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30th October 1970
Page 31
Page 31, 30th October 1970 — road and workshop
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Which of the following most accurately describes the problem?

Eenchwise: lathe sense (37)

SO FAR in this series, in attempting to ease the path of the trainee in his approach to lathe work, I have where possible avoided entering too far into technology. I want him to have a go rather than worry too much at the start and then find his interest fading.

Nevertheless, some of the more hidden or finer points can now be examined to good purpose. particularly various metals and their behaviour when under cutting-tool lead. Obviously, in vehicle repair some materials to be machined will be in their new state and easy to work, others will have been reclaimed by welding which can cause tool trouble.

Let me first look at aluminium, for this material is now playing a bigger part than ever in component manufacture, and is also more responsive to welding techniques than it was a few years ago. Also, this material machines far better today, as the old trouble of metal building up on the top face of the tool is far less in evidence. However, where an old part is being turned this fault can occur and as the build up continues, so can the surface of the work be torn up.

One way around this to some extent is a slight work speed increase, as aluminium does require high speed for a good finish. The tool shape is relevant to this problem and tool rake and angle can often be increased to good purpose; both these factors play an equal part in avoiding surface tearing. But I would certainly recommend in addition that an ample flow of cutting oil is put right on the job. Water-soluble oils are frequently used and work well with paraffin added—in fact I would think paraffin gives a better finish.

Aluminium and its alloys can contain silicon despite their soft nature, and this is the reason for the tool losing its edge surprisingly quickly. And a dull tool can also be the cause of surface tearing.

Sand and slag Turning to cast iron, I will just deal with the points that can worry the turner, and these concern sand and slag; whether they are there because of cheap welding or the result of poor practice in the foundry does not matter, the point concerning an operator here being that they can cause the best tools to lose their edge in seconds. With cast iron the same tools are used as for steel, except that top rake can be zero or negative: cutting speeds are also similar to those for steel. However, when setting up an 3ccasional new casting for a modification, or dealing with a reclaimed one, particularly when about to cut a surface not previously machined, the feed for the roughing out cut should be coarse.

Therefore place the tool well below the surface and as the chips form this will break the surface ahead. Also the tool point has a better chance to travel below the sand layer, so the first roughing cut can be taken at anything between 0.025in. and 0.060in. and the tool will still survive, whereas a light skimming cut can bring instant trouble.

Care is needed in selecting a cutting or cooling fluid, as the dust can become a grinding paste, but a coolant is needed to keep this dust down as well as cool the tool. The trainee turner may be somewhat puzzled when using a normal soluble oil, as this quickly turns to plain water—or so it seems. What happens is that the swarf will filter out the fat, leaving the water behind. Therefore normal oils are unsuitable and a naphtha-base solution is required as this will hold together in passing through the swarf.

Quite a lot of copper-based alloy is in use in vehicles and as far as the turner is concerned he will find it handles in many different ways, some easy, but some quite troublesome, and the more pure copper you meet the more careful attention it will require. Pure copper is stodgy stuff to work.

Fine feed Tools for this material are again similar to those used for steel, with similar angle and rake. Work speeds, however, need to be high and a fine feed is called for. When dealing with a new casting the same approach should be made as with cast iron. ie a deep first cut to get below the abrasives left in casting. This first cut should, however, be taken at a moderate speed, but speed can be increased once the roughing cut has exposed clean metal.

Cutting oil needed A cutting oil is needed and this may vary with the alloy, but generally a paraffin-based solution will work well although. if dealing with pure copper which can cause a build up on the tool, a sulphurized mineral oil is recommended. It may be that the garage turner will at some time deal with a magnesium alloy, and this material is one that is at variance with all others he would normally handle for it has a flash point in the region of 538deg C. and there is a risk of fire under certain conditions where the swarf can ignite quite quickly—something like an incendiary bomb in that water will make it burn more fiercely. In the event of the material igniting it must be smothered with sand, earth, or iron swarf if that's handiest. Feeding ample cutting fluid to both the tool and the work is a safeguard as wet swarf is less likely to ignite.

The heat for ignition can be generated from too high a cutting speed, and it pays to keep the cutting speed below 500ft per minute, as above this speed very fine chips or dust may be produced that will increase the ignition risk. It pays to apply a fairly heavy cut to produce sizeable chips which arc far less likely to fire off. To avoid the chance of friction from tool rub, rake and clearance can be slightly increased provided that the tool cutting edge is well supported. But above all maintain a sharp tool for this material and on no account be sparing with coolant.

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