Gibs, and overhang would have been my first suggestion. The cross slide can be locked down, if not already. I almost always just leave the compound at whatever angle it was last used at. But some guys want it straight pointed at the chuck (which baffles me some, on a small lathe, as you already have the ways pointed that way) or, if you are working at sneaking up on very fine tolerances, putting it at around four degrees from parallel to the bore, gives you aprox one tenth of the infeed, that you put in to the compound screw. Do the trig and get back to me with exact numbers, if you need them. It's a pretty handy and simple, useful thing once in a while! Trig too!
A close look at the QCTP Holder, for a radius in the back of the slot, throwing things off would not hurt, though I kinda lean away from it as 'cause'. Easy to whistle a chamfer on the cutter holder, to account for it if there.
Counter to all the advice that one gets to have the tool on exact center height, I would suggest experimenting with a touch lower, or higher, to see if that changes the cut conditions. Taking a look at a chart that explains edge treatments, will explain better than I will, but if the edge treatment includes a purposely beveled or rounded off edge, plus a layer of plating, you may not be cutting with the part of the edge, that you thought you were!
A question I have used a lot when training apprentices... "Did you do the Math?" Typical carbide speeds are right around 4 times the revs that HSS cutters are happy at. Surface feet per minute!
Not all edge treatments on Carbide inserts will give good results on light cuts or at slower speeds. Feed rate depends on the radius of the cutter tip. A larger tip Rad., and you can use a larger feed rate. The trade is that a larger tip Rad., will also put more force in to effect, pushing against both the work, and the cross feed. Pretty much every tool catalog show the expected optimum cutting conditions that the maker expects to produce the best return on investment for a shop owner. Lots online too, if you know what insert you have. Also found on the insert pack, often, if you have that
My usual advise to a newb, is to stay away from 1018, as it takes speeds and feeds that seem pretty scary, to get really good results on a consistent basis, vs many (usually not very much!) more expensive metals you can buy. 12L14 is almost as nice as Brass to cut, 4140, 4340 and similar have a pretty good machinability rating too. 303 for stainless. If buying aluminum for turning, I REALLY like dealing with 7075 in the higher -T numbers, even with it's ability to corrode in some circumstances, while you watch!
6061 is cheap and available get -T4 or -T6, 2024 if you can find it, again, at he higher temper numbers.
Chatter is akin to music. It's a series of vibrations, that complement the ones before and after. Some times, you happen across the cut conditions that make that funky music, and the way out, is to change the cut conditions.
