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Poor surface finish

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blowlamp12/05/2014 14:03:42
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1215 forum posts
82 photos

Let's see some pics of a few different test metals and the tool you're using to do the cuts. I can get that pattern with any lathe by using a blunt, too large tip radius, not at centre height tool, or some hard to machine material or at too high a speed.

Martin.

Bikepete12/05/2014 16:16:59
225 forum posts
16 photos

It may be a bit of a last resort but I'm wondering if that could be 'single phase motor vibration' - a pattern from torsional vibrations feeding through from cogging. Bit impractical to suggest trying a three phase alternative but as per this PM thread:

**LINK**

you could maybe try with belts very slack to see if that helps... assuming other owners of the same machine don't see it (also on single phase) it's possible you have an especially poor motor in that regard...

WALLACE12/05/2014 19:20:01
304 forum posts
17 photos
It's definitely cyclic.

Could you not work out the frequency from the diameter, rpm and spacing of the pattern ?
If it's a multiple of 50hz, then I would have thought - like Bikepete - it was motor related.

Or... another idea.... a off centre cog somewhere in the feed gear train ?

Ringing you'd probably hear.

W.

W.
Martin Cottrell12/05/2014 23:03:58
296 forum posts
18 photos

Thanks again guys for all the suggestions. Further to my phone call to Warco last week it was suggested that I tried turning something between centres to see if the pattern was replicated using that turning method. So, with that in mind I set aside Saturday afternoon for a play. I started by fitting the new 5-3mt reducer into the cleaned headstock taper and then fitted a brand new centre into the reducer. On turning on the lathe there was a visible wobble at the tip of the centre, my DTI registered a run out of 0.005" so I set over the top slide and skimmed the centre until it was running true. I have a centre drilled & surface ground test bar which I put between centres to check the tailstock alignment. Applying the DTI to the top of the bar highlighted that the tail stock centre is higher than the headstock, rising by 0.003" over 6" of travel towards the tailstock, not sure what I can do about that but that's another problem for another day.

Anyway, whilst the DTI was set up, I decided to mount it at the rear of the test bar and check for any movement of the needle as I ran the lathe at its slowest speed, happily there was only a barely perceptible movement of the needle, well below 0.001". I then decided to apply powered feed ( 0.003"/rev) to the carriage to mimic the path of the tool and immediately the DTI needle went crazy showing deflections of around 0.010-0.015" so rapidly that the needle was almost a blur. Disengage power feed and the deflection stopped, feeding the carriage manually caused the needle deflections again, infact just turning the carriage hand wheel to the point of taking up any backlash without actually moving the carriage gave the same result.

It appears to me that the motor is causing a severe vibration which is somehow transferring to the carriage when it is set in motion. By this time I had run out of available spare time to continue experimenting but have in mind to check the saddle gib adjustment again and perhaps to remove the motor belt completely and see if the problem replicates with just the motor running on its own. I am not sure that this phenomenon would cause the helix pattern on the work but it does seem a step in the right direction! Any comments would be gratefully received!

Regards, Martin.

blowlamp12/05/2014 23:47:42
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1215 forum posts
82 photos

Still looks like chatter to me - as I said before, try turning some different metals with sharp tooling.

Martin.

Chris Trice13/05/2014 00:34:41
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1362 forum posts
9 photos

Try turning it at a much slower speed. If you've got slack in the saddle allowing .010" to .015" of movement, you need to seriously adjust your gib strips and/or feedscrews.

WALLACE13/05/2014 01:20:10
304 forum posts
17 photos
What's it like if you manually feed ? I'm not familiar with your lathe but my l5 has a rack and pinion drive on the saddle that can be used to take cuts - the trick is to always keep one hand on the hand wheel so it's always moving at a consent speed with no stoping.

If that's ok, then it's something to do with the feeds - and I'm still thinking a gear that's been bored off centre somewhere...

Chatter you can usually?hear - I once bored out a 3" diameter stainless bar and that rang like a bell - continuously !! I've generally found that chatter produces a finer pattern but that could be dependent on the lathe, tool geometry, speed etc etc so wouldn't guarantee it...

W..
Nigel McBurney 113/05/2014 10:33:48
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623 forum posts
3 photos

I dont know how experienced you are,but I would suggest you try to find a really experienced turner for a second opinion ,if they cannot get good results then I think that you should dump the lathe on warcos doorstep stating that the machine is not fit for purpose, the tailstock height difference is a fault that should reject a lathe, I googled the lathe type and it appears tp have a vee and flat bed,a saddle traversed on this type of bed should not cause any of the deflection described particularly when traversing by hand .

refering to the comment on chatter and ringing when turning thin tubular shapes, this will occurr on any lathe,its the nature of the bell like shape, the work needs some vibration damping applied, during my aprenticeship I was making some 3 inch dia micrometer drums in brass . I bored them out ok but when turning the od and the taper where the numbers go it really sang and vibrated ,foreman showed me what was apparrently an old trick,fill up the Inside Dia with a thick greasy rag and ram it tight with a screwdriver handle ,vibration ceased, Of course depending on the job ,internal chatter on a thin bore is more difficult to cure ,a damping material can be clamped on the outside dia, or a larger piece of material used,bore to size,vibration should not occurr with the thicker wall material ,then turn the od to size.

Chris Trice13/05/2014 11:19:03
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1362 forum posts
9 photos
I turned up some rocket engine bells (non working) in aluminium and the ringing was deafening when boring them. Two thick rubber bands wrapped around the outside killed most of it.
Martin Cottrell16/05/2014 23:47:22
296 forum posts
18 photos

image.jpgHi all,

Well, after more investigating today I think I have finally nailed the problem and as I suspected it is motor vibration that is causing the problem. I started by repeating the previous experiment but removed the motor drive belt to eliminate the gearbox drive train and spindle bearings from causing the vibration. The vibration was still just as severe when the motor was started so as I final trial I slackened off the motor retaining bolts so that the motor was no longer securely clamped to the back of the lathe bed. On starting the motor again there was no vibration and the DTI needle stayed motionless as I traversed the saddle along the bed. I refitted the drive belt with the motor still loosely mounted, refitted the 3 jaw and chucked the original test material and took a test cut using the same tool, speed, feed and depth of cut as before and to my relief obtained what I feel is a very acceptable finish! The image above shows the original poor finish with the improved finish on the smaller diameter using the same tool & settings as described previously.

I shall now contact Warco again on Monday and see if I can extract a new motor from them!

Regards, Martin. ( no longer p***ed off!)

Neil Lickfold17/05/2014 00:13:30
575 forum posts
102 photos

A part from motor vibration, also check the simple things like the pulley running true on the motor. On my lathe I had to bush and rebore the pulley. I have a piece of ruber between the motor and mount plate . I also have some rubber washer made also to help it become isolated. Now I can have the motor running and the dti does not move on the work piece.

I also changed the one piece belt to a segmented belt. When it comes around to replacing the current head stock belt, it will be replaced with the segmented belt as well.

My lathe had a problem with the adjustment of the 1/2 nut alignment on the lead screw. I put a dti on the screw and adjusted it , so the screw was not pushed in an up or down position. This did help over all .

Neil

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