Setting x offset without presetter or taking skim cut

[pmurdock]

Ok. So I've got a big piece mounted in the lathe. Can't take it out of the chuck. I need to change tooling and add a big boring bar and ID threading tool.

I can no longer use the presetter and I can't take skim cuts to measure x offset.

First the boring bar - it's a kennametal - I go to the lathe book and find the dimensions. The centerline dimension to tip shows as 0.64" so I go to the offsets page and set x offset as the centerline (-20.7913) and then if I add (-1.28) to the machine centerline it gets me with a thou or two of the proper offset for the boring bar. BUT why did I have to enter 2X the length to the point I would have thought I only needed to enter -.640 which is what the kennametal specs show.

Second - I try the same method with the ID threading tool and it is off by 0.1xx" or something like that - no way to calculate why it's off that much. Shouldn't I just be able to take the machine centerline and adjust the x offset by the mfg specs on the distance from tool centerline to cutting tip?


Cheers
Paul

[Geof]

The centerline to tip dimension of 0.64 is a radius dimension and the machine offsets work on diameter, hence the 2 x 0.64.

Maybe the threading tool is not dimensioned to the tip but to the root of the thread.

[pmurdock]

You are correct - but I'm still wrapping my head around why the offsets uses diameters since we're basicallly measuring from machine x zero.

the kennametal catalog LT (laydown threads) says nothing about how "F" dimension is setup (ie from root of thread or tip of point). Although I can believe the root theory since threading inserts come in all different sizes they have to base it off a constant value so perhaps that is the root value and then each insert has an added offset (labeled as "EX") based on the TPI (thread per inch) value of the insert.

I'll test the theory out mon. and see if it works..

This all came about because I use an insert with 8-48 tpi partial profile and the threads were never working out because I always cut the root of the threads based on the max or min. diameter, which doesn't take into account the 1/8 or 1/4 height of the thread lopped off according to the UN standard and the partial profile insert has a root radius that only matches the 48tpi - so you actually have to cut it deeper.

hope that made sense.

cheers!
Paul

[Geof]

Makes a lot of sense. I find switching between full profile and partial profile a nuisance sometimes because the tip is not in the 'correct' place.

I find it easier using diameter not radius because diameter is what you measure.