Fixturing forgings and castings, part 1: the basics.

Part of a series of articles on fixturing odd shaped parts, such as forgings, castings and secondary operations. A few guidelines I have developed over my 40 years of machining.

Fixturing forged or cast parts for machining is always a challenge. The non-regular, constant changing surfaces defy solid and consistent points to clamp on, Yet consistency and rigidity in loading is critical to achieving precision results and reducing headaches in subsequent operations.

Key point to remember when considering fixture design

-The more consistent your operation results are at the outset, the easier it will be in subsequent operations.

Too often, preparatory operations (such as milling a flat for banking or drilling locating holes) are given less attention. This is when attention is most needed. This is when the time needs to be invested in such factors as:

  • Establishing consistent banking surfaces
  • Establish point or holes for solid fixturing for your “big cutting” ops
  • Establishing datums and references for inspection.
  • Establish areas where banking will take place, and clearance areas (where banking should not take place)

-The banking surface: Consistent banking opposite to where force is being applied.

For example,if you will be drilling, you want optimum support underneath the part, ideally surrounding the area to be drilled. (This applies to all machining operations that apply downward force.)

-Don’t have more banking surfaces than you need. Rarely should you have more than one for any axis.

If there is more than one surface on the banking side of the stock forging/casting, you want to bank only on one surface, typically the one that has the greatest surface area and is centered to the part (see figure 1 below).

This is a common issue for new fixture builders. As an example if, you have a part that has a .250 step, the fixture builder will build in a .250  step. Exactly .250.

But parts are rarely that consistent. If the step on one part is .248″ the one surface will be used for banking. If the next part has a .253 stop. you will now be banking off the other surface, and more than likely skewing the part in the fixture.

Surfaces not used for banking should have clearance, (unless you have good reason to need support under a second surface, such as very large part).

figure 1. An example of how NOT to over-constraint. If clearance was not established under the lowest point (on the right) of the workpiece, then any excess of material would cause in to not sit flat with the intended banking surface on the left. You have just introduced an unplanned inconsistency.

 

 

If you do determine you need to have multiple banking surfaces, you will need to insure that the step distance(s) between surfaces remains consistent-virtually impossible in forgings or castings.

-When clamping irregular shaped parts three point for clamping is ideal.

This will be discussed in the next installment. To be continued…

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