Boring tool height

Not by anyone who knows anything about machining. A turning tool should never be above centre height. That's a sure fire way to bugger the cutting edge.

I assume this is to allow the tool to dig

Turning: At or just below centre height.

Boring: At or just above centre height.

Oops. I got that A over T didn't I? I did actually mean the other way round, but brain connected wrongly with fingers.

Cliff.

I think the other way round. There are two arguments - clearance angle and "dig in". On outside work, raising the tool above centre height reduces the carefully chosen clearance angle (eventually to the point where the cutter won?t cut any more). If the cutter is above centre then if it "catches" in the work it will deflect downwards into a greater diameter and cut deeper - making things worse - at best you get poor surface finish. On those arguments the tool should be exactly on centre height to get correct clearance and slightly below if anything (though it has to be spot on for a facing cut not to leave a centre "pip".

Boring is the reverse. Dropping the tool below centre reduces the clearance angle. If the tool digs in it is dragged into a deepening cut. Thus it should be exactly on centre or a hair above.

Woodturners are all taught to "scrape" (negative rake cutting) below centre height on the outside and above on the inside of a bowl for the same reason - a "catch" is not going to develop into a disastrous "dig in" if it forces the cut depth to be smaller.

Gentlemen,

As an engineering apprentice many many many moons ago we were always taught that the tool regardless of internal or external turning should be on centre height and its not difficult to set up either.

Mart>> It is normally reckoned that when turning externally the tool should be >> at

So did I but practical experience taught me that it is often difficult to be that precise. An outside tool placed above centre heigth or an inside tool placed below may induce chatter and give a rough surface, so I used to aim at the safe solutions.

True for turning but a bit different for parting off

SLIGHTLY above centre height enables the frontface of the tool at completion to push off the tiny residual pip and complete the parting off.

If the parting off tool is below centre the residual pip remains and tries to to drag the tool tip forwards. If an inserted tip carbide parting off tool is in use this can sometimes drag the insert out of its location.

Although we always talk about "at centre height" what we really mean is when the setup allows the toolpoint to move truly radial to the spindle axis. In this case the working front clearance angle and top rake are those that are ground into the tool.

As soon as the tool tip is not "at centre height" the tool tip no longer moves radial to the spindle axis and this changes both the working clearance angle and working top rake. This change in angle is inversely proportional to cutting diameter - tool height makes little difference when cutting large diameters but becomes critical when cutting a say 1/8" workpiece.

The change in angle is arcsin 2h/D. For an 0.005" error in tool height and a 1" diameter workpiece the change in angle is only

1/2 deg which can be safely ignored. However it rises rapidly as workpiece diameter decreases - at 1/8" diameter the angle change is 4 1/2 deg which is enough to severely compromise the working front clearance angle.

Jim

When the center height is difficult to find, such as when turning steel water pipe with a rounded slanting-edged bit, I start slightly low and raise the tool until it cuts well. I doubt that I ever set it more than half a mm above center, and then only to make a clean parting cut as mentioned by others. For boring the pipe I start slightly high so the bit will deflect out of the cut.

Those skew-cutting HSS bits came from a second-hand store; they weren't my idea. They shave cheap steel like pipe smoothly and easily.

jsw

Does nobody swing the tool post around and use the centre in the tail stock!!

I do.

I do, but I have a "Swiss Multifix" tool post that rotates completely around. Don't know how well that works with other types.

jsw