Carbide inserts

After many years of using only HSS tools in my home workshop I have recently been spurred into trying carbide insert tooling by posts on this forum. My first tests with polished TCGT inserts were very encouraging indeed, and for finishing cuts on unleaded mild steel i got a better finish than I have ever seen with HSS. However, I'm aware that polished TCGT inserts are intended for aluminium and plastic, so I would be interested to know what the issues are in using them for cutting steel. I have tried coated TCMT inserts, but I can't take cuts as fine as with the GT inserts, and the finish is not as good.

I would also appreciate some guidance on what cutting speeds to use with carbide inserts (TCGT or TCMT). Can I just use some multiple of the speed I would use with HSS for the same task?

Hi Mike,

I have used carbide insert tools for many years, especially when machining work with hard spots. Carbide tools can be run two times the speed of HSS tools or more, just make some tests with vaying speeds. My chips often come out blue. The inserts intended for Aluminium works well on other metals too, as you have already discovered. The special polished inserts may cost a bit more, so use them for the finishing cuts, and cheaper inserts for roughing.

Thor

That's good information.

Many thanks Thor.

Mike

Mike You should have no problems using the **GT inserts on steel Just be careful not to chip the edges as they are very sharp and can chip easier than **MT inserts which have a stronger edge. Go to the thread Jasons tips, and you will see a few pics of various ** GT and a **GX coated and uncoated.

Thanks Raymond. I wasn't sure if there was some non-obvious reason for not using GT inserts on steel, but it seems not.

Mike

Mike, www.shop-apt.co.uk/ Large range of inserts etc and good service. Lynne.

I will second that from Lynne,, APT, excellent service, Nice inserts, also there is

Riley Shutt [ Walter, Sandvik, Arno ect ] and Cutwel [Korloy ] both those are also first rate.

Edited By Raymond Anderson on 08/04/2016 07:42:15

Carbide inserts can be run at considerably faster surface speeds than HSS tooling, but whether this is a nice to have or essential to get a good finish depends upon the material. For instance EN8 needs to run fast with inserts to get anything like a good finish. Likewise the depth of cut versus finish with inserts also depends upon the material. The low carbon steels seem particularly bad in this respect; possibly leading to the blanket statement that inserts don't work with small DOC. As a contrast it is possible to get a good finish on 303 stainless with a 2 thou DOC.

Generally feeds need to be higher than most MEs use to get the chip breakers on the inserts working properly. The aim in industry is to produce short, easily removable, chips. A long length of swarf or a birdsnest looks impressive, but in my case it usually gets in between the tool and work, fudging the finish along the way. I never use less than 4 thou per rev and particularly for roughing I'll be at 12+ thou per rev.

This thread may be of interest as to the behaviour of different materials when turning:

**LINK**

Andrew

You should push them at about 150% the standard speed, particularly harder metals but watch out for flying blue shards.

Clive

+1

By hard experience I have found that carbide inserts don't seem to like cutting less than about 4 thou/rev. Below that they chatter and/or break. I usually run between 4 and 8 thou per rev, the softer the metal the higher the feedrate.

Andrew

And don't forget you can machine hardened steel in the hardened state, although for this you almost certainly need to be running at max speed. If it's not glowing, it's not softening the swarf - which is how it works.

I think Neil was talking about parting tool inserts? Not the polished inserts the OP was talking about. I've certainly found the polished inserts much more fragile than more common types. Like many of us on here with hobby lathes I'm not interested in what industry does.

I've been using CCGT/MT inserts for a while, they work great on a mini-lathe.

I was probably pushing 3 thou DOC when parting, which is quite a chip with a 3mm tool.

At full diameter that's ~0,7 cubic inches per minute, which would be the limit for plain carbon steel with my 0.5hp motor, although EN1a is an easier proposition and as soon as the cut starts the diameter is reducing together with the removal rate.

I've achieved a sustained rate of removal of over 0.5 cubic inches per minute on plain turning (that made blue chips!) I think I've shown the pic below of a 3mm cut. Normally I'd use a shallower DOC (1mm) & 4mm feed at ~1100 rpm for rapid stick removal.

It all shows that a properly set up mini-lathe can pretty much use all the available power when used with carbide. Bit like the bumble bee that can't fly

Neil

Well, I think my questions have been pretty comprehensively answered!

Thanks to everyone who responded. I was particularly impressed with Andrew's detailed test results in his linked thread.

Just need to digest it all and do some tests of my own now.

I don't have any problems at all running the inserts the OP bought with any reasonable feed from very fine to what they will stand. People can have lathe problems which will mean coarser feeds or deeper cuts what ever type of tool they are using. From my experience tips are much more likely to chip when a lathe has this problem and it's rather noticeable. The answer is to adjust it if possible to obtain a more rigid set up.

In terms of sharpness they are no different to the standard style of tip in the same shape.

The only limitation I have found is depth of cut but I suspect that's partly down to the machine I am using them on. For max removal rates with deep cuts on many materials HSS makes more sense anywa ground to a shape that suites that sort of thing.

The photo I posted of the finish was turned at 500 rpm with a very fine feed for a mini lathe. It was a bit over 25mm dia cut depth circa 1/2mm. I wouldn't be happy with blue swarf on mild steel because I would expect a poorer finish. The first cut I took produced an awful finish. I knew it would because the cross slide was way too loose. The rest was ok including headstock bearings. 1/2mm should have been and was adequate to remove any remaining play. Probably more than needed on this particular lathe but if I had been working to a size it is the sort of cut that I would probably take and it's no good finding out that there were problems after the cut has been taken.

Where the tips should really score is tip life as there is no coating to wear off. It can be rather surprising how quickly this can happen when turning aluminium in particular and once it's gone bang goes the finish.

John

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Ajohnw, Hi John, Here is a CCGT tip that has seen a lot of use [only the first of the 2 proper corners ] It has been used mostly on En 1 and 8 and very little Aluminium. As you might be able to see it has lost it coating at the used corner [and did so a bit past] but the tip is still very sharp and still gives an excellent surface finish [please note, I always use flood coolant for steel] So I can't agree on your statement that once a coating wears off the finish goes down. Certainly not all tips are created equal, and that may have a bearing. This particular insert is Arno CCGT 090304 TiCN/ TiALN coated.

Cheers.