texture on stainless casting parts after lathing 2

[MechatroPro]

I've seen this special texture on the surface of stainless steel parts a lot, however I'm not sure what these marks are called or why they appear on the surface after the part is machined. the texture is easily felt under hand. so I'm wondering what they are and why they show up on the surface. please check the picture, the part is made of 316SS (CF8M).

 

[moltenmetal]

It looks to me like the machinist has to do a better job of picking speed and tool feed rate combinations or use a more rigid machine. The materials experts may weigh in if they feel that this is actually a material defect related to casting, but I've seen plenty of cast CF8M parts machined with excellent surface finishes before, and I've seen 316SS barstock machined with that ugly surface finish before too and poor choice of speeds/feeds was the cause in that case.

 

[arunmrao]

The casting appears to be defect free,the poor finish has been contributed by the machinist for not having used the right tool and feed. Cast stainless steel castings when machined look bright and shining, it is aways a pleasure seeing one in progress.

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

Yes the effect has to do with the coarse casting structure and the tough austenitic matrix, but I would agree with MM that better machining practice would go a long way to mitigating it.



"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Ron]

Tool chatter.

 

[tbuelna]

I had a somewhat similar issue with parts CNC turned from 3/8" dia annealed CF vacuum-melt 440C bar (AMS 5618). The speed/feed rates the vendor told me he used were within the recommended values I could find. He also said he used carbide inserts with edge geometry recommended by the vendor for this material. Machined surface is 5/16" dia x .35" long and turned on a Mori-Sekei slant bed with bar feed. Never determined the cause of this problem, but it was a rough machined surface that later got removed, so I didn't look into it further. Just speculating, but I think the problem was likely due to worn cutting inserts.

 

[EdStainless]

In alloys such as 304 and 316 (including cast versions) people under estimate the effects of surface work hardening.
The first pass may look good but the material that is being cut on the second pass is 50% stronger so the tooling and speeds are all wrong for it.
Most people do not take deep enough cuts when machining stainless.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[MagBen]

Looks to me so-called "orange peel" defects.
As many indicated, machining was likely the direct cause. However, the root cause goes to the material itself (but not necessarily to blame the material).
In the first case (316 SS), the large grains is the root cause.
In the second case (440C), the remaining austenitic soft phase in the hard martensite hard matrix is likely the root cause. Carbide inserts put so much press in the material.

 

[btrueblood]

Using tooling that is not stiff enough will exacerbate the problem as well. If your parts are "singing" (making audible noise) during the cut, you will see that pattern on the parts when you stop the lathe.

 

[berkshire]

We used to call this " Lathe Chatter " , it is not the mark of a good machinist .
In my opinion you need to give this job to somebody who knows what they are doing.
B.E.

You are judged not by what you know, but by what you can do.

 

[Maui]

OP, did the machine operator use carbide lathe tooling or high speed steel? Were these turned dry or was a lubricant used?