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How do you use indexable cutting tools.

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George Tilley13/09/2010 12:31:50
14 forum posts
Hi Guys,
 
I have a C3 lathe from Machinemart and I am very happy with it for the work I want to do. HSS and carbide brazed tip tools work very well but I cannot cut at all with indexable tools. The tool just skips along the outside of the workpiece leaving barely a mark. This happens on BMS, brass and aluminium. Is there a different cutting technique required for indexable or ??.
 My tools were not expensive but I would expect them to do something.
 
Thanks,
George   
Peter G. Shaw13/09/2010 13:09:16
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1531 forum posts
44 photos
Well, assuming that the insert actually has a cutting edge, then is the height correct, ie not too high? That's all I can think of.
 
Regards,
 
Peter G. Shaw
George Tilley13/09/2010 13:11:37
14 forum posts
Hi Peter,
 
Should it be at the same height setting as HSS tools?
 
Tools are brand new but sold as economy.
 
Regards,
George
Andrew Johnston13/09/2010 13:15:16
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7061 forum posts
719 photos
Hi George,
 
Here are some random thoughts on using indexible inserts.
 
1) In general inserts are not as sharp as HSS tooling and need a decent depth of cut (minimum several thou) and/or a rigid lathe/workpiece combination. I've never used a C3 but I suspect it comes into the category of not particularly rigid?
 
2) To get a good finish on steel with inserts you need to run at high surface speeds, normally at least 400fpm, so 1500rpm plus for a 1" diameter bar
 
3) Turning aluminium is best done with polished inserts, or you end up with BUE, which ruins the finish. BUE = built up edge, ie, swarf building up on the cutting edge of the insert
 
4) Brass should be pretty forgiving. My experience of imported indexible tooling is that the holders are ok, but the inserts are rubbish. Throw them away and buy some decent ones. I use Greenwood among others for indexible inserts.
 
5) If the material you are turning is small diameter it may be flexing away from the tool, try tailstock support?
 
Hope this helps.
 
Regards,
 
Andrew
Tigermoth13/09/2010 13:20:45
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33 forum posts
Hi Andrew
That sounded very helpful - I am about to go down the route of indexable tools simply due to the ease of replacement of the cutting edges.
Bob
George Tilley13/09/2010 13:40:17
14 forum posts
Hi Andrew,
 
Thanks for your comments.
 
 I will get some decent inserts and have another go. Do inserts cut at the top or bottom edge as this will alter the tool height setting?
 
The C3 is not too rigid but works fine on HSS or brazed tip tooling so I was hopeful that indexable would work as well.
 
Cheers,
George
 
 
KWIL13/09/2010 14:02:39
3681 forum posts
70 photos
Inserts cut on the top edge just like any other lathe tool. I agree you do have to raise the speed but they will cut just like any other tooling. A friend held the opinion that you could not take small cuts so I gave him a mild steel test piece that was 0.0004" different in diameter on the outer end of a 5/8" diameter bar. This was done on a Myford S7. Needless to say the swarf was rather fine!
 
In general machining to size it is not unusual to take off the swarf nearly or actually blue.
Andrew Johnston13/09/2010 20:23:57
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7061 forum posts
719 photos
Bob - Glad my random ramblings were of some help!
 
To elaborate further on the useful comments by KWIL:
 
1) It is perfectly possible to take small cuts with inserts, but you do need a fairly rigid setup, and it does seem to depend on the material. Brass, aluminium and cast iron are fine (sorry about the pun), but steel, particularly stainless, can be more of an issue.
 
2) All the above depends on using inserts with small nose radii, say 0.2mm. My experience with roughing inserts, with nose radii on the order of 1mm, is that they are less inclined to take fine cuts.
 
3) I totally agree that if you're working the machine and inserts properly the swarf should be coming off blue; or hotter, I have seen dull red. As I understand it, for steel at least, about 70-80% of the heat from the cutting process should be dissipated in the swarf.
 
Regards,
 
Andrew
ady13/09/2010 22:43:30
612 forum posts
50 photos
I've found that carbide doesn't give such a good finish, and is less adaptable for fast small ad-hoc jobs, which a hobbyist-dood does a lot of, it needs a lot more set-up care and time to produce a nice finish.
It's also more brittle than HSS and can fracture where HSS would simply flatten and be resharpened with a quick brush on the grind wheel.
Carbide also needs to be run at what I would call silly-speed to work properly, it can be a bit of an eye opener compared with HSS, with hot swarf flying off all over the shop.
Cast iron is a nightmare, a storm of bits but HSS blunts too easily.
 
Carbide is fantastic for heavy roughing out work for anything like stainless and cast iron, or if you've got a bucketful of metal to remove as fast as possible on a limited cut depth hobby machine.
I find that negative rake works great on stainless and cast iron btw.
 
Carbide probbly gives a good finish in a more industrial scenario when you have to run off 100 of an item and once you've got a proper setup you plug in a new insert after x units and get a consistent result.
On the other hand if it's a one-off basic metal item...a quick spin with the HSS at a relatively low speed has it done and dusted with a luvly finish.
 
HSS grinds easier, so is faster for form tools and making weird thread types, and when it blunts or breaks, it regrinds faster.
 
Both have their place in your arsenal, and neither can really replace the other at what they do best.
 
If the tool is skipping you need more stiffness, a more acute angle on the tip(maybe negative rake), or more speed.
Don't put too much pressure on a carbide tip, it fractures relatively easily.

Edited By ady on 13/09/2010 22:59:04

Andrew Johnston13/09/2010 22:58:44
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7061 forum posts
719 photos
Hi Ady,
 
I'd be interested to know more about your experiences with negative rake tooling. Is this on a lathe? I know that industry uses negative rake indexable inserts for milling, as they are often stronger, but I understood they needed a lot of horsepower. It hadn't occured to me to look at negative rake tooling for a lathe.
 
Regards,
 
Andrew
ady13/09/2010 23:10:05
612 forum posts
50 photos
I just stuck a bit of metal under a standard tool (which had become worn) so it was now skewed at a negative angle...wound it in there and if it worked...great.
 
Above centre seemed to be better for boring, I just listen to the tool cutting and fiddle about with it until I won't disturb the neighbours at 3am.
 
Carbide is amazingly hard so as long as you can get some bite pressure on an edge, any edge, it will cut virtually anything, it doesn't even have to be very sharp, although obviously sharper is better.

I've got an old Drummond/Myford M series, which a few of the old lathe books say was a very stiff machine for it's size.
The backgear needs fixed, but it still cuts metal as a direct drive and as far as power is concerned you could stop the chuck revolving with your hands if you really wanted to.
 
I've set it up so nothing breaks when I overdo things, it just stops and the belt rubs.

The saddle is very stiff as well, I bored out the cross slide(got 2 of them) cut a relatively long cross slide leadscrew nut and tonked it in from the back, this has made a big difference with no backlash so far.
I think the original leadscrew nuts were a bit marginal, war austerity measures and all that stuff from 1940.

Edited By ady on 13/09/2010 23:34:16

Andrew Johnston14/09/2010 20:54:27
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7061 forum posts
719 photos
Thanks ady, I'll have to give that a go when time permits.
 
Regards,
 
Andrew
ady14/09/2010 22:48:15
612 forum posts
50 photos
Certainly in my limited experience, it appears that stiffness is far far more critical than power when two materials come up against each other on a lathe.
 
carbide vs the outer skin on stainless steel on my unit made this really quite noticeable.
 
Once through the skin, even stainless machined quite normally.

Edited By ady on 14/09/2010 22:50:24

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