Do you know which version?
I think many brands of biscuit jointers (possibly including Makita's one) went through a cost reduction redesign recently, when the patents expired and it became a free-for-all. The average price has certainly fallen rather a lot.
I have the earlier Makita 3901 model and its plunge has so far been excellent (one of the reasons I bought it, in fact). It also has a different fence design to the more recent version. If his is a newer one, I would think about contacting Makita. I'd also check round it carefully, in case it has fallen off the bench at some point and landed badly. Any dented corners, chipped paint or cracks in castings or plastic?
On mine, the castings are aluminium (not white metal), and the machining is high quality. I have a feeling the plunge bars have bushings too. If it did fall off the bench, the main casting (to which the plunge bars attach) would probably bend rather than shatter, so it could be significantly out of true and I might not notice initially.
And finally, although you probably do need a lubricant on the bars, exactly what rather depends on what materials are being used. I have a similar problem with my old 1/4" Bosch routers (POF 500A and 600A). The bars are mild steel and the sliders are white metal: they jam up when you try to plunge them. I even bought a new base and saddle set (basically everything bar the springs and bolts), but that did it too. There is probably a lubricant that works well, but I haven't yet found out what!
E.
