Tablet Take-Off

For non-round tablets, tablet take-off orientation can be a critical component to achieving high production speeds without tablet defects.  There are a few key attributes to consider for sorting out what could otherwise be a confusing combination of press, tablet and tooling geometries.

First, identify the preferred tablet orientation relative to the tablet take-off bar.  In most cases, it is best to align the tablet take-off bar parallel to the long axis of the tablet.  This presents the least impact stress on the tablet since the contact area between the tablet sideband and the take-off bar is maximized.  Tablet flight during ejection is also more controlled since this minimizes tablet rotation as it transitions off the lower punch cup and down the ejection chute.

In some cases, a small amount of angle (typically 5°) where the trailing edge of the tablet makes contact first with the tablet take-off bar can help “break free” the tablet from deep lower punch cups by introducing a torsional moment.

In cases of multi-tip tooling, there is often a trade-off between ideal tablet take-off orientations and the size / number of tips that you can fit on the tool.  Please contact your tooling supplier for details on optimizing tip geometries for your application.

Second, to determine the appropriate punch keying angle between the punch key and the punch tip, there’s a few pieces of information regarding the press turret that are essential.

· Direction of press rotation (clockwise or counterclockwise)

· Punch guide keyway orientation (measured in degrees) to the pitch circle diameter (PCD).  In many cases, presses have the keyway in-line with the direction of rotation, so this is 0°.

· Slant of the tablet-take-off bar normal to the PCD.  For tablet take-off bars that are curved, the angle to be measured is from a theoretical line tangent to the curve at the intersection point with the PCD.

Thankfully, most rotary tablet presses in the field are well documented and understood so punch keying angles are known or published in the TSM (Tableting Specification Manual).  In cases where this is in doubt, just these three data points from the press will suffice for the tooling designer to calculate the right keying angle for your punch.

Take-off bar design varies depending on the press manufacturer with some preferring straight bars and others curved.  For punch cups with very deep cups, selecting a take-off bar with a small vertical angle orientation (bar touches the tablet at the bottom of the tablet’s sideband first) can help rotate the tablet up and out of the cup avoiding tablet breakage.   

Lastly, when checking your punch drawing’s keying angle to confirm tablet take-off is correct, it is useful to understand differences in drafting conventions between U.S. and non-U.S. companies.  Punch drawings use descriptive geometry to represent 3 dimensional objects on two-dimensional (orthographic) planes.  In the U.S., the convention of third angle projection is used where objects are projected onto the exterior face of an imaginary three-dimensional box.  Non-U.S. companies typically use the convention of first angle projection where objects are projected onto the opposite inner surface of an imaginary three-dimensional box.  In this case, the viewed surface of an object that is drawn is said to be projected through the object and onto the view plane.  As a result, the keying angle may appear to be reversed when comparing U.S. to non-U.S. drawings.  Proper drawings should denote the drafting standard as shown below:

For further information on tablet take-off and punch keying angles, please contact Natoli Engineering.

Todd Martin

Sr. Technical Service Engineer