Calculating Starting Sizes for Seamless Tubing
Most producers prefer to calculate the starting size for any seamless tubing application. But for purposes of illustration, here is a hypothetical example. In this case the finished part size is 6.500 inch outer diameter (OD) by 5.000 inch inner diameter (ID), less than 3 inches in length. Let's assume that the tubing is cold drawn and finish annealed (about 950F). In this example, the machining sequence starts with chucking on the outside.
Example I-Chuck OD Required: Finished part size of 6.500-inch OD x 5.000-inch ID, cold drawn and finish annealed seamless tubing; finish machined length less than 3 inches. Machining sequence: chuck OD, bore ID, turn OD.
| Procedural Steps | Calculations |
Illustrations |
|---|---|---|
| 1. Determine critical machining data | L Finished machine size |
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| 2. Determine nominal 0D | 2. Maximum finished Add tolerance (6.500") Add cleanup (+ 0.023 ) allowance (+ 0.080) NominalOD (6.603") |
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| 3. Determine intermediate tube ID (before eccentricity adjustment) |
3. Minimum finished ID = 5.000" Subtract tolerance (0.040) Intermediate = 4.960" |
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| 4. Determine minimum wall required |
4. Nominal 00= 6.603" Subtract intermediate ID ( 4.960") = 1.643" (twice minimum wall) Divide by 2 to get minimum wall =0.822" |
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| 5. Determine eccentricity allowance factor (F.) |
5. Applicable wall thickness tolerance = ± 7 1/2%
Fe = 1.00 (wall thickness tolerance) |
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| 6. Determine nominal average wall |
6. Min. wall/Fe (0.822"/0.92 ) =0.888" |
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| 7. Determine nominal tube ID (eccentricity adjustment is now included because of step 6) |
7. Nominal tube 0D = 6.603" Minus twice nominal average wall = 1.776" Nominal tube ID= 4.827" |
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| 8. Tube size to order when machining true to OD |
Use nominal tube dimensions |
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Ordering size: 6.580" 0D (+0.23" - 0.000") x 0.888" average wall (±7 1/2%) |
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