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Trouble turning brass by a very inexperienced engineer

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Vincent Shaw-Morton12/05/2010 19:34:43
17 forum posts
15 photos
Hi Everyone,
well my  new lathe is now in my workshop (new to me that is) and I am trying some initial turning of the brass bushes for a flywheel axel on a polly model.
I am encountering a problem, yet  I have used the lathe before without any trouble.
Anyway the problem is that on turning the brass rod I find I am getting a strange very noticeable ( to the eye and feel) rippling effect. The ripple peaks at around 1/8 intervals, but the actual finish is ok. I am using plenty of neatcut cutting oil and a hss/silver steel ( I can't remember which material, but it's a standard cutting tool that can be ground) The actual turning speed I haven't yet established, but should be able to do so soon, but it is as fast as the lathe will go (sorry about the vagueness on the speed) I am feeding by hand at a nice slow speed and have tried taking cuts of between 5-20'. The rod sticks out from the chuck about 1/2".
Any clues as to my problem?
thanks
Vincent
 
JasonB12/05/2010 19:56:47
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25215 forum posts
3105 photos
1 articles
Brass is usually cut dry.
 
Don't know what your top speed is but for that diameter 1000rpm should be fine.
 
Brass also likes a very sharp tool with little or no top rake (top surface of tool should be flat not ground to point upwards.
 
Make sure everything is rigid and that the tool is not sticking out the tool post too much.
 
Do you get it under power feed you may be lifting the carrage/slides as you turn the handle
 
And finally is it brass as Bronze or gunmetal are the usual materials for this type of thing.
 
Jason

Edited By JasonB on 12/05/2010 19:58:13

Rob Manley12/05/2010 19:57:18
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71 forum posts
14 photos
Is this facing the rod or machining the outer diameter?
Jens Eirik Skogstad12/05/2010 19:58:35
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400 forum posts
22 photos
The lathe tool has wrong angle of rake. Grind the tool who has 0 degree rake and do not use cutting oil.
 
The drill for brass/bronze must have 0 degree rake to prevent dangerous drilling. Grind away the sharp edge of drill to a 0 degree rake.
 
You need the book "The amateur lathe" by L. H. Sparey, there are a lot of information to use the lathe.
Terryd12/05/2010 20:51:01
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1946 forum posts
179 photos
Hi there,
 
Brass is normally very forgiving when turning and normally it is a lovely metal to work with.  Although you should use a zero rake tool I often can't be bothered to change my tool and still get great results using normal steel cutting tools with 7degree top rake, but it should be sharp (I usually touch it up with a slipstone).
 
You should cut dry, not take too small a cut and feed the tool faster than you think.  A slow feed can cause 'rippling' on shallow cuts.  Once you get this effect it will repeat itself and you need to get your tool cutting below the rippled surface to renew it and get a true surface.  don't ass k how I know
 
 
Terry 
John Shepherd13/05/2010 09:01:45
222 forum posts
7 photos
I think Terry is spot on about feed speed as I have noticed the same thing but I wouldn't rule out a non riged set up as a contributory factor as well. Are you locking the compound slide?
 

On the subject of using the right tool for the job I was the same as Terry and tended to use what was in the toolpost from the last job to a degree. Then I got a quick change tool post and now try to follow some good advice I was given: - ‘always remove the cutting tool from the machine when you have finished the job in hand - that way you will then select the right tool for the next job' This covers a whole range of tools with removable cutters including drills, jigsaws etc.

 

Edited By John Shepherd 1 on 13/05/2010 09:04:08

Terryd13/05/2010 11:54:22
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1946 forum posts
179 photos
Hi John,
 
I would of course use the correct tool under normal circumstances.  I don't have interchangeable toolpost but I do have a 4 way indexableable one.  What I was trying to emphasise that brass is so forgiving a material in most of it's normal states that it is not essential to use a zero rake tool.  I understand the theory behind tool grinding and rake angles depending upon the shear characterisitics of the material etc, but samples of the same spec material can vary in practice so we can only ever consider the theory as a compromise otherwise we would be regrinding tools all the time to obtain the perfect rake. 
 
There is often a discrepancy between the theoretical, and empirical observation (as in many aspects of life). and I often find that most engineering operations are a compromise between both theory and empirical evidence.  Many excellent practical engineers I have met and read, often contradict accepted practices and theory.  Examples of questioning accepted theories are in both Tubal Cain's works and those of GH Thomas amongst others.  There is a discussion of brazing vs silver soldering elsewhere and the theory of dezincification of brass has been aired.  Tubal Cain points out that  there have been many examples of brass boilers in the model and toy world which have served for many many years without problems, (Wilesco and Mamod boilers for example).
 
Having said that  of course I adhere to ideal unless experience has shown me otherwise.  What works is what matters.  At the end of te day it's horses for courses!
 
Terry
Ian S C13/05/2010 12:30:05
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7468 forum posts
230 photos
It wouldn't be unusual to have two bits of brass, one machines perfectly, while the other grabs. Both will usually cut well with zero rake, same thing happens when drilling, there was an artical in ME a few years back. Ian S C
Circlip13/05/2010 14:32:47
1723 forum posts
What is the swarf like?? turnings or fine needles ??
 
  Regards  Ian.
V8Eng13/05/2010 19:22:11
1826 forum posts
1 photos
For what its worth here are a few things that gave me trouble when first working brass, they all created the symptoms you describe:
 
Incorrectly centred tools.
Unsharp tools.
Tools extended too far from toolpost.
Large areas of workpiece not supported.
Speed completely wrong.
If feeding by hand on hard materials a lack of constant pressure can occur sometimes.
 
All this may seem pretty basic, but it got me at first.
 
 

 

Edited By V8Eng on 13/05/2010 19:27:35

Vincent Shaw-Morton14/05/2010 06:38:51
17 forum posts
15 photos
PROBLEM RESOLVED.
Hi all and many thanks for coming back to me, problem resolved.
The solution was definitely covered in your replys.
It was a combination of a relatively blunt tool, use of neat cut oil and too fast a feed speed.
I ground a new tool and made sure it had no top rake on it. The neatcut was aggrivating the problem, causing the tool to sip on the work, pushing the metal ahead of it. Once the wave had built up in height it would dig in and cut, this would reduce the wave height, the tool would once again then slip pushing the metal before it until the wave was so big the tool would once again dig in, slip, push, cut, slip, push, cut etc etc
A really sharp tool with no top rake, no cutting oil and a slow but steady feed speed and I am getting a mirror finish. BTW my spindle speed was 1340, as fast as my lathe will go.
Many thanks
Vincent
John Shepherd14/05/2010 07:55:51
222 forum posts
7 photos
Vincent
Glad you have sorted it out. Its typical that a combination of small things cause a problem!
 
Terry
I hope you don't think I was being critical about using the wrong tool - I was just comparing what you said with what I (and I suspect many others) do and get away with. It  was also a good lead in to passing on the advice I was given about removing the cutting tool after the job in hand.
You may have noticed I did say I 'try' to follow the advice, which I do never the less recognise as good practice.
 
I agree that there are many compromises made in the home workshop because of the machinery, tools and materials we have available. At the end of the day it is the results that count as your last sentence clearly points out.
 
John S

 

Edited By John Shepherd 1 on 14/05/2010 08:11:07

thomas oliver 217/05/2010 19:15:29
110 forum posts
Use CZ121 free-cutting brass in future.  Free-cutting mild steel also available (EN1A) and aluminium alloy - cant remember latest spec number.  All make life a lot easier. 
Stub Mandrel17/05/2010 19:20:39
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4318 forum posts
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 What I was trying to emphasise that brass is so forgiving a material in most of it's normal states that it is not essential to use a zero rake tool.
 
 I think in another thread on here I told how I had been having some trouble with stainless steel - until I realised I was using my flat topped brass cutting tool...
 
Neil

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