Two cobots, the same 900 mm arm, twice the difference in precision — and the reason is not the reach

Last week the Universal Robots range gave us a tidy rule: pose repeatability degrades as the arm gets longer, almost in proportion. Put the Techman Robot catalogue next to it and the rule falls apart. Techman's TM5-900 and its TM16 both work within roughly a 900 mm radius. The TM5-900 is quoted at +/-0.05 mm repeatability, the TM16 at +/-0.1 mm. Same reach, half the precision. The number that changed is the payload: 4 kg against 16 kg. And once a Techman arm leaves the TM5 series, the tolerance stops moving at all — the 900 mm TM16, the 1,100 mm TM14, the 1,300 mm TM12 and the 1,300 mm TM20 are every one of them rated at +/-0.1 mm, whatever their length. So precision at this manufacturer is a property of the platform, not of the geometry. There are two families: a light one that holds 50 micrometres and a heavy one that holds 100, and choosing a shorter model inside the heavy family buys nothing in accuracy. That is a different engineering answer to the same physics, and it changes how a cell is specified. At Universal Robots a buyer trades reach for precision. At Techman a buyer trades payload class for precision, and the reach comes free. The pattern repeats inside the heavy family in the way speed is spent. The TM12 and the TM20 share a 1,300 mm arm and weigh 32.8 and 33 kg — two hundred grammes apart — but the TM20 carries eight kilogrammes more. It pays for them with time rather than metal: its first two joints turn at 90 degrees per second where the TM12's run at 120, and average tool speed falls from 1.3 to 1.1 metres per second. The TM12 is, quietly, the fastest arm the company sells, because a long arm converts a given joint rate into more travel at the tool. One more number is worth pulling out, because it runs against the industry grain. Every arm in this range is rated for an ISO Class 3 cleanroom, a stricter class than the Class 5 that only a single Universal Robots model reaches and the Class 6 available on its larger ones. For semiconductor and optics work — where the whole question is whether the machine may stand in the clean space at all — that is the line in the specification that decides the purchase, and here it does not depend on which model is chosen. A note on where the figures come from. Techman publishes no specification table on its own product pages, and its printed catalogue carries the numbers as an image with no text layer. The full data set is documented instead by Omron, which has resold these arms under its own brand since the two firms allied in 2018; its comparison table was last revised in October 2024. Where the two sources disagree — Omron rounds the TM5 reaches to 700 and 900 mm, Techman states 946 mm for the long variant — the manufacturer's own figure stands.