When I break a warp thread during weaving, it is always the thread on the far right (no matter if it is a floating selvedge or the first thread in the draft). Although the placement of the broken thread makes it relatively easy to repair, it would be better if I could avoid having to repair it at all. I weave on an 8-shaft jack loom. I am right-handed and have tried to be careful not to put more force into my shuttle toss from that side, and I’ve paid special attention to keeping the tension on all the threads in the warp. Do you have any ideas why this may happen and how I can resolve it? Thank you!
Essentially, you are describing two factors to address: one is the breaking thread, the other is the fact that it is only on one side. I'll start with the second one. Many times weavers will tell me that the selvedge on one side is much better than the selvedge on the other. There are two usual reasons for that. One is that however the shuttle is being inserted, there is more drag on one edge than on the other. The other, which I've found to be more likely since they have usually made an effort to insert the shuttle the same way on both sides, is the twist of the yarn. Yarn twist causes the turning weft thread to snuggle into the edge warp thread on one side where the twists are in the same direction, but to be pushed away slightly on the other side where the twists oppose each other. This, although by itself it shouldn't cause breakage, may be related to the reason your problem is only on one side.
Any breakage of an edge thread, however, is due to too much draw-in, and too much draw-in is always a result of not having enough weft slack in the shed. My guess is, especially since you mention "toss the shuttle," is that you are sending the shuttle more or less straight across the shed, which restricts the amount of weft inserted. So then, when the beater pushes the thread to the fell, there isn't enough of it to take the over-and-under path it should take as it interlaces with the warp threads. The breakage on one side could be related to yarn twist in that somehow the twist on the yarn is slightly untwisting on that side and is more likely to fray apart. This can be a factor especially if you don't beam the floating selvedge.
A temple can always help prevent draw-in, as I've mentioned in previous posts, but it will NOT prevent breakage of selvedge threads if there is not enough weft angle in the shed. If you use a temple, always set it at the width of the warp in the reed, insert it in the cloth RIGHT next to the last (selvedge) thread on each side (if you set the teeth a few warp threads inside the selvedge it will push the remaining threads, away leaving a gap), allow a weft angle of thirty degrees, close the shed to prevent the weft angle from flattening (unless the yarns make this unwise), and beat.