Lathe run out, how accurate?

[dobbslc]

I've just been fiddling with my lathe to try and get it set up a bit better.
I've put in a length of mild steel with a dead centre and run a pass along it.
At first it was tapered to the tune of 0.1mm! tapering to the tailstock.
I've shimmed up the front tailstock foot a bit at a time and got it to 0.06 then 0.05 and now to 0.015
Is that a decent kind of run out? It's a Chester Chinese lathe by the way so not a heavy old Boxford or Colchester!
Admittedly it's not a full length of bar at 12" long but it's all I've got.

 

[scott0031]

You don't adjust a tail stock by height, it's done by moving it left or right of the center and most tail stocks have adjustment for this built in. The only way you would need to adjust height if there is excessive wear somewhere or if something bad is going on. If .015 (assuming you are talking metric here) is good enough for what you want then leave it alone. It's very common in the trade to rub parts down to final size or if there is a slight run out. In theory you should be able to get it spot on but it's probably more than good enough for what you require. You will find especially by the way you have shimmed the tail stock that those tolerances will change from part to part. To set up a tail stock properly it should be done with the part between centers using a dog and catch plate. Reason materials tend to move when cut and chuck can bias the part, this does not happen between centers because the part is free to release any stress as its not being clamped by a chuck and restrained.



Regards,


Scott

 

[Pete.]

0.015mm over 300mm is excellent.

Why are you shimming the tailstock upwards? It's more likely that it needs the sideways off-set adjusting.

 

[Rig Pig]

If the tailstock needs to be adjusted up the bed probably needs levelling, unless it's well worn.

 

[dobbslc]

My bad wording sorry!
I meant I've been shimming the bed of the lathe under the front foot on the tailstock end!

I do have adjustment of the tail stock but haven't touched that.
I was just getting a large amount of taper on everything and wanted to try to improve things as best I can.

 

[scott0031]

Really need some clarification here. If you put a bit of bar in the chuck say 3 inches turned length and a good sturdy diameter (no centre) and turn it and do skim cut are you saying the work is coming out tapered? I appreciate its a chester (which model by the way, would help to know the actual model) and probably not the best but its not wise to shim up the bed as you could be well be getting those results by stressing the bed.... which is a big no no.

 

[Pete.]

Yeah don't twist the bed - unless it's to un-twist it! You'll reduce the bearing on the saddle ways and promote wear.

Level the bed first with a machinist's level. If you want to make a cheap one then there are some vials going cheap here or you can DIY one same as this guy here.
Once the bed is level (or straight at least) then turn a test bar between centres and adjust the tailstock until you're turning parallel.