PDA

View Full Version : Question regarding welded aluminum fixtures



Chuck Wintle
02-04-2014, 8:24 AM
We use custom welded aluminum fixtures for the manufacture of certain items and the flatness of the bottom is critical. Across a 6 foot span it flatness is spec'd to .004 inch or less. Can such a fixture warp if placed outside and subjected to heat and cold fo the summer and winter?

Jim Creech
02-04-2014, 8:37 AM
Generally speaking, yes it can.

Mike Lassiter
02-04-2014, 8:55 AM
That's a pretty small tolerance over a 6 foot span. I would think your fixtures would have to be made from very thick aluminum and be extremely rigid to be able to meet that tolerance on a good day and even inside building expansion from temperature changes could affect meeting that tolerance. As such, I think being outside in hot/cold would change the tolerance but not permanently. That is if fixture is rigid enough not to become warped from not being supported as it is in use.
Aluminum has a high expansion rate, and what you have at 60 degrees will not be what you have at 80 degrees. Keep in mind we are considering a 6 foot span with a 0.004" tolerance.

Mike Lassiter
02-04-2014, 9:22 AM
I just looked up the linear thermal expansion coefficient of aluminum in the 4th edition Pocket Ref which I took and did calculation on scientific calculator to see using your 6 foot length. Based on chart and 72 inch long aluminum a 20 degree temperature change would change that length 0.018". I based this on difference between 60 & 80 degrees. Formula is (linear thermal expansion coefficient from table (0.0000128) x initial length (inches) x (final temp - initial temp).

So not knowing anything except your stated 0.004" tolerance; a 20 degree temp change makes a 0.018" difference in the 72" length of aluminum.

mike holden
02-04-2014, 9:57 AM
As an ex-quality geek for automotive manufacturing, Yes you can store your fixture outside as long as it is stored on a known flat surface, granite surface plate, steel surface plate, ground concrete surface plate, etc. However, proper storage of the fixture would not be proper storage for the surface plate, so practically speaking, no, you cant store the fixture outside and expect to reuse it without repair. It is not good practice to create fixtures without allowing for proper storage, and as an ex cost estimator, that storage, equipment and the square footage it consumes, should have been factored into the cost. Sounds like someone cut corners on this project, or simply did not understand all the factors involved.
Mike

Jeff Erbele
02-04-2014, 5:59 PM
We use custom welded aluminum fixtures for the manufacture of certain items and the flatness of the bottom is critical. Across a 6 foot span it flatness is spec'd to .004 inch or less. Can such a fixture warp if placed outside and subjected to heat and cold fo the summer and winter?

Yes it could warp, but based on the information provided it is impossible to determine if it will or will not. It depends on a lot of factors such as the type of aluminum, the structural design, welding techniques - opposing weld beads - short welds - back stitching etc., preheating & stress relieving by peening or heat.

On the plus side of thermal expansion due to a temperature rise, in theory thermal contraction should occur due to a temperature drop. Your fixture should return to the same size it was at ambient shop temperature for example.

Thermal expansion is one thing, warping is another. Certainly the dimensions will change in width, height and length with a temperature change but maintaining flatness depends on the design (mass, supports, truss, gussets) and welding procedures.