Surface Finish CNC Turning

Surface Finish CNC Turning

Category : Beginners Useful Stuff

CNC Turning Surface Finish

Ever wondered what feed rate is best to give you a particular CNC Turning surface finish? Or do you just guess?

Well look no further. From now on you’ll be able to work out what feed-rate will give you the surface finish you require when CNC Turning.

Oh and there’s a formula where you can work out what surface finish you’re going to achieve without a fortune teller.



We are going to use um Ra.

The um stands for microns.

The Ra stands for roughness average.

Here is a full explanation if you really want to know. 

CNC Turning


You can buy a flat block like the one above which you can use as a comparison to see what surface finish you’re getting.

The section above is for CNC Turning surface finishes.

So you can already see that 12.5 is like a plowed field and .4 is like a babies arse (as my father used to say) .

So if you’re fixing a fence post then 12.5 might be OK.

Surface Finish The Dreaded Formula

surface finish

First of all what feed rate would you need to obtain a certain CNC Turning surface finish.

CNC Turning

Now what if you’re already machining the part. That means you know the feed rate. What surface finish will you get?

CNC Turning


Our tip radius is .8mm

CNC Turning

We want to achieve a 3.2 um Ra  CNC Turning surface finish

CNC Turning

So here’s the formula.

CNC Turning

Let’s do this bit first.

8 x .8 x 3.2 = 20.48

Now let’s divide it by 1000

20.48 / 1000 = .02048

Now lets square root the answer.

Square root of .02048 = .143

So the feed-rate you need is F.143

CNC Turning

OK… You Came Back. You Ready to Carry On?

Anyway let’s see how you do it backwards.

surface finish

So imagine you had been turning these imaginary bits with a .8mm radius insert at a feed-rate of .143 (F.143)

First of all square the feed rate .143 x .143 = .020449

CNC Turning

Now do the bottom bit 8 x .8 = 6.4

surface finish

Now divide the top by the bottom.

surface finish

.020449 / 6.4 = 0.00319515625

Then simply multiply this by 1000

0.00319515625 x 1000 =3.19515625 (3.2)

That’s 3.2 to you

surface finish
A surface finish comparator

It worked backwards!!! Phew what a relief.

Surface Finish, Now Let’s Think About This

So to get a 3.2 finish with a .8 tip radius you would need a feed rate of .14

Now let’s do the same calculation with a .4 radius tip.

surface finish
Calculate surface finish

So did you work it out?

Here’s one I prepared earlier.

The answer is .101

And if you can be bothered to work it out with a 1.2 radius tip.

The answer is .175

.4    =  .101
.8    =  .143
1.2  =  .175


So as the tip radius gets bigger you get to increase the feed rate!

Thanks for watching and reading

If you have been affected by any of the issues in this post or need CNC Counselling then contact me.

Oh yea we do CNC Training too not to mention Classroom Training.

Or call us

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Look no further Contact CNC Training Centre



Jeevan manikanta

April 21, 2022 at 7:52 am

what is the RPM we can use for 0.8 radius corner with feed of 0.15 to achieve the 3.2 Ra

Ankit Sharma

May 24, 2022 at 4:30 am

um is not micromilimetres it is micro metres or simply microns. Please correct


    May 30, 2022 at 9:58 pm

    Thank you

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