Large Flange Machining - Raised Face Surface Finish Problem ASME B16.5

[Veero]

We are a welding company and our sub contract machinist is having problems achieving a consistent finish with a spiral raised face we have clad. The flange is approximately 2300mm in diameter. OD limit of raised face machining is 1850mm, ID limit 1650. Due to its size rotation is extremely slow.

According to ASME B16.5 the finish must be between 3.2 and 6.3µm.

The achieved finish seems to vary in bands, going from one finish to the next as you move in from the OD of the RF. It goes from an acceptable finish in a band approximately 25mm wide, then the individual groove width seems to widen (as if the tool is cutting deeper), then reverts back to the acceptable finish.

The result is a striped finish, with alternating bands of fine, then coarse, fine then coarse.

This description is relative. Our inspector agrees that the finish is all within the acceptable band but since it varies in these concentric 'stripes' we anticipate our customer will not accept it.

We are scratching our heads, the machinist is adamant that the tool tip traverse is correct and there is nothing untoward with the machine and has repeated the operation using a DTI gauge to check the motion of the cutting tip.

Can anyone shed any light on why this might be happening? I am wondering if some sort of resonant frequency is being induced but am unable to find any helpful references on the subject.

Any ideas or information greatly appreciated.
Veero

 

[looslib]

Is the cutter hitting hard spots in the flange?
Is the depth of cut varying?
Is the workpiece rigidly mounted to the table?



"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[Veero]

I do not think there are any hard spots in the flange surface. The machined finish is far too consistent to be hitting any defects.

Please see the attached photo. Hopefully you can just about see what I mean about 'stripes'.

It looks like the wider spaced grooves are from the tool tip cutting deeper, but the machinist used a DTI gauge to check there was no deflection on the tool arm during its traverse across the face during machining.

 

[BrianE22]

My actual machining experience is limited but it looks like tool vibration to me. Some of the factors would be depth of cut too small, feed rate too low, tool tip radius too large, RPM or high or to low.

There are some experienced machinists that hang out at

http://www.practicalmachinist.com/vb/general/

that might be able to offer further insight.

 

[unclesyd]

I would be carefuL in reworking based on what I see compare to some it is a good gasket landing surface.
I would look to something mechanical as the pattern is very consistent from edge to edge.
At first look it is like a tool change problem, but probably not as the Machinist would notice this.
Are you using insert tool bits?
Did anyone check the surface prior to machining the grooves.

What are the materials, flange and overlay metal.
What process was used to used for the overlay?
Was it deposited as stringers or a weave?

 

[racookpe1978]

I've got to admit I don't know exactly what is causing the pattern, but the following can be looked at:

It is a regular, repeating, sinusoidal pattern with a peak-to-peak "distance" of 12-14 revolutions on a relatively slow-moving workpiece being turned. Free or random "vibration" (of the part, of the tool, or of the toolholder, or of the machine base or mounting/clamping mechanism) would be less regular in my opinion. Vibration would also be "faster" or closer together. (Vibration would tend to mar (affect) one groove over a short 1 or 2 inch length of that one groove.

It could be a gradual buildup of waste metal under the tool that regularly gets broken off. Key word would be "regularly" getting broken off or removed, but this looks like it the change back to normal is smooth - not quick.

I tend to think of a bend or curve or rough spot on the tool feed screw mechanism or keyway: something is letting the downward pressure on the tool relax and then return to normal every 12-14 revolutions. Or regularly deviate back and forth (high pressure then low pressure) about the "normal tool pressure" on the scribing tool that you want.

Rotation speed of the part seems constant.

If CNC, is your hydraulic pressure constant?

 

[BillPSU]

Typically you should address vibrations such as you are experiencing with first a good tooling setup.
Use shortest tools possible.
Rigid clamping of the work piece and cutting tools.

Reduce tool cutting pressure
Change rake angle of tool from negative to positive or use negative/positive inserts.
Change lead angle of tool from zero or negative angle to 15 degree positive angle.
Utilize a relatively small tool radius,(typically .032R)

Check machine condition.
Check slide adjustment. (Adjust gibbs if equipt)

Add vibration dampening material to part and/or tools.

Change spindle speeds within reason. Change feed rate.(Increase feedrate)

Choose which ever changes you desire based on what you are willing to try.

Bill