Hardness & Surface Roughness

If by constant parameters you mean feed and speed of the cutter, you may be overworking your tool and machine. For harder materials, you need to reduce feed and speed.

Overworking, or overloading a mill will cause the machine to labor excessively in the cut, which will show as a rough finish.

sorrry 2 interupt sir

but he is asking about specimen hadrness

not about

machine n tool...

do u think der is ny discussion needed 4 relating overloading...friction...all dat??

i think they r variables which can affect parameters definately

but plzz sir can u explain y u said so..?

coz i dont think dat friction...load... etc can higher d roughness

actually its depends on hardness...as he had explained na

dan what

Hello HaiIm Max.

I would disagree with your conclusion that harder metal gives higher surface roughness value.

But since we dont know what metals are involved (ductile, brittle, gives BUE, doesn't give BUE, annealed not annealled different microstructures, etc) it is difficult to explain what is really going on in your test. Especially since milling is not a single point process.

In single point turning, the formula for theoretical surface roughness Ra(theoretical) is f^2 /32 rn where f= feed rate and rn is tool corner radius.

Note that this expresssion does not call out hardness, cutting speed, feed rate, rake angle, depth of cut, or a host of other factors.

FEED RATE, (f squared) Not HARDNESS is the greatest single determinant of surface finish on a machined workpiece.

I don't know of a formula to predict surface finish using any other parameters, and none using hardness.

Your failure to identify the materials involved is a serious complication to our sense making.

In my experience with various plain carbon and alloy steels, generally speaking to improve as machined (ie not burnished or polished) surface finish one can:

1) switch grades to a steel with Sulfur, Lead, Bismuth, Phos, Nitrogen or other free machining additives;

2)Reduce feed and increase speed.

3) Increase Rake Angle on tool;

4) Up to a point, increase hardness;

5)Alter microstructure (and hardness) by annealling;

(IS it the hardness, or the microstructure, that is what gave you the results you cite?)

6)change tool coating/ metalworking fluids;

Please note that these are for normal operations in process surface finish; abnormal surface textures can also be caused by different tool failure modes and inconsistent BUE (Built Up Edge);

I respect the fact that you are reporting the results that you obtained from your work; but i disagree that your claim of harder metal gives higher surface roughness value is valid. Surface roughness is primarily a function of (and negatively correlated with) feed rate and tool radius.

Respectfully, Milo