Thread: vibration during turning

Originally Posted by nadroj

Try HSS.

Jordan

Nailed in one line, speed and horsepower as well as machine/bearing rigidity are what you need for negative rake tools.
Where do you live Nadroj,we might be neighbours?
Will

Carbide tips look to have some shape to them at the cutting edge, that changes the angle at the cutting point.
Could be even negative? Even though the flat base of the insert is at a positive angle.
Not an expert on this, just an observation.
I do have a few carbide tipped tools, but rarely use them due to the versatility of HSS.

Jordan
Penrose

> Carbide tips look to have some shape to them at the cutting edge, that changes the angle at the cutting point.

What you see could be just a chip breaker. Chip breakers have little to do with the cutting geometry itself.

> Could be even negative? Even though the flat base of the insert is at a positive angle.

Inserts for negative rake can often be used from both sides (from a triangular insert you get 6 usable tips, 3 on top and 3 on bottom). If you look at such insets from the side, they are rectangular. Such inserts ONLY work in negative rake toolholders, that present the insert with the tip sloping down towards the workpiece. Without this downwards sloping, there would be no clearance angle below the cutting edge. Then the tool only rubs and chatters.

Negative rake inserts are sold very cheaply, it is the simplest to make inserts, and it is the oldest inserts there are, since some 60 years ago it was not yet possible to make intricate inserts like nowdays. The problem is when people buy such cheap negative inserts, maybe with the view to flip it over and use twice the number of cutting edges, and then install these inserts into toolholders designed to take positive rake inserts. Without the downward sloping seat. The result can be complete frustration with insert tooling and a return to HSS.

For lightweight lathes with low HP motors, positive rake insert tooling works as good or better than HSS. For example CCMT or TCMT inserts are excellent for roughing and finishing steel etc. For Aluminium and plastics and even wood CCGT-ALU or TCMT-alu with 20 to 30 degrees positive rake work wonder, these are as sharp as a freshly ground and carefully honed HSS. I also uuse these for fine finishing steel, they are so sharp that you can take off cuts of just a few microns.

Sharpness matters a lot on a manual lathe. The rule is, that you cannot take a depth of cut that is less than the radius of the cutting edge itself (not the tip radis, that is another thing, I mean the radius of the cutting edge itself as seen under a microscope). That is why it is not possible with a carbide insert designed for roughing, to take off a very fine chip on a manual lathe, like necessary to turn something like a ball bearing seat that requires an accuracy of better than 0.001mm. If you try, the insert just rubs and leaves behind an ugly irregular finish. On a cnc lathe the same insert can produce a nice finish and hold accurate dimension, because you program it to take not less than 0.1mm for the finishing cut, and the machine is rigid enough to do that with accuracy and without the need to take control measurements as you approach final dimension. On a light lathe you need to approach final dimension slowly and take measurements, and to do this you do need a very sharp tool, either HSS or a very positive rake polished type insert like a CCGT-Alu.

Sharpness also matters to reduce or eliminate chatter. The less sharp an insert, the greater the cutting forces, the more chatter. But also, the less sharp an insert the longer it lasts on a powerful heavy lathe, and believe it or not, also the better the surface finish - but only on a very rigid heavy lathe. Negative rake inserts very often have on purpose chamfers or radiusses on the cutting edge to make it blunt.

Undortunately the literature from the many insert makers is directed to where 99% of their customers are - powerful heavy industrial CNC lathes. Their recommendations do rarely apply to smaller manual lathes. Believe me, if your lathe is less than say 250kg and 2HP and less than 3000rpm top spindle speed, only sharp and positive rake inserts will work well for you - either that or good old HSS.

- Uncoated inserts are always sharper than the same insert coated. There is little benefit in coating ouside industrial use.
- Either use flood coolant, or no coolant at all. Using drip or brush on coolant thermally shocks the carbide. The cutting edge of such shocked insert under a microscope looks full or microcracks, which greatly shorten the insert life.
- If the lathe allows it, try to use inserts at about 3 times the surface speed you use for HSS. Take as deep a cut as reasonably possible. Inserts hate too shallow cuts. When roughing, chips should be very hot and blue, carbide just loves this condition. When finishing to a very precise diameter and best possible surface finish either use special polished supersharp high top rake inserts, or use HSS.

Great info Chris. I'm going to track down some positive rake inserts and a new holder. Where do you get your -ALU inserts from?

cba... That is a 5 star essay.
I shall name the essay ... The Idiots Guide to Carbide Tooling.
Well done and thankyou.