Kilayim, Chapter 9, Mishnah 6. This mishnah is very similar to the previous one, which dealt with clothing merchants. Here the mishnah deals with craftsmen who sew garments that may contain shaatnez, at a time when they are sewing for non-Jews.
The words of the mishnah:
"Tofrei kesus tofrin k'darkan" - those who sew garments may sew in their usual manner, even though that manner includes resting the garment on their knees and the like.
"U'vilvad shelo yiskaven bachamah mipnei hachamah u'vageshamim mipnei hageshamim" - provided that his intention is not, in the heat of the sun, to shield himself from the sun, and in the rainy season, to shield himself from the rain.
The basis of the leniency:
Everything depends on the principle of 'eino miskaven': the craftsman's intention is to make it comfortable for himself to sew the garment, not to wear it. But if his intention is to wear the garment, that is certainly a problem.
And understandably, the same point raised by Rabbi Akiva Eiger regarding the previous mishnah applies here as well: here too we are speaking of a situation in which there is no 'p'sik reisha', meaning that it is not certain that he will ultimately come to wear the garment in the course of his work.
The mishnah concludes in the same way as the previous mishnah: "V'hatznuim tofrin ba'aretz" - those who are scrupulous in the fulfillment of the mitzvos would take care to sew with the garment resting on the ground rather than on their knees, in order to make certain that they would not come to the prohibition of shaatnez.
In summary: those who sew garments may sew in their usual manner, provided that their intention is not to benefit from the garment as protection from the sun or the rain, and provided that it is not a 'p'sik reisha' that they will come to wear it. The scrupulous, however, would sew with the garment resting on the ground, in order to distance themselves completely from the prohibition.