If understand correctly that its cross-slide infeed you are talking about, it sounds like maybe you might be setting the thrust bearings a little too tightly – there has to be a bit of freeplay in order to have a working clearance so that nothing 'binds' – in practice a little backlash doesn't affect the accuracy as one is always putting cut on in the same direction..

As above, if tightening up the thrust bearings causes jerkiness then back off the nut until it is smooth again. The issue with older lathes is that there will be uneven wear of the leadscrew. The tool movement per turn of the handwheel will vary depending on where you are on it. The two ends tend to be nearly as new but in the middle it will be worn. This wear is magnified on a lathe as it doubles when the diameter is checked. The simplest solution is to approach the desired size then measure what you have and adjust the next cut to suit. Repeat until the desired diameter is achieved. The other issue is that any difference in centre height of the part to the tool tip will cause a difference between the target diameter and the actual diameter. This is hardly noticeable with large diameters but becomes a problem as the diameter gets very small.

The final thing is how sharp is the tool? You cannot remove 0.001" (I'm using inches because you said it was an old Colchester) with a blunt tool, it will rub. Trying a second cut by putting on a further 0.001" may still rub. The third attempt may start cutting but will now take off 0.003". This gives the appearance of jerky movements.

Martin C

I should have said it's when I'm hand feeding the cross slide so no gearing engaged.

While you may not have engaged the power cross feed, there will probably be gearing within the apron still engaged with the cross slide screw when you are manually rotating the handwheel.

A perusal of the lathe parts manual exploded diagram of the apron should give an idea of the gearing arrangement for your Colchester (you didn't say which model). The probematic Harrison at work gives a jerky action with the PCF gearing disengaged – it tends to "load up" & then jump, which makes setting an accurate dimension WRT the DRO difficult. As I said earlier, I am pretty certain that this is caused by a bent shaft within the apron.

Dropping the apron isn't usually difficult as such, just a bit of a pain as the feed shaft, spindle start switch/clutch shaft (if fitted) & lead screw have to be withdrawn to free it. Only with the apron dropped will you be certain that nothing is engaged with the cross slide screw.

Nigel B.

Have ordered new thrust bearing as these were cheap and strangely metric

A bit of a strange one – the parts list for a Colchester Dominion I found online doesn't show a thrust bearing there & in a list of bearings used shown later in the manual all are Imperial types.

Not to say that newer or older machines than the manual I found were not different though.

Nigel B.