turned finish

[david kitchen 1Participant @davidkitchen1]

I have a Chester CUB620 which I bought six years ago using advice from L.H.Spareys book.

Recently the finish on all materials is very similar to that shown in the photo of a piece of aluminium.

Regardless of tool type HSS or tipped, speed or rate of cut, the result is always very similar.

The finish seems ridge free when facing.

The sample in the photo shows the finish, from the right. The first 6mm fed using the hand wheel. the three sections from the right were cut at .5mm depth of cut feeding at .033, .01, .013 respectively. The spindle speed 800 rpm. The cutting tip Sumitomo CCGT09T304 NAG-H1 which is made for aluminium. Tools are held in a T2 dovetail toolpost.

The cross slide jibs are adjusted up and I can't find any play in the saddle.

I suspect the feed mechanism and would be pleased to hear from anyone who has dismantled the saddle/apron assembly.

I have asked Chester and a local precision engineers.

Sorry to have to ask such a lot, but I can't think of any one else to ask

[david kitchen 1Participant @davidkitchen1]

[KWILParticipant @kwil]

When you say you cannot find play in the saddle, is it snug in the vertical plane. ie it cannot lift due to a bent leadscrew?

[Brian WoodParticipant @brianwood45127]

I had effects like that some time ago on my Myford. Running a toothbrush along the leadscrew and then cleaning the clasp nuts to allow correct depth bedding again cured it.

Brian

[Brian WoodParticipant @brianwood45127]

Hello David,

I forgot to add in my comment that because you have observed this behaviour as a recent departure from normal, it would point to something along the lines of my experience rather than some dramatic event leading to an eccentric or bent leadscrew.

In those cases I would expect the markings to have a bias along one side of the bar and be absent on the other as there would be a cyclic component involved to affect the cutting at the tool face. Another clue is that things behave as expected when facing, the markings are only apparent when the leadscrew joins in.

I hope I'm right

Brian

Edited By Brian Wood on 10/03/2013 16:20:02

[_Paul_Participant @_paul_]

Perhaps a bit of swarf in the drivetrain or possibly a dry or binding bearing? as the results are consistent despite your varying feedrate it may be in the primary part of the geartrain.

To help narrow it down have you might want to consider turning the gears by hand to feel for any tight spots.

Paul

[Speedy Builder5Participant @speedybuilder5]

Does the saddle have an adjustable slide underneath to stop it lifting when turning in the reverse direction (Like Boxfords) and is there someting wrong with this 'inverted' slideway. You say 800 rpm and tipped tool – thats a bit slow for a 1" bar ?

Do the tips have chip breaker so that you don't get long spirals of swarf that could damage the turned finish?

Are you cutting dry and what are the tool tips designed for.

Are the 'rings' due to cyclic vibration – you can sometimes hear this as a droning noise. Look at the headstock bearings / lubrication. When we turned thin walled tubes, we used to slip a bit of rubber inside the tube to stop it vibrating.

If you turn between centres, do the rings still appear.

So much could go wrong – we will all be intrested in what caused them in the end.

[Ady1Participant @ady1]

Intruiging

Looks like a leadscrew thread issue, try cleaning your leadscrew nut up, and leadscrew

That shouldn't affect a well adjusted saddle though, maybe the speed of cut varies slightly

—-

I've had marks like those those two brighter rings appear from time to time, on an otherwise perfectly good cut

Not 100% sure what causes them either, maybe some form of reasonance

My bed isn't exactly pristine either so that may be responsible

[HopperParticipant @hopper]

Saw a similar one to this somewhere recently and it turned out to be the headstock thrust bearing was set too loose.

It's worth a try.

[ScottParticipant @scott]

David

I had marks exactly like that on my C3 and it turned out to be because the stub shaft of the self-act gear was out of true. It wasclimbing up the gear teeth and then dropping back again as the carriage moved along. I had to bore and bush the apron to fit it in straight . That's clearly not the case with yours if it has just started happening but if, as Gray has suggested, the pitch of those marks corresponds with the pitch of the leadscrew I reckon a good clean of the leadscrew and the apron gears would bring an improvement (and check that the carriage gears are in the correct plane and their shafts are not bent or out of line).

Good luck

Scott

[Sub MandrelParticipant @submandrel]

I've seen this effect when using the topslide set over to machine a taper. It corresponded to whether I was turning the handle towards or away from the work, compounded by a slightly loose gib strip.

This makes me think that the comments on a dirty leadscrew/geartrain may well be part of the problem, but probably also worth cleaning and adjusting all the bearings surfaces as well.

Neil

[Ian S CParticipant @iansc]

I have a similar effect with my 1326BH lathe (its Taiwanese), but not always, and some times feeding by hand helps. I think it is vibration from the single phase motor, and somewhere back in the 90s there was an artical in ME about isolating the motor from the lathe bed by inserting a sheat of rubber between the motor's foot and the mounting plate, and using flexable plastic bushes around the bolts, the bolts are reduced a size or two to fit, link type belts help also.

Does your lathe have a separate drive rod and lead screw, I feel that using the lead screw for feed is somehow wrong, others will disagree, and I have no experiance with that form of feed. Ian S C

[Ed DuffnerParticipant @edduffner79357]

I was getting the same effect as the multiple, evenly spaced rings on a home made lathe. This was due to the lead screw being slightly eccentric and feeding through a threaded bush(half nut) in the saddle which had a radius on the internal shoulder. As the lead screw rotated it was forcing the bush against this radius and causing the saddle to rock.

It was slightly remedied by removing the radius on the bushing(half nut) allowing the bush to float around the eccentric curve. The lead screw eccentricity is caused by the bush that it passes through being drilled off center. This bush is part of the saddle feed winding handle.

[david kitchen 1Participant @davidkitchen1]

Thank you all for your suggestions, but there are a few things that I didn't mention.

The lathe has a 3ph 2hp motor run from a Newton Tesla inverter set up with variable speed and reversing.

The lead screw is fully covered and is only used for screw cutting. The feed has a seperate drive rod.

David

[Gray62Participant @gray62]

I have experienced this on my GH1330 lathe, and has also been documented by LatheJack (search the Albums for Chinese Torture), we tackled the problem in slightly different ways but the cause was the same.

The feed shaft drives a worm in the apron, where this worm is mounted, the feed rod passes through driled holes in the casting, no proper bearing surface, I found that the worm was not a great fit on the shaft and the shaft was moving around as it passed through the apron.

The solution was to machine a new worm with a better fit on the feed shaft and to put bearing on either side of the worm to support the feed shaft as it passes through the apron casting. This has pretty much cured the problem.

regards

Graeme

[Brian WoodParticipant @brianwood45127]

Hello David,

I've shot my bolt I'm afraid, over to others for more suggestions. I do think there is a clue in this being a fairly recent departure from the norm, what else might have happened or changed to bring that about?

It is probably just detective work now to eliminate things that will bottom it. I'm sure we would all like to know the answer when you have it!.

Brian

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