There is always a lot of discussion about run out when it comes to chambering barrels however regardless of how well you clock in the barrel prior to taking that first cut with the reamer you need a tail stock that is aligned to the head stock because it is your tail stock that carries the reamer and reamer holder so getting this correctly aligned is important. I say important as opposed to critical as there is a glut of reamer holders designed to take out radial and angular errors when using a reamer and they all work to varying degrees however it is important to make sure your machine is as near to perfect as you can get it.
I turn a fair amount between centres and I make a point of checking the diameter of the material at both ends during an intermediate cut. Friday I performed the check and noticed around 0,1mm negative error on the diameter per 300,0mm length (.004″/12″) Which is enough to send me off for my check gauge to see what was happening.
The gauge is very simple, a round disk with a 25,0mm spigot machined on one side. The outer diameter has an M6 tapped hole aligned with the centre axis of the gauge to attach a Mercer 0.0001″ Dial indicator. To use I pop into a collect chuck, start the machine and take a fine cut off the major diameter to ensure it is concentric to the spigot and then attach the indicator. Making sure the bed and underside of the tail stock is clean I use an unused good quality #3 MT Adapter in the tail stock, move it into place so the indicator is touching the MT, lock the tails stock in place, zero the indicator at the 270 degree point, then flip it over to the 90 degree point and read the error. In theory both reading should be zero however if they not you need to make some adjustments.
Most lathe tail stocks have two hex head screws in the base which in turn screw into a block that sits on the underside and tightening/slackening one screw in conjunction with the other pulls the headstock to the left or right when viewed by above. I use two T handled hex keys and on mine turning the front screw clockwise whilst turning the rear screw counter clockwise gives a positive reading and reversing the procedure gives a negative reading.
Negative and positive is best explained thus, place a length of material between the headstock and tail stock and take a cut, now measure the diameter at the head stock and again at the tail stock, if the second reading is greater than the first then you have a positive error, if it is less you have a negative error and from this you know which was to adjust the tail stock. What I must point out is this is counter intuitive in that you would expect to be pulling the headstock towards the cutter when adjusting from the front when you are actually moving an internal floating block so the direction of movement is reversed.
Back to business, three minor tweaks and a test cut showed zero error at 300,0mm and that was to 5 decimal places so happy days 🙂
Finally, yes there are other ways of checking tail stock alignment, some a simple as a steel rule suspended between centres to using a ground steel bar however this is the way I take care of the job, moderately ‘Old school’ however it works very well for me.