Guides / Materials

Acetal Rod, Plate and Tube: Typical Stock Sizes, Tolerances and How to Pick

October 9, 2026

Almost every machined acetal part starts as a standard stock shape: a rod, a sheet or a tube. The stock size decides how much material is removed, how much waste there is and how long the job takes to start. A design that fits a common stock size is cheaper and faster than one that needs a special order.

How acetal stock is made

Most acetal rod, sheet and tube is made by extrusion. Molten resin is pushed through a die and cooled, and the cooling rate sets the structure. Large diameters and thick sections cool slowly in the middle, so they have more residual stress and sometimes a small void along the centre line. Cast and compression-moulded forms exist for very large sizes. The material properties guide explains how the grades differ.

Stock comes as homopolymer or copolymer, and in natural, black and a few other colours. See the copolymer versus homopolymer guide for how the choice affects machining and the final part.

Typical size ranges

Form	Typical range stocked	Common form
Rod	About 3 to 150 mm diameter	1 m or 3 m lengths
Sheet and plate	About 1 to 100 mm thick	1000 x 2000 mm and similar
Tube	About 12 to 150 mm outside diameter	Limited wall choices
Square and flat bar	Several widths and thicknesses	Cut from sheet or extruded

These ranges are typical and each supplier stocks a subset. Ranges overlap in the middle, and extremes at both ends take longer to get. Imperial sizes are stocked widely in North America, and metric in Europe and Asia, so a drawing in one system may be cheaper to source in the other if the part tolerates a small size change.

Tolerances on stock

Extruded stock is held to commercial tolerances, much looser than a machined part. The values depend on the maker and the size, and these are typical patterns:

Item	Typical pattern
Rod diameter	Oversize by a fraction of a millimetre up to a few millimetres, with a wider spread on larger diameters
Rod roundness	Slightly out of round, more on large diameters
Sheet thickness	A percentage of nominal, often wider on thin sheet in relative terms
Sheet flatness	Some bow, more on large, thin sheets
Tube wall	Wall thickness and concentricity vary

Precision-ground or centreless-ground rod is held much tighter and is used for small, automated turning, such as on a Swiss lathe. See the Swiss turning guide. Ask the supplier for the tolerance on the size you need rather than relying on a table.

What oversize means for machining

Because stock is oversize, there is always material to remove. For a turned part, a rod 1 to 3 mm over the finished diameter gives enough to clean up the skin. For a milled part, a sheet a few millimetres thicker than the finished thickness lets both faces be machined flat.

Design around this:

• Pick the nearest stock size above the finished diameter. A 40 mm part from 45 mm rod wastes more than one from 42 mm rod.
• Check availability. Common sizes are stocked at many shops. An unusual size can double the wait.
• Plan for clean-up on both faces of sheet. If you want a 10 mm plate flat on both sides, start from 12 mm.
• Use the skin. If the stock surface is acceptable, leaving it on saves time, but only where tolerance and finish allow.

The oversize adds to the cost and the lead time. See the cost guide and the lead time guide.

Residual stress and large sections

Thick plate and large rod carry stress that is released as material is cut away. A part machined from the centre of a thick plate behaves differently from one cut from the edge. The bigger the section and the thinner the finished part, the more it moves. The annealing guide covers stress relief, and the flatness guide shows how to hold a plate that wants to bow.

For large rods, shops sometimes use a hollow starting shape or cut the part in two stages to relieve stress. For critical parts, ask whether the stock is annealed from the maker or needs an extra step.

Void along the centre of rod

Large extruded rod can have a tiny pore or hollow along the centre line, from shrinkage as the core cools. It is usually harmless for non-critical parts but can show up when a bore is cut through the centre or when the part is a pressure or sealing component. Natural rod shows it more easily when cut. For critical parts, ask the supplier whether a stock form with a controlled core is offered, and cut a test slice from the bar before committing.

Tube versus bored rod

Approach	Strengths	Weaknesses
Buy tube	Less waste, faster	Limited sizes, concentricity varies
Bore rod	Both diameters controlled, wide size choice	More material removed, more machine time
Bore thick-wall tube	Compromise	Availability

For bushings and sleeves, boring rod is common because the inside and the outside both need to be concentric and the wall controlled. A tube with a close wall can save a lot of material if one is stocked in the right size.

Designing to stock

Small changes to a drawing can open up the stock list:

• Move a diameter to the nearest common size when the function allows.
• Allow the shop to choose between homopolymer and copolymer.
• Accept natural or black.
• State a maximum overall length, not an exact bar length, so the shop can cut from standard lengths.
• Say if substitution with imperial or metric stock is acceptable.

For the full list of items to include in a request, see the RFQ checklist.

Ordering stock for a first run

Stock is bought in lengths and sheets, so waste is part of every job. For a first run, a shop usually works out how many parts fit in a bar or a sheet, and orders accordingly. A few points help the estimate:

Item	What to consider
Bar length	Parts per bar after allowing for parting and a gripped end, usually 20 to 50 mm of waste per bar
Sheet nesting	Part outlines plus saw or cutter width and clean-up allowance
Spare material	A small allowance for setup scrap and a failed first part
Minimum order	Some suppliers sell only full bars or sheets, which affects small orders
Offcuts	Useful for later small parts, if stored with the lot number

For a prototype order of a few parts, stock cost can exceed machining cost, because a full bar or a cut piece is bought for a handful of parts. Asking whether the shop already holds the size can cut cost and lead time. If a part can be made from a size already on the shelf, say so in the request. The cost guide shows how material, setup and machine time share a quote.

Checklist before you request a quote

• Give the finished size and say how much oversize is acceptable.
• State the grade, colour and any substitution allowed.
• Note a stress-critical or void-critical requirement.
• Say if ground rod is needed.
• State the quantity so the shop can decide on whole lengths or cut pieces.

Send the drawing and we match it with up to three shops that machine acetal and reply within 24 business hours.

Frequently asked questions

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