Supporting > Tooling & Machines

A good tool: test bar

<< < (2/2)

Chipswitheverything:
There is an assortment of factors that can lead to problems with turning acceptably parallel along the length of a workpiece, especially on the relatively lighter lathes that we tend to use in the home workshop. 
Has the lathe been levelled when it was first set up?, and checked now and then for any twist in the bed?, particularly if it is mounted on a wooden bench or stand. If the machine is on raising blocks, such as the ones that many Myford lathes ( including mine )are mounted on, then the adjustment is easy, via the jacking screws.  Otherwise shims may be needed under the bolted down feet.   If an accurate precision level can be used, or borrowed, then the job is easy, otherwise the procedure that the Myford handbook suggests, with a parallel bar, and DTI can be used.  Unless you are quite confident that a twist or strain has not been introduced into the lathe bed, then other adjustments are not really possible to accurately determine.

 The tailstock set over can be adjusted to neutral in various ways.  A test bar with centres, of known quality, supported by good centres in h'stock and t'stock, and checked with a DTI , is a useful way.  Or a centred BMS bar, say around 10" in length, 3/4 " dia., roughed down to leave a couple of collars upstanding , one near each end, can be supported by centres and driven by a "dog" , and very light cuts taken over the collars at the same undisturbed tool setting, to help adjust the tailstock set over to achieve equality at either end.  A quick check can also be made by mounting a DTI on a mag base on the lathe faceplate or catch-plate, and doing a turn-around test against a centre in the tailstock barrel.  Dave

vtsteam:
Ditto on first checking if lathe bed is twisted, first, via levels.

If in doubt, then, you can check a pre-ground test bar for bow, and for center hole accuracy by supporting between centers, and putting DTI at each end in turn, and the middle. Rotate the bar for each test. Both ends should remain stable on the DTI. If not the bar's center at the end with the error is incorrectly drilled. If stable at both ends, but reading varies in middle, the bar is bowed. If all check out then do the longitudinal tests per 55fairlane and others above.

ps. use a known good dead center for the tailstock end -- there are some problematic live centers, I've found.

Navigation

[0] Message Index

[*] Previous page