Hello. We have a part that has sur 1

[Nikhil1508]

Hello.
We have a part that has surface finish of 0.4 Ra value on face and O.D. It has max diameter of 8.5mm and length 20mm and has a few steps in between. Material is SS303. Can we achieve this Ra value on a CNC lathe machine. If so, what kind of parameters (feed, depth of cut and spindle speed) should we machine at to achieve this finish? Also what kind of insert will be suitable.
Thank you in advance.

 

[mfgenggear]

Nikhil
No it can not be done.
Might get good finish with a screw machine. Generally this is a ground and lapped and polished surface. A lathe can turn a 63 micro. A polish maybe a 32 micro.

 

[rothers]

Good quality turning can achieve 0.4 Ra. Polishing is only required for 0.2 Ra and less.

 

[EdStainless]

A good finish on 303 is a fools errand.
There will be tearing from the original cutting process.
This is free machining grade and it will be loaded with inclusions, that is why the chips break.
9 time out of 10 when someone says they have a good smooth surface on SS they did it by smearing the surface.
It may beet an Ra but you have further degraded the already poor corrosion resistance.
This material work hardens a lot.
So when you turn you want to take all of the material in one pass.
Lots of HP and slow infeed.
If you make a second pass it will be on material that is much harder than original.
And the tooling for that second pass will be very different from the first.

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P.E. Metallurgy, consulting work welcomed

 

[mfgenggear]

Edstainless Is correct. It is easier to obtain a better finish with machinable heat treated stock. That can be heat treated. Such as 416 ss, or precipitation harden stainless Like 17-4, stainless feeds and speeds for roughing requires a deep of cut like .o15 inch , if you try to take a cut of .ooo5 inch
It will not cut properly. It will work harden and rub. Also a lot will depend on the tolerance of the diameter. Precision tolerance of less .0005 should be semi finished turned heat treated then
Finish ground to hold Precision tolerance and surface finish.
If only a small one time run, and a wide open tolerance, try to hold a 32 micro, polish with fine grit emery . But will take a while. Thus it is not economical for large lots.
There is electri polish to obtain a fine finish and polish . Check with a supplier to obtain the details.
Not given the geometry makes all suggestions a best guess, but an ther option is center less ground which is often used on Precision pins.

 

[mfgenggear]

As standard practice roughing slow rpm, heavy depth of cut, and medium infeed,
For finishing light depth og cut , high rpm, slow infeed rate such as .0005-.001.
Contact a carbide tool insert sales engineer to
Obtain starting feeds and speeds. With their inserts. Or if a small lot try micro 100 carbide cutter, and modify them with diamond grinding wheel to meet your geometry clear requirements.

 

[drawoh]

Nikhil1508,

Is that 0.4[μ]inches or 0.4[μ]m?

According to the Machinery's Handbook 0.4[μ]m is completely do-able in a lathe.

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JHG