迹字石油化工大學⊙ FErrors between parts To design a machine, one must not only be sure that parts will not break, one must be sure parts will fit together with the desired accuracy EXample: You cannot create 4 matching holes in two components So you oversize the holes But then the clearance between the bolts and the holes means that the components do not have a unique assembly position This is the fundamental challenge in designing machines For a limited number of parts and dimensions, basic accounting methods can be used to keep track of interferences and misalignments These methods often assume "worst case tolerance For complex assemblies, advanced statistical methods are required
F.Errors Between Parts • To design a machine, one must not only be sure that parts will not break, one must be sure parts will fit together with the desired accuracy – Example: You cannot create 4 matching holes in two components • So you oversize the holes • But then the clearance between the bolts and the holes means that the components do not have a unique assembly position! • This is the fundamental challenge in designing machines • For a limited number of parts and dimensions, basic accounting methods can be used to keep track of interferences and misalignments – These methods often assume “worst case tolerance” – For complex assemblies, advanced statistical methods are required
迹字石油化工大學⊙ G. Errors can be cumulative EXample: You create a lazy tongs mechanism, and it works great! Fully extended, its reach matches that predicted by the spreadsheet (thats BIG John next to the tongs)
G.Errors Can Be Cumulative • Example: You create a lazy tongs mechanism, and it works great! – Fully extended, its reach matches that predicted by the spreadsheet (that’s BIG John next to the tongs):
迹字石油化工大學⊙ BUT When retracting notice that some links are tight, while the end links are still spaced, and Cannot be closed by the actuator. This is due to the slop (backlash)in the joints, so the tongs do not fully retract, so you may not be able to pull that asteroid in far enough We need to learn MoRe about accuracy and repeatability,so we can think ahead about how our machines will design BEFORE We build them There is a LoT more to engineering than just stress analysis
– BUT when retracting, notice that some links are tight, while the end links are still spaced, and CANNOT be closed by the actuator. This is due to the slop (backlash) in the joints, so the tongs do not fully retract, so you may not be able to pull that asteroid in far enough…. •We need to learn MORE about accuracy and repeatability, so we can think ahead about how our machines will design BEFORE we build them – There is a LOT more to engineering than just stress analysis!
迹字石油化工大學⊙ H Error Tolerance Budgets Errors in parts and the assembly are controlled through the use of Error Budgets and Tolerance Budgets Error budgets attempt to predict how a machine will perform when it is assembled and running Each module is represented as a rigid body and has a coordinate system assigned to it Error budgets account for errors, geometric, thermaL., in each modules degrees of freedom(3 position and 3 orientation errors) Tolerance budgets attempt to predict what the final assembled shape of the machine will be given geometric errors in the parts Will the parts even have enough tolerance to make sure they will all even fit
H.Error & Tolerance Budgets • Errors in parts and the assembly are controlled through the use of Error Budgets and Tolerance Budgets – Error budgets attempt to predict how a machine will perform when it is assembled and running • Each module is represented as a rigid body and has a coordinate system assigned to it. • Error budgets account for errors, geometric, thermal…, in each module’s degrees of freedom (3 position and 3 orientation errors) – Tolerance budgets attempt to predict what the final assembled shape of the machine will be given geometric errors in the parts • Will the parts even have enough tolerance to make sure they will all even fit