Brass Round Bar Sizes: Choosing Diameter and Length for a Lathe Job
Choose brass round bar diameter and length using imperial-to-metric conversions and a worked example that accounts for facing, parting and workholding.
Choosing a brass round bar size means deciding on both diameter and usable length. The finished part’s dimensions are only the start: facing, parting and workholding also consume stock. A little planning before ordering can prevent a seemingly suitable piece becoming too short halfway through the job.
This article explains brass rod sizes, imperial-to-metric conversions and a practical length calculation for a small lathe project. Always check the actual product specification and your machining setup before committing to a size.
Brass rod and round bar: compare the dimensions
Suppliers often use “brass rod” and “brass round bar” for circular stock. Those names do not tell you the tolerance, grade or surface condition. Compare the listed diameter and specification, rather than assuming the word rod describes a precision finished shaft.
For turning, the starting diameter needs to accommodate the largest finished feature and whatever machining allowance the process requires. If the finished outside diameter is close to the nominal bar size, check the stock tolerance before expecting every surface to clean up.
Imperial brass round bar sizes in millimetres
One inch is exactly 25.4 mm. The table gives mathematical conversions for several familiar sizes. A supplier may round its display values: 3/8 inch, for example, may appear as 9.5 mm rather than 9.525 mm.
| Imperial diameter | Exact conversion |
|---|---|
| 1/4 inch | 6.35 mm |
| 3/8 inch | 9.525 mm |
| 1/2 inch | 12.7 mm |
| 5/8 inch | 15.875 mm |
| 3/4 inch | 19.05 mm |
| 1 inch | 25.4 mm |
Metric and imperial sizes that look close are not identical. A 6 mm bar is not 1/4 inch, and a 10 mm bar is not 3/8 inch. Keep the drawing’s units visible in your notes and convert deliberately.
How much length does a lathe job need?
Start with the total finished length of the parts, then account for the operations that remove material. Also allow stock for the planned gripping arrangement. Useful questions include:
- How much will be removed when facing the ends?
- How wide is the parting tool’s cut?
- Will you saw individual blanks before machining?
- How much stock must remain in the chuck or collet?
- Will the part need a second setup to finish the reverse face?
There is no single allowance that suits every lathe or component. Plan the sequence with your actual tools and workholding.
A worked stock-length example
Suppose you need four spacers, each finishing at 20 mm long. For this planning example, assume 1 mm of extra length per spacer for final facing, a 2 mm parting cut for each spacer and a separate 25 mm gripping remainder.
Four parts × (20 + 1 + 2 mm) + 25 mm = 117 mm of starting stock.
These are illustrative allowances, not instructions for how far to grip any particular bar. Check them against your machine, tool width and proposed sequence. A 100 mm piece would not cover this example; a longer piece would be needed. Before using a longer bar, consider its support and protrusion rather than assuming extra length can simply hang out of the machine.
If you change to sawn blanks, redo the calculation using saw kerf and the facing required on each blank. The calculation changes with the process.
Short lengths or a longer piece?
Short lengths can suit a one-off model engineering job and keep unused stock manageable. For repeated components, compare how many finished parts each option yields after cutting losses. Several short bars can create more unusable remainders than one suitably managed longer piece.
Store the remaining stock with its grade and nominal size. An unlabelled offcut is harder to use confidently on the next drawing, even if it still measures close to the diameter you want.
Ordering CZ121 brass round bar
Browse our CZ121 brass round bar range and select the diameter and available length required. Workshop Standard’s material length limit is 1000 mm. Each quantity represents one bar of the selected length; check the listing’s current options.
For example, our 1/2-inch (12.7 mm) brass round bar has a stated diameter that can be compared directly with your drawing. If you are still choosing the alloy, read CZ121 vs CZ108 brass before ordering.