Surface Finish (C_f) and Tolerance (C_t) Impact on Component Cost

Basically the finer the finish and the tigher the tolerance the greater the cost. Ideally from a cost impact point of view it is best for the designer to accept the finish and tolerance capability of the primary process. Part performance and functioning usually dictates that this is not possible so consideration must be given to what kind of seconary processes will be required to enable the component to perform as desired. This is discussed at length in Chapter Six of the book "Process and Operation Planning: Revised Edition of the Principles of Process Planning, A Logical Approach" - by G. Halevi - ISBN: 1-4020-1653-0  

The chart below shows the relationship between Tolerance and Surface Roughness.

Tolerance ± mm			Surface Roughness Ra (μmm)
	<.005			>0.20
	.010			0.32
	.015			0.45
	.020			0.80
	.030			1.00
	.040			1.32
	.050			1.60
	.060			1.80
	.080			2.12
	.100			2.50
	.150			3.75
	.200			5.00
	.250			8.25
	.350			9.12
	.600			12.50
	1.000			25.00

The next chart shows the surface roughness capablity of various manufacturing processes. Variations on this chart can be found almost anywhere. We think it originated in Machinery's Handbook!

Surface Roughness Chart - Shows the roughness average for different manufacturing processes in micrometers and micro-inches. The values are shown with a typical range and a less frequent range for each manufacturing process. As a general rule the further to the right of the chart means a more costly process.       Average Range             Less Frequent Range Manufacturing Process Roughness Average (Ra) Micrometers (Micro-inches) Forming Processes 50 (2000) 25 (1000) 12.5 (500) 6.3 (250) 3.2 (125) 1.6 (63) 0.80 (32) 0.40 (16) 0.20 (8) 0.10 (4) 0.05 (2) 0.025 (1) 0.012 (.5) Sand Casting                                                     Hot Rolling                                                     Forging                                                     Permanent Mold Casting                                                     Investment Casting                                                     Extruding                                                     Cold Rolling, Drawing                                                     Die Casting                                                     Metal removal or cutting processes 50 (2000) 25 (1000) 12.5 (500) 6.3 (250) 3.2 (125) 1.6 (63) 0.80 (32) 0.40 (16) 0.20 (8) 0.10 (4) 0.05 (2) 0.025 (1) 0.012 (.5) Flame Cutting                                                     Snagging                                                     Sawing                                                     Planing, Shaping                                                     Drilling                                                     Chemical Milling                                                     Elect Discharge Machining                                                     Milling                                                     Broaching                                                     Reaming                                                     Electron Beam                                                     Laser                                                     Electro-Chemical                                                     Boring, Turning                                                     Finishing processes 50 (2000) 25 (1000) 12.5 (500) 6.3 (250) 3.2 (125) 1.6 (63) 0.80 (32) 0.40 (16) 0.20 (8) 0.10 (4) 0.05 (2) 0.025 (1) 0.012 (.5) Barrel Finishing                                                     Electrolytic Grinding                                                     Roller Burnishing                                                     Grinding                                                     Honing                                                     Electro-Polish                                                     Polishing                                                     Lapping                                                     Super Finishing                                                       50 (2000) 25 (1000) 12.5 (500) 6.3 (250) 3.2 (125) 1.6 (63) 0.80 (32) 0.40 (16) 0.20 (8) 0.10 (4) 0.05 (2) 0.025 (1) 0.012 (.5)