# @stem_antics on Instagram

- **Type:** Video
- **Original URL:** https://www.instagram.com/p/DcBcvvvRRc4
- **Gondola URL:** https://gondola.cc/posts/69054405-stem-antics-instagram
- **Thumbnail:** https://img.gondola.cc/tr:w-,h-,fo-auto/postThumbnails/4dadf94502.jpg
- **Posted:** 2026-08-14T13:45:02.000+00:00
- **Account Owner:** Stem Antics (@stem_antics) — https://gondola.cc/stem_antics

## Caption

Cutting a round tube so it wraps perfectly around another round tube is one of the least intuitive jobs in fabrication. The joint looks trivial. The geometry is not.

Where two cylinders intersect, the line of contact is a saddle curve, known in the trade as a fishmouth or cope. Only one case stays simple: equal diameters meeting at any angle produce a true ellipse, which is why an elliptical template works on same-size tube and fails on everything else. Change the diameter ratio and the curve becomes transcendental, with no tidy closed-form flat pattern. Unroll it and you get an asymmetric wave plotted point by point.

Handrail work lives inside this problem. UK guidance converges on circular handrail of roughly 32-50mm outside diameter, and a continuous rail demands a joint at every post, return and transition. Every one of those is a cope.

Machines attack it four ways:

- Hole saw notchers clamp the tube in a swivelling vice and drive a saw of the mating diameter straight through. Cheap, fast, restricted to circular profiles and usually 0-60 degrees of mitre.
- Abrasive belt notchers press the tube against a contact wheel ground to the mating radius. Superb finish, no chip evacuation problem, kinder to thin wall.
- End mill notchers cut with a rotating cutter on a tilting head and hold real tolerance on structural sections.
- Fibre laser tube cutters treat tube rotation as an extra axis, driving the head along the true intersection curve and adding weld bevel in the same pass.

That last method earns its keep on thin wall. At the horns of a fishmouth the cut runs almost tangent to the surface, leaving a feather edge that burns through under TIG before the root has fused. Good programs blunt those horns deliberately, trading a fraction of a millimetre of fit for a weld that survives.

Geometry decides the joint long before the arc is struck.

#stemantics #fabrication #tubenotching #manufacturing #weldingengineering

## Stats

- **Views:** 28,868
- **Likes:** 144
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- **Comments:** 2

## Tags

tubenotching, manufacturing, fabrication, weldingengineering, stemantics

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