Machine Stock and Metal Castings
As a casting supplier, we review and approve all casting designs prior to being sent over to the foundry for pouring for obvious reasons. The obvious reasons being that the design is able to be casted and for that we check things like wall thickness, parting lines, and internal cores. However, over the years we have found that it is a common mistake for engineers to forget to add machine stock to the necessary areas of the casting. For the most part this is not a major problem, but there are instances where this can create quite the headache.
Machine stock on a casting refers to extra material added in the areas that will require machining after the casting is poured. To do this, machine stock must be accounted for in the design and/or CAD drawing of the casting. This is where we usually discover the problem, during our review of the CAD drawing and then compare it to the required machine tolerances. Machining is simply the removal of material to make the part meet the desired dimensions. As you can guess, to machine a casting there must be excess material there to be removed. If there is no excess material whatsoever, then machining simply cannot be performed. If there wasn't enough machine stock added, it can make it very difficult and tedious for the machinist, which will just increase machine time and costs. Also, an insufficient amount of machine stock could result in the casting being scrapped during the machining process (if too much material is removed the casting is useless), further increasing overall costs. The amount of machine stock to be added depends on the dimensions that must be met during machining. At CPM Industries, we supply the castings as well as the machining, so our engineers can help make that determination for our customers.
There have been instances where the machine stock was not added and could not be added without further modification to the casting design. This usually happens on areas of the casting that are a tight squeeze and adding more material would basically make it impossible to machine. To solve the problem, the design of the casting must be edited to accommodate the machine stock. If it is simply not an option to alter the design, then often times we will suggest pursuing an alternate casting method, such as investment casting. Investment casting offers much better dimensional accuracy than sand casting, but also comes with higher price tag.
The images below are a before and after of a aluminum casting that we machined and supplied to a customer. As you can see, there was a lot of material removed during machining, especially on the two ends that are tapered.