Turning to size - right everytime (well nearly!)

[Metal Mickey]

This isn't one for the 'old sweats' more for the like of me 'novices'. Its not even mine but it does work and its so simple! Well it nearly always works and is 200% better than I have achieved before.

Get on with it man I here you say....OK, how do you turn down to size every (nearly) time? You take the cuts you would normally make until you get near to size. Then measure the work piece and halve the amount to take off. You keep doing this until you are a thou away when you can either take a light cut or polish off as it were.

It seems simple but it does really work! I read it in a recent UK model engineering magazine (don't ask me which). Anyway, try it and you will be surprised. I was. ;D

 

[Twinsquirrel]

Simple but effective...... thanks MM :bow:

 

[Cedge]

MM
When I first began chasing dimensions, a machinist warned me that those little marks on the dials were for getting "in the neighborhood". Then he suggested measuring and cutting by halves. You're right.... it works a treat. I have DRO's on both machines but I still measure as I get in close, just to keep everything honest, and I still cut by halves. It's a technique that has saved my bacon more than once.

Steve

 

[rake60]

Professionally the last cuts have names.

You rough it close (to within .060") then take a
"Pre-Finish Cut" of .015" per side. There's still .030" left.
You mic that result for size, taper and check the finish.
Adjustments are made for the "Finish Cut" based on those observations.
Those machines are rock solid rigid and set up using the best optical and laser
devises available.

At home it's a little different. Especially when I'm boring with a small boring bar.
I'll leave .060" then run that rough cut again to remove any adverse tool spring effects.
From there I take a "Pre-Pre-Finish cut of .005 on a side. A "Pre-Finish cut to confirm the
the result of the "Pre-Pre" then cut it to finish.

Sometimes it works out perfectly!
Sometimes I end up making a wooden dowel to be fitted with emery cloth to make
it perfect.

That's OK to say!
We're not on the clock here!

Rick

 

[zeusrekning]

Ricks got it. I also try to leave the same amount on my pre-pre finish cut, my pre finish cut, and my finish cut so that tool flex is consistent.
Zeusrekning

 

[Metal Mickey]

Thank you Rick for the professional terminology and approach. Its nice at least to hear the pro's methods to use. I learn much on this forum every time I log on.

Mike

 

[kf2qd]

One other point needs to be made about how the pros do it. The pros are generally using larger, heavier and stiffer machinery than we get to use in our home shops. The toolpost on a pro machine weighs half of what my mini lathe weighs. Heavier machines mean I can work to the dials and take a measurement on that next to last cut and finish to size. A pro machine doesn't have much flex because the pro machine needs to get things done without any time in there for screwing around to get it to come out right.

A pro machine needs to work like the Warn&Sqeaky 0AB and 1AB I ran many years ago. You only got to measure the finished parts, and they could pile up fast if you didn't stay with it. And it had to run a full 8 hours.