finish when turning

[pmdevlin]

Very basic question here, when I'm turning on the lathe, doesn't matter what material, brass, aluminium, the finish is very, well best described as scratchy.

I have just use a new index, doesn't matter what speed, by hand or using the auto feed (which is marginally better) the finish is poor, I then have to finish with different grades of emery cloth, which means the finish is then dull. I remember my mentor, when he turned, the finish was always crisp and perfect.

I am relatively new at this, until now, the finish wasn't that important, but now I need to turn some stainless shaft and have a nice finish as its for an rc project which needs exact tolerances, so I want a good finish.

Any ideas appreciated

thanks

Paul

 

[goldstar31]

Paul
It is a well known saying but the finish is no better than the finish of your lathe tool.

As you are a newcomer, might I suggest that you read the remarks in Conrad Hoffman''s suggestions on his site.- Advance Tool Sharpening ?

There is a lot of blether about sharpening ALL the faces of a lathe tool. In fact, a lathe tool need only be honed for the Depth of Cut. It might be a few thousands of an inch! The rest of the tool is for all sorts of reasons like rigidity and cooling but it doesn't need to be honed. Use a cutting fluid if you want but I settle for the old fashioned lard oil for steel.

If you hone- and I don't write grind, you need a very fine stone. Perhaps Arkansas or a fine diamond one but you should aim in a reflecting your thumb nail in the cutting edge. If you angle your tool with less than 3 degrees to almost rub, you should create the finest steel wool which will crumble if rubbed between your fingers.

Does this help?

Regards

Norm

 

[Hopper]

Quote : " I use a new index"

Presumeably you are using a small home hobby lathe. You may find High Speed Steel tool bits give a better finish. Most small lathes do not have the rigidity to do well with indexable carbide tools. Also good quality brand name indexable inserts may help, as may using the ground-finish inserts some manufacturers supply for turning aluminium etc. They work well on steel on small lathes.

 

[pmdevlin]

Hi Norm,

thanks for reply. My concern would be actually sharpening the tool by hand, and losing any specific cutting angle. I realise we are in the machining world, but having to sharpen a new tool, is that common practise, have I naively expected too much. As I cut, I take only a very light pass all the time, probably nerves thinking I am going to jam the lathe! I have a very very fine polishing stone, maybe use that?

Hopper, yes hobby lathe, Ill try some hss bits that I have, thanks!

Paul

 

[goldstar31]

Hi Paul
Thanks for your reply. As someone who never 'took up the tools as a job', I can understand your problems very clearly.
No one seems to explain that there is little or no difference in the various cutting tools in the workshop. Whether you are drilling a hole, milling or using a lathe, the same cutting action and the same demands are made. A mill is only a drilling machine and a drilling machine is only a lathe turned at 90 degrees. Consequently, a cutting edge will work if it . is say 5 degrees or 7 degrees regardless of the machine. In a small amateurs shop you need a SHARP edge because you haven't got the rigidity and horse power to push metal away. In Production terms, such things are important. You and I are concerned with using one tool to replace using a lathe and getting things to two thous oversize and then grinding the last bit to a 'mirror finish' The foregoing is 'old practice' but our tools are of that generation.
Further simplifying matters, you should be able to have a lathe tool which will remove the hairs on your arm - or laughingly, the pubic hairs on a flea!

To get a mirror finish, you can do it either by cutting followed by a succession of finer and finer abrasive grits or putting the mirror finish onto the tool and getting a tenth of a thous off a part with a honed tool. What you will not get- or I cannot, is a mirror finish from a lathe tool which has had a buffing from something which is not dead flat.

Apologising for probably telling you how to suck eggs, you can get surprising results with a flat topped lathe tool-having got the top mirror finished on a bit of dead flat plate or a bit of glass and then honing the cutting face. Once you UNDERSTAND just how little one needs, you can concentrate of adding the improvements.

I hope that this is constructive.

If you want me to try to explain how to get cutting angles with a cheap 6" double ended grinder, then ask

Cheers

Norm

 

[django]

Always try to take at least .005" deep finishing cuts. Lighter cuts can result in the tool pushing away from the work then skidding on and then digging into the work surface giving a terrible finish. Hope this helps PS All other factors must right as well eg sharp tool on centre height, rigidity, speed and cutting fluid.

Paul

 

[Niels Abildgaard]

Hello Paul

Can You show us pictures of Your misery?
Will help getting help for You

 

[pmdevlin]

makes sense norm, especially the bit about not having the hp! Its a chester conquest, I got second hand a couple of years ago, but as time has passed, my expectations have increased, probably due to friends asking for that "little job" and its always that job that you want to do very well

I'm using (probably) cheap metal, with cheap indexable cutting tools. Today as I'm off work, I went rummaging through the tools etc I was gifted when my mentor passed away a couple of years ago. There are some hss tools (6mm I think) I don't even know which way round they go in the tool post, maybe Ill post a pic for you all to advise!

Django (great film by the way!) you are probably on the money here, scraping along is what I am doing, maybe I need to be bold and actually cut something on one pass!

Now I'm getting fussy with the lathe, I see it has play and slop that has developed, especially when parting so its probably time to give it some tlc, I don't even know where to lubricate it!

 

[Charles Lamont]

Always try to take at least .005" deep finishing cuts. Lighter cuts can result in the tool pushing away from the work then skidding on and then digging into the work surface giving a terrible finish. Hope this helps PS All other factors must right as well eg sharp tool on centre height, rigidity, speed and cutting fluid.

Paul

This is just not true. With a properly sharpened and set high speed steel tool, I can often take a cut of less than 0.0005" in decent material.

 

[goldstar31]

This is just not true. With a properly sharpened and set high speed steel tool, I can often take a cut of less than 0.0005" in decent material.

This is quite achievable and seems to be now forgotten and better. It is possible to get that 'barely perceptical swarf' which the old timers like Cleeve published.


Years ago, the late Jim Early and I tried to republish and were threatened with legal action by the then new owners of Model Engineer.

And a Merry Christmas to all our readers:wall:

​

 

[deverett]

If you are worried about your tool sharpening ability, have a look at GadgetBuilder's Vertical Shear tool. This can give a very good finish.
http://www.gadgetbuilder.com/VerticalShearBit.html

Dave
The Emerald Isle

 

[goldstar31]

If you are worried about your tool sharpening ability, have a look at GadgetBuilder's Vertical Shear tool. This can give a very good finish.
http://www.gadgetbuilder.com/VerticalShearBit.html

Dave
The Emerald Isle

What rarely seems to be asked is 'How does one remove a tenth of a thous' when all one has is a lathe with thous engraved on the feed screw dials? Obviously, moving a slide one division ie a thous, means that the lathe will cut TWO thous. Can we improve on what we have got?

Well, I have lathes long past their sell by date- and arguably a bit shakey.

What can be done without a lot of effort to remove a gnat's cock of metal rather than 2 whole thous? I doubt that few of us have the gear to remove an exact tenth of a thous but removing that tiny dusting of metal is practical.

Whilst there is a mathematical formula developed out of the 6th proposition of Euclid, it is doubtful that no matter how clever we are, our machinery will fail us. Well, like mine??????

However, the math or maths suggests that if we set the top slide over by 'about' 5 or 6 degrees, that whisker of metal can be removed by using the top slide only.

Does this help?

Norm.

 

[Hopper]

There are some hss tools (6mm I think) I don't even know which way round they go in the tool post, maybe Ill post a pic for you all to advise!

You might get yourself a copy of LH Sparey's classic book "The Amateur's Lathe". It has the best illustrated rundown on grinding various HSS lathe tool bits that I know of. The rest of the book is worth its weight in gold too. Essential reading for every aspiring (and existing) lathist.

If you are going to use a lathe, you need to know the basic principles of how it cuts metal. Might as well start now!

Also, on depth of cut with indexable carbide tools, often these will give a better finish with a deeper cut than with one of just a few thou. They seem to like a bit of work to do and not to be just rubbing on the job. But I find that HSS finishes better on my ancient hobby lathe that is not the most rigid or wear-free of critters.

 

[django]

This is just not true. With a properly sharpened and set high speed steel tool, I can often take a cut of less than 0.0005" in decent material.

What isn't True? Also Why would you even want to be taking half thou cuts when you can polish it off in a matter of seconds? Sounds to me like your not taking the right finishing cut in the first place? Maybe the old adage of measure twice cut once should be applied.

 

[Charles Lamont]

What isn't True? Also Why would you even want to be taking half thou cuts when you can polish it off in a matter of seconds? Sounds to me like your not taking the right finishing cut in the first place? Maybe the old adage of measure twice cut once should be applied.

It is not true that one needs to take a 5 thou cut to get a good finish. I know what I am doing. Well, sometimes. Evidently we work differently, which is fine.

 

[django]

It is not true that one needs to take a 5 thou cut to get a good finish. I know what I am doing. Well, sometimes. Evidently we work differently, which is fine.

I didn't say that you needed to take 5 thou cuts only try to to avoid the rubbing that can occur when using indexable tips especially on poor quality steel, that's all.

 

[Blogwitch]

Try taking a 2 thou cut on something like Delrin and see what sort of mess you get, then try 20 thou and be amazed at such a great finish on the same material. That is mainly due to getting rid of the heat generated by friction, the shallow cut can't do it, the deep one gets rid of it in the shavings.

This is the same with most materials, a decent cut usually ends up with a good finish while shaving doesn't. The only time I have come across anything different is when I am boring on both the mill and lathe.
I always try to do a fairly deep cut, say above 5 thou, sometimes a lot more, to get down to exact size.
This is with HSS BTW, with moulded tungsten, you will be very lucky to get a decent finish on a small machine with a shallow cut.

John

 

[bazmak]

Never tried to take off 1 tenth thou,why would i.If you need great accuracy
then turn to +1 or 2 and polish off Simples

 

[goldstar31]

Never tried to take off 1 tenth thou,why would i.If you need great accuracy
then turn to +1 or 2 and polish off Simples

I was following through the drawings of the Acute tool and cutter grinder and noted some tolerances were 'tight'. Whether these are needed to be held remains to be seen.

However, when it comes to boring, the expedient of 'papering' is difficult- to say the least. I recall several 'nasties' when boring cast iron for such things as the Quorn and the Mark 1 version of Versatile Dividing Head and- OMG- the Westbury mill drill when I split the bored casting. They nipped:fan:

Way, way before this, I was obliged to sample in tenths to do the statistical analysis - to set up production machinery. You know this three standard deviations from the mean malarkey calculations. Not everyone's cup of tea but I recall messing about with 'real' engines- the 12" to the foot variety when 3 thous meant - knackered!

Now where did I leave my 'guessing stick'?

Norm

 

[bazmak]

agree Norman,i too had to work in tenths way back when we all did proper engineering.My point being that with home engineering and cheap chinese
lathes I have not yet nor do I see the see the need to work in tenths
If I need to make a cylinder and piston to better accuracy then I would do the bore and then the piston to suit. As an apprentice with unlimited equipment and measuring equipment its not difficult apart from failing eyesite and shakes
Merry Xmas