CNC Turning Surface Finish

CNC Turning

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 micro millimeters.

The Ra stands for roughness average.

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

You can buy a flat block like the one above which you can use as a comparison to see what 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.

The Dreaded Formula

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

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

So………….

Our tip radius is .8mm

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

So here’s the formula.

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

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

Anyway let’s see how you do it backwards.

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

Now do the bottom bit 8 x .8 = 6.4

Now divide the top by the bottom.

.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

It worked backwards!!! Phew what a relief.

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.

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

Interesting

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

Thanks for watching and reading

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