What’s up guys? Michael with Michael Talks Metal back for the 145 th time to talk about my favorite topic…. Metals.
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Our topic for today involves some of the things that can affect your manufacturing plan for any given steel part. Machining allowances for steel bars and mechanical tubing…..Aerospace standard AS 1182
Most raw steel products are permitted to have limited surface imperfections from the manufacturing processes at the mill. The allowable depth of these imperfections is restricted to the depth of the machining allowance. Hot working and thermal treatments can also result in a decarburized surface. While allowable in the raw material, we don’t want these imperfections in our finished parts.
The machining allowance establishes the amount of stock to be removed from the surface of the mill material to assure any surface flaws or decarb present will also be removed.
We need to emphasize ” removed from the surface”….. other factors can make this more complicated than it would seem to be.
For a simple example:
Let’s say our part is a 3 inch diameter shaft 3 ft in length and will be made from a hot rolled alloy round bar. AS1182 tells us that the minimum stock removal should be .156″. Our bar stock then needs to be a minimum of 3.312 inches in diameter (3 + (2 x .156)). But we also have an allowable tolerance for straightness of up to .125 inch per 5 feet of length. When our bar is not straight, we will not have equal removal on all sides along the length when turned on centers. An extra allowance needs to be made for straightness to assure the minimum amount of stock is removed all around; in this case another .150 inch. So it takes a 3.462 minimum size hot rolled bar to assure a flaw free surface in our example.
The longer the part is, the more thought should go into the tolerances for shape and straightness and their potential effect on uniform stock removal.
Some mill products have already removed some or all of the required stock in operations like rough turning, grinding, and polishing. Cold drawn products without stock removal have improved size tolerances, but any imperfections remain.
Machining allowances also vary by mill product and nominal size.
SAE AMS has published an Aerospace Standard for recommended stock removal that applies to Aircraft Quality and Premium Aircraft Quality steels. Aerospace Standard AS1182 applies to materials ordered to AMS material specifications.
So for the 145 th time CHECK THE SPECS!!!!!!! TWICE. This is Michael with Michael Talks Metal. Thank you for watching. Michlin Metals is a full service value added distibutor and supplier of all things metal and steel. Check the website for more info: www.michlinmetals.com If you missed last weeks video, click here. Still here and haven’t subscribed, click here.