How can I keep a deeply drilled hole straight?

It hadn't occurred to me to suggest these before, but having thunk the thought...

https://en.wikipedia.org/wiki/Gun_drill

...of course, you need to consider how expensive a solution is actually justified by the requirement .

Or if you have enough space put a bigger hole in the part and sleeve it.

regards Martin

I remember sitting in the library of the old Manchester Museum of Technology reading a learned treatise explaining how it was always more accurate to turn the work piece and hold the drill steady. Not sure I can reproduce the analysis, but it was along the lines that whatever error in cncentricity was introduces at one side of the hole was balanced by an equal and opposite error half a revolution later. Same in principle goes for alignment errors, either of bed or tailstock.

Of course this is also a value judgement, not a quantitative statement, as evrything depends on the rigidity of the set up as well as the tool.

So stick the work in the lathe, drill first, big as you dare in one swipe (the bigger the drill the stiffer it is), then ream to size and finish.

It is embarrassing that I posted twice. I am getting old and probably a bit soft in the head. At least I gave the same advice twice. I think my affliction is called CRS: can't remember s....

Managed to measure my tailstock misalignment. It's 'as delivered' by the supplier - I've never adjusted it since new.

Using this method the spindle centre and a half-centre clock nearly 0.02mm out with the ram protruding 60mm.

Measuring deviation over distance, the ram travels 0.035mm down and 0.08mm right over 80mm.

My DTI is a 0.01mm type. All the measurements are rounded up to the nearest 0.005 guesstimate.

For what I'm doing, a straight rather than an accurate hole, the alignment figures feel 'good enough'. Am I right or are my tailstock alignment figures scary bad?

Ta,

Dave

Edited By SillyOldDuffer on 15/09/2017 17:08:59

See if there's paint or crud between the two parts, especially at the back tilting it forwards.

The L/R error is easy to introduce when adjusting back and forth, and rather fiddlier to get rid off but it can be done.

Always clamp it with 'normal force' before making a measurement.

Neil