Lathe turning speed in relation to different metals

Hi Guys,

Currently I am going back to machining fundamentals.

Reason: If you don't have a grasp of these you are ill prepared.

So OK you are told different speeds for a given diameter of say mild steel, silver steel, brass or aluminium.

But can any one explain why it is advised to go at a given speed for a harder / softer metal.

Chris

The "best" speed is due to the relationship between the mechanical properties of the material (shear and yield strength, Youngs modulus, etc) and the mechanism of chip formation, also often limited by the properties of the cutting tool.

Practically speaking, it because that that is the speed that "works".

There is a lot of science involved. Basically it is how the material being cut reacts with the cutting edge of the tool. If you compare the same material with different cutting edge profiles it will give you some idea. Tool manufacturers such as Sandvik have numerous solutions and in production machines it may take numerous trials with a particular machine to get to an optimum tool life. You can search online and find many articles on the science behind it all. With small hobby machines and a single type of tool for everything much is trial and error, what works for doesn’t for another.

I would also add it's also the speed that removes the most metal in the least time at the best cost of cutting tool. They assume you have at least a Harrison 300, some tables list the HP require to achieve the speeds and full flood coolant.

Hence Mr Wheelers question if you saw it in time.

Nice to know for college courses but not really relevant to the man in a shed, ultimately 'it depends'

Edited By Dave Halford on 31/07/2020 20:47:26

"Hence Mr Wheelers question if you saw it in time."

The mysterious disappearing post! I wondered about that as it vanished almost in front of my eyes, without trace!

Sense and sensibilities ensuring inclusivity and diversity I guess?

fpm is directly scalable from a Sherline to a Holbrook but that is where the similarity and theory end and why cutting speeds and feeds are recommended and not compulsory, dependant on tool material and geometry, machine rigidity, the way the work is held etc. The chances of getting a consensus on the OP question is about as likely on here as agreement on which lathe is best.

Paul.

Someone has gone through the pain of testing how materials machine with different tool materials and cutting angles, clearances and speeds. They are not set in stone but provide a good start point, they will always be a compromise so feel free to deviate. If your tool fails prematurely then you have gone the wrong way, if the finish you desire is not being produced then you need to determine where your problem lays. In my experience the recommended speeds, feeds and angles are not far out. It is probably better to go with the recommendations of hobby authors than full on industrial machine recommendations.

Mike

On a small hobby machine it's often suck it and see

If you do a big piece cutting can be not very good on the outside and improves as you whittle it down

Not so easy to see subtle differences on a larger powerful machine because it compensates for more issues

On the other hand a unimat sl shows you everything, a unimat is great for showing you good tool grinding angles since you have almost no stiffness

Edited By Ady1 on 01/08/2020 08:02:59

I'm sure you can find more than enough explanation in books - remember them? However, in the home workshop, what works is what matters... Suck it and see.

Suck it and see also applies in industry. For example a lot of high temperature alloys can be machined but since very few businesses use them there is not real "knowledge". So manufacturing engineers have to find cutting speeds. The easiest way is to put a piece in a lathe.........

JA

.

Presumably removed by a moderator, as deletion of the content [by Mr Wheeler] would leave a trace.

I didn’t see it

MichaelG.

Try Sparey,s book, still good after all this time.

Simple Direct No bull.

Mr Wheeler did wonder where, and why, it had gone!

Mr Wheeler, I thought it was incisive and excellent advice-but it disappeared.

Than you for those that answered sensibly which was at a basic level the stock materials yield and strength have most to do with book suggested speeds.

I see nothing wrong with asking what others may take as obvious or just get on with it type of questions. The marriage of handed down knowledge with hands on suck and see experience is likely to produce superior results to just having a go.

I am a man in the shed in no rush to go any where. Those who like being rude or have a propensity to be intolerant on this forum may be impressing a certain type whilst alienating another.

Chris

I also thought it was good advice although very much straight to the point, a very similar comment has been made but not in such a direct manner.

Emgee

Chris,

The answers to most of your general lathe questions are, as Larry suggests in Spareys book and others, perhaps Jasons and Neils 'how to' series in the mag would be more to your liking.

The reason why most replies involve 'suck it and see' is that we have found out not all small lathes behave the same. I know you feel like you are missing knowledge that would make you better at turning and you are, it's what comes from trying stuff out on your machine.

Agreed, a little diplomacy often helps.

Chris

I find it much easier to get a good finish on a small lathe with a VFD as I can alter the speed while taking a cut. The finish will change with rpm.

But as mostly said above, it's trial and error, I know there are books and recommendations etc...but I'd rather make some swarf first, and then post the results and questions on the forum, probably you will get more meaningful answers.

Edited By ChrisB on 01/08/2020 15:52:27

Chris Tick Tock,

I hope you did not think I was being smart or rude with my reply, this was not my intention.

The reason I mentioned Sparey,s book is because, like you, I had little or no idea regarding cutting speeds/materials ect, but this book explains it all in plain terms. If it is now out of print, more,s the pity, since it,s a goldmine of information for beginners [like me ] No doubt there are many other good book available, I just never felt the need to look any further than this one since it seems to cover almost everything.

No doubt others here will advise you on the subject but the speeds given by Sparey are ;

Stainless Steel 50ft per min

Carbon Steel 60

Cast Iron,, Mild Steel Wrought Iron 80 ft per min

Brass 200

Aluminium and its Alloys 300.

To calculate revs Cutting speed divided by quarter of work diameter= r-p-m

Sorry if I sounded like a Smart-Arse, that is far from the case, I can assure you !

some people find the following to be helpful as a starting point for feeds and speeds: **LINK**

you use a ruler, to indicate the correct speed, like so: **LINK**