I would try using a small drill to loosen the chips in the coolant hole. J ust drill a bit then shake out the chips and repeat until clear.
Carpenter Tech has an online book on machining stainless steel. I would ch eck there for recomendations on carbide grades. I suspect it will be hard er to find inserts with a lot of clearance. Carpenter tech book also has r ecommendations on how much relief cuttinc tools should have. Stainless req uires more clearance to prevent cold working.
I would try using a small drill to loosen the chips in the coolant hole. Just drill a bit then shake out the chips and repeat until clear.
Carpenter Tech has an online book on machining stainless steel. I would check there for recomendations on carbide grades. I suspect it will be harder to find inserts with a lot of clearance. Carpenter tech book also has recommendations on how much relief cuttinc tools should have. Stainless requires more clearance to prevent cold working.
As I understand it the issues I have to address are the relatively low speed, power and rigidity of my 1965 leather-belt-drive lathe. The C5 inserts I bought last time chip easily. Is C2 a better choice?
I don't run fast or hard enough to need coolant. For boring I squirt a little oil in the hole with a needle oiler between passes, or cut dry.
I generally use what I have found. My understanding is that C2 and C5 are good for cast iron as they are very hard. And C6 is not as hard but is stronger. So I think C6 would be your best choice. But pay attention to everyone else. I am less than an expert.
'Sorry I missed what it is Jim wants to use carbide for. Gunner's response is both sensible and accurate, although a good HSS will outperform Stellite on a small lathe, and Stellite isn't worth looking for. It isn't as hard as the high-cobalt HSS grades. Its big virtue, before carbide became reasonably priced, was its heat tolerance.
My 1945 SB 10L does not like carbide, either. The bed just isn't rigid enough, even though the machine has been lightly used and is very tight.. I use some, mostly with abrasive materials like fiberglass. But it doesn't work well with steel.
HSS will handle anything I'm likely to run on this lathe. It does great on 303 stainless, if that's what Jim was asking about.
I use HSS almost exclusively, except for special shapes that are hard to find or make but are common in carbide.
The impertus for the boring bar hunt was a cup fixture I was making to hold 1/4-20 screws threads-out in a collet, 14 pieces of 1/4-20 x
new, larger leveling feet for my sawmill. The oak logs I'm cutting into beams now are twice the weight I designed for.
My lathe rarely drills straight due to tailstock abuse, despite starting the hole with a center drill in this holder:
so I take a boring bar pass through a hole to recenter it before drilling to final size. The only small enough boring tool I could find in the drawer was a tiny second-hand HSS internal threading bit. A Micro 100 bit that bores small enough costs ~$25, so first I tried the second-hand tool store and found an Iscar equivalent and the larger one I mentioned.
The lathe came from a trade school and the tailstock spindle was oval from being used as an anvil horn. The dealer swapped in another spindle from his junk drawer, and I cleaned it up very lightly with an MT2 reamer until it would grip a drill chuck arbor.
The tailstock assembly may not have been the lathe's original one since its clamping plate under the bed was crudely shop made as though the original had been lost or broken, and I had to separate the base and shim it up slightly to center height. I decided to just use it and take note of problems before making irreversible changes. Most of the small deep holes I drill are for non-critical axle grease passages or retaining screws.
Supposedly the trade school instructor used a little of his yearly maintenance budget to buy school-color light blue paint to hide damage and spent the rest on whiskey. I stripped and refinished most of it and bought or made replacements for missing/broken pieces.
Parts from scrapped SB Heavy Tens do appear occasionally. I bought a partly stripped headstock to cobble up a temporary large-diameter wheel lathe. The only critical part it was missing was the back gear lever + cam bushing, which I copied from mine. The taper pin was positioned differently, clearly not drilled on the same jig as mine.
It's still a nice hobbyist lathe as long as I'm not on the clock or making parts to other people's specs. I can adjust my own designs to its strengths and weaknesses.
Those are good generic recommendations, but, unfortunately, there is a huge variation in the performance of carbides, grade-to-grade, among different manufacturers. Within the grade designations, whether the American "C" grades or the ISO "P" grades, there is a lot of latitude allowed to steer a grade toward one type of application or another -- wear resistance versus interrupted cuts, for example -- even within a single grade designation.
When you're buying generic carbides, especially in odd lots and in bulk, there's no way to tell what you really have, in terms of performance, until you try them. If you buy graded carbide from a major manufacturer, you still find big variations in performance from one manufacturer to another, because there are many variables in manufacturing carbide inserts.
When I covered tooling for _Machining_ magazine, I'd often ask the engineers at the big tooling companies (Iscar, Sandvik, Kennametal, Hertel, etc.) what they'd recommend for use on my old South Bend. The answers generally were their equivalent of C6 (P30), or something like that. I really wanted to try the micrograin and superfine grades, because they'll take a much sharper edge and they're used a lot at positive rake angles. My thought was that they must be tough to handle positive rake. But that turned out to be wrong. They're very strong, but not tough. So I never got a chance to try them in steel. I do have a couple I got from Sandvik, that I use when I'm cutting fiberglass ferrules for fishing rods. (I also have a polycrystalline diamond tool that I use for that work when I can. It's a plain triangle.)
Anyway, if you really want to use carbide on an old lathe that lacks either the rigidity or the tightness of a modern lathe, you really ought to call and ask the techies at one of the major tooling companies. If you don't want to pay those prices, then try to find some quality C6 at a discount, and cross your fingers.