Cutting Speeds/Feeds

Now am totally confused - easily done these days.

On another thread there was a link to Tomstechniques.com website in the USA, a very informative site, very interesting for the beginner, and on one of his videos he discusses cutting speeds. So for mild steel - to which I took to be the BDMS or Black MS we use here in the UK - he suggests a cutting speed of 100ft/min, giving an RPM of 640 for a 5/8in, or 16mm, cutter.

However, I have been going by Harold Halls 'Milling, a complete course' book, and by 'Model Engineer's Handbook', who suggest cutting speeds for a 5/8in or 16mm cutter achieved by RPM of 375 and 385 respectively, which has worked pretty well in my mill I have to say.

But why is there such a variation? And what RPM do the team think is a good One for this size cutter?

Furthe query on feed rates, the milling feed rates recommended I see suggest (to me) very fast feed rates, up to 3in/min. What formula determines feed rates?

Chris

Edited By ChrisH on 02/04/2014 10:51:58

Because different people have different views! Personally I'd run a 5/8" HSS cutter at around 600rpm for low carbon steels, ideally with flood coolant.

Feedrates are set by the number of flutes, the rpm and the load per tooth. So if we say we want 2 thou per tooth with the above cutter, and it has three flutes, we get a feedrate of 3.6"/min. That's pretty slow, I'd probably be running twice that, but I have an industrial size vertical mill. So you have to adjust the feeds and speeds to suit the mill, starting with a sensible load per tooth. If the mill is small you may well end up with lower speeds and feeds in order to maintain a proper tooth load while not exceeding the machine capability.

In summary it all depends on your mill. Obviously there are formulae for feeds and speeds, but they are not set in stone for all mills and cutters. You need to adjust the input numbers to suit your mill.

Regards,

Andrew

In the home workshop I recommend 500 rpm for a 1/2" dia HSS cutter ,1000 rpm for a 1/4" dia 250rpm for a 1"dia and so on. Colin

As already stated, it's worth remembering that industrial speeds are maximum limits and are geared towards maximum production even if at the expense of shorter tool life, since tools are cheap and machine time expensive. Also those for HSS presume flood coolant. In the home workshop the priorities are usually reversed, and flood coolant isn't universal. Reducing to half or two thirds of industrial speeds makes a very considerable increase in tool life. The only caveat with carbide is to run fast enough to stay out of the built up edge zone.

The short explanation is that if you run a cutter too fast the biggest danger is not feeding it fast enough and making the teeth rub and quickly blunting them. Many authors will suggest slower speeds that work perfectly well and make hand-feeding a rather less traumatic affair.

Another explanation is that faster cutter speeds & feed rates = faster metal removal rates and therefore higher loads on the machine. Small hobby machines may not be able to give their best under such loads and results will be disappointing. My experience is that nothing will tell you more about the 'happiness' of the cutter than feedback through the handles. It is this that sets manual machining apart from CNC where he 'tables' are of far greater importance.

Neil

Thanks for all the responses, it has helped me no end

Andrew, I came back and read your reply after trying a run at 600 rpm for the 16mm end mill, 3-4mm cut at 9mm deep, using a feed rate I later measured at just a tad over 3in/min using pretty near flood cooling using neat cut oil. I say pretty near, the table floods if I use too much, but I need a good flow to cool the tool/job and stop the smoke, so it's a balancing act. I am pleased to say it worked out very well, then read that it was roughly what you had recommended, so very pleased to have found that. Good surface finish too.

I am still without a lot of mill experience so it's very early into the 'touchy-feely' learning curve, but I know what you mean John from other disciplines, so learning, but not there yet!

I guess it is all suck it and see, what works and what doesn't, and being aware of the difference between commercial and home workshop practices.

Cheers

Chris

Edited By ChrisH on 03/04/2014 13:52:09

80 to 100 ft per min on mild steels is a good guide for hss cutters,any faster and the cutter life is rapidly reduced,you can go a lot slower,it takes longer though the cutter life is extended,

I would point out Chris that, at the start of the paragraph detailing the speeds, I say "As a starter I would make the following recommendation;" Whilst a few lines further down, having detailed the speeds, I say "With experience you will no doubt be happy to increase these values, probably quite substantially "

Harold

Harold, delighted to have you posting on this forum again! I have followed your book on milling and have had no cause to disagree with what you have written; things like speeds that you have suggested have served me well so far.

It was just reading/seeing on another website quite different speeds quoted made me question what might be right or not; realising that the home workshop machines would not have the capability to match the more 'professional' machines were slower speeds the more appropriate? I feel this is indeed the case here. I feel this is quite possibly so as it was significant that the speed and feed rates I employed in my last post were only achievable on my mill if I used a lot of coolant/ lubricant, but I was looking to see if I could increase my work rate as I am a very slow worker! The use of neat cut oil enabled a faster work rate, whether I would continue down that path in the future is debatable as it is more messy and takes longer to clear up as you go along, so it's swings and roundabouts. I spend less time machining and more time cleaning up!

As I said earlier, I am still at an early learning stage, not only in how to do stuff but also in what is good/achievable for my machines, but it is very interesting and informative to hear what others, more experienced than me, have to say on what works for them, and seeing how that might be applied in my case.

Chris

Edited By ChrisH on 03/04/2014 23:01:06

Edited By ChrisH on 03/04/2014 23:03:06

Last night Harold Hall had posted a reply on this thread to which I replied.

This morning his reply has gone, it's just not there at all. Makes it look as if I made it up.

What is going on?

Chris

Hi All

A while back I made a basic excel spread sheet to calculate speeds and feeds.

You can unprotect and modify the sheet if you like, or just change the settings in the yellow boxes

Set the pink boxes to your machines speeds and feed rates.

The Green boxes indicate available combinations of speed and feed for the settings chosen.

**LINK**

Regards
John

Edited By John McNamara on 04/04/2014 16:43:35

You can find threads on other forums revolving around cubic inches per horsepower. You can up the speed, which means you have to up the feed rate but then your machine just doesn't have the power to do that much cutting. Amateurs are best served with fairly small cutters so this doesn't happen.

Hi Bazyle

Do you think 700W per cubic inch of metal removed as used in my calculator is correct? the figure is a little arbitrary. a blunt cutter, workpiece toughness, among others will change the amount of power needed.

The white boxes are speed and feed combinations that are within the feed per tooth range specified

Note: This tool is machine specific, you set it up for the feeds and speeds available on your machine the pink boxes, as I have done for my Shizouka mill. If you have several machines set a different one up for each using the settings available for that machine and rename it to your machine .XLS. 

Regards
John

Edited By John McNamara on 04/04/2014 17:57:18

> What is going on?

It was all a dream, Chris.

Actually I can see Harold's post, I don't know why you can't? It hasn't been blocked or edited.

Neil

G'day, I can see it all the way here in Australia.

You didn't hit the 'ignore member' button by mistake?

Regards * Danny M *

There's a lot that is a dream here Neil!

I still can't see the post Danny, but if it's visible in Oz, well, I must have hit the 'ignore member' button, perhaps when I edited the post on my iPad - (big fingers, little spaces, clumsy operator, late night, the worse for drink, etc etc).

Well, you live and learn, never knew what the 'ignore member' button did before!

Chris