[信息與通信]tolerance_spreadsheetii-資料下載頁
2025-01-03 23:23本頁面
【正文】 static root sum square) Nominal Tolerance Z contr. Shift contr. Casting ~30 *() *() Housing 30 *() *() 30 + 30 = 0 RSS(above) Sum(above) RSS + SUM = Therefore, for a SRSS analysis, the gap is calculated to be 0+ The casting dimension would need to be = 29. 781 to guarantee SRSS 6 sigma fit! SRSS Splits the Tolerances into a Z(normal) ponent (which is RSS’d) and a shift ponent (which is added). (Adding the shifts assumes a worst case shift direction on all tolerances.) The sum of these two ponents is the SRSS Best Approach for the simple example 177。 102 30 177。 .173 See spreadsheet Why Decide on best Approach by tolerance contribution rather than dimension count? ? Shifting means is the reason for deciding between approaches. ? An analysis with many dimensions will have means who’s mean shifts will likely negate each other…. however if one of the tolerances has a high contribution, its mean shift will most likely be very impactful…. …. So dimension count is not the most accurate method to determine which approach is best. A more plex example: Using the standard loop equation method ? Problem: Plastic housing with a chassis fixed to one edge. There is a pcb screwed onto the chassis with a 2mm screw in a 3mm hole. The concern is the gap between the pcb and the housing. ? Solution: Generate the loop by entering each nominal. Note the sign conventions used. Also note that the pcb is biased (.5mm) against the edge of the screw. See the spreadsheet for results. The results are that the biased gap must be (or the unbiased gap is ) a c d e b Gap? centered biased A more plex example: Using TFA method (Tolerance Focused) ? Problem: Same as prior problem. ? Solution: A TFA approach involves first identifying all the tolerance contributors ( housing width, chassis hole to edge, pcb hole, screw diam, pcb hole to edge.) All the gaps in the system are entered ( screw to pcb.) There is no need to have exact nominals or to enter proper sign conventions for a loop equation. The final result is the required gap to be six sigma. In this case … the same value as achieved with the (slower) conventional loop equation approach. See spreadsheet. a c d e b Gap? A real world example Stingray pad design: ? Evaluate how much tolerance a pad between the spacer and shield would need to absorb. Place pad Between spacer and shield A real world example (cont) Stingray pad design: Six sigma pression pad needed between shield and spacer See spreadsheet
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