Mill Fixturing:  Easy as 1, 2, 3

Lunch counter stools may not be an everyday topic with machinists, but here’s a question for the guys in the shop.  Which is more stable – a three-legged stool or a four-legged stool?

It may come as a surprise, but a three-legged stool is more stable.  With four legs, there is always the potential for rocking.  That is eliminated with three legs.

The same concept is used in stabilizing a part for milling.  If a part were to float in midair, it would have six degrees of movement – up and down, right and left, backward and forward.  The trick is to restrict all of these movements while machining the part.  A method of fixturing must be used to “fix” the part in place.

There are many types of fixturing that can be used when milling.  Whatever type is used, it is recommended that the 1-2-3 rule be applied.

The 1-2-3 rule should be used when designing a mill fixture because this concept restricts movement in three planes (all six directions of movement), creating a stable part.

Looking at a machine bed, an unrestricted part can move all around.  Use the following points to stop part movement when machining.

Point 1:  Use a single endwise stop point.  This is best used when abutting the short end of the part.

Point 2:  Use double side stop points.  These two are best used along the longer side of the part.

Point 3:  The three points on the table indicate the plane that the part will occupy as the part sits on these three points.

 Now the part will be held properly for machining with no extra movements allowed.  This procedure works great for smaller parts, but larger parts may need extra support.

Fixture Jacks

To give larger parts additional support, it may be necessary to use a threaded Jergens jack screw.  However, when the jack screw is installed under a part and clamped down, there is a tendency to bend the part with any excessive force.

To counter this potential bending, use an indicator on the top face of the part when installing the jack screw.  

 Once the indicator moves, the plate is correctly supported.  The clamp should then be located directly over the jack screw to pinch the part.

It is important to apply the clamp straps directly over the table location pins to avoid bending the part piece.

Using this method provides true location with full support for any heavy-duty machining.