tnmg inserts

Can someone explain to me (not much experience of inserts and find all the terminology confusing), are all TNMG inserts negative type??? Also are the generally all the same dimensions apart from thickness across manufacturers?

Thanks

Edited By mark smith 20 on 14/09/2017 12:01:03

A good code list here> **LINK**

Every time this subject comes up, we see the same hyperlink. The problem being that this is for American (imperial) nomenclature. For metric ISO nomenclature which has different numbers after the letters, you need to look elsewhere such as Korloy, Sandvik, Iskra etc.

Sorry - busy right now but perhaps someone can post a link? Probably something like this. See page 16.

Murray

Edited By Muzzer on 14/09/2017 12:37:44

This one is pictorial for the identification of inserts you have or have seen -

http://www.engineeringsupplies.co.uk/pictorial-insert-guide-i-10.html

This one is for holders so you can identify the insert type required

https://www.cutwel.co.uk/learn-the-turning-tool-iso-code-system

This is helpful too -

http://www.helmancnc.com/general-turning-insert-nomenclature-for-cnc-dummies/

That's 3 minutes and 20 seconds worth of Google searching 

- Nick

Edited By Nick Hulme on 14/09/2017 13:36:11

Thanks all for the links ,i think i understand a bit more now after reading those. You have to understand i don`t own any carbide tipped lathe tools at all and this is for a different project.

One further question ,i see lots mentioned as for roughing or finishing etc... Does that just have to do with the sharpness of the inserts /coatings.? Is it impossible to get a good finish with a insert stated as suitable for roughing? Are the roughing inserts tougher in some way.

Carbide inserts are optimised for industry where time and energy are money. Roughing cutters have edge profiles designed to remove metal as economically as possible. As this usually involves metal being peeled off with a wedging action you can imagine that the finish left behind might not be good. At the other end of the scale, a finishing cutter is designed to leave a good surface on a particular metal. It can still remove lots of swarf if pushed, just not as efficiently.

In my home workshop I've found that I can usually get a reasonable finish from any of the carbide tips I own by altering the depth of cut, rpm and feed speed for best results. It takes a bit of experimentation and wouldn't do in a factory, but it works for amateurs with time to learn and chaps who already know what they're doing. When 'reasonable' isn't good enough for the job in hand, I switch to HSS.

Dave

Edited By SillyOldDuffer on 14/09/2017 15:04:07

Not knowing whereabouts in the world the original poster resides makes being always right a little difficult.

You only need to read the 'practicalmachinist' forum to see that the professionals have exactly the same problems. It tends to be with the more esoteric materials, but "how do I get a good finish on...." threads are quite common.

I'd surmise that at least some of the difference between roughing and finishing inserts is to do with the chip breaker. Roughing tends to be at higher feedrates than finishing, so producing thicker chips which break differently.

If I understand the cutting process the chip wedge doesn't actually touch the cutting edge of the tool, but rubs on the top surface inland of the edge. The edge simply rubs on the already cut surface. The finish depends not only on speed but also the material. Some of the sticky low carbon steels are the worst, like EN3. Upping the speed can transform the finish. If you run fast enough the interface at the shearing of the chip is approaching red hot, and the metal surface is very soft. So the insert edge rubbing on it simply smooths it and leaves a good finish.

Andrew

Got a friend, an engineer of long standing who decided that he would "get all modern" , his lathe is a rather large DS&G, it takes a 20" chuck. The lathe is used a lot for boring hydraulic cylinders, and after trying just about every carbide tipped tool he went back to HSS.

Ian S C

Exactly. Unless you are taking a stupidly teeny finishing cut which sort of rubs material off, lathe tools drive a crack up the material a little ahead of the actual tip. Counterintuitively the power per unit volume of material removed actually falls considerably for a fair range once over a certain critical level due to the heat involved in shearing the metal softening it ahead of the shear. This level almost invariably involves too much heat for HSS tooling to withstand but carbide tools are designed and engineered to work in this region. Ideally most of the heat comes out with the chip leaving the workpiece relatively cool. Obviously good for precision but it also greatly helps getting good finish. Much less energy locked up in smooth surface than in rough one. Which is why liquids of any depth always end up with a flat (relative to the local gravitational field) surface. So if you take all the heat of cutting energy out with the chip it almost has to leave a smooth surface behind. Chip-breaker geometry pretty much defines what happens here so, obviously, the tool shape is pretty closely optimised to the material being worked and the operating conditions. Which is why there are so many types of tip.

At the lower speeds used with HSS the shear profile tends to be similar to that of the tip of the tool so a nicely polished tool gives a better finish. Aluminium rated inserts along with sharp and super sharp shape sharp varieties generally work more than adequately at HSS speeds but they do tend to be fragile. Other types its in the lap of the gods. Pretty much anything with positive rake will work outside its design range but finding the out of range sweet spot can be difficult.

The built up edge which most of us have cursed when working with aluminium is the most common example of a HSS tool working in the softened metal region. Was working with a "too good to bin" off-cut of "horrorminium" earlier this week. Ploughed field being a kind description of the finish at anything other than the teeniest cut. Took me far too long to remember that upping the speed'n feeds with flood coolant was the right way to get a nice finish off a heavy cut. Came out a lovely satin finish. All due to changing the thermal and energy issues at the point of cut.

Clive.

Edited By Clive Foster on 15/09/2017 16:11:40

Edited By Clive Foster on 15/09/2017 16:12:36

At the risk of carping ....

The references provided by Nick are very useful, but what I would like to know is 'I want to face some aluminium, which tip should I use?' Or 'I want to get a good finish on some steel', or 'I want to part of some brass' ....

I get that you can work out sizes and stuff from the numbers, but why would I pick this rake angle or that tip radius?

Most of us have a lathe in a certain size and power range - surely it's possible to have some simple recommendations of which tips to use for what?

My approach at the moment is to buy something from ARC or other suppliers and hope it's general purpose enough...

Iain

Iain, well said ive learned somethings from the links given,but a kind of simple guide for home users would be helpful.

The inserts come in a bewildering array of shapes and radiuses ,rakes etc... even for just one type.

Like why use a positive or negative rake, etc.......

Life isn't like that. There are just too many variables to allow hard and fast rules. The metal itself has a significant effect; was it bought against a material standard, or did it come from a scrap yard; do you really know what it is?

For general turning I use CCMT inserts for everything except aluminium alloy, where I use CCGT inserts. Larger nose radii tend to be more robust, but need bigger minimum DOCs. I use 0.8mm for roughing, 0.4mm for most turning and 0.2mm if the part is small and/or I need "sharp" internal corners.

Spindle speeds are heavily dependent upon the material, some cut fine at low spindle speeds, others absolutely require high surface speeds to get a good finish. Low/medium carbon steels, particularly EN3, are some of the most finicky. Feeds need to be higher than many people use; I almost never go below 4 thou/rev and I'll be at least double that for roughing.

In the end you need to experiment and build up your own experience and rules of thumb. And even then one can be surprised every now and again.

Andrew

I've not found a simple answer to the question, rather a mass of guidelines aimed at experts! I think the fundamental issue is that manufacturing is about cost, and managing it involves factors irrelevant to small scale operations. There are several trade-offs well worth the effort in mass-production that result in a multitude of alternatives.

I've not found anything that links insert types simply to these various trade-offs. What follows is what I've gleaned. I don't know that it's right and offer it in the hope that someone more knowledgable will correct it!

The two main factors that matter to a cutting tool are the rake angle and the relief angle. And the effect of rake and relief also vary with cutting speed and depth of cut. These factors interact will the material being cut and tweaking them is more-or-less advantageous depending on how hard or soft the metal is; for instance copper is very soft compared with chilled cast-iron.

The rake angle determine how 'sharp' the tool edge is where it hits the work. More positive angles are 'sharp', negative angles are 'blunt'.

• Positive rake is better for 'soft' metals. (In this context, Mild and low carbon steels are at the hard end of 'soft'.) So use a moderately positive rake on most steels, and make the rake progressively sharper i.e. more positive for Brass, Aluminium and Copper. Sharp tools are not good for taking deep cuts, but are better for finishing.
• Negative rake is used for very hard metals, for interrupted cuts (like rounding the corners off a square plate in a lathe), and for roughing. Negative rake inserts are often better value than positive ones because both sides of the insert can have cutting edges. Also, in industry, deep fast cuts with a blunt edge uses less energy to remove metal than a positive rake insert.

Relief angle seems more to do with finish, and the ideal relief angle also depends on the metal being cut. The idea seems to be that the tool should rub slightly just after the cut to smooth it whilst allowing space for swarf to escape. In practice the relief angles of inserts vary between 5° and 14°. 5° relief angles are used on hard metals like stainless steel, and 14° on soft metals like Copper. Relief angle doesn't seem to be very critical, and an insert with 5° to 10° relief should do most jobs.

Bottom line on this is that the bewildering range of possibilities may not matter much to the average home user. I've found that:

• 'Sharp' high positive rake inserts recommended for Aluminium work well on mild steel and brass at relatively slow cutting speeds producing ribbons of swarf.
• The Moderate positive rake inserts of the type easily available from Arc and others are good for mild-steel and equally effective on Brass and most Aluminium Alloys intended for machining.
• Fast heavy cuts with negative rake inserts are impressively good at removing steel quickly. The swarf comes off as a hot spray of chips not ribbons.

More important I think, than choosing the 'correct' insert is having a machine tool fast and powerful enough to exploit carbide. On a smaller machine (<2000 rpm and/or <1HP), I think you'd be better off with HSS. That said, I suspect an ML4 cutting with the sharp inserts designed for Aluminium, might do perfectly well on all the usual metals.

I think not needing to cater for Industrial optimisations explains why many of the hobby suppliers only stock a small subset of carbide inserts. They are the type having Rake and Relief suitable for 'average' work and they can be expected to do a good job in most circumstances. I used them exclusively on a mini-lathe with good results. On a bigger machine it's more obvious that other shapes of insert are useful but I'm pretty sure almost everything I do could be done with the small subset I started with. (I mostly turn mild-steel, aluminium and brass; sometimes stainless, cast-iron and graphite (with HSS); never Copper.)

In conclusion, I suggest 'ordinary' inserts are fine for most hobby work, and it's not worth worrying about the rest unless you're planning something special. Professional use is a different ball-game entirely.

Dave

Edited By SillyOldDuffer on 17/09/2017 12:54:24

My two 'pennorth worth is that it isn't all that easy to get a super finish with carbide tools. Good, yes , but not excellent. I have tried experimenting and following a few tips given on this forum. The results have been a bit disappointing.

In my humble opinion, you cannot beat a round nose HSS tool that has been properly ground and then honed. I can take sub thou finishing cuts with these and the "swarf" is more like cast iron powder. The finish is almost mirror like on most steels.

I am not trying to be provocative and am always willing to learn, maybe you can get such a good finish with a carbide insert, always willing to try again!

Andrew.

I thought that the advice for brass was almost zero rake? Ah well, must have been doing it wrong all this time.

On a more general point all these discussions about tooling versus good finish, or otherwise, are rather pointless without any sort of proper measurements of Ra or Rz.

Andrew

I wouldn't disagree Andrew; I had much the same experience. I had variable results with carbide until Andrew Johnston explained the need for a high feed-rate as well as fast rpm and deep cuts. Even now I'm better educated I still have to experiment with the metal in-hand for best results.

With carbide you have to balance rpm, cutting depth and feed-rate and there's a point at which the finish suddenly jumps from indifferent to excellent. The only thing I can say about finding the balance is that it's HOT - well beyond anything that an HSS tool would cope with. I sometimes wonder if people brought up on HSS have trouble with carbide because all their instincts and training scream 'too hard, too fast' long before carbide is anywhere near its sweet spot.

Even though I mostly use carbide, HSS is essential in my workshop. It's particularly good at edging up to a dimension and a good finish with fine cuts. Carbide does better when you land directly on target with a deep cut, which isn't easy!

Dave

Thanks, guys for this. Whilst I appreciate there's a lot of 'it depends', some kind of starting is helpful.

Andrew. My decoding of the magic words CCGT and CCMT indicate that they are both Diamond 80 deg, 7 deg clearencem double countersink, single sided.

The difference appears to be the tolerance, but confess that I can't quite decode what that means.

Iain

Well, I did say that my take on the subject could be wrong and invited corrections. Also I plead guilty to over-simplification by using words like 'hard' and 'soft'.

I use positive rake on brass all the time. It works OK, but if anyone is doing it wrong it's far more likely to be little me than you Andrew!

Dave

It refers to the accuracy to which the insert is made. Remember that inserts are sintered from particles and binder in a mould. The CCGT inserts are polished, have higher rake angles and sharper edges, so need to be made to a better tolerance, possibly with smaller particles. As you surmise both inserts have the same shape and will fit the same holder. Here's one I took earlier, CCMT on the left, CCGT on the right:

Now back to work.

Andrew