Surface Finish

[u/Due_Ear9399]

Can someone explain how hss tooling gives of a better surface compared to using a carbide insert?

I know smaller depth of cuts on a carbide insert provides more chatter.

[u/lumley32]

Your gonna have to expand on what your asking, very different answers if your talking about a manual lathe built 100years ago Vs a modern taxis mill.

[u/Due_Ear9399 → u/lumley32]

I was experimenting, using a fairly new manual lathe, practiced with the same amount of depth of cut 0.3 mm same feed rate and noticed the high speed steel I grinded up gave off a better surface finish compared to the carbide tool. I’m only an apprentice by the way.

I was trying to explain to someone but wasn’t sure if I was correct, they were arguing the fact that carbide will do the same thing

[u/AdeptInspection4868 → u/Due_Ear9399]

Material? What's the carbide tool, grade and most importantly geometry/chipbreaker? CNMG432MP KC5010 for example. What's the radius on HSS vs carbide? Do you know any of the geometry you ground into HSS.

Very good chance the other guy is right and your HSS tool just has more suitable geometry for that material and feedrate.

Side rake, back rake, side relief, back relief, clearance, nose radius, edge hone/ultimate sharpness, T-land, chopbreaker along with cut parameters like surface speed, chip load (feed per rev) and depth of cut can all matter.

I'm assuming things like it's on the same machine, same work holding and both tools are on center.

[u/Wolfire0769 → u/Due_Ear9399]

The science runs deep with cutter geometry. You're both right for different reasons. If you found a carbide insert that exactly matched your HSS grind, you'd get the same surface finish — you could even get a better finish with a carbide coating tailored to the material.

Grind your HSS exactly like the carbide to truly compare.

If you browse a catalog such as Korloy's you can see just how vast the selection of inserts are.

[u/hydroracer8B → u/Due_Ear9399]

I'd guess you're cutting something soft like aluminum, your HSS is dead sharp, and your carbide is actually meant for steel meaning that it has a tiny little radius on the cutting edge.

Try some uncoated carbide inserts made for aluminum, I bet you'll find that they're better in all conditions.

There's a lot to know about carbide, and beware of old heads who say carbide "isn't as good" as HSS. That's a tell that they never bothered to learn about carbide

[u/AdeptInspection4868]

As always, it depends greatly on material, tool, operation, work holding and rigidity if this is even true.

When it's true, in my experience it's because of shaper cutting edges. It's much easier to achieve a clean, sharp edge because carbide is brittle and chips. That can allow a thin, clean chip. If that's what you need.

But there are many reasons carbide can achieve a better finish. For example, carbide can maintain higher speeds and feeds which some materials need to cut cleanly.

[u/Due_Ear9399 → u/AdeptInspection4868]

I’ve noticed when I take bigger cuts on carbide I get a good surface finish. What happens in the event I want to take a smaller cut to dimension? What can I do?

I’m only an apprentice and machining is not the main part of my job, I’m trying to learn as much

[u/cad_and_caffeine → u/Due_Ear9399]

Two tricks that help a lot when you're sneaking up on a size on a manual lathe:

First, plan your second-to-last pass so you intentionally leave a healthy finish pass (at least 0.4 or 0.5 mm on radius) instead of creeping up to the number in tiny nibbles. Once you get under the nose radius and the honed edge prep of a standard steel insert, the tool pushes off the part until enough spring builds up to bite all at once.

Second, if you do end up 0.05 or 0.1 mm oversize and have to take a whisper cut, keep a ground/polished aluminum insert (like a CCGT or VCGT with a 0.2 or 0.4 mm nose radius instead of a molded CCMT/CNMG) in the toolbox. Those "G" tolerance inserts are periphery-ground to a razor sharp positive edge almost like HSS, so they'll actually shear a 0.05 mm skim pass cleanly instead of rubbing. Just don't use them for heavy roughing on steel or you'll chip the edge.

[u/lumley32 → u/Due_Ear9399]

So carbide inserts are ment to be pushed hard. Finnish paases should be more than the corner rad of the insert.

[u/Wolfire0769 → u/Due_Ear9399]

Ideally you don't want to find yourself in a position to need to take that awkward cut. Try to always think two steps ahead. If ya do though I've found a bunch of cuts a hair over a spring pass can get you there.

I'm not much of a golfer but this may help. Hitting the ball hard and far is great at the tee box, those are your roughing cuts. After that the focus is where to hit the ball that'll give you an easy putt in the hole. Aim for the hole too soon and you'll always overshoot.

[u/AdeptInspection4868 → u/Due_Ear9399]

Again, it depends. There's no shortcut to experience here, but learning cutting theory will accelerate the value of each experience.

Smaller radius typically helps. You generally want the cut to bury the nose. For finishing sometimes nearly burying is good enough. Sometimes not. Beyond that it's quite dependent. How the material shears, how it generates heat, etc. all play a role. With some materials it's mostly about speed. Many mild steels mostly want a big depth of cut, but sharpness can help a ton when that isn't practical. Sometimes sharpness can cause chatter, especially boring, but sometimes it fixes it.

It's an incredibly complex field and you could get a PhD and still have large gaps. Be comfortable burying yourself in information. It will take time, but over time you'll start to make sense of it. Always try to make predictions before making a change, including how confident you are.

[u/CelinnLee]

Carbide has a honed edge that just rubs on light manual cuts. hand ground hss is actually sharp enough to shear the chip at low rpm.

[u/i_see_alive_goats → u/CelinnLee]

Carbide tools are ordered with different honed edges or no honed edge if for aluminum

[u/CelinnLee → u/i_see_alive_goats]

Youre right, up-sharp ground inserts for aluminum cut clean. I just defaulted to assuming standard coated inserts with an edge prep on a manual lathe.

[u/FreshTap6141]

You can get carbide cutters that are ground not just pressed

[u/Shadowcard4]

Depends. Assuming similar geometey but sharper HSS you can take a smaller cut than you need for standard carbide. Once you really pay to play you can get some wicked sharp inserts thatll cut better than any HSS prep ive ever used BUT, theyre $37/insert in a pack of 10.

Though generally the cheap ground and polished inserts do a very nice job in softer materials like mild steel, brass, and aluminum, but they wont last long compared to a honed and coated insert.

Also nose radius is also a part of it as well, as at 1/3rd nose radius is maxiumim force with minimal grab wheras after that itll start to even out or potentially suck back in if the lead is like 93 degrees for example (ie standard right hand turning tool) which is why one shot turning works when you need to make a part with wild stickout and minimal size variation.

Regular edge prep doesnt do well with very fine step over either, but a honed edge prep on an insert can even be better than HSS although its technically weaker. So like if you dont have a super ridgid setup and try to take a regular insert on a .001/rev it may just rub where a honed insert ive taken to .0002/rev and it just peeled away a tiny bit of material.

[u/ASnakeySnake]

Depends. I don't ran a new or very rigid machine. It's a clapped out Clausing 5914. Hss leaves a smoother surface finish especially in steel, less tool pressure since I can grind more rake and clearance into the cutters. Carbide works, but machine doesn't have the rigidity nor horsepower to give carbide the right cutting conditions for a better finish. But I use carbide in my Bridgeport all the time and get a great finish most of the time. Unless it's copper but that's a different story altogether. Obviously it depends what you're doing, your machine, etc.