Why is the nominal inside diameter not enough?
Because it is a result, not a dimension anyone controls. The inside diameter is the outside diameter minus twice the wall, and each of those has a tolerance. A tube that is ordered by outside diameter and wall has an inside diameter that varies from piece to piece, and from one end of a piece to the other.
Take 25.4 mm outside diameter and 3.0 mm wall. The nominal inside diameter is 19.4 mm. Now suppose the outside diameter is ±0.13 mm and the wall is ±10%. These numbers are illustrations, not values from a standard.
- Smallest inside diameter: (25.4 − 0.13) − 2 × 3.3 = 18.67 mm
- Largest inside diameter: (25.4 + 0.13) − 2 × 2.7 = 20.13 mm
The two ends are 1.46 mm apart. An adapter that slides into the smallest tube would sit in the largest with a gap of 1.46 mm plus its own tolerance. A gap that large is a problem for alignment and for any joint that relies on the fit. The remedy is to order a tube with a controlled inside diameter, or to bore or ream the tube end before the adapter goes in.
What is out-of-roundness, and why does it matter?
Ovality is the maximum outside-diameter reading minus the minimum, taken at the same cross-section. For a nominal 20.00 mm tube that reads 20.10 mm at its widest and 19.92 mm at 90°, the ovality is 0.18 mm, or 0.90% of the nominal diameter (an illustration of the calculation, not an acceptance limit).
Three controls are often confused: outside-diameter tolerance, ovality and wall-thickness tolerance. A tube can be within its diameter tolerance and still be oval, and an oval end will not take a round plug evenly. Thin walls also distort more easily in forming, cutting, clamping and measuring, so a tube that must accept a close-fitting part should have its dimensions and its measurement method stated.
Does a welded tube have a seam inside?
It can. A welded tube is rolled from strip and welded along its length. ASTM A513, the standard for electric-resistance-welded carbon and alloy steel mechanical tubing, covers round, square, rectangular and special shapes made from hot- or cold-rolled steel. Whether the weld bead is removed on the inside depends on the product form, so ask. A bead inside the end stops a plug seating, and it moves the inside diameter locally. Seamless tubes have no seam, but they have the same tolerances to check.
What is the seating shoulder, and how deep should the adapter go in?
Many adapters have a flange or step that stops against the tube end. The shoulder gives a fixed insertion depth and holds the adapter square to the tube before it is joined.
The insertion depth decides how well the adapter holds the tube's axis. A deeper plug lines up better and shares the bending load with more of the tube wall. A shallow plug leaves the joint at the tube end as the only support. There is no one number for the depth, because it depends on the load and the joint type (Q1-04). Set it in the drawing, and check that the thread inside the adapter is not cut through by the end of the plug.
How does the tube limit the thread you can use?
By the wall that is left on the adapter. The adapter's outside diameter is the tube's inside diameter, and its thread cuts into that. The wall around the thread is
(tube inside diameter − thread major diameter) ÷ 2
Take a tube of 1.000 in. outside diameter and 0.095 in. wall. Its inside diameter is 0.810 in.
- A 5/8-18 thread (major 0.625 in.) leaves (0.810 − 0.625) ÷ 2 = 0.0925 in. (2.35 mm) of adapter wall
- A 3/4-16 thread (major 0.750 in.) leaves (0.810 − 0.750) ÷ 2 = 0.030 in. (0.76 mm)
The second is a thin skin around the thread. A thin wall around a thread is both weak and hard to weld without distortion (K09-Q1-04). If the thread you need does not leave a sound wall, the answer is a larger tube, a smaller thread, or a different attachment (Q1-01). The pitch and the hand of the thread are separate choices (K06-Q1-07, K01-Q1-01).
Which tube standard controls the fit?
The one that defines the product you buy. Tube and pipe are different products with different dimension systems. If a drawing calls for tube but the request cites a pipe standard, resolve the conflict before the quotation. For mechanical tubing, ASTM A513 is one example, and it covers welded tubing; seamless mechanical tubing and tubes made to European standards have their own documents.
Whatever the standard, say four things in the order: the standard and its edition, the nominal outside diameter and wall, which dimension is controlled (outside or inside diameter), and the ovality and measurement method if the fit is close. Read the tolerance table of the edition you order. The tolerances are listed by size and product form, so take them from the edition you order.
Does a vehicle tube change the sizing?
The method does not change: measure the real tube, with its weld seam, and size the adapter and thread to it. What changes is who chooses the tube. On a vehicle, link and chassis tubing is often fixed in advance by the vehicle maker or the chassis design, so the adapter is chosen for the tube, not the other way round. Measure the actual inside diameter at the end where the adapter sits, and check the wall against the thread limit above before you order. If the tube is chromoly, tell the supplier, because the material affects the welding procedure (K09-Q1-04). The chromoly tubing buyer's guide and the tube adapter buyer's guide cover the tube and the adapter.