How Mechanical Tube Sizes Are Calculated
The Timken Company has pioneered a method to determine the most economical tube size, which insures that all part dimensions will clean up during machining. This program has the capability to determine what we call a "guaranteed" tube size for either full or bright metal cleanup on the outer diameter and/or the inner diameter, based on the condition of the tube.
The guaranteed tube size is based on the part's critical finished dimensions and critical machining position. The required critical dimensions are:
- maximum finished OD
- mimimum finished ID
- maximum finished part length
All applicable tolerances and surface finishes ("machined" or "ground") should be included. The critical machining position is based on whether the tube is chucked on the tube OD or the tube ID during the initial machining operation.
Critical finished part dimensions and machining position influence the amount of cleanup, the size tolerances and the tube eccentricity variables. These variables are then factored into the tube size calculation.
Cleanup Allowance
"Cleanup allowance" must be added to the finished OD dimension or subtracted from the finished ID dimension to provide for the elimination of surface imperfections, decarburization and camber (out-of-straightness). Other allowances specific to the application may be needed. (See table)
Examples of Tube Size Calculation
Finished Part Dimensions
4.995 inches ± .005 inch OD x 4.005 inches ± .005 inch ID x 7 inches Part is cut to length and held on one end. Allowance for camber is made for parts over 3 inches, measured from the end of the part to the face of the chuck.
Examples below are for a hot rolled tube using Timken Company cleanup allowances.
| Operation |
Formula |
Operation |
|
| 1 |
Determine nominal tube OD
(see Figure 2) |
Maximum machined OD
Plus cleanup stock
Plus camber allowance
Plus OD negative tolerance
Nominal tube OD |
5.000 in
+ 0.054 in
+ 0.027 in
+ 0.028 in
5.109 in |
127.00 mm
+ 0.054 mm
+ 0.027 mm
+ 0.028 mm
129.77 mm |
|
| 2 |
Determine minimum
tube ID
(see Figure 3) |
Minimum machined OD
Minus cleanup stock
Minus camber allowance
Minimum tube OD |
4.000 in
- 0.042 in
- 0.027 in
3.931 in |
101.60 mm
- 1.07 mm
- 0.69 mm
99.84 mm |
|
| 3 |
Determine minimum
wall required
(see Figure 4) |
Tube OD (Operation 1)
Minus tube ID (Operation 2)
Divide by 2
Minimum tube wall |
5.109 in
- 3.931 in
1.178 in
÷ 2
0.589 in |
129.77 mm
- 99.84 mm
29.93 mm
÷ 2
14.96 mm |
|
| 4 |
Determine average wall
required
(see Figure 6) |
Minimum tube wall
Div. by the complement
of minus tolerance
Average tube wall |
0.589 in
÷ 0.925 in
0.637 in |
14.95 mm
÷ 0.925 mm
16.17 mm |
|
| 5 |
Determine nominal tube OD
(see Figure 6) |
Tube OD (Operation 1)
Minus average wall doubled
Required tube OD |
5.109 in
- 1.274 in
3.835 in |
129.77 mm
- 32.34 mm
97.43 mm |