Hello,
we are looking for precision reference beams that will stand vertically and support weights up to 2Kg. We would like a flatness spec. of 20mil or less over 6', and low weight => aluminum, fiberglass or carbon fiber might be best. Standard aluminum extrusions, and fiberglas pultrusions, cannot meet the flatness spec. Can anyone suggest a supplier that might meet our needs ?
Thank you,
Declan Hughes
what about using a secondary grinding operation to flatten an aluminum extrusion??
The camber and twist tolerances allowed for aluminum extrusions are pretty loose. You can pay a price premium and get parts made to half or a quarter of commercial tolerances.
Actually, they're not made differently, they're just inspected. Truth is, extrusions are _usually_ much better than commercial tolerances. So you could just buy them to commercial tolerances, inspect them yourself, and throw away the ones that aren't good enough.
I've seen aluminum pieces of moderate size leveled on a surface grinder. It works okay if you put a dam around the table and keep the workpiece completely submerged in soluble oil coolant. Caution: The grinding swarf, once dry, may be very easy to ignite.
-Mike-
My understanding about aluminum is that it tends to have a lot of internal stress, so machining one side leads to new bending. The flatness spec. for Bishop Wise Carver extrusions is 0.3mm per 300mm, and 1.5mm twist over 2000mm (=> about 1.7 degrees assuming a 50mm width). I would like to get a flatness spec. of 0.5mm over 2000mm, and less than 0.5 deg twist over 2000mm .
Declan
Clearly you're right for serious machining operations, but my understanding is that taking very small amounts off (which is what you'd do in a flattening operation) shouldn't affect the internal stresses. A good machinist should be able to give you a definitive answer...
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