Finer bore finish needed

[CarbideBob]

I have heard guys get better finishes with HSS steel tooling when it comes to softer materials.

Why is this?

 

[WakelessFoil]

Why is this?

They say you can get HSS sharper than a pressed carbide insert. I've seen guys grind old drill bits into makeshift boring bars and get a pretty nice finish on metals like aluminum or brass.

 

[Orange Vise]

Problem is surface footage.

0.25" is pretty small. If your lathe spindle has a 2Kish max rpm, that's only 130 SFM. I'd want to be at 500+ to finish brass, i.e. 10K rpm or more. Also, the spring pass isn't helping.

I think one of the abrasive solutions is your best bet.

 

[Orange Vise]

They say you can get HSS sharper than a pressed carbide insert. I've seen guys grind old drill bits into makeshift boring bars and get a pretty nice finish on metals like aluminum or brass.

That wouldn't really apply here because nothing in the 1/4" ID range will use a carbide insert. These are solid ground bars.

A ground carbide bar will be just as sharp as a ground HSS bar. They're both made the same way, just using different types of grinding wheels (diamond vs CBN), but typically with the same grit wheels, 240-320. The carbide bar will be 3X more rigid, all else equal, although that might be unnecessary here since the bore's length:diameter ratio is 1:1.

As for carbide inserts, there's also a big difference between positive ground inserts like a CCGT/CPGT vs an unground CNMG. Not only are the ground inserts obviously sharper from the grind itself (especially the nonferrous inserts), but they're only PVD coated (1-3 microns thick) or uncoated, whereas the typical unground inserts are CVD coated (5-20 microns thick) with an additional PVD layer (another 1-3 microns) on top. CVD adds significant tool life, at the expense of sharpness because all corners are effectively rounded over.

 

[bikemutt]

Thanks for the follow up replies.

The second boring bar actually did a pretty good job at the slowest change gear feed rate I could get.

I figured maybe I could take it into the end zone by finishing with a roller burnishing tool. I found a used Cogsdill in the correct diameter range, replaced the mandrel and rollers since they were worn out. The incremental improvement in finish was not impressive.

That said, I dug up a formula in an old thread for the predicted RMS surface finish given nose radius and feed rate. Further research led me to a similar formula predicting Ra given nose radius and feed rate. Based on the minimum feed rate I can achieve, and that I used, the predicated Ra in micro-inches is 47 which is less than ideal for roller burnishing; both Cogsdill and Elliott indicate 80-120 Ra for an ideal pre-finish. Burnishing needs enough hills and valleys to be successful. Before I write-off roller burnishing, I'm going to try for a 100 Ra pre-finish, see if that gets me there.

Beyond that, I'm also adding an independent power feed so I have more control over speed and feed. There may be a combination which lets the boring bar do the job in a single step.