Aluminium 2024 T351 bowing when machining 'open pockets'

[Danjones777]

Hi all,

i am a relatively new CNC milling programmer, 22 years of age and i have kind of been thrown in at the deep end with minimal practical experience, i have studied the theory and worked on our CAD/CAM system extensively, but practical experience is something i lack. my boss has given me the task of machining some trial parts, the first of which i did yesterday. the part is clamped to a fixture plate. my part was dimensionally perfect until i loosened my clamps and found my material had 'bowed' at either end by roughly 8mm. the part is 2200mm x 240mm x 35mm, the finish thickness is 2200mm x 240mm x 32mm. i skimmed the top with a 160mm facemill and skimmed an equal amount from the bottom, roughly 1.5mm off each side to get my material flat and down to the correct thickness. there are then two 'open pockets'/ slots that are 2200mm x 60mm x 21mm deep that leave the part in almost a 'T' shape. the way i went about milling these slots was to use a 25mm 6 flute endmill with 5mm depth of cut and 15mm width of cut. leaving 0.25mm on all faces to finish with a 20mm endmill.


like i said, the part was dimensionally perfect.. what is the reason that my part has warped when i have released the clamps? is there any way that i can avoid this happening without sending the material off to be de-stressed/annealed?


i lack experience in all honesty and this part has a flatness tolerance of 0.1mm that i dont know how to achieve. if anyone on here with more experience than myself could offer me a direction to go that would be fantastic.

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[Jim Dawson]

First I would talk to the boss and see if he has any input on the problem.

The reason it warped is because the machining operations relieved the stresses built up from the manufacturing processes. I have had some success in clamping only from the side or center and allowing the material to ''float'', but on a piece that size it would be difficult to do at best. Setting it up in 2 or 3 vices rather than clamping it down to the table might be an option.

You proposed the best answer: Sent it out to be stress relieved.

[mactec54]

Hi all,

i am a relatively new CNC milling programmer, 22 years of age and i have kind of been thrown in at the deep end with minimal practical experience, i have studied the theory and worked on our CAD/CAM system extensively, but practical experience is something i lack. my boss has given me the task of machining some trial parts, the first of which i did yesterday. the part is clamped to a fixture plate. my part was dimensionally perfect until i loosened my clamps and found my material had 'bowed' at either end by roughly 8mm. the part is 2200mm x 240mm x 35mm, the finish thickness is 2200mm x 240mm x 32mm. i skimmed the top with a 160mm facemill and skimmed an equal amount from the bottom, roughly 1.5mm off each side to get my material flat and down to the correct thickness. there are then two 'open pockets'/ slots that are 2200mm x 60mm x 21mm deep that leave the part in almost a 'T' shape. the way i went about milling these slots was to use a 25mm 6 flute endmill with 5mm depth of cut and 15mm width of cut. leaving 0.25mm on all faces to finish with a 20mm endmill.


like i said, the part was dimensionally perfect.. what is the reason that my part has warped when i have released the clamps? is there any way that i can avoid this happening without sending the material off to be de-stressed/annealed?


i lack experience in all honesty and this part has a flatness tolerance of 0.1mm that i dont know how to achieve. if anyone on here with more experience than myself could offer me a direction to go that would be fantastic.

Try a cast material instead of extruded if you can, this has less stress in it an won't move as much

You can stress relieve them in you shop you only need a oven that can reach 350 max

How to machine

Don't clamp down the plates if they have a bend in them already, you have to shim as best you can, clamping them down to straighten them will only make it worse

Rough cut the pockets close to size, stress relieve, you may need to stress relieve more than once before you finish machine, this is not easy on large extruded plates