Greg.
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5th August 2014, 01:33 PM #7SENIOR MEMBER
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I will give this a shot Ew. I have some of those nice Ally inserts on hand. The carbide that I was using did an admirable job given its nose radius of 04. That said, I buggered up the part so it is back to making it again.
Shed, I have some HSS on hand that I will also try. Last time I tried some HSS I was a bit disappointed, despite it being "professionally" ground (aka RDG Tools pre-ground set).
Good to hear Greg. I intend on trying this out to see how I go.
Thanks for picking that up. I made a typo. The radius is in-fact 04 (DCMT 070204).
I have to agree with this sentiment. The amount of time I have poured down the drain picking through the carbide catalogs and on Ebay has not really paid off. It seems hit and miss for hobby folks. I have tried to buy a few different brands and types of inserts in search for something that works well. I will likely purchase some DCMT with the 02 radius to see how it goes. The 04 in the grade above give a really nice shine for light cuts. That said they tend to top out at 0.1mm. Anything beyond that and it is a fine line of getting it perfect and having it cut, rub and bugger the part.
Something about having silicon carbide getting into my way oil has me avoiding this unless absolutely necessary. I know that one is meant to cover the ways with paper towel etc, but it still has less appeal than using a nice sharp tool to finish things off.
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5th August 2014, 02:02 PM #8Cba
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No chance with a standard all-purpose carbide tool. The rule is this: you cannot take a depth of cut that is less than the cutting edge radius. Note that is NOT the tip radius that matters here - I am talking about the sharpness of the cutting edge. Or in other words, not if the knife tip is pointed or rounded, but how sharp the blade is. Any coating of the insert will only make matters worse, as coating means more rounded cutting edge.
For depths of cuts that shallow, you can use a freshly ground and honed HSS tool. Or if you wish to use insert tooling, you go for an uncoated DCGT-alu. Unlike DCMT, these inserts are polished to a mirror finish, they are razor sharp (I mean it, they will cut your skin like a scalpel), and they have a highly positive top rake of between 20 and 30 degrees. In the CNC world these are used to turn Aluminium (hence the -alu suffix). In the home shop you can use them for the finest finishing cuts also in steel and even stainless. You can take off microscopic chips with depths of cuts even under 0.01mm. Without rubbing. Ideal on a manual lathe to approach high precision dimensions, like ball bearing seats, that industrially would be ground not turned.
However, DCMT resp DCGT is a very pointed tool, together with the high top rake that makes for a very very fragile tool. If you do not need that pointed shape to access an intricate cut, you would be better off using a CCMT tool, respectively CCGT. CCGT is still fragile, if you accidentally hit the tip it will break. That is the downside of these supersharp polished high positive top rake inserts.
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5th August 2014, 02:52 PM #9SENIOR MEMBER
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Just for the record, it is the nose radius that largely determines the minimum DOC. Further, any coating on an insert is completely immaterial in this regard. Anyone who actually uses coated inserts, instead of looking up how they think they work on Google, will know that the coating just wears off the insert with use. Though I can't see that being a problem with some
Here's some more information on the above topic, I really like the Sandvik educational material, and sometimes read through it when I want something to read (hotel rooms are boring, depressing places ... well unless you're in the type paid for by the hour!! , I will go a year or more before turning on a TV in my room, so this type of things is great.). Insert nose radius That page is an extract from a more comprehensive discussion, I can't recall which paper it is, but check "general turning" or similar. Rather than wading through catalogues, I think it would be a much better use of time would be to have a good read through the educational technical info, and then you will have a much better idea of what to look for in an insert. Sadly as much as I like their technical documents, I find their actual catalogues a horse's butt to navigate. To the extent that I rarely buy them and need to source from another manufacturer, but the specs will be similar.
I've been scratching around trying to find my high rake inserts I use, and buggered if I can find them. They fit a turning and facing tool that I tend to mainly use these days. Very handy being able to do both with the one tool. I'm pretty sure the inserts are Kennametal, high rake, diamond shaped, and I would have bought the ones with the minimum nose radius. As for specific numbers I won't be able to help until I find the packet again (it was sitting on my desk right next to me until only a week or so ago). As mentioned above, you can then hone the insert to be even sharper and smaller nose radius with a hand held diamond hone if you wish. I tend to touch up my inserts a lot, as I'm such a tight wad with money!
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5th August 2014, 04:44 PM #10Cba
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Pete, I am afraid but simply googling up a Sandvik catalog is not going to help you much to run a light hobylathe. I am talking from experience here - many accepted rules for industrial machine tools do simply not apply in practice to light and not so rigid hobbylathes.
If you have never tried these highly polished high positive rake tooling like CCGT-alu, DCGT-alu, TCGT-alu, please take my word for that these will work exceedingly well for fine and finest finishing of steels, with depths of cut below 0.01mm. They will work just as good as a freshly ground and honed HSS but last longer.
Tip radius affects surface roughness, it does not limit doc. In the extreme, you can use a round nose tool to take off a miniature 0.005mm chip, as long as its cutting edge is razor sharp.
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5th August 2014, 05:01 PM #11SENIOR MEMBER
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I'm not sure what part of this you didn't understand:
I've been scratching around trying to find my high rake inserts I use, and buggered if I can find them. They fit a turning and facing tool that I tend to mainly use these days. Very handy being able to do both with the one tool. I'm pretty sure the inserts are Kennametal, high rake, diamond shaped, and I would have bought the ones with the minimum nose radius. As for specific numbers I won't be able to help until I find the packet again (it was sitting on my desk right next to me until only a week or so ago). As mentioned above, you can then hone the insert to be even sharper and smaller nose radius with a hand held diamond hone if you wish. I tend to touch up my inserts a lot, as I'm such a tight wad with money!
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5th August 2014, 07:14 PM #12
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5th August 2014, 07:17 PM #13Cba
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Pete, you are a master at twisting words. At least when you run out of arguments. I can assure you that Sandvik has zero interest in the particular needs of home shop hobby machinists. They make their money with industry. What you fail to understand is, that small light machines do not respond well to the same rules that work big industrial CNC machines. If you think otherwise, fine, please yourself.
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5th August 2014, 07:29 PM #14Member
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That's a hard task!!
The best might be a tool post grinder, or a cylindrical grinder. I understand not everyone has that luxury.
A very sharp HSS tool with coolant would be enjoyable or a file and emery is what I might do, you know, as we were all taught not to do!!
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5th August 2014, 09:04 PM #15SENIOR MEMBER
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Thank you for telling what I do/don't understand. Holy Cow Chris, this is like pulling teeth! How am I running out of arguments? I posted the link to why it works this way, from the company who makes them, and describes in detail why it works this way. Geometry is geometry, absolutely nothing to do with whether the lathe is in a home shop of production environment. Meanwhile you've come up with ...? Our machines are nowhere near as rigid/powerful as a big industrial machine, and often don't have the same speed capability, but geometry is still geometry and the rules don't change. Don't believe me? Leave Google alone for 5 minutes and actually use your lathe. Chuck a thin piece of stock with a sharply pointed triangular tool in it (ie very small nose radius), but one where one cutting edge is close to perpendicular. Run it one way, then run it the other way. Same lathe, same tool, same amount of sharpness, but one way will deflect the work more than the other way. That's the way it works, see for yourself if you don't believe me.
For this type of work you really want the cuts to be in line with the lathe's axis, if you use a large nose radius the tool will want to push the work away. I too have a large radius tool I use where I want really good surface finish, it will also take fluff (indeed I ground it to make a test bar). Useless on smaller diameter work. I'm literally in the middle of dealing with this very issue right now, and stopped just for dinner. 3.00 mm shafts that I need +/- 0.05 mm to fit bearings. At 3 mm, deflection of the work is a big problem unless it's approached properly. As per usual, I'm just using my usual high rake inserts that I polish up. I've had equal success with HSS. It doesn't matter what the cutter is, it's all about geometry. All carbide does is allow the work to turn faster, and at 3 mm that's not worth squat to me, nor I'd suggest to the OP at 10 mm. I just use the same insert I use for pretty much anything except brass. However just for fun I'll go back to work now and use a HSS tool for the same thing, no biggie, it's all about geometry. High rake, sharp tool, small nose radius, low feed, job done.
Hopefully the OP has got something of benefit from all this?
Edit: Ok I'm done for the night at the lathe, and tried all manner of different HSS and carbide combinations, in rubbish steel rod. 2500 rpm and fine feed (sorry I can't give a figure for that off the top of my head). The HSS tool I thought might work, did, but not as well as I thought it may. Mind you it's a weird geometry, and I can't even recall why I ground it that way, but sharp, medium to high rake to a fine point. Better than that was the high rake insert tool I mainly use these days; good finish, less deflection of the work. About the only thing that's bad about them is there's no chip breaker on them, so chip control could be an issue when power feeding. I can't see it being too bad for us using manual lathes but thought I'd mention it. Manual feeding I'll just manually break the chip. It's not too bad on steel, but aluminium can be truly painful as it can string badly. The other issue with it is it won't handle a deep DOC, I haven't looked at why, but it just does't cut at all. I still need to investigate what's going on there. The broad rounded tool Chris mentioned above? Couldn't get it to run at all. Not reliably anyway. Chatter is the main thing with very broad tools like that, this puppy was no exception, and she was singing like a bird! No great surprises there I guess. On larger work it may be ok, but everything needs to be rigid. I re-ground that for aluminium facing a few weeks back, so it would possibly run better with less rake. I tried various other inserts that I have, and they were "ok" but not as good as the high rake one. So from that I'm sticking with my high rake inserts that I polish up, but it's always worth experimenting, as I've been surprised before.
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