G'day William,
Once again, a good job on showing us how you to things, thanks...!
I don't recall in this issue as been discussed with you or not but, I wonder what you use to assist a smooth run on the runners underneath the sled..??
I mention this because, on many of the sleds I make through the years to work in various type machines, selecting first a hard/dense wood for those runners, is the best start as they don't wear as fast nor they move as much with the humidity (swelling) however, if the sled gets a lot of work, the runners do ware and therefore compromising the sleds accuracy, sometime a sled doesn't need to be accurate but, in your case, it does...!
So, and to try maintaining its accuracy and smooth operation, I rub some wax on the runners and a little on the top flat that frictions on the metal table, its does makes things very smooth, while tight in those groves for accuracy.
Another substitute is, some normal soap, any type of soap really, it wares a little faster but a simple thing to fix...!
I just though in mentioning this, in case some folks here, didn't know about it...!
Cheers
George