Hi Mike, for the most part model engineers don’t operate at the speeds and feeds which justify the use of carbide insert tools. So for most steels I generally use HSS, particularly if you have uneven or broken cuts, at least if you damage the tool its quickly re-sharpened.

Although TC is very useful for difficult materials like stainless steel, phospher bronze and screwcutting, parting etc. They are as you have found to your cost very easily damaged. There are grades of insert that are more resistant to shock and abrasion like those for machining cast iron. I’m not up to speed on the grades required, you would probably be better to speak to someone like JB cutting tools if you want to continue using TC tips for this type of work.

Andy.

Having bought, over the years mostly on ebay a large quantity of shapes and sizes of inserts, the big brands commonly have their suitability to sustain interrupted cutting on the box. This is important for industrial users and is generally lacking on the cheaper brands. It helps to have a variety so that you can find which are best for any particular job. I have bored soft chuck jaws with inserts intended for aluminium successfully to get a fine finish and got away with it, your metal might be simply too hard for your CCMT’s. If boring a difficult job, it is important to have the absolute minimum of unsupported bar projecting to maximise the stiffness, having a movable stop on the bed allows close work without the danger of a crash.

On 4 August 2024 at 22:13 Fulmen Said:

If carbide couldn’t handle interrupted cuts, why would carbide mills exist?

Carbide can be somewhat counterintuitive in that they don’t like too slow speeds. Interrupted cuts combined with cooling is another source of problems as the thermal cycling can cause excessive wear on certain qualities.

If you can, run carbide at the recommend speed (RPM). Non rigid lathes can handle axial forces better than radial forces. So reduce the depth of cut and if that is not enough, reduce the feed/rev to avoid/reduce chatter.

If I run out of options, I also use an insert for aluminum on steel and stainless. Because they are very sharp and mine have a 0.2 mm tool nose radius, the cutting forces are much lower then when using inserts for steel. If that also doesn’t help, i try a HSS tool. On my non rigid lathes, I use inserts intended for finishing most of the time (VP15TF).

As a general rule I try to minimise interrupted cuts as much as possible, with any type of tool, and the larger the radius the less I like them.

That is not just a matter of what the tool can handle, but also the hammering effect on the lathe. I don’t often use fly-cutting on the mill, for the same reason.

If I need machine a chain-drilled surface I always remove the worst of the ridges first, by filing or (less often and certainly not as first choice) grinding.

On my manual shaper, I file a chamfer on the approach edge first to take some of the shock out of the action.

Regarding speeds and feeds, I must admit I do not sit down with a calculator first! I judge the action by watching and listening; and quite honestly have never found any real problems using carbide tips at modest speeds.

.

One exception is some mild steel of unknown parentage and even less quality-control, originally the tie-rods for a big cable-drum. It tears dreadfully with carbide but a sharp HSS tool, cutting oil and experimenting with speeds and feeds brings acceptable results for the intended purposes (not fine-finish moving parts, and often painted).

The alleged steel stocked by a certain DIY supermarket chain is similar, and sometimes so rough and approximate to diameter it has to be skimmed before die-threading, but again, I use this only for non-critical applications.

On a model it would be for parts that on the full-size were likely to have been just brush-painted over hot-rolled or forged surfaces – tie-rods, brackets and the like.

If you need an interrupted cutting insert the info is in the grade number along with the metal type. Obviously premium makers only.

Fulmen-

There is far less impact between the work surface and cutting edge in milling, than in turning with interrupted cuts; and the cutting geometry in milling is quite different from that of turning.

There are big multi-point face-milling cutters with inserted carbide tips, but the tips are designed for that duty.

One other thing to watch if you feel the need to go for an insert intended for interupted cuts is they tend to be *N** not *C** so you will need to buy additional tool holders to suit the neutral edge as they don’t fit the common 7deg edge of the “C” type holders.

Also likely to only come in the larger sizes so if you are limited for tool height then the associated larger shank holders may not fit. Ditto if wanting to bore a small hole.

On 4 August 2024 at 20:17 Andrew Crow Said:

Hi Mike, for the most part model engineers don’t operate at the speeds and feeds which justify the use of carbide insert tools. …

A bold assertion from Andy!

But carbide’s ability to remove metal faster than HSS isn’t the main reason many prefer it to HSS.  It’s because inserts are easier to use and/or more convenient than HSS!  Reasons include:

• No need to buy a grinder. (Cheap modern ones are no good – a heavy industrial unit with a suitably hefty tool-support is needed.)
• No need to learn grinding.  Many find acquiring grinding skills difficult, despite the persistent claims of HSS fans that it’s easy!
• Most common tool-forms are available off-the-shelf, with many inserts supporting up to four cutting edges.  Thread forms are particularly useful.
• Once the tool-holder is set to height, inserts can be swapped and rotated without readjusting anything.  Not possible with HSS, though similar flexibility can be obtained by buying a Quick Change Tool Post and several tool-holders.  (Whilst HSS is a cheap  material, spending money and wasting time is no object when it comes to using it!)

Inserts aren’t perfect.  They’re consumables, not meant to last forever!   Not a problem because, when they wear out, inserts are quickly replaced without disturbing the workflow.  Stopping to resharpen HSS is much more disruptive.

Early carbide was brittle compared with HSS; this is much less true today.   Like Jason I don’t have much trouble doing interrupted cuts, certainly not enough to condemn carbide outright.

In practice I use both, carbide preferred at least 80% of the time.   Main reason I switch to HSS is that it’s easier to get a good finish on difficult materials – carbide is fussier than HSS about rpm, depth of cut and feed-rate.   The other reason is special form tools; HSS can be custom ground – cutters aren’t limited to standard shapes.

Bottom line, though carbide delivers best results when driven harder than HSS,  that’s not essential, or the main reason I prefer it.

Dave

This was a fairly brutal interrupted cut:

Cleaning up these chain drilled bores in hot rolled steel:

I used a CCMT insert at 370rpm, 0.05″ DOC and 8 thou per rev feed. I got through one edge per part. The noise was horrendous, with plenty of chatter, but it got the job done. The inserts didn’t chip but the edges wore away over time.

I only chip an insert if I do something daft, like run the insert into the chuck when everything is stationary.

Andrew

There’s multiple grades of carbide, and those letter designations to identify replaceable carbide tips doesn’t mean there all exactly equal in performance between the multiple brands. Each different manufacturer almost for sure uses various binders and possibly slight differences for the same grade of carbide and it’s letter designation. Even the sintering processes are unlikely to be exactly the same between each manufacturer. So there’s multiple reasons MikeK may have had his CCMT inserts chip and others mention having no issues. Yes rpm and feed rate is important, depth of cut even more so. Double the depth of cut with everything else equal, and the shock impact levels the carbide tip sees will be much much higher. Depending on the steel alloy, the welds might be easily machined, but that heat might well have hardened or created a heat affected zone. With harder metal, the carbide will chip much more easily. Even a small tip radius verses a large one will chip easier. Braised tip carbide will also be more chip resistant than the replaceable simply due to the better support behind the carbide that type of cutting tool has.

He didn’t mention it, but infrequent applications of coolant or cutting oil can also cause micro fracturing with carbide that also helps the carbide to chip.

I have a fly cutter specifically designed to use the round RNG round carbide tips. Obviously it’s never once seen anything other than an interrupted cut. And like Jasons examples, boring heads excel at producing accurate partial internal radii. But I still reduce my DOC compared to doing a continuous cut when boring. And most of my boring bars are either solid carbide, braised or have replaceable tips.

I don’t agree that most amateur’s don’t use the extra rpm carbide is capable of, While I personally chose to never use the maximum, I do try to stay within 85%-90% of it whenever possible. I also read the chip color when machining steel. Turning light to medium blue after the chip has left the work piece is my cutoff point for maximum rpm with carbide, and what the manufacturer says for rpm may or may not be true for what I’m doing. There maximum rpm listings are only handy but suggested starting points anyway. Chips turning dark blue to purple is for commercial work in my opinion where speed and bulk metal removal is more important than the best cutting edge life. Afaik, there’s in general at least 6 grades of carbide hardness. I don’t recall seeing any C1 carbide used for cutting tools, but I believe C2 & C3 are used for wood router bits. C4-C6 is likely the grades of replaceable tips we might be using. But there could easily be more I have no knowledge of. It’s my understanding that the higher the amount of cobalt binder used, the more chip resistant the carbide becomes. But more binder reduces the tools effective cutting edge life span. Pick which one you want because you can’t get both in the same piece of carbide.