Carbide tipped lathe tools

I have just acquired a set of these with brazed tips. They are painted blue and all the tips have the paint on the sides of the tips whilst the top face is polished. To me that then seems as if the cutting edge has one face painted and the other clear.

My elementary question is 'should I be preparing these tools in any way before using them or just use them and let the work prepare them?' i.e. remove the paint from the side face.

Many thanks

Martyn

The "El-Cheapo" sets of brazed carbide tools are not always the best quality. Probably easiest to give them a go and see if they work. Don't bother to remove the paint, it will quickly wear off. If the tools cut you are onto a winner if they don't cut then they can be re-profiled but you will need a green grit or diamond grinding wheel to do it.

John

Thanks John, very clear advice which I will follow up. Of course, hope I will be on to a winner!

Thanks

Martyn

Yes!

For some reason most people (and some suppliers) are not aware that these tools are usually supplied as unfinished blanks NOT finished tools.

A close look will probably show you that there is no front clearance on the inserts.

This is the main reason why people think they are useless, and the problem is exacerbated because they don't normally come with any finishing instructions. To get decent performance you need to use a diamond or green-grit wheel to put suitable clearance on the cutting edges and probably round the tip slightly. ideally put a polish on all sides with a diamond slip.

This will give you a tough tool ideal for jobs HSS can't touch and where atypical insert would crumble, but it is NOT a process I would recommend to a beginner.

Tooling supplied like this is how carbide is usually supplied for heavy duty work, though pre-finished ones are available they are generally £££££

Also, watch out for cheap ones with forged shanks, those with properly turned shanks generally have far better geometry to start from.

Neil

Edited By Neil Wyatt on 18/02/2018 18:51:08

Well I didn't realise that. I knew they were often poorly shaped but assumed this was just poor manufacturing. I shall have to have another look at the ones languishing at the back of the drawer!

John

Thanks Neil, will approach this with a bit of care and thought. Having just successfully ground two HSS tools I realise how worthwhile it has been to have made a half decent adjustable rest for my grinder. A decent rest with diamond/green grit will be the answer but will take time!

Martyn

Thanks also Neil.

A while ago I bought a set of nearly 40 TCT brazed 12mm shank tools, at package price close to £1 per piece. Good price, but I was not very impressed by turning results. Then on closer examination, i found out that the front and side faces variably and in many cases not at all finished, often completetey rough and unground with the cutting 'edge' unble to contact the work before the rough carbide body below.

After grinding the tool on a green grit wheel (quite a tough process!) the tools did cut well, but they did seem generally somewhat more fragile than expected - I was turning some stainless 304 at the time, and if the tool height was not fully correct, the tool tip frequently broke nearing the centre of the facing cut.

I also found that with a 9-piece set of TCT boring tools: again although the tools appeared to be ready ground, the front face was ground in many cases at such an angle that the body of the carbide fouled the work before the cutting edge could make contact, however one oriented the tool. Again, regrinding the tools made them usable.

I am inclined now more to use insert tooling. However I did find on using a holder with CCMT insert from one of respected suppliers on this site, that it was completely unsuitable for any sort of light cut, having a remakably round surface where I expected to find a cutting edge, such that there was no sharp edge to cut the work before the insert body contacted it! I assume it is intended for very high speed heavy work, such that the tool forces its way through semi-molten work surface - is that correct, and the alleged reason for not using insert tooling on small lathes? Certainly not suited to any light finishing cut.

The CCGT tips work well, and can be used on mild ferrous material (including interrupted cuts), but don't necessarily expect a very long tip life, but a useful one.

Martin

CCMT tips require a depth of cut of about 2/3 the tip radius, so for a typical 0.4mm radius insert that's maybe a minimum of a 25 thou cut - no good for shaving cuts!

The 'official' line is that smaller tip radiuses are a bit fragile for 'hobby use' - they are easier to chip but i do use them.

CCGT tips are much sharper and although intended for non-ferrous metals many find they work well on steel. They can give a mirror finish on aluminium alloys.

Neil

No I doubt that story, if turning is creating that much heat it would make any kind of accurate work impossible, and more than likely create a lot of unnecessary stresses in the work itself.

Michael W

OK, a bit of an exaggeration in normal work, although it is not unusual to have blue swarf when turning ferrous material at speed, implying temperatures in the region of 300C

But in the case of turning very tough or hardened material (in which I have as yet no experience), I believe it is fairly usual for the local work temperature at the cutting edge to be near red heat - that is how it becomes soft enough to be turned!

I have a couple of boron nitride tipped inserts - I must try one on somthing hard some time

Martin

Yeah I thought it definitely gets hot, but not anywhere near melting the work in usual cases. (The most extreme example of this probably has to be friction welding, but that isn't usually creating the shape of the item.)

one of the reasons stainless is such an annoying material that chews its way through tools is mainly because it's such a poor conductor of heat, meaning all the heat gets dissipated through the tool instead, thereby deforming the cutting edge quicker.

Michael W

It is not unusual for the metal at the shear zone to be at red heat. If the chips are not straw/blue you're not pushing hard enough! There is an advantage to running fast. SInce the metal at the shear zone is hot it is also easier to shear, so the power needed to remove a given number of cubic inches per minute goes down slightly. In theory most of the heat leaves in the swarf. However, each bit of swarf cools very quickly, so the shear zone is likely to be hotter than the colour of the swarf might indicate.

For hardened material, >60Rc, the rule of thumb is shallow DOC and high feedrate. As Martin says it is important for the shear zone to be red hot.

The CCMT inserts with smaller radii seem to wear more quickly, but I've never had a problem with them chipping. As standard I use 0.4mm radius inserts, but use 0.8mm if I have a lot of heavy roughing to do, and I use 0.2mm if I need a small internal radius on the work.

Minimum recommended DOC varies from insert to insert. For the CCMT inserts I use it is about half the nose radius. That's only 0.1mm on a 0.2mm nose radius - tiddly! It's a myth that carbide inserts will not take small cuts; they will. But of course there are caveats. First, the material has an effect. For instance bronze is reluctant to accept small depths of cut. On the other hand 303 stainless steel has no problem. Second, the finish can be worse with small DOC. Oddly for 303 stainless steel it doesn't seem to be much worse. However, for plain low carbon steels (which is what I assume most people use) the finish can be significantly worse.

However, it's much easier just to take sensible cuts when finishing. If there is 20 thou to go, take a 10 thou cut and be done with it.

Andrew

I actually didn't know that either, but it would explain why with the painted boring bars, the cutting faces are not even close to centre height of the body, meaning that even on a really robust machine the vibration and rubbing would be horrendous.

Michael W