Machining does not achieve a surface roughness of Ra 0.1

[august jsc]

I'm looking for a way to machine a spherical product made of SUS304, with a required hardness of 60 HRC after heat treatment. I've selected a specialized turning tool designed for machining hard metals, but I still can't achieve a profile tolerance of 0.05 and a surface roughness of 0.1. Can anyone identify the issue? Thank you very much!

 

[Lnewqban]

Welcome, @august jsc !

Could you describe that “specialized turning tool“?

 

[Baluncore]

Turning in an NC lathe?
What insert style, material and grade?
What diameter sphere?
What RPM and feedrate?

 

[august jsc]

Welcome, @august jsc !

Could you describe that “specialized turning tool“?

Thank you for your attention; this is the chip and program I used for machining.

 

[august jsc]

Turning in an NC lathe?
What insert style, material and grade?
What diameter sphere?
What RPM and feedrate?

I used CNC lathe
insert is CBN
Sphere diameter is 38.1 [+-0.03mm], inside 30[-0.008 +0.03]
RPM from 150 and feedrate 0.15

 

[Lnewqban]

Thank you for your attention; this is the chip and program I used for machining.

Have you tried different velocities?

 

[august jsc]

Have you tried different velocities?

I tried increasing or decreasing the spindle speed and feed rate, but the result was that my chips became chipped. This product has undergone nitriding.

 

[Baluncore]

This product has undergone nitriding.

Nitriding does explain your problem with surface finish.

Nitriding is usually done after the machine cutting, because it operates at low temperatures, so does not distort the product as significantly as other heat treatment processes, such as case-hardening.

To machine a nitrided product, you should be fine-grinding to size, then polishing to the required surface finish. I would use a grinder, mounted on the tool-post of a lathe.

You might consider burnishing with a small hard sphere, as a tool in the lathe. That will smooth and harden the surface. Burnishing is productive with materials that work harden, such as stainless steels. Burnishing also removes tension in the surface, which prevents the initiation of surface cracks.

 

[august jsc]

Nitriding does explain your problem with surface finish.

Nitriding is usually done after the machine cutting, because it operates at low temperatures, so does not distort the product as significantly as other heat treatment processes, such as case-hardening.

To machine a nitrided product, you should be fine-grinding to size, then polishing to the required surface finish. I would use a grinder, mounted on the tool-post of a lathe.

You might consider burnishing with a small hard sphere, as a tool in the lathe. That will smooth and harden the surface. Burnishing is productive with materials that work harden, such as stainless steels. Burnishing also removes tension in the surface, which prevents the initiation of surface cracks.

Thank you very much for the information you shared. However, even right after heat treatment, achieving a surface finish with a roughness of Ra 0.1 and a geometric profile tolerance of 0.05 was already very challenging. After nitriding, it has caused uneven dimensions on the product. Regarding the use of a grinder mounted on a lathe, could you provide me with some images or examples of grinding materials?

 

[Baluncore]

After nitriding, it has caused uneven dimensions on the product.

That is where a grinder should be used, to remove about 25% of the nitriding depth.

A belt or motor driven abrasive stone, mounted in a CNC lathe, replaces the single point cutter with a grinder, tangential to the surface. That improves the surface finish by virtue of the contact geometry.

Most CNC grinders are used for grinding cylindrical surfaces and machine tapers. Some are able to process internal and external profiles. There are CNC lens milling, grinding, and polishing machines available.

Special purpose machine setups are often needed for spherical grinding. How you do the job depends on the detail of the part and the tools you have available.
Google: CNC spherical grinding machine

 

[august jsc]

Thank you very much! Thanks to your advice, it seems I can address the issue after nitriding. However, I still have a problem achieving a profile tolerance of 0.05 after heat treatment. Could you review my program and the type of tool I am using and give me your opinion? I suspect that my program and tool might not be optimized, which could be why I am unable to control the roundness.

 

[Baluncore]

I don't think my reading your G code will help, as I do not have a simulator, or know your CNC machine capabilities. All my precision machines are retired from toolrooms, and are manually controlled.

I think you should consider the possible reasons for unpredictable dimension and finish. A hard tool, cutting a hard material, may change the depth of cut as the material surface hardness changes with depth. When tool deflection is similar to the depth of surface hardening, you can expect aggravated variation in the depth of cut.

You might lower the tool cutting edge slightly, to reduce push-back from the work. If the cutting edge is too high, it will rise to, and then burnish the surface, damaging the relief face of the cutter. That plunging path can be initiated cyclically, by interrupted cutting due to eccentric distortion.

Maybe you can release most of the distortion prior to nitriding.
1. Anneal the stock before machining.
2. Machine the first approximation, sufficiently oversized to handle any distortion.
3. Heat soak to the nitriding temperature, allowing the distortion to relax after removing the bulk of the material.
4. Finish machine to approach the final dimensions.
5. Nitriding.
6. Grind to final dimension and finish.

 

[august jsc]

I don't think my reading your G code will help, as I do not have a simulator, or know your CNC machine capabilities. All my precision machines are retired from toolrooms, and are manually controlled.

I think you should consider the possible reasons for unpredictable dimension and finish. A hard tool, cutting a hard material, may change the depth of cut as the material surface hardness changes with depth. When tool deflection is similar to the depth of surface hardening, you can expect aggravated variation in the depth of cut.

You might lower the tool cutting edge slightly, to reduce push-back from the work. If the cutting edge is too high, it will rise to, and then burnish the surface, damaging the relief face of the cutter. That plunging path can be initiated cyclically, by interrupted cutting due to eccentric distortion.

Maybe you can release most of the distortion prior to nitriding.
1. Anneal the stock before machining.
2. Machine the first approximation, sufficiently oversized to handle any distortion.
3. Heat soak to the nitriding temperature, allowing the distortion to relax after removing the bulk of the material.
4. Finish machine to approach the final dimensions.
5. Nitriding.
6. Grind to final dimension and finish.

Thank you very much for helping me.