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Poor finish using indexable lathe tools on steel

RFI re above

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Peter Turvey15/04/2019 20:52:04
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16 forum posts
7 photos

Any advice regarding cause of poor finish when turning EN1A Freecutting steel using indexable carbide tooling?

Using standard Greenwood type 1 general purpose inserts in a Myford Super 7B. 10mm shank in Dickson toolpost

Cutting dry

Turning 3" max diameter EN1A steel bar down to about 2 1/8" diameter

Speed 290 RPM

Feed 0.0069" per revolution seems to be getting best finish

Cut depth between 5 and 10 thou (deeper cuts at higher feeds causing jams and belt slips)

Checked all gibs are tight but not over tight.

Chips coming off anything between long blued spirals and fine particles depending on cut and feed rates.

Getting better finish on facing cuts

(Component will be a part of a bearing cone puller for our 1914 Stanley Steam Car, so not a need for high finish - but a useful turning exercise)

20190415_174104254_ios.jpg20190415_174016569_ios.jpg

martin perman15/04/2019 21:15:18
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1519 forum posts
64 photos

I've just finished a project for a friend machining 75mm dia EN1A at a higher speed, 1000 rpm roughly, than you 0.5mm cuts at a slow feed rate and got a good finish.

Martin P

T.B15/04/2019 21:15:40
16 forum posts
3 photos

Looks like a CCMT insert from the photo , If it where me i would double the spindle speed and depth of cut.

On my S7 with CCMT inserts the best finish is with at least a 20thou depth of cut and a fairly fast feed to get a decent chip.

Tony Pratt 115/04/2019 21:17:34
838 forum posts
2 photos

Speed is way too low, 1000 RPM would be nearer the mark.

Tony

David Standing 115/04/2019 21:17:49
1193 forum posts
45 photos

Reduce your tool overhang too, if you can.

 

 

 

Edited By David Standing 1 on 15/04/2019 21:18:08

Chris Evans 615/04/2019 21:17:57
1377 forum posts

Up the speed, I would be running at 600 to 850 RPM with those tips. For a good finish when only taking a small amount off consider using a carbide tip with the "GT" ground tooth style rather than the usual "MT" moulded tip general purpose style. The GT tips are sold for cutting aluminium but work well as light duty on steel.

The tips you have should be OK if the feed and speed are upped.

XD 35115/04/2019 21:17:59
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1210 forum posts
83 photos

Try the other end of the insert or a new insert and see if it improves , make sure the tool is on centre .

Peter Turvey15/04/2019 22:01:01
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16 forum posts
7 photos

Thanks for the quick feedback everyone, will give your suggestions a try when I can get in the workshop again later this week.

I got my initial ideas on speeds and feeds from the Greenwood website cutting data page by the way.

Baz15/04/2019 22:16:30
166 forum posts

Could also try a drop of coolant or neat cutting oil

Andrew Johnston15/04/2019 22:20:20
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4497 forum posts
520 photos

Read this thread:

**LINK**

In summary:

  • Run at a much higher spindle speed
  • Bigger depth of cut
  • To compensate for lack of power you can reduce the feedrate to, say, 4 thou per rev
  • Reduce tool overhang; the insert should only just clear the holder

If lack of power is still a problem try using an insert with a smaller nose radius if possible. Smaller nose radius means lower DOC and feedrate is possible while still getting a good finish.

Andrew

Neil Wyatt15/04/2019 22:29:06
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15705 forum posts
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73 articles

Nothing to add really - as others suggest you are being too cautious for TCT.

Even 10 thou is quite shallow, you should ideally aim for 2/3 tip radius minimum.

Neil

Hopper15/04/2019 23:32:42
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3396 forum posts
66 photos
Posted by David Standing 1 on 15/04/2019 21:17:49:

Reduce your tool overhang too, if you can.

A big +1 on this.

Clive Foster16/04/2019 00:34:06
1639 forum posts
45 photos

Start by finding out what the official makers data-sheet figures for speed, feed and depth of cut are. Carbide insert tooling does generally need to be worked hard to get a good finish but that doesn't always mean very high speed. Some of the ones intended for use on manual machines have surprisingly low surface speed requirements.

For example I have some rhombic inserts made by Sandvick grade H10F, type CNMG 0432-23 whose book surface speed range is 85 - 100 ft/min. Go much faster and the edge disappears. Feed is 5 to 9 thou per rev and depth of cut range 14 to 145 thou, 100 thou is suggested as a typical depth of cut to start with. Basically you play with the depth of cut to get nice short tightly curled blue chips leaving a good finish.

I have others specified for 500 to 1000 ft / min which is more what folk expect and one finishing type that works best at approaching 1,500 ft/m albeit with a recommended feed of around 6 thou per rev at 14 thou depth of cut. Somebody read the ft/min as m/min when purchasing. Ooops! So they need to go three times as fast as I expected. Difference between mildly scary and seriously frightening. Chip blizzards are bad enough in brass. Blue hot steel is much worse. Awesome finish tho'.

Clive

Edited By Clive Foster on 16/04/2019 00:34:34

Neil Wyatt16/04/2019 09:11:16
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15705 forum posts
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Also, if you do want to take light cuts, try ground (e.g. CCGT) inserts.

Neil

JasonB16/04/2019 09:59:28
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Moderator
15038 forum posts
1532 photos

Have a look at my post at the bottom of this page to see how DOC and feed affect the finish, the harder the tool is worked the better the finish you get.

Greenwoods speeds do seem rather tame, quick look at Korloy for a CCMT at 3" in general steel is 750rpm.

Also if you can only take a cut of 10thou before you get slip and jams then I'd say something is amiss with the machine.

Rik Shaw16/04/2019 11:26:36
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1240 forum posts
336 photos

That tool is stuck out way to far. Bring it back inboard till the shoulder behind the insert is level with the toolpost and have another go.

Rik

XD 35116/04/2019 11:50:29
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1210 forum posts
83 photos

Has anyone zoomed in on the insert tip ? Looks chipped to me and if it is no amount of feed rate change will make it work . I typed in greenwood type 1 insert into google and couldn’t see anything of use , in have had issues recently with cheap inserts that won’t cut anything and i put it down to dodgy inserts .

If you need to do things on a tight budget stick with HSS otherwise be prepared to fork out for known quality carbide inserts - the cheap junk isn’t worth it , been there done that got burnt !

JasonB16/04/2019 12:07:09
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Moderator
15038 forum posts
1532 photos

XD, this is the Greenwood tools type 1, they use a simplified number system for people that don't understand the code, it's a CCMT 06 02 0 (whatever radius is selected) they are not bucket quality and should work fine.

Too much swarf on the htip to see if it is chipped or not

XD 35116/04/2019 12:31:34
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1210 forum posts
83 photos

Ok thanks Jason , I’m not familiar with that brand .

To me looking at the finish and zooming in on the photos the finish looks to be tearing and the tip looks like the radius is gone - when things go awry i look at this first ,Its a simple thing to change and check .

Ian.

Nigel Graham 216/04/2019 14:59:22
83 forum posts

On a couple of points raised...

1) The standard designations look alarming with all those letters and numbers, but are fairly easy to find and to follow, and it may be worth keeping a copy with other references, like tapping-size tables.

I don't know our regular suppliers' criteria when selecting inserts to sell to us, for our use in conditions far more variable than in industry, including our using manually-operated machines not always in the first flush of youth (the machine-tools, not us!).

However, most carbide tips are really for production machines with very rigid structures and automatic feeds, clearing metal at very impressive rates and capable of very high surface finishes. In this area, their manufacturers' catalogues, intended for the precision-engineering trade, quote insert or insert-facet lives of typically 20 minutes at the tip's intended material, speed and feed! (I believe insert life and price is accounted for in costing the work and preparing quotes.)

2) Showers of swarf... make some form of guard or screen easily placed and removed, to keep the swarf where it belongs.

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