Ok so looking at what's available for basic boring bars, and finding the sets of small bars with carbide tips. Is a carbide tool a good choice for an Atlas lathe with QCTP? I keep reading that carbide is not good on basic hobby lathes. ??
I assume you're referring to those 9 piece Chinese brazed carbide boring bar sets you can get for about $10.00 or so. These will work fine for your job since you're not going very deep. Like all brazed carbide tools, they are not very sharp out of the box so use your diamond stone to hone it to a sharp edge. I like a very tiny nose radius on mine to improve the finish. Most of these tools have a zero-lead so they are not ideal for closed bottom bores but for a counterbore, they will be fine.
Boring is not the same as external turning. The cutting forces are the same but in the case of boring, the bar is what deflects the most. Accordingly, use the shortest bar with the biggest diameter that will fit in the bore and you should be fine.
You will find that a small lathe will bore just as accurately as a larger lathe. The issue you face is cutting speed. Unlike a carbide insert, the cutting speeds for a brazed carbide tool is not published so it comes down to experience. I actually used these bars on my Sherline lathe to bore all sorts of stuff and while they are not the most accurate bars, they get the job done. I run them at the top speed my lathe will achieve, about 2800 rpm and have had good results. I rough at about a 0.020" deep depth of cut and feed to get as curled chip as I can get. I finish with a 0.005" depth of cut and this works well for me.
What you need to do is take a spare work piece and figure out what the change will be in inside diameter for a given depth of cut. The most important sample cut will be a finish cut. Say you dialed in a 0.005" depth of cut and it produced a 0.009" change in ID. Knowing this, you can rough until you hit 0.018" ID and then dial in your 0.005" cut and measure again. If it consistently produced a 0.009" change in ID, dial in another 0.005" and you should come in on size. If it didn't give you what you expected then change your final depth of cut.
You would do the above regardless of the kind of bar you use - inserts, solid bars or brazed bars. The trick is to know how your bar cuts for the given set up you have on that job. Remember that coolant makes a big difference in the chip form, finish and how the bar cuts. A bar will cut one ID without coolant and another ID with coolant so experiment and try. Also, if the bore has to be accurate, let the part cool to ambient temp before taking your measurement before that final pass or two.
