General rules of thumb for surface finish with carbide inserts?

[Sincerd]

I've been struggling to get a good surface on the lathe and mill. Since the lathes a little bit easier to control, what are good rules of thumb for insert tooling on the lathe?

Will a sharper angle insert produce a better finish than a large radius?

Will faster speeds produce a better finish than slower?

What angles of the insert matter the most for surface finish?

For me, it seems I get the best surface finished taking somewhat deep cuts of .050 or more under a pretty high feed rate. I think it's a situation where I don't know what I don't know.

Mostly I'm running mild steel a36, 1018. I Have been able to get decent service finish in stainless and some kind of mystery tool steel.

 

[BajaFire]

General rule of thumb - faster spindle speed (SFM/RPM) for finish passes. Theoretically a larger nose radius gives better finish (more overlap between cusps on a lathe, for example). However, that large radius causes increased tool pressure, so in the name of rigidity, a smaller radius with a finer feedrate is often used.

One of the reasons you get good surface finish in deep cuts is increased heat. Hotter the chips coming off (and harder the part), shinier the possible surface finish. In lieu of depth, finish cuts trade increased speed to get enough heat. Combined with the lighter depth of cut, you have less tool flexing => good finish AND good precision.

It is difficult to get shiny/mirror finishes on A36 and 1018.

 

[Sincerd]

General rule of thumb - faster spindle speed (SFM/RPM) for finish passes. Theoretically a larger nose radius gives better finish (more overlap between cusps on a lathe, for example). However, that large radius causes increased tool pressure, so in the name of rigidity, a smaller radius with a finer feedrate is often used.

One of the reasons you get good surface finish in deep cuts is increased heat. Hotter the chips coming off (and harder the part), shinier the possible surface finish. In lieu of depth, finish cuts trade increased speed to get enough heat. Combined with the lighter depth of cut, you have less tool flexing => good finish AND good precision.

It is difficult to get shiny/mirror finishes on A36 and 1018.

Any idea why sometimes I get almost like a smearing on the surface? Looks like it's rubbing, but it happens even if I'm taking pretty deep passes sometimes. Guess the feed is too fine? I thought it was that I was running too fast.

 

[Booze Daily]

If it’s on deep passes it could just be the chips scuffing the surface. .05/.06 finish pass on low carbon steel with the right rpm should be able to get you a 63 finish no problem.
A prickly or mottled surface, usually on alloys, is not enough rpm.
There’s only about 10 rules for machining and there’s about 10000 exceptions to the rule.

 

[henrya]

Good answers so far. Look at the insert manufacturer’s web site for your insert and follow their suggestions for DOC, SFM, and feed. You might need a different insert for better results. Also keep in mind the material you are having trouble with is not known for best surface finishes. If it suits your parts, try some 12L14 for nice cutting and finish.

 

[john.k]

on a mill ,a single wiping insert in a cutter ......on a lathe ,a similar principle....the brilliant DVD finish is actually a rough cut smeared over into a bright grooved finish....not satisfactory for any kind of mechanical bearing surface ,but looks real good.

 

[eKretz]

Low carbon steel can be difficult. Best to take either a fairly heavy cut and get the speed up or to use a very sharp insert with oil or coolant. I used to finish 1018 at 1200+ SFM all the time.