Towne Technologies, Inc.
6-10 Bell Ave., P.O. Box 460, Somerville, NJ 08876 USA
Phone: (908) 722-9500 Fax: (908) 722-8394
DESIGN RESTRICTIONS FOR CHEMICAL MILLING
Chemical machining removes metal from all exposed surfaces simultaneously. As a hole is created by etching, the newly created walls of that hole are exposed, and the etchant acts on those surfaces too.
It is not possible to concentrate the cutting force in just one direction, as would be possible with a drill or conventional milling machine.
This basic characteristic of chemical machining necessitates some design constraints when creating a part that will be made by chemical milling. Some of these limitations are described here.
Minimum hole size:
Due to the lateral etching, the minimum hole diameter is slightly larger than the material thickness. Depending upon the type and thickness of the metal, the minimum hole size is usually somewhere between .0005" and .002" greater than material thickness. Materials greater than .020" thick are likely to require an even larger minimum hole size.
In some cases it is possible to get even closer to the material thickness, but the edge quality and uniformity may be reduced.
Generally, we etch equally from each side of the metal. This gives us a 50/50 etch ratio.
Often a design requires that we etch more from one side, and less from the other. For example, a design may require that we etch 90% from one side and 10% from the other (a 90/10 Etch ratio).
Unlike a mechanically drilled hole, the walls of an etched hole are not straight. They will have a slight bevel. As the etching goes down into the metal it also goes sideways, so the hole will be slightly wider at the surface than in the center of the metal. The lateral distance from the surface of a hole to the edge of the thru hole is called the "knife edge". The amount of the knife-edge will vary depending upon several factors including material, material thickness, hole size, and etch ratio.
In the case of some very fast etching metals such as copper, brass, and beryllium-copper, the bevel can go in the opposite direction, leaving a concave wall (see figure 3). This can be considered "negative" knife-edge. This profile can sometimes also be achieved in slower etching metals such as molybdenum or stainless steel, provided that the minimum hole size is significantly greater that the material thickness.
The following cross section views show the bevel and knife edge as they relate to the etch ratio.
Due to the beveled edges encountered with chemical milling, it is important to understand where critical dimensions will be measured.
The standard procedure is to start and end each measurement at the tip of the knife edge. In the case of a hole, the measurement would be across the area where the light would pass if the part were back lit. Likewise, the land areas will be measured where the light would be blocked. See "Thru Hole" and "Land Area" in Figures 1 and 2. If the surface dimension is more critical, it must be specified on the drawing.
Tolerances are normally set by the demands of the intended use for the finished part. Setting the tolerances tighter than they need to be can increase the cost of the part. Less expensive artwork can sometimes be used, if tolerances are not too tight.
There is no absolute rule for determining the best achievable tolerances in all cases. There are many variables that affect tolerances including the type of metal, the size of the part (or sheet of parts), the thickness of the metal, the smallest feature size, density of the pattern, cost restrictions, and others.
As a general, "unofficial" guide, the following "rule of thumb" may be used to estimate anticipated tolerances:
Parts, or sheets of parts up to about 6" square and a minimum of .004" thick, can usually be held to a tolerance equal to 10% of the material thickness.
For similar material up to about 10" x 10". The tolerance is likely to be about 15% of material thickness.
Larger sheets may need 20% of material thickness.
Please note, as stated before, this is just a general guideline to tolerances. Any specific case may be different.
In many cases we can beat these tolerances, especially if there is only one critical feature size. Some metals are more controllable than others, and hence allow tighter tolerances.
In most cases the critical feature size repeats in many places on the same part, such as an array of same-size holes. In general, the range will be more uniform than the tolerance might imply. Within one piece, the actual hole sizes may tend to be all around the same area, whether that be near the nominal, or mostly toward the higher end of the tolerance, or mostly toward the low end. Within a batch of parts the entire tolerance band may be used.
It is the nature of chemical milling to etch laterally in all directions. For this reason sharp corners are not possible.
All inside corners (such as the corners of a square hole) will radius by an amount equal to the material thickness. For example, in .005" Stainless Steel a .010" x .020" rectangular opening would be fully radiused at both ends.
Outside corners (such as the outer corners of square mask) radius somewhat less. Generally the radius is equal to about 75% of material thickness.
In some cases this radiusing can be reduced, but not always.
Temper and Physical Characteristics:
The temper of the metal has some effect on the etching characteristics. For any given metal, some tempers may etch better than others, but the difference is rarely enough to warrant using a temper other than the design requires.
When no temper is specified, we usually use the temper that has the best etching and handling characteristics.
When heat treating is require, it is commonly done on the raw material beforehand. This avoids damage or deforming of the finished parts during heat treating. Of course, it is still possible to heat treat the parts after chemical milling when required.
The chemical milling process does not alter the temper or physical characteristics of metal.