Tool geometry for gunmetal boring bar

I am in the process of making a between centres boring bar and was wondering what kind of geometry the HSS cutting tool will need for a GM casting (5" gauge loco cyl).

I know that for brass you don't need any relief angle on the top face for plain turning so I was hoping if someone could tell me if this is wise for a boring tool intended for GM, or whether it is wise to have some form of relief on this face?

Thanks

The angle on the top face is more usually termed as 'rake', sometimes indiscriminately as 'top rake', or more carefully defined as 'side rake' or 'back rake' depending on which face is raked away from the feed face.

There's generally no need to be too exact with rake angles - something between 5 and 10 degrees works for most materials.

Although received wisdom is (or was in the 1970s when I was learning) that discontinuous-chip higher zinc brass like CZ121 would normally use zero degrees rake, my own belief is that using that might increase the risk of chatter and squeal as it increases tool loading and can encourage an intermittent elastic oscillation.

For that reason, plus the fact that gunmetal's often leaded and therefore a little more ductile I tend to use the same few degrees as I would for most other materials.

I don't think any of this is a really exact science unless you're in a tight cost/capacity/productivity calculation. The sharpness, rigidity and clearances of the tool are generally more important than the rake.

This just what I think works well on what I do. Others may think otherwise.

Edited By Mick B1 on 11/01/2023 13:13:38

Providing the tool does not rub and is nice and sharp I don’t really think the angles are that critical, somewhere between 5 to 10 degrees will do the job.

I agree 10 degree top rake,plenty of front clearance to avoid the tool rubbing on the bore,as its cast bronze there can be hard surface inclusions which will take the edge off a nice sharp hss toolbit,so take a roughing cut through the bore with either a hss tool or a brazed on carbide tip, and also rough face each end of the bore to remove the cast surface, cutting speed 100 foot per min, cutting between centres is not as rigid as boring with work held in the chuck,so the larger the tool tip radius will increase the risk of tool chatter,a sharp tip rad will incurr less chatter,too but can result in a rougher finish,though the finish is more dependant on how good the tool tip is honed with a medium India oilstone.to get a good finish on gunmetal te tool must be sharp.

+1 on about 10 degrees side rake and plenty of front clearance to allow for the curvature of the 'ole. Plus, no back rake.