Surface finish has lines in???

[jeff46]

Hi Guys,

I own an old colchester student lathe, the bull-nose type, it works well however When turning a parallel piece I am getting faint lines in the surface finish approx 6mm apart along the full length of the piece, it's almost as though the traverse is stalling or something every so many revs where it leaves a visible line on the work.
Incidentally the cutting tool is positioned at the rear of the work therefore the work turns clockwise and the cut is taken from the back not the front as in conventional set-up, the traverse direction is from tail-stock to chuck.
I can't actually see any jump or slowing of the saddle but the marks on the work indicate something is wrong???, everything seems oiled and free moving so I'm at a loss as to what may be causing this.
Not sure how to check the thread piece that links to the feed bar when you engage the traverse or if there is any adjustment in these things.
Help please, any suggestion welcome.

Jeff

 

[Cole2534]

Inspect the gears in the drivetrain, one may be missing a tooth.

 

[jeff46]

Thanks Cole,
will check them when i get home.

Jeff

 

[jeff46]

" If the tool is at the rear, I assume you mean upside down, I think the tool is lifting "

No mate, the tool is the right way up the chuck turns in reverse.

Thanks
J

 

[billmac]

Do you get the same finish problem with the tool in the conventional position and normal rotation?

 

[Clive603]

Missing tooth in the drive train sounds far too drastic but uneven crud build up or the activities of the swarf packing gremlins could affect tooth thickness somewhere in the drive train by just enough to momentarily change feed velocity sufficiently to leave a witness line. I bet it looks horrible but is pretty much impossible to feel.

Is the line pitch the same for all feed rates or does it vary? If it is a drive train problem contemplation of this point may give a clue.

In principle you could identify the source by fairly tedious means starting by verifying that the line is always in sync with itself on successive cuts then working back through the drive to see what else is synchronised. Verify synchronisation by using the spindle clutch to start and stop the auto feed with bed stops set to define start and end positions. Obviously its not safe to run right up to a stop but you can get close enough to ensure that after you disengage the feed and wind the saddle back to the start position stop by hand for the next pass all the innards will be in the same relative position as they were on the previous pass.

Positively identifying the cause and going for a specific fix may well be more trouble than going straight to the "nuclear" option of a major strip, scrub, lubricate and re-adjust service. In your position I'd start by stripping out, clean and carefully setting up the end gear train, not forgetting the banjo mounting surfaces. If that didn't cure it I'd do the whole saddle assembly and scrub the bed, not forgetting underneath where the anti rise bearings or gib strips run. (Its taken nigh on 40 years but I've finally realised that girding up for the big strip and just doing it is actually much quicker than all the time wasted futzing around a puzzle problem. Generally a puzzle problem is a good indication that a full go through and spray of gremlin repellant is about due. Fix one puzzle and the shop gremlins will have another for you in a month or so!).

Clive

 

[wlbrown]

surface finish

HELLO,
does the machine have a feed rod, or is it driven
by the lead screw? could this be from driving it on
single phase power?
wlbrown

 

[Peter.]

Could it be a build-up of crud on the carriage traverse pinion or rack?

 

[Conrad Hoffman]

Is there a rack involved anywhere? 6mm sounds close to the pitch of a drive rack and maybe there's a problem with the pinion or the distance. Since you know the 6mm figure, it should be possible to look at the ratios of everything in the drive train and locate the problem fairly quickly.

 

[HuFlungDung]

It could be a filthy grease-packed-with-chips rack under the front lathe slideway. Or a filthy pinion gear that is engaged in that rack.

It could also be a worn out bushing supporting that pinion, which has allowed the pinion to sag and run out of line in some cock-eyed manner which subverts the proper rolling action of the gear involutes.

 

[Forestgnome]

There's something my lathe does that's similar. The pinion gear for the carriage handwheel has a fair amount of backlash, and my handwheel isn't balanced, so every time my handwheel crank comes over the top it bumps the carriage forward a tiny bit when the wheel falls. You can test it by applying a little drag on the handwheel with your hand as the carriage moves, which prevents the bump to see if it gets rid of the problem..

 

[jkruger]

I have had this same situation with several older machines. I would bet this is it.

 

[ortho]

Ditto. I'd remove the crank handle temporarily just to see if that's the problem.

 

[oldbikerdude37]

There's something my lathe does that's similar. The pinion gear for the carriage handwheel has a fair amount of backlash, and my handwheel isn't balanced, so every time my handwheel crank comes over the top it bumps the carriage forward a tiny bit when the wheel falls. You can test it by applying a little drag on the handwheel with your hand as the carriage moves, which prevents the bump to see if it gets rid of the problem..

I bet you are right on the money.

 

[Tony Wells]

I've dealt with that on quite a few lathes with wear, and it was the first thing that popped onto my head, except the 6mm sounds pretty close for one rev of the carriage handwheel. I'm not that familiar with the specific lathes we are talking about though.

 

[EPAIII]

After carefully reading the two posts by the original poster, I can not see where he has clearly stated what direction these lines are running. I think that should be established before attempting any possible solutions.

So, are these lines running around the diameter of the work: therefore they form circles around it? Or are they straight lines that run the length of the work?

Each of these different problems would have different possible reasons and different solutions.

 

[jkruger]

Ditto. I'd remove the crank handle temporarily just to see if that's the problem.

No need to pull the handle, just hold some friction on it with your finger to keep it from slopping over and bumping. After 46 yrs this has become a habit for me.

 

[jeff46]

Guys,

Thanks for the many possible answers, the lines by the way do run around the diameter every approx 6mm.
I have tried the holding the handle but this didn't make any difference.

When I arrived home I took of the end guard to check the drivetrain, on close inspection the gear on the shaft that comes out of the head of the lathe (the one that is the actual drive) has 40 teeth, two are partially broken incredibly they are completely opposite each other, think this may be the problem.

Now I have to find a replacement????? anyone have one of these by any chance?? it has a 5/8" hole in the centre approx 3" outside diameter with 40 teeth.
Maybe I should put on a new thread???
I live in the UK.

Regards
Jeff

 

[Clive603]

Google for Colchester Student Spares or variations there-of and you will find several trade suppliers of both new and used parts. Bede Tools list some new gears at merely "suck your teeth" prices but by part number only so you'd need the parts book to find out if they have yours. Best guess is order of £20 to £50 delivered should net something suitable.

Be wise to check the mating gear too. Partial breakage usually means something got picked up so it may have been hurt too. Running against a pair of partially broken teeth probably wasn't good for it either. Time for a scrub out too just in case the bit that did the damage is still hanging around awaiting its chance for a repeat performance.

I've seen similar damage, although not to an opposed pair of teeth, caused by a slightly bent shaft and by a damaged ball bearing. Bearing case was interesting as part of the cage had disintegrated so some of the balls were free to wander around, presumably enough to upset clearances.

Clive

 

[jeff46]

Just found the order part number from colchester it's No 6160 change gear 40 tooth, unfortunately they inform me it's now obsolete and no longer available, any chance anyone got one lying around?????
Alternative to that I assume I could purchase the 3 gear set??? Colchester say they have plenty of the 21 tooth gears which would require linking gears of specific tooth size, not sure how i'd find that out??.

May end up having the damaged teeth welded then file to shape as a last resort.

The change gear is 40 tooth with and 8 spline centre hole.

Jeff