Need a perfect 8mm hole - to ream or bore?

[BillTodd]

Cheap 'jobber' bits can be OK. I used a one to drill a 5/6mm 8" through a steel spindle (the 5mm was riveted into the end of a 6" long 6mm rod - I didn't want to wreck a good bit). Just check how they cut and make sure the end is ground correctly.


Boring bars should only be used to cut forwards (make sure you don't drag them back on the final pass).

As Robin said, you should take nice even cuts up to the finished diameter. The finishing pass should be the same depth (since the bar will always bend while cutting; if you try to take finer cut the bar will bend less and you'll take off more than you thought you were going to and probably over shoot the diameter)

My guess your new boring bar is only ground to an approximate shape, so it's ready to be sharpened (since its final shape is determined by the material you are going to cut)

The tool angle should be correct: the tool should cut with the very tip of the tool: the top, front and underside should all slope away from the part.

When you cut, the material has to shear apart and slide along the face of the tool, The material will cut easier if this shear face is at an acute angle. However, as the shear angle gets more acute, the tool gets weaker. The cutting tool shape is a compromise between shear angle and strength (and cutting force).

Soft/weak materials (ones that shear easily) will cut with a low force, so the tool can be acute. Stronger/harder materials need more force to shear, so will need tools the a more obtuse angle (for strength). Beware strong materials that are also soft (e.g. copper and its alloys) they will grab the tool.

You can cut strong/hard materials like steel with a knife like tool - you just need to take a very very fine cut.

The tool should be sharp: I.e. the cutting faces should be smooth, flat and at the same angle all the way 'till the last molecule :naughty: Don't confuse the words 'sharp' (meaning the edge is not rounded) and 'acute' (a tight, narrow angle).

Bill