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Poor surface finish

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Martin Cottrell05/05/2014 21:02:03
296 forum posts
18 photos

image.jpgHi all,

Today I finally got to set up my new (bought last May but only just installed) Warco GH1232 lathe. I spent quite a long time levelling the lathe then decided to put a piece of 1" mild steel in the 3 jaw chuck and have a play.

To cut a long story short I am getting an odd spiral surface pattern on the workpiece as shown in the photo. The best way to describe it wouldn't be like the appearance of the red & white spiralled signs that you occasionally still see outside Barbers shops.

I have tried a newly sharpened HSS tangential tool and also a new carbide insert and both give similar results. I've also tried substituting the mild steel with brass but still get a similar result. Changing speeds, feeds and depth of cut seems to make little difference. Gibs all seem correctly adjusted, tool securely clamped in a heavy duty QC tool holder and set nicely on centre height. The spiral pattern shown occurs when using power feed but a similar but not so regular finish occurs when feeding the tool manually with the saddle feed wheel.

The piece of stock shown is approx 1" diameter and the finish shown was with the HSS tool running at 300rpm 0.003"/rev feed & 0.010" depth of cut. Any ideas as to how to improve the finish would be gratefully received.

Regards, Martin.

Phil P05/05/2014 21:14:27
518 forum posts
137 photos

Is the pitch of the helix pattern on the work-piece the same as that of your leadscrew ?

My Myford was doing something similar exept the pattern was a series of rings on the same pitch as the leadscrew but not a helix, I messed around with the leadscrew and halfnuts for ages with no improvement. It eventually turned out the saddle gibs needed adjusting and the problem disappeared.

The helix pattern suggests some vibration, maybe in the head stock bearings ?

Make sure you eliminate or correct each problem logically one at a time if you can.

Phil

Rick Kirkland 105/05/2014 21:29:12
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175 forum posts

Martin, I bought a 1236 in January and upon taking a half decent cut I experienced serious chatter and similar patterns on work. This was all due to none of the gibs and slides being adjusted correctly, in other words they were all loose and the problems dissapeared when I got it adjusted correctly. Hope this helps.

Rick

Saxalby05/05/2014 21:40:56
156 forum posts
14 photos

Hi Martin,

I had exactly the same problem when I first got my Boxford ME10. The problem was caused (at least in my case, by motor vibrations through the drive belts. I found that if I slacked of the tension the helix pattern disappeared. But then there was insufficient power to the spindle. I read a suggestion that the linked belts, NutLink is one brand, gave a smoother transmission than the solid belts fitted as standard. Fitted the link belts and problem gone.

Regards Barry

Martin Cottrell05/05/2014 21:56:59
296 forum posts
18 photos

Thanks for the speedy responses guys! You've given me a few things to check out which I'll try over the next couple of evenings and report back.

Kind regards, Martin.

Ian S C06/05/2014 12:08:29
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7447 forum posts
230 photos

Martin, Go to the top of the page and on the left you'll find a window with Keyword in it, put in poor surface finish, and go right and click on go, the bottom two (out of 4) threads are worth looking at. Ian S C

Martin Cottrell07/05/2014 22:53:44
296 forum posts
18 photos

Well, I've spent this evening checking jib adjustments, headstock bearing adjustments and various combinations of slack & tight drive belts and cannot get the surface finish to improve although the pitch of the helix changes between a slack or a tight belt. I have even tried running with just one belt instead of two as one appears to be slightly tighter than the other when fitted together. I've tried both manual and power feed with the saddle and neither makes any significant difference. Running my DTI across the helix shows a difference in depth from crest to trough of just under 0.001".

Any further suggestions would be gratefully received. In the meantime I think I will phone Warco tomorrow and see if they can shed any light on the problem.

regards, Martin. ( Fairly p*ssed off at present!)

Nick_G07/05/2014 23:08:22
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1808 forum posts
744 photos

.

A bit of a random one here.

With it being a new machine. Is there oil in the headstock.? Oil acts as a damper as well as stopping gears eating each other.?

Nick

Martin Cottrell07/05/2014 23:12:39
296 forum posts
18 photos

Hi Nick

Yup, plenty of oil.

Martin.

Ed Duffner07/05/2014 23:21:43
730 forum posts
61 photos

Hi Martin,

I had similar marks on MS and aluminium when I had movement in the spindle on a home made lathe. I was wondering though if you would get the same pattern if you take a cut with your compound slide?

...or whether the gearing on your saddle might be meshed too tightly with the rack causing a slight flexing as the teeth mesh along the rack?

Cheers,
Ed.

Ady108/05/2014 00:51:33
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3463 forum posts
513 photos

Yup. spindle trubbles

Looks just like engine turning

Neil Wyatt08/05/2014 09:46:21
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Moderator
16655 forum posts
687 photos
75 articles

All obvious things have been suggested. ED's suggestions are worth following up.

The problem is we've all seen that pattern, as it can be caused by a multitude of issues - anything that causes the cut to vary over a regular interval.

Neil

Nigel McBurney 108/05/2014 10:18:24
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614 forum posts
3 photos

Why should a new lathe require adjusting? as you have checked most of the commonly known faults have a look at the three jaw chuck, the severe chatter on the work is quite common on well worn or strained chucks where the jaws are gripping at the back of the chuck but not gripping at the front,of course it should not happen with a new chuck ,the chuck jaws may not have been ground parallel to the axis of the chuck. ( perhaps the far eastern makers have a rubbish grinder or operator)I have assumed that you have fitted the chuck correctly onto the spindle. First try fitting up the four chuck,mount the steel bar and turn at the same speed and feed and see what results you get . you could also try mounting the bar between centres and turning, though with 1 inch bar between centres speed should be reduced to 200/250.rpm.

If you achieve a good finish then its the 3 jaw chuck at fault, to confirm it place some paper between the jaws and work piece at the front of the jaw and see what the bar machines like. I have a 10 inch four jaw on a Colchester master, I bought the chuck second hand and for years it has only been used to hold castings and irregular parts,customer wanted a quick job so to save a chuck change I used the chuck to grip a 1.5 inch bar and got a result similar to the chatter in the photo.The work was only gripped at the back of the jaws, change chuck back to near new 3 jaw and back to normal .One may ask why have I got a crap chuck ,well I also have a Colchester Triumph which I bought at an ex gov sale and it came with a brand new set of chucks ,faceplate ,centres etc so I do have a lot of options ,though I find the Triumph hard work being well past retirement ,the Master is my preferred lathe.

Martin Cottrell08/05/2014 21:40:15
296 forum posts
18 photos

Some great replies here chaps, thankyou, & together with a conversation with Michael at Warco I have a few more avenues to explore. Unfortunately work dictates that I will not have time to play until the weekend now but I will keep you posted on any progress (or lack of)!

Thanks once again but don't run off as I will probably need more help in due course!

Kind regards, Martin.

julian atkins08/05/2014 22:38:19
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1219 forum posts
353 photos

hi martin,

i would try dropping the speed and checking that your lathe tool is well honed and isnt rubbing and double check the tool is at correct centre height. ive never used carbide insert tools so cant comment on whether yours is working correctly. ive never had this problem with steel, but a few times with large lumps of hard drawn phos bronze when ive been a bit 'gun ho'.

cheers,

julian

Ian S C09/05/2014 11:31:05
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7447 forum posts
230 photos

I managed to get a similar pattern on a hole I was boring today. I was happily boring the hole out from the drilled 28 mm, to 35 mm with a HSS tool cutting dry, the finish was good, but I thought maybe it will be better if I lubricate it with soluble cutting oil for the last cut of .005", and then is when it happened. Instead of cutting oil, I should have taken the tool out and sharpened it. Ian S C

Nick_G11/05/2014 23:13:49
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1808 forum posts
744 photos
Posted by Martin Cottrell on 05/05/2014 21:02:03:

I spent quite a long time levelling the lathe

Ok it's level. - But do all 4 points where it touches the ground have similar pressure on them. (the head stock end will obviously have more) A lathe can be level with 3 of the 4 points under pressure. But if 1 has very little or none the lathe can vibrate very badly.

Nick

Nigel McBurney 112/05/2014 09:44:08
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614 forum posts
3 photos

A lot of precision machine tools (lathes ,grinders ,jig borers )sit on three feet so there is no chance of bed distortion or rock. If a modellers lathe vibrates when sitting on 3 out of 4 feet ,it probably the lathe is at fault rather than the mounting, its usually the spindle assembly or chuck out of balance ,If the workpiece is out of balance then the operator should balance the work or reduce speed.

Nick_G12/05/2014 10:38:39
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1808 forum posts
744 photos
Posted by Nigel McBurney 1 on 12/05/2014 09:44:08:

A lot of precision machine tools (lathes ,grinders ,jig borers )sit on three feet so there is no chance of bed distortion or rock.

I understand the point you are making. But surly the foot positioning is then a different pattern of triangle to allow for this.?

Nick

Ian S C12/05/2014 13:17:39
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7447 forum posts
230 photos

I'd put a shim under that foot, there's a chance that the bench could be a few thou out at one corner, maybe the bench needs the shim. Ian S C

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