Photo Chemical Machining is accomplished by selectively masking the metal, then spraying with an acid. As the droplets of acid impinge on the metal erosion occurs. This process removes metal without creating burrs and does not work harden the material. Other advantages are that most metals can be processed by Photo Chemical Machining, and the temper has no bearing whatsoever. Photo Chemical Machining does have unique characteristics that must be considered to achieve the optimum results. Some of these are presented here, but it would be prudent to contact your Vaga Industries representative if in doubt.

UNDERCUT - As the acid droplets impinge on the metal and begin to cut through, a certain amount of side erosion occurs. This is referred to as undercut and is the main factor governing the tolerances that can be held. As the thickness increases so does the undercut, and in turn so do the tolerances that can be held. HOLE SIZE AND METAL THICKNESS - As a general rule, the diameter of the hole cannot be less than the thickness of metal. This relationship varies with the thickness of metal to some degree.

MATERIAL WIDTH BETWEEN HOLES AND METAL THICKNESS - The metal thickness puts limitations on the material width between holes. Minimum widths are shown in Table 1.

Table 1
Thickness	Spaces between holes (w)
less than .005"	At least metal thickness
Over .005"	At least 1.25 times metal thickness

Table 1. Space between holes

TOLERANCES

Center to center dimensions - Center to center tolerances are dependent upon the photo-tool used for masking the part. The center-to-center tolerance is proportional to the linear distance involved as shown below.

Table 2
C/C Dimensions (inches)	Tolerance
1.0" or less	+ .0002
1.0"- 3.0"	+ .0004
3.0" - 6.0"	+ .0008
6.0" - 10.0" (in)	+ .0008

Table 2. Center to center dimensions

INSIDE CORNER RADIUS AND METAL THICKNESS - The minimum inside corner radius will normally be equal to the thickness of metal. If the metal is .005" then the radius will be .005".

OUTSIDE CORNER RADIUS AND METAL THICKNESS - The normal radius of a sharp outside corner will be 2/3 the metal thickness. Special adjustments can be made if a sharper corner is required. EDGE BEVEL AND METAL THICKNESS - A small amount of etching or erosion does occur laterally. In most instances the metal sheets will be sprayed from both sides. The resulting edge profile will resemble the drawing shown in Fig. 3. The bevel "A" should be approximately .1 x thickness (T).

In some applications etching from one side is preferred. In that case "B" will be approximately .4 x thickness (T) as shown in Fig. 3.

Fig. 3. Etched from both sides (A) and one side (B)

Table 3
PROTOTYPE AND SHORT RUNS
Approximate flat size	Thickness, "T " inches
	0.002	0.005	0.010	0.020	0.040	0.060	0.080
2" x 2"	± .0005	± .0005	± .001	± .0015	± .002	± .004	± .005
8" x 10"	± .001	± .001	± .001	± .002	± .003	± .004	± .006
12" x 18"	± .001	± .002	± .002	± .003	± .004	± .005	± .008

Table 3. Tolerances on prototype and short runs

Table 4
PRODUCTION RUNS
Approximate flat size	Thickness, "T " inches
	0.002	0.005	0.010	0.020	0.040	0.060	0.080
2" x 2"	± .0005	± .001	± .002	± .004	± .005	± .006	± .007
8" x 10"	± .001	± .001	± .003	± .004	± .005	± .006	± .007
12" x 18"	± .001	± .002	± .004	± .005	± .005	± .007	± .010
18" x 24"	± .002	± .002	± .004	± .005	± .006	± .007	± .015

Table 4. Tolerances on production runs

Table 5
RELATIVE FORMABILITY OF COMMONLY USED METALS
MINIMUM RADIUS FORMABILITY - 90°
Material	Alloy	Temper	Across	Grain Direction 45°	Parallel
Low carbon Cold rolled Steel	No. 1	hard	2.0 x	3.0 x T	flat
	No. 2	1/2 hard	1.0 x T	1.5 x T	1.5 x T
	No. 3	1/4 hard	SB	0.5 x T	1.0 x T
	No. 4	soft	SB	SB	SB
	No. 5	dead soft	SB	SB	SB
Stainless Steel	301/302	annealed	SB	SB	SB
		1/4 hard	SB	0.5 x T	1.0 x T
		1/2 hard	0.5 x T	1.0 x T	1.5 x T
		3/4 hard	1.0 x T	1.5 x T	2.0 x T
		hard	1.5 x T	2.0 x T	3.0 x T
Berylium Copper	25	anneaied	SB	SB	SB
	25	1/4 hard	SB	SB	SB
	25	1/2 hard	0.5 x T	0.7 x T	1.1 x T
	25	hard	1.0 x T	2.0 x T	2.2 x T
Brass	No. 6	annealed	SB	SB	SB
		3/4 hard	SB	SB	SB
		hard	SB	SB	0.5 x T
		x hard	SB	0.5 x T	2.0 x T
		spring	SB	1.0 x T	3.0 x T
		x spring	0.5 x T	1.5 x T	3.5 x T
Phosphor Bronze	Grade A	annealed	SB	SB	SB
		1/2 hard	SB	SB	SB
		hard	SB	SB	1.0 x T
		x hard	SB	0.5 x T	2.5 x T
		spring	SB	1.0 x T	3.0 x T
		x spring	0.5 x T	1.5 x T	3.5 x T
	Grade C	annealed	SB	SB	SB
		1/2 hard	SB	SB	0.5 x T
		hard	SB	0.5 x T	2.0 x T
		x hard	SB	2.0 x T	4.0 x T
		spring	SB	3.5 x T	5.5 x T
		x spring	0.5 x T	4.0 x T	7.5 x T
SB = Sharp Bend . . T = Thickness of material The values shown above are suggested for minimum bend radius. Prototypes shouid be made to insure structural integrity.

BEND RADII Table 5 shows minimum internal bend radii for different type metals. The radii shown are generally accepted throughout the industry, but prototypes should be made to insure structural integrity.

Edge to edge dimensions - There are many parameters involved in determining dimensional tolerances. One of the first should be cost. If a tolerance can be opened up to production parameters and not affect fit, form or function it should be used. In some cases this option is not available, so additional quality control measures must be employed to guarantee acceptable parts. This naturally increases cost.

FORMING

TOOLING HOLES - In many instances Photo Chemical Machining provides a practical method of manufacturing formed metal parts. Tooling holes can be located very accurately in relationship to the part for secondary forming.

HAND FORMING - Bend lines as shown in Fig. 4 can be etched into one side of a part. This allows the easy hand forming of sharp bends. Naturally there is a loss of structural integrity.

Fig. 4 Hand forming with bend lines