Ultem Machining, brittel material?

[UchidaDS]

Please see attached picture.
Not sure why it crack. It seems that the crack start at internal slotting area.
It does not crack when we receive the part, but after a while, it break into two pieces.
1. This might due to the stress concentration at one point?
2. I assume that: When the bottom surface of the machine part is not flat to the ground (let say flatness is about .012", bow in one direction) , when install it to the base, due to the strain it start cracking. I think Ultem is kind of brittle after looking at the cross sectional view.

What do you guys think?

 

[MikeHalloran]

The cross slots have almost cut the part in half before you do anything else to it.
You've got stress raisers everywhere.
A little bow is all it takes to complete the fracture.

Taken purely on its own, lacking context with which to defend or justify it, it's an awful design.

Sorry. You asked.



Mike Halloran
Pembroke Pines, FL, USA

 

[UchidaDS]

Which area are you indicating of the stress raiser? Lacking context? Please point out.
Based on my observation, the Right side has more meat but somehow the crack seems to be started at that area.

 

[MikeHalloran]

Stress raisers:
Counterbored slots, everywhere, intersecting, from both sides, with small/no radius at the bottom 'corner' of the c-bore.

Do you anneal the material before or after machining?
Use a machining coolant that's known to not affect the material?



Mike Halloran
Pembroke Pines, FL, USA

 

[KENAT]

I believe you can create Ultem parts using Fused Deposition Modelling.

If the material is critical rather than the manufacturing process, you might consider that. It may also free you up some on the geometry to eliminate some problematic features.

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[UchidaDS]

No, no anneal for that material.
How could you have a radius at the corner of the c-bore? I don't see this in many application.

 

[UchidaDS]

I do know how to add radius, but just don't see this is practice.

 

[patprimmer]

The pictures are not good, but from what I can see, Mike is spot on.

The crack seems to start from a sharp corner and the surface finish shows some indications of being pushed by the cutters at times.

You have created stress risers by both extreme changes of section thickness and by creating very sharp edges in high load areas.

You should make sure you use very sharp tools with moderate rake and lots of back clearance. Sabic should have a technical information sheet on this.

You should also use your cutters have a rasius at the edge of the cutting surface.

You must anneal the parts.

Reducing section thickness in some areas might help with load sharing, but I will presume you are locked into the part geometry for reasons required for fit or function.

Regards
Pat
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[UchidaDS]

patprimmer: You are exactly right that I am locked to the part geometry due to the function and contraint of the space.
Patprimmer and Mike: What is the minumum radius?
See attached.

 

[patprimmer]

The minimum is about 3mm. OK not possible, therefore the biggest you can accommodate without destroying fit or function. A chamfer on a mating part may allow an increase in radius. Any radius at all will help a lot.

Regards
Pat
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[MikeHalloran]

The minimum radius is: "all you can stand".
I.e., you don't want a screw head bearing on the radius, and you want the c-bore diameter large enough to leave a visually noticeable radius at the bottom of the pocket. I.e. the outer diameter of the flat part at the bottom of the c-bore would be derived from the max screw head diameter plus an allowance representing the screw position tolerance, plus a little fudge.


Mike Halloran
Pembroke Pines, FL, USA