Lathe Tool Bit - a victim of many sins

Recently in the frustration of being unable to get a smooth surface finish on a small lathe job, I took the unusual (for me), action of taking a look at the tool bit.


The job was to turn some small specialised bolts for a GoPro Camera mount, and despite taking all the usual actions of adjusting tool angle, speeds and feeds I could not get an even vaguely acceptable finish

Left click the camera images below for a high resolution view...

Even after sanding with 400 grit paper, the finish looked like it had been cut with a thread cutter...

[caption id="attachment_108" align="aligncenter" width="300"] The tool post of the AL 250G lathe[/caption]

Rotating the tool-bit gave instant results, you'd wonder why I did not do it earlier. I guess the real answer is  looking at the tool bit with an untrained eye everything looked normal. In fact with a bright light and it still did not look cracked, broken or crashed.

In this post we examine the tool-bit more closely, and hopefully capture a few lessons along the way.

[caption id="attachment_109" align="aligncenter" width="300"] The tool with carbide insert mounted in the tool post[/caption]

The lathe is a small AL250G 10" swing with a 500mm bed. The tool bits are Sumitomo Electric Co AC830P, general purpose bits (data sheet attached below).

[caption id="attachment_105" align="aligncenter" width="300"] Inserts are small, about 12mm[/caption]

A close visual with an untrained eye did not notice anything particularly unusual ...

[caption id="attachment_97" align="aligncenter" width="300"] the damaged tip is closest to the centre of the photo[/caption]

But if we get out the macro-lens and a proper camera, things become more obvious ...

[caption id="attachment_94" align="aligncenter" width="300"] The damaged tool-tip is on the left after rotation[/caption]



Even at this point, the tip did not look sufficiently damaged to consistently score the finished surface. Varying the speeds, depth of cut made little to no discernible difference



[caption id="attachment_106" align="aligncenter" width="300"] Andonstar USB microscope[/caption]

Time to break out an even closer look, the microscope really starts to shed light on the problem. Side by side the damage is obvious

[caption id="attachment_107" align="aligncenter" width="300"] Left - old tip, right new tool tip[/caption]

It looks as though just the surface coating has been removed but a close look reveals edge damage ... the real culprit

[caption id="attachment_98" align="aligncenter" width="300"] damage to the edge of the tool-tip...[/caption]

The damaged area is very small... overall 0.58mm (22 thousandths of an inch) as I can best measure it ...

[caption id="attachment_101" align="aligncenter" width="300"] Front edge of tool-tip (side view)[/caption]

after doing a little research, (resources below), I have since learned that my tool bits have been the victims of multiple sins.... too much heat, too much speed etc etc ...

I guess the lessons here are obvious, hope you found this interesting, please feel free to leave a comment ...



Resources

Sumitomo Data Sheet - AC830P
Sandvik Tool Tip Wear