- Joined
- May 2, 2011
- Messages
- 816
It is easy to become obsessed with accuracy on older machines, but the truth is, unless you know what the dimensional accuracy of the machine was when it left the factory, you cannot know how far it has deviated from that point. In order to make any useful measurements on bed wear it is absolutely useless to start from the 3 jaw chuck which is very likely the most worn and inaccurate part of the machine. Even a brand new 3 jaw chuck of good quality will run out .003" to .004". If you really want to do an objective test, you need to start from the machine spindle, not a chuck. Remove the chuck and carefully clean both the headstock and tailstock tapers Get a test bar, and fit this between centres of known good accuracy (or new, with a certificate of tolerance), and fix a good DTI in the tool post. Move the compound slide until the V ways are fully hidden, then tighten the gyb screws to lock the top slide. Zero the DTI on the test bar and lock the cross slide or tighten the gybs. Run the DTI along the side axis of the test bar, then adjust the tailstock set over till you get the least deviation possible on the side axis. If you now run along the top axis you will get a reading of the fall between the headstock centre and the tailstock centre. Make a note of this. Move the DTI to the headstock end of the test bar, lock the carriage, and rotate the lathe slowly by hand, and note down the deviation Mark the high and low points with sharpie on the bar. Move the DTI to the tailstock end of the test bar, and repeat, turning by hand and note the reading. Now set the test bar so that the marked high and low points are on the top and bottom faces of the bar, so that the finger of the DTI runs along the null point of the front axis (face) of the bar. Wind slowly and gently, if you wind too hard the pressure on the gears will move the carriage slightly, remember that cutting force pushes the saddle onto the bed Vees. Note down the deviation. Move the carriage to the tailstock end of the bed, where the wear will be least and try to rock the carriage across the lathe using A LIGHT push pull motion. Remember there are no cutting forces present to push the carriage hard down on to the bed, and you are trying to detect front to rear movement on the carriage without forcing the carriage up the vee ways of the bed, note the deviation, and repeat at the headstock end, and in the centre. You now have some idea of the wear in the bed, but remember, the harder you push, the more deviation you will see, but what you are seeing is not necessarily wear, it is just the carriage moving because there is no cutting force present to hold it down onto the vee ways.
Now you have a simple choice, either spend time and money correcting it, or live with it. Personally, I live with it unless it begins to affect what I make, then I find the problem, and find a way to work around it. The only thing a super accurate lathe does is grant you bragging rights in the bar, I know many turners who turn out accurate professional work as employees using machinery that is far from new. Most accuracy comes from the operator knowing his machine, and making allowances for any wear present. I used to use a 3 jaw chuck which ran out about .015" if you put a bar in it and tightened it up. With some care and tapping, I could get the error down to about .006", but if the component I was making was being turned on all surfaces, finished and then parted off the bar, the fact that there was .006 difference in concentricity with the bar left in the chuck is completely irrelevant. This is why you should always start with a bar slightly larger than your finished dimensions and turn all to size, rather than start with a bar with the finished outside diameter. Even a brand new quality lathe is built to tolerances, and I know that my worn machines are still more accurate than I am! Remember if everything was turned to 0.0000 (were it possible to do this) as soon as the temperature changed, it would seize up!
Now you have a simple choice, either spend time and money correcting it, or live with it. Personally, I live with it unless it begins to affect what I make, then I find the problem, and find a way to work around it. The only thing a super accurate lathe does is grant you bragging rights in the bar, I know many turners who turn out accurate professional work as employees using machinery that is far from new. Most accuracy comes from the operator knowing his machine, and making allowances for any wear present. I used to use a 3 jaw chuck which ran out about .015" if you put a bar in it and tightened it up. With some care and tapping, I could get the error down to about .006", but if the component I was making was being turned on all surfaces, finished and then parted off the bar, the fact that there was .006 difference in concentricity with the bar left in the chuck is completely irrelevant. This is why you should always start with a bar slightly larger than your finished dimensions and turn all to size, rather than start with a bar with the finished outside diameter. Even a brand new quality lathe is built to tolerances, and I know that my worn machines are still more accurate than I am! Remember if everything was turned to 0.0000 (were it possible to do this) as soon as the temperature changed, it would seize up!