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Richard Smith 1420/01/2022 21:11:55
6 forum posts
1 photos

I've got the use of an old Colchester lathe but the crossfeed is giving problems. When trying to set the size of cut the handwheel does not seem to move the tool in by the correct amount almost in little jerks. I have stripped and cleaned slide and leadscrew and all seem to move smoothly. If I nip the nut at the back on to the two thrust bearings the movement becomes notchy. Cant see anything wrong with the bearings. Anyone experienced this before?

mgnbuk21/01/2022 07:51:57
1175 forum posts
71 photos

Probably something in the apron gearing - the Harrison VS330 at work does the same & I'm pretty sure that this is due to a bent shaft in the apron.

I know that a collegue ran the saddle under power into the bed stop due to a misunderstanding of how the machine operated - a Colchester we had at the time (Mastiff, IIRC) had a slipping clutch in the apron that stopped the saddle on the bed stop, but the Harrison doesn't have such a clutch. A shear pin failed that time, but the notchy cross feed "appeared" around the same time. Investigating this further is a long way down my list of priorities, though.

Nigel B.

DiogenesII21/01/2022 08:13:06
516 forum posts
202 photos

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..

Clive Foster21/01/2022 09:35:11
3103 forum posts
107 photos

Seen notchy issues in similar screw feed systems from bent shafts, worn bearings and a thrust bearing slightly tilted in its housing.

The only specific lathe one was a bent shaft.

Well apart from the totally WTH case where the ball thrust race at the back end of a taper turning SouthBend Heavy 10 had been assembled with two ball cages between the thrust washers. How that level of brain fade happens on re-assembly I shall never know.

Clive

Martin Connelly21/01/2022 09:51:37
avatar
2123 forum posts
222 photos

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

Richard Smith 1421/01/2022 17:59:54
6 forum posts
1 photos

I should have said it's when I'm hand feeding the cross slide so no gearing engaged. I have done quite a bit of turning and understand backlash. I thought it was a bad tool shape to start with until testing when not cutting. I've tried loosening the thrust bearing nut to give slight backlash and the handle then loses its notchy feel. Putting a depth gauge against the slide shows that it's movement is still not constant but jerky and not corelating with the dial. The slide itself is smooth with no binding when the lead screw is removed. When doing larger movements eg 0.5mm the dia and slide movement is confirmed by the depth gauge.

When I take a repeat cut without altering the cross slide I would expect the tool to remove powder but it leaves no trace.

To me the only things involved here seem to be bronze nut, screw and thrust bearing. Thoughts?

mgnbuk22/01/2022 11:18:50
1175 forum posts
71 photos

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.

Martin Connelly22/01/2022 15:33:37
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2123 forum posts
222 photos

If it is due to a bent shaft as suggested above then the notchiness would likely appear at the same point in the rotation of the leadscrew, so is this the case? If you plot the actual movement versus expected movement on a graph is it cyclical or random? At one end, centre or other end of the leadscrew?

Martin C

Richard Smith 1425/01/2022 23:20:45
6 forum posts
1 photos

Thanks for all the help so far.

I checked and the notchiness at the start and middle of the leadscrew but still there. I stripped the leadscrew out again and tried rotating the handle without it, it is now smooth so I think this would discount the gearing. I rolled the leadscrew on a saw table and it appears flat. Have ordered new thrust bearing as these were cheap and strangely metric, although its a Colchester Dominion Lathe. I will report back when fitted.

fizzy26/01/2022 09:35:46
avatar
1840 forum posts
120 photos

the cross slide on my L5 bears no relation in terms of scale printed on the dial to what it actually does - its a genuine L5 dial. For 10 thou dialed in it moves just shy of 8 thou. Thankfully I have a good DRO !

mgnbuk26/01/2022 11:29:30
1175 forum posts
71 photos

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.

Richard Smith 1426/01/2022 18:31:26
6 forum posts
1 photos

A bit of a strange one - the parts list for a Colchester Dominion I found online doesn't show a thrust bearing

The lathe has a taper turning attachment and the thrust bearings fit into this so perhaps a later modification.

Richard Smith 1403/02/2022 18:53:17
6 forum posts
1 photos

Well put new thrust bearings in and the cross silde is now much smoother, all the notchiness has gone. Just having a problem when now when cutting a taper but will start a new thread on that, thanks for the help everyone. Bearings were only £10 a pair!

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