Structure Frame with Machined Components - how to guarantee tolerances?

[Mateus_R]

Hi everyone,

I have a question about how to construct a structure frame that has attached several machined components that has some tolerances to respect (For example: attached in the structure there is 4 supports to connect other component using pins, and the hole where the pins are going to be fixed have to be in tolerance with the other supports).

I think we can do it in two ways:
1) Machine all the components, weld them in the respective piece of structure and then weld all the frame together. To weld all the components i would use a jig to respect tolerances and dimensions. In this case I'm afraid about the thermal stresses induced by the welding process.

2) Use raw components and weld everything. After welding, machine the components (For example: the diameter of the tubes where the pins are going to be placed), in order to guarantee tolerances and avoid thermal stress due to the welding process. The problem here is to machine components that are now part of a big structure and to do it, its requested some special tools (like a drill with an arm, or something similar).

The 2) option is reasonable?

Thank you for your comments.

Kind Regards.

 

[dhengr]

Mateus_R:
Your second approach is usually better, but so are reasonable tolerances and some adjustability on both sides of the connection. With some good fabricating experience and engineering judgement, you start to learn which method works best, or what you can get away with, under various conditions. You still likely have to design-in some adjustability on your frames, near these connections. Pay particular attention to the fact that your structure will deflect and those dimensions will change as soon as you start loading it, so this is kind of a nebulous process. And, you have to know where you can take advantage of various things. The fact that your structure is somewhat flexible might allow you to pull/push it around a bit to make things fit, without over stressing anything. Many times you do rough machining to get most of this done, hole sizes, flatness, etc., then do the welding of the frame, and finally do the finished machining which involves lighter passes and equipment. There are magnetic based drills and mills which attach directly to the frame to do some of this kind of light work. Sometimes you heat treat (stress relieve) fabrications after welding and rough machining to get rid of the welding residual stresses, and then do your final machining.

 

[CANPRO]

What kind of accuracy do you require and how big is the structure? If your schedule permits, I would erect the steel frame as accurately as possible, then survey the finished structure and then machine bearing plates to go between the structure and your equipment. This method would depend on the how your equipment needs to be supported, but it would allow you to erect the structure with reasonable tolerances and then machine just a few manageable pieces to bring the final structure within the tighter tolerances.

 

[Mateus_R]

Thank you for your answers dhengr and CANPRO. It clarifies some of my questions about the overall process.

 

[dhengr]

Mateus_R:
Sometimes, you can shop build a sub-frame which will carry your equip. and has the tighter tolerances applied under better conditions. This frame just drops into the structural framing then, like any other framing members, with min. impact on the needed tolerances. Often, you design bearing points with sufficient stiffness so they can be shimmed and bolted down vertically like any other item, and with normal structural tolerances. Then, for lateral location and fixity you just weld some shear blocks btwn. the equip. feet and the actual structure. Talk to a few millwrights about what they want, and need, and do. They are usually the guys btwn. your design and the final installation.

 

[Mateus_R]

Thank you for your help.