Strength of Aluminium base plate

[gm2612]

I have a fixture designed in which the base plate is Aluminium. Two clamps (diagonally oppiste) will be used to hold the job. Clamping is done using M6*0.75 Allen bolts. Aluminium base plate is of 20 mm thick, in which 10 mm tapped hole is provided for clamping. Clamping of allen bolts are done with hand tightening, which equals to 60 LBS torque. After two or three times of using the fixture (unclamping, fixing new job, and clamping) I find that the tapped holes in aluminium base plate got loosened. Is there any basic design problem in this fixture? Can anyone help to resolve this issue?

Thanks and regards,

GM

[RWOLFEJR]

Might want to consider steel threaded inserts or possibly use studs and nuts?

[gm2612]

Thank you RWOLFEJR. Your reply is very usefull for me. I would also like to ask this question, is there a possibility of the thread in the tapped hole of Aluminium base plate getting weakened due to low shear strength of the Aluminium, as we apply 60 Inch. LBS torque for tightening?

Thanks and regards,
GM

[jboggs]

It is never a good idea to expect a tapped hole in aluminum to withstand repeated usage at any torque. If a threaded hole in aluminum is going to be used repeatedly then always use some kind of thread insert such as Helicoils, Keenserts, or other devices.

[gm2612]

Thank you very much jboggs, and RWOLFJR.