HSS vs Carbide.

HSS is much tougher than carbide, and takes a significantly better edge. So why doesn't everyone use it all the time? Because carbide is harder,
and so wears better. HSS goes blunt quicker.
Also, carbide retains it hardness to higher temperatures than HSS, so you can push it harder and cut metal faster.
This hardness comes with several pricetags though - first, and least important, carbide is hard to sharpen. Replaceable tips mean the difficulty of sharpening doesn't matter much, as the sharpening is done by the factory. Also with a little practice you can shape and sharpen carbide on a "green grit" or far better a diamond wheel, so this can be overcome in full except for a bit of effort.
Second, while carbide is very hard in compression, it's tensile strength is very low. So for instance never use a carbide tool below center - the forces will tend to pull it apart, and it will crack. It's only strong when it's in compression.
This also means that the types of edge you can give to carbide are limited. A sharp edge will often have areas where the forces are in tension, so you can't do that sort of really sharp edge in carbide.
The inserts used in big tools (and therefore the ones which are readily available) usually have rounded edges partly for this reason, to prevent tensile forces and keep the carbide in compression.
However some types of carbide inserts, eg inserts intended for machining non-ferrous metals and aluminium, try to give the carbide more tensile strength and the inserts shapes which, while sharp, are still mostly in compression when cutting.
For a small lathe, these inserts are the best to use as they are reasonably sharp - not as sharp as HSS because the geometry is limited by the need to keep the carbide in compression or nearly so, but getting close.
But for a really sharp edge, HSS wins out. There are some metals which I can cut with HSS which I just can't cut with carbide. Okay the HSS edge doesn't stay sharp for more than a minute or so, but - I can't cut it any other way. so that's what I have to do.
Third, carbide is brittle. This matters for interrupted cuts, it matters a whole lot, it's what will cause the beginner and probably even the experienced the most tip breakages. You have to slow down a whole lot when making interrupted cuts with carbide, so much so that overall HSS is usually faster-cutting on small machines.
I have gone on a lot about the disadvantages of carbide, but so far not about the disadvantages of HSS. For the small lathe owner, there are only two - the minor one is that it can't be used on some hard alloys, which only applies to a very few people - and the major one is that the knife edge goes blunt quicker than carbide.
People, eg those who are concerned about output rate, those in business, or just impatient people, can find this need to resharpen HSS regularly incredibly annoying.
It's not necessarily bad, it depends on the work you are doing and other tings - in some cases it's not a problem at all, in others it's a deal-breaker.
For a small hobbyist it's usually not a big deal, eg if you are machining aluminium you may have to sharpen once every few hours of hard working with a single edge, and maybe once every few days for more normal work.
Unless you are in production of hard alloy parts, or your smallest machine is a Bridgeport, I'd suggest at least giving HSS a try.
So what do I use? On the minilathe, carbide for the everyday first attempt, because it doesn't need sharpening - though for me an average material might be EN24, with copper, inconels, hastelloys and monels at the harder-to-machine end, rather than the average brass or ally for most people.
(I do most of my work in hard materials - when I cut ally it's like, wot?, it's almost not there, knife through butter, and so on. What, only cut to a hundredth? Feels like I could easily get to a micron, a tenth of a micron if you like :)
I use HSS on the minilathe for unusual tools, or unusual metals, and when I want the best finish and results. I also often use HSS for parting, unlike many people, for the toughness.
On the mill, it's mixed carbide and HSS. HSS by preference for interrupted cuts (there are more interrupted cuts in milling than in lathe work) and for some finishing work, carbide for speedy removal of lots of metal.
Just my 2d-worth,
-- Peter Fairbrother
why is a 6mm HSS endmill so much more expensive than a 6mm HSS drill?
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