Open Forum: Needle Point Grinding
January 21st, 2013
In 1946, a set of government standards was established to produce acceptable criteria for hypodermic needles. These standards, called "GG-N-196" are still in use today. Precepts of correct bevel length for various gauge sizes are divided into three classifications: A,B, and C bevel lengths. An "A" bevel is the longest and sharpest and is often used in applications such as blood collecting. To obtain the long, sharp point, a compromise is made--the tip is very delicate, and can be easily deflected and damaged. A "C" bevel does not have a point as sharp as the "A" bevel, but it is more sturdy and less delicate. A common example of the application for "C" bevel type is in piercing a rubber stopper. The length of a "B" bevel falls between "A" and "C". There are more applications for the "B" type bevel, as it is relatively sturdy and sharp.
A standard needle point or bevel as applied to "GG-N-196" has three grinds: a primary grind and two secondary grinds. The primary grind angle is the largest factor in determining the finished bevel length. To determine the primary angle, the formula is as follows:
Tangent of <(X)=
|primary grind length|
Since the inception of "GG-N-196", an explosion of needle point applications has occurred. Many needle point styles have been developed that are not addressed in the "GG-N-196" specifications. Examples of these needle types are: Trephine, Cournand, Veress, Huber, Seldinger, Chiba, Trocar, and Francine. The expansion of applications and improvements to product function has led to diversity in the development of new point styles. For example, a Huber point may be selected in an application where an anticoring needle point is required. The anticoring point cleanly pierces through a septum without coring, with a reduced risk of blocking the needle, which coring can cause. As the applications for specialty needles expand, variations and modifications will continue to be made to improve product performance. Today, a manufacturer can assist the design engineer with design input and in the development of product specifications. Careful consideration should be made in choosing the appropriate type of hypodermic tubing and needle design. For example, "Bright-Draw" tubing might be selected for an application where red blood cell damage during flow through the needle is of concern. Type 316 SS or inconel may be selected over the standard 304 SS if the application involves a corrosive environment. The inside diameter surface finish is not a critical factor in determining needle sharpness, but the hardness of the tubing is important. Soft tubing will not grind as easily as needle temper tubing; it has a tendency to smear and load the wheel. The ground edges of soft tubing will be rougher, affecting needle sharpness. Needle sharpness is often an important factor. This is usually measured as the force required to penetrate a thin membrane. Statistical tools can be used to calculate the sharpness of a production run. Among the variables that contribute to needle sharpness are: Grind geometry, grinding wheel choice, grinding coolant, material removal rate and tubing choice. In addition, lubricating coatings and other post grinding operations can improve sharpness values. Process capability studies and PFMEA (Process Failure Mode Effects Analysis) can aid in dealing with potential problems on new manufacturing projects. It is important to try to isolate and analyze each variable as it affects the use and performance of a hypodermic needle.