Lathe cutting tool height

An interesting read, Michael, and it confirms my opinion - you pays your expert and you takes your choice. I suppose that in the final analysis it's what works for the individual on their lathe that is the right answer.

It reminds me a bit of the endless discussions in cycling magazines about why there is a left-hand thread on the left-hand pedal (I think I've remembered it the correct way round) with all sorts of theories proposed, contradicted, refuted and argued over. The real answer is that our forebears found that if they used a right-hand thread, the pedal would unscrew itself and fall off.

George

And for the same reason you need a long spanner to remove either pedal which will have self-tightened in use.

Hmmm, I can't swallow that! The idea that experts know nothing has become popular recently, but I think it's a dangerous myth. Whilst experts aren't infallible, anyone who has studied a subject in depth usually performs far better than the average Joe.

Industry don't accept that what works for the individual on their lathe is the right answer. Far from it: they carefully match carbide inserts to the material, and choose from a huge range of shapes, sizes and angles. The information may not be on the web, but how to optimise cutting has been scientifically studied for at least 150 years. The personal experience of Model Engineers and books published 80 years ago aren't "State of the Art".

To me advice about putting the cutter 5° above centre height and also on centre height both make sense. Fig 50 in John Purdy's link underlines this is an "it depends" question, without a single clear answer.

Fig 50 explains why the same tool is positioned 5° above and later on the centre-line. The change is necessary because repeatedly sharpening the tool alters its geometry. The forged tool shown in Fig 50 is a particular old-fashioned shape. It starts life at 5° and is clamped in a horrible rocking tool-holder that allows it to be dipped down to 0° as it approaches end of life after multiple regrinds.

Forged tools of that old-fashioned type have long since been replaced by plain rectangular HSS tooling that simply sits flat. The original advantage of hooked forged cutters has gone because HSS does the job without needing that special form. And getting rid of the rocker type tool holder was a major benefit - they're not very rigid. Modern tool-holders only support HSS and Carbide at a right-angle to the job.

I think 5° over centre was excellent advice before HSS, and was still useful guidance up to 1940. During WW2 the need to improve production rates caused a rapid switch to HSS and Carbide, and a preference for cutting on centre.

Both approaches work, and 5° above could well be useful when a less rigid machine has to take heavy cuts or tackle a difficult material. South Bend's Guidance isn't the law though, it was best practice at the time, hence worth knowing. Coming up to a century later, I think cutting on centre or slightly above is appropriate for most people most of the time. I often experiment for best results, but I always start just above centre.

I hope we all agree cutting below the centre-line is a bad idea!

Dave

Dave, I think you are correct that this is an approach that is based on past technology and cutter material. I think that trying to follow this rule of 5° above centre would result in a lot of tool height setting unless you are only working on a small range of diameters. This makes me think it was aimed at a production line where someone may have had a large batch of one part to work on at a time. It would cause problems if you are trying to work with the dials or with a DRO to achieve a dimension, with CNC or where overall dimensions may change from bar stock size to finished size at one setup as you are only making a one off part (like a lot of us on this forum are likely doing).

Martin C