Acrylic tube dimensions and tolerances — why the numbers can be right and the tubes still will not fit
A tube with a 20 mm outside diameter has to go inside another tube with a 20 mm inside diameter. The two numbers on the drawing match. That does not mean the tubes will slide together smoothly.
This is easy to overlook in a tube enquiry: the customer has specified the tube dimensions, but has left no clearance between the two parts.
To establish whether acrylic tubes will fit together, we need to consider the clearance, out-of-roundness and the dimensions that actually matter in use, as well as the outside diameter, inside diameter and wall thickness.
Outside and inside diameter differ by two wall thicknesses
For round tube, using nominal dimensions:
In this article, 60 × 5 mm means a 60 mm outside diameter and a 5 mm wall thickness. The nominal inside diameter is therefore:
60 − 2 × 5 = 50 mm.
There are two easy mistakes here. One is subtracting the wall thickness only once. The other is calculating a 50 mm inside diameter and assuming that it will smoothly accommodate a tube with the same 50 mm outside diameter.
The second calculation gets the bore size right, but leaves no clearance for the fit.
A smooth sliding fit needs clearance
From our experience with this kind of tube-in-tube fit, when the aim is reasonably smooth insertion, we generally recommend starting with at least 0.5 mm clearance on each side, then checking the actual fit.
“On each side” means the gap between the outside surface of the inner tube and the inside surface of the outer tube. For nominally round, concentric tubes:
So 0.5 mm on each side means a 1 mm difference between the diameters, not a 0.5 mm difference.
This figure is our experience-based recommendation for smooth insertion; it cannot by itself guarantee a smooth fit for every pair of tubes. Extruded plastic tube has some ovality: the actual cross-section is not a perfect circle. The clearance calculated on paper should not be assumed to exist in every direction on the real parts.
The dimensional calculation is the starting point. An actual insertion check is still needed.
Mark the dimensions that actually control the fit
We also receive drawings with a very tight tolerance on every feature, but no indication of which dimensions actually matter to the assembly.
Dimensions vary during plastic extrusion. In volume production, treating every feature as an equally tight control priority leaves little practical room for adjustment in the design and process.
We ask customers to distinguish between:
- Critical dimensions: a change directly affects assembly or use, so stable control of these dimensions takes priority.
- Non-critical dimensions: some variation can be allowed while still meeting the requirements of the application.
With that distinction clear, we can review the actual geometry and allow for acceptable variation in non-critical features, giving priority to keeping the critical dimensions stable.
The permitted range for non-critical dimensions also needs to be agreed. The point is to match the tolerance requirements to what the part does, rather than putting the same tight tolerance on every number in the drawing.
To confirm an assembly fit, we need the mating part and its drawing
As we explained in our tube enquiry checklist, the basic specification and application are enough to start a conversation about a standard, non-custom product. Not having a drawing should not prevent that first enquiry.
When an order has specific assembly requirements, further confirmation is needed. We generally ask for the physical mating part and its corresponding drawing.
The part is used for a fit check. The drawing lets us confirm that the mating part itself matches the customer's design dimensions. Together, they establish what we are checking the fit against.
We also carry out fit checks at intervals on the production line to check whether the tube being produced meets the assembly requirements.
A real example: changing the outer tube from 60 × 5 to 60 × 4
A customer planned to insert a tube with a 50 mm outside diameter into an outer tube specified as 60 × 5 mm.
On nominal dimensions, the outer tube also had a 50 mm inside diameter, leaving no clearance for the inner tube. We spotted this and discussed it with the customer, who changed the outer tube to 60 × 4 mm.
| Dimension | Original design | Revised design |
|---|---|---|
| Inner tube outside diameter | 50 mm | 50 mm |
| Outer tube outside diameter | 60 mm | 60 mm |
| Outer tube wall thickness | 5 mm | 4 mm |
| Calculated outer tube inside diameter | 50 mm | 52 mm |
| Clearance on each side, assuming round, concentric tubes | 0 mm | 1 mm |
Reducing the outer tube's wall thickness by 1 mm increased its inside diameter by 2 mm. This provided a nominal clearance of 1 mm on each side between the tubes.
After the change, we received no reports from the customer of a problem with this fit.
The example shows why the two mating parts should be considered together when choosing a tube specification. Looking at each tube's dimensions in isolation makes it easy to overlook whether there is room between them for assembly.
Send the dimensions and the fit requirements together
If the tube has to fit another component, include the following with your enquiry:
- What the tube will do, and how the two parts are assembled.
- The outside diameter, inside diameter or wall thickness of each part, with the meaning and units of each dimension clearly labelled.
- Which dimensions are critical, and the permitted variation for each feature.
- The physical mating part and its corresponding drawing for confirmation.
Send us the section and the fit requirements
If you are not yet sure which dimensions need priority, start by telling us how the parts are assembled. We can discuss the tube specification in the context of the actual fit, and identify where it may bind and where clearance is needed before production.