For the people that are using inserted tooling for turning, what cutting speeds are you using for aluminum and mild steel?
Looking at the manufacturers guides it seems like they recommend much higher cutting speeds than most manual lathes can muster. I assume they are for CNC applications on massive, very stiff machines.
I'm looking for a guideline I can use in a home shop on a midsized lathe (PM1340-GT).
Another question, if the above lathe drops more than 50 RPM when in the cut am I working it too hard?
Dan, this is a great topic with complicated answers. I'm no carbide expert but when I looked into inserted carbide cutters I realized that there is much more to the subject than I thought. Looking at my notes, here is what I considered at the time:
- The lathe - rigidity of the lathe, tool holding system, horsepower, speeds available, feed speeds and degree of control.
- The insert - geometry, grade/coating, nose radius, chip breaker, rake (neg, pos).
- Material - each has its own cutting speed. Each insert manufacturer will have their own SFM recommendations for each material.
- The cut - continuous, interrupted, coolant used or not
I'm sure the list is not complete but its the meat. Everything on the list is inter-related, and that's the thing many of us don't think about much.
Most of us own smaller, less rigid lathes but some, like you, own bigger, more rigid and powerful lathes BUT almost none of us own lathes that are FAST enough to use the insert as intended. Cutting speeds for a common insert like a CCMT insert in mild steel can vary from 700 to 1700 SFM, depending on the grade and coating of the insert. Let's say you have a 1.5" OD work piece of 1018 mild steel and you're using a PVD coated insert that you're feeding at 0.004 ipr. Cutting speed is about 1300 SFM so rpm will be around 3300 rpm. For aluminum with a CCGT insert, you're looking at about 1900 SFM for an rpm of about 4800. Depending on the chipbreaker and the depth of cut you're taking, cutting loads will vary but if the motor is powerful enough and the lathe is rigid enough then the insert will cut as intended.
I think a lot of us know that an inserted carbide tip will cut at a slower speed but few will cut with the finish and tool life they should. Some inserts have very sharp edges and some have edges that are formed/pressed with a beveled edge. It really depends on which insert you're using.
The other factor that comes into play is the nose radius of the cutter because this has a direct impact on cutting forces and finishes. To reduce chatter, you really have to at least bury the nose radius in the cut and if you use a chip breaker then the required depth of cut for the chip breaker to work as intended is even heavier. So, it also depends on the nose radius/chip breaker configuration, too. In addition, if you don't take an adequate depth of cut you get more deflection and that leads to difficulties in holding tolerances.
When you consider all the factors that go into using these tools properly, it isn't as cut and dried as what your speed should be. And if the speed drops by 50 rpm during a cut, I would suspect the tool is not cutting as it should, the tool may be deflecting and the resultant cutting loads are too high.
I started looking at this subject many years ago because I figured it was easier to switch out an insert than it was to grind a HSS tool, and it is. But it didn't work well at the speeds I had available, nor was my little lathe rigid enough to sustain the cutting loads the insert produced at the depth of cut the nose radius and chip breaker required. I have a larger lathe now but these factors still apply.
As an aside, I once had an opportunity to see how a CCGT insert worked on a Sherline lathe with a 10K spindle on aluminum; that showed me what that insert is truly capable of when the speeds are appropriate, even if the depth of cut is not ideal. It matched HSS is accuracy and finish, no question. Now I know speed is important with these tools.
Anyway, this is why I prefer HSS. The edge actually lasts longer than a carbide insert does (when used improperly by me on my lathes) and it cuts better in just about every way. I can hone an edge faster than I can change an insert anyway so this works for me.
