Measure the test taker's knowledge about the root-mean-square method and the root-mean-square method.

Measure the test taker's knowledge about the root-mean-square method and the root-mean-square method.
Definition:

Root mean square method: the root mean square method is a statistical analysis method. It is used to extract the square sum of the dimensional tolerances in the dimension chain and then open the root to obtain the tolerances of the key dimensions.

Calculation formula:

;

It is assumed that the Ppk indicators of each dimension are uniform (such as 1, 1.33, 1.67), and the process is in the center.

The formula is as follows:

// According to the formula, the size of the target and parts must be 1.33 PPK and the process is in the center.

That is to say, in the case of design, the Assembly personnel is the yield rate of 4σ.

However, this is not the case. It also leaves room for optimization for other statistical tolerances.

Root mean square method:

Question:

The A dimension value and tolerances are 54.00 ± 0. 20, B is 12.00 ± 0. 10, C is 13.00 ± 0. 10, D is 16.00 ± 0. 15, E is 12.50 ± 0. 10. Use the root mean square method to obtain the nominal value and tolerances of key dimension X. (This question can be compared with the extreme value method)

Computing process:
① Calculate the nominal value of X:
DX = DA + DB + DC + DD + DE
= 54.00 + (-12.00) + (-13.00) + (-16.00) + (-12.50)
= 54.00-12.00-13.00-16.00-12.50-
= 0.50mm
② Calculate the tolerances of X

Key points of the root mean square method:

Note on the assumption that RSS is used: ① the premise of using RSS to analyze the statistical tolerances is that the size values of the manufactured parts are concentrated in the center value, and the output is normally distributed;
② If a separate tolerance in the tolerance overlay analysis is produced under the process control of ± 3 σ, then the result of RSS tolerance overlay analysis also represents ± 3 σ, that is, the input process control level also represents the output project control level;

Complete process of using the root mean square method for tolerances analysis: 1) view the general process of the tolerances analysis:

1. Define the target dimensions and judgment criteria for the tolerances analysis (the most difficult and easy to ignore step for the complete tolerances analysis );

2. Establish closed dimension chains;

3. Convert asymmetric tolerances into symmetric tolerances;

4. Determine the positive and negative dimensions of the dimension chain;

5. Calculate the nominal value of the target size;

6. Select the Method for Analyzing tolerances

7. Calculation of tolerances;

8. Judgment and optimization;

2) design by flow in sequence, and use the root mean square method for process 6.

1. Define the target dimensions and judgment criteria for the tolerances analysis (the most difficult and easy to ignore step for the complete tolerances analysis );

For example, the target size is the assembly Gap, and the judgment standard is Gap> 0. These must be included in the target summary table of the tolerances analysis.

2. Establish closed dimension chains;

3. Convert asymmetric tolerances into symmetric tolerances;

4. Determine the positive and negative dimensions of the dimension chain;

D = A + B + C + X, so the target size X = + D-C-B-.

5. Calculate the nominal value of the target size;

Nominal value gap: dGap = + 46.00-10.00-15.00-20.00? = 1.00

At this time, you can use 3d software to check whether the gap is 1, on the premise that your 3d drawings are drawn based on symmetric tolerances.

6. Select the Method for Analyzing tolerances;

Select the root mean square method.

7. Calculation of tolerances;

Nominal clearance tolerances:

Therefore, the target size is Gap = 1 ± 0. 58; (the value calculated by the extreme value method is 1 ± 1. 10)

The maximum value is 1.58, and the minimum value is 0.42;

8. Judgment and optimization;

Because the target size is determined by Gap> 0, the results of the tolerances analysis meet the requirements.

However, the root-mean-square method requires process control on the part size. Especially when the extreme value method is deemed unqualified

9. Mark the drawing of the tolerances analysis result, and mark the corresponding symbol in the statistical tolerances method.

The root-mean-square method is the statistical tolerances method. The statistical tolerances must be marked for the assigned parts. For example:

In addition, the quality management should be improved to ensure proper quality control.

Use the root mean square (RMS) method. Note: 1. tolerances analysis tool:

① Manual. (It is not recommended)

② Use an electronic data table.

Tolerances analysis table version 1:

Tolerances analysis table version 2:


Tolerances analysis table version 3:

Tolerances analysis target summary table:

// You can send the author email zjc9915@qq.com to ask for these forms.

③ Use the tolerances analysis software, such as VisVSA ?.

The author hopes to use the front-end for a software for analyzing tolerances, which can be used by everyone.

2. No matter what method you finally take, use the Extreme Value Method first.

Which method is used to obtain the value of the tolerances on the drawing? Be careful.