Hollow bar is valuable because it can put a machine shop closer to the finished geometry than solid round bar. But that advantage depends on selecting the right starting size.
A vague RFQ such as “316L hollow bar, about 5-inch OD” leaves the supplier without enough information to determine whether a stock size will clean up to the finished component.
The most useful hollow-bar RFQs define the outside diameter, inside diameter, machining allowance, length, quantity, grade, and documentation requirements together.
OD: Define the Outside Envelope
Outside diameter is usually one of the first dimensions a buyer provides, but the important question is whether it describes the raw stock requirement or the finished component.
If the finished part will be turned down, the stock OD needs enough material for cleanup and any runout or surface condition the machining plan must accommodate.
Sending the finished OD together with the preferred starting OD can help a supplier understand the intent rather than treating the raw dimension as an isolated number.
ID: The Dimension That Creates the Hollow-Bar Advantage
The inside diameter is where hollow bar can save substantial machining compared with solid bar.
A starting bore closer to the finished bore reduces the volume of stainless that must be drilled, bored, or trepanned away. For a repeat part, that can reduce cycle time, chips, tooling consumption, and machine load.
The starting ID still needs enough allowance to clean up to the finished bore. If the finished ID is critical, provide it or attach the drawing.
Wall Thickness: Useful, but Avoid Redundant Ambiguity
OD, ID, and wall thickness are mathematically related, but tolerances mean the purchasing specification should identify which dimensions control acceptance.
If the supplier is being asked to meet an OD and ID, do not assume a nominal wall statement communicates a separate tight wall tolerance. If wall thickness itself is the critical characteristic, make that clear.
The applicable product specification and customer drawing should define how dimensions and tolerances are evaluated.
Machining Allowance Should Be Intentional
More allowance is not always safer.
An oversized OD and undersized ID create a heavier wall, which means more material to remove. Too little allowance risks failing to clean up the surface or achieve the finished dimension.
The correct allowance depends on stock condition, straightness, tolerance, setup, chucking method, part length, required finish, and the machine shop’s process.
For important or repeat parts, let the machining team define the required starting envelope rather than choosing a hollow-bar size only from catalog dimensions.
Length and Saw Allowance Matter Too
A tube or hollow-bar RFQ should include the required raw length and the tolerance on that cut.
If each blank will be faced to length, provide enough saw allowance for the operation without purchasing unnecessary material. If several different blanks come from the same stock length, provide the cut list so the supplier can evaluate yield.
For repeat production, quantity by release and annual usage can also help determine whether a stock-and-release approach is practical.
Grade and Specification Complete the Dimensional Request
Dimensions alone do not define the material.
Include the exact stainless grade, applicable specification, required MTR, and any NACE, DFARS, EN 10204, country-of-origin, or customer-specific conditions.
Scan Tube Steel lists mechanical tube/hollow bar in 304/304L and 316/316L, with sizes from 2-inch OD through 10-inch OD and ASTM A511, NACE, and DFARS capabilities.
A Hollow-Bar RFQ Template
A clean request can be written in one block:
Product: stainless mechanical tube / hollow bar.
Grade: 304L or 316L, as required.
Specification: ASTM A511 or project requirement.
Starting OD: state the required dimension.
Starting ID: state the required dimension.
Finished OD/ID: include when useful for sizing.
Cut length and tolerance: state the blank requirement.
Quantity: pieces, total footage, or production release quantity.
Documentation: MTR and any additional compliance requirement.
Delivery: destination and required date.
That information lets the supplier compare the finished geometry with available stock instead of sending several rounds of clarification questions.
For the next machined hollow component, send Scan Tube Steel the starting and finished dimensional envelope with the material and documentation requirements. The goal is to source a size that gives the machine shop enough cleanup allowance without paying to machine away unnecessary stainless.
Source References:
https://www.scantubesteel.com/stainless-steel/tube/mechanical/
