The Cambridge Turning Trials

[Anonymous]

Raymond: I'll be interested in seeing the photo. What radius do you use on your inserts?

Sam: Thanks, it's interesting that 'hard turning' is not new. Without wanting to prempt my experiments, I am expecting the ordinary insert to turn the hardened steel, albeit without a long life. The CBN insert is simply to try it. They exist, I haven't used one, so why not give it a go. However, I suspect it's not something I will be using on a regular basis, as they're pretty expensive.

Regards,

Andrew

[Sub MandrelParticipant @submandrel]

Little bit of random information – turning a blank for a 3 1/2" poly-V pulley from 1/2" thick aluminium alloy chequer plate. A high-rake tool gave a very poor finish tending to dig in, but a hand ground 1/8" radius round nose tool with angles for mild steel gave a mirror finish. Great chip formation with both tools and no welding of material on the tip. Roughing cuits started at 20 thou, finished at about 8 thou, all dry.

Suggests that high-rake tools don't suit the really tough grades of aluminium alloy.

Neil

[speelwerkParticipant @speelwerk]

I have no experience with "hard turning" but this is what Hembrug write about it on its website.

**LINK**

[Anonymous]

Here are the results from attempting to turn fully hardened silver steel. I used two inserts, in both cases with a 0.4mm radius nose:

1. Koroloy CCMT09T304HMPNC3120 – general purpose positive rake carbide insert for steel

2. Koroloy CCMW09T304 grade KB320 – a CBN insert for steel and cast iron with zero rake

The test bar was 25mm diameter. For both inserts I ran the lathe at 1200rpm (about 100m/min), with a feedrate of 0.1mm/rev and a depth of cut of 0.2mm. In both cases the finish was very good and the swarf came off bright orange. After use the carbide insert showed definite signs of distress on the cutting edge, whereas the BCN insert didn't look like it had been used. Under the magnifying glass the carbide finish looked fairly uniform, but not perfect. The CBN insert finish looked completely uniform; results borne out by the surface roughness measurements. These were (Ra in µm):

Carbide: 1.03 0.85 1.35 Average = 1.08

CBN: 1.51 1.44 1.45 Average = 1.47

The swarf came off as a complete tangle of very fine ribbons, just like a very fine metal pan scourer. I've never seen swarf like it; very fine, but completely uniform, and quite brittle. Here's a picture of the swarf and test bar, the CBN turned end is on the right:

Using my hardness file set I estimate that the end turned by the carbide insert is about 55Rc, ie, it has lost a little hardness. For the CBN end the hardness seems to have gone up (!), as now the black file, 65Rc, will not touch the surface; I wasn't expecting that. It makes some sort of sense though; the whole point of 'hard turning' is to replace cylindrical grinding as the finishing process. If the work ends up softer afterwards it rather defeats the object of the exercise.

As I understand the theory the idea of hard turning the idea is that at the cutting edge the metal is well above red heat, and so is soft. While in theory, again, most of the heat is supposed to come out with the swarf I don't quite see how the surface of the parent metal stays hard, as what quenches it? May be the key is the fact that the CBN insert has zero rake?

Well, that has been a most interesting experiment. The CBN insert certainly lived up to expectations, producing a superb finish without affecting the hardness. Mind you at £25 for a single edge non-regrindable insert it damn well ought to!

Regards,

Andrew

[Sub MandrelParticipant @submandrel]

Food for thought!

Neil

[VersabossParticipant @versaboss]

Gents, maybe it is worth mentioning that CBN inserts can often be found much cheaper on the usual auction site(s).

A happy user,

Hansrudolf

[Raymond AndersonParticipant @raymondanderson34407]

Andrew,

I have just today remembered that I was going to put up a photo of EN24T. I have

put a photo in my album of a piece that I am working on at the moment. The insert used was

Walter Tiger tec WPP01 the nose radius was 0.4 and the depth of cut used was 1.10 the feed rate was 0.182 and rpm was around 350 at 70Ø.

Regards,

Raymond.

Ps, No coolant was needed or used.

[jason udallParticipant @jasonudall57142]

Just a thought ever tried USA CUT 55 at home?

All the benifits of EN16T ( apart from weldability) but free (well relatively) cutting.

[Chris TriceParticipant @christrice43267]

Very good thread (how did I miss this first time around?). Just a quick note, I was trying to bore out a 1/2" hole in a piece of EN8 and couldn't get a really good finish. The boring tool was as short as possible to do the job and the whole set up was made as rigid as possible. Despite various speeds, still no joy so I put it down to the shaft of the boring tool flexing. Then in a moment of exasperation, I changed the tip for a polished Sumitomo one intended for aluminium and blow me down the next pass gave me a finish like glass. The moral of the story is don't be afraid to experiment.

[chris stephensParticipant @chrisstephens63393]

Hi Chris,

Goes to show that not all tips are equal, just 'cos it's carbide does not mean it's going to be perfect every time, but Sumitomo are one of the better ones (provided you get one suitable for your own needs of course).

chriStephens

[Anonymous]

Raymond,

Thanks for the results on EN24T, looks like an excellent finish was obtained. I assume that DOC and feedrate are in mm, pretty impressive if not?

Regards,

Andrew

[Raymond AndersonParticipant @raymondanderson34407]

Hi Andrew,

Yes, the feedrate and depth of cut were in mm, sorry for my oversight. If you ever get the chance to try a Walter Tiger tec insert I think you will be very pleased with the results.It is actually Walter Titex, Walter being the indexable section, and Titex the Drilling and end milling section.

Regards,

Raymond.

[HasBeanParticipant @hasbean]

Gents,
I know this thread is a tad old but thought it best to add to it rather than start a new one.

Now I don't get a lot of time these days in the workshop (back of the garage) what with work, kids etc so I relished the chance to have a play this afternoon (won't bore you with the details).

Whilst turning some black bar I was too heavy handed and chipped the insert, CCMT type. Being tight on time instead of replacing it I found another holder with a DCMT insert of the same type and used that.

Now I immediately noticed an improvement in the surface finish! The speed and feed was the same, same insert type, radius and chipbreaker, so what had changed?

I then went off at a tangent and dug out some of the various inserts I had that were effectively the same apart from that they were either CCMT or DCMT shapes.

Each time the DCMT inserts gave a better finish.
But Why?
The same radius, chipbreaker design according to the box (looks the same under a magnifier) etc. Even a small depth of cut to offset the entrance and clearance angles!

All I can think of is the chipbreaker design makes a difference because it's squeezed into a 55 deg angle instead of a 70 deg angle? I don't know, just grasping at straws here!

Any thoughts Gentlemen?

Paul

[KWILParticipant @kwil]

Although both tips have a support under them from the toolholder so insert support is similar, what overhang from the toolpost was involved with each item? Was the toolholder projecting further in the case of the CCMT?

[HasBeanParticipant @hasbean]

Aha, well done that man, hadn't thought of that. I shall go and check!

 

Oh well, the DCMT holder has about 8mm more overhang than the CCMT holder, probably set it like that so I can use it closer to the tailstock I guess.

Edited By HasBean on 05/09/2015 17:26:26

[MuzzerParticipant @muzzer]

Simply describing the insert as CCMT only tells part of the story. There is also the grade of material used for the insert and perhaps more importantly the shape of the cutting edge ("chip breaker" ).

Have a look at this technical guide from Korloy. Scroll down to page 30-31 and you can see the variety of chip breaker profiles available.

Without knowing more about the inserts you have been comparing, it's difficult to be surprised that they cut differently.

Murray

Edited By Muzzer on 05/09/2015 19:43:17

[HasBeanParticipant @hasbean]

Hi Murray,

I usually use Mitsubishi cermets, grade NX2525. The CCMT boxes I have don't have a chipbreaker suffix but the DCMT ones do, FV, which is a finishing type. Under the magnifier they appear the same and are both sold as having the same chipbreaker by the supplier.I also tried some Walter and Korloy inserts of the same grades and chipbreaker type as marked on the box and appearance.

Must say I also noticed a different sound when cutting, seemed 'quieter' for want of a better word.

At the end of the day I know there is a difference between them but being a nosey bu**er I just wish I knew why!

Still, I've learned something and now have more options so can't be all bad!

Paul

[JasonBModerator @jasonb]

Could a thou or two difference in tool heights be making a difference? Can you measure them with a height gauge of the lathe bed or cross slide. I'm assuming you have the two in different holders and are just swapping holders.

J

[Ian S CParticipant @iansc]

Have you dressed the edge with a diamond lap? If not, try it particularly with the noisy one.

Ian S C

[MuzzerParticipant @muzzer]

I don't know what speed you are using but they like high surface speeds. Getting the chips red hot within the work zone where the metal is shearing is critical. Have you tried varying the speed?

[HasBeanParticipant @hasbean]

I have used different holders but also swapped the inserts on the same holder. Speeds? Well I tend to use the lower spindle speeds but I do get a better finish all round on higher ones just that when the lathe sounds like a turbine it frightens me a bit

My theory is, although I'm happy to be proven wrong, is that the same chipbreaker design must be different when formed on inserts with different included angles, I really can't think of any other variable?

Paul

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