﻿ Calculating Starting Sizes for Seamless Tubing - Questions & Answers - SSTTAC # 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
0D = 6.500"
ID = 5.000" 2. Determine nominal 0D
2. Maximum finished
Add cleanup (+ 0.023 ) allowance (+ 0.080)
NominalOD (6.603") 3. Determine intermediate
tube ID (before eccentricity
3. Minimum finished ID = 5.000"
Subtract tolerance (0.040)
Intermediate = 4.960" 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" 5. Determine eccentricity
allowance factor (F.)

5. Applicable wall thickness tolerance = ± 7 1/2% Fe = 1.00 (wall thickness tolerance)
Fe = 1.00 - .075
Fe = 0.925 6. Determine nominal
average wall
6. Min. wall/Fe (0.822"/0.92 ) =0.888" 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" 8. Tube size to order when
machining true to OD

Use nominal tube dimensions
from steps 2, 6, 7: 6.603" OD x 4.827" lD x 0.888" average wall
Only two of the three cross-sectional
dimensions (0D, ID, or wall thickness)
can be specified when ordering
tubing.
Normally ordered by specifying the
OD and wall dimensions. 