Chris

In 1, are you confusing "volume of material removed" by a larger endmill with cutting speed ?

A larger cutter will always win on volume of material removed provided adequate machine specs are suitable.

Emgee

Agree with Mick.

Chris's first point is ambiguous: 'A bigger end mill can cope with higher speeds removing metal.' I'd restate it: 'A big cutter can remove metal faster than a small one.', which breaks the misleading link to 'speed' as used in No.2

It's cutter speed over the metal in metres per second that matters. Surface speed depends on RPM and tool diameter. To get the same surface speed a small diameter cutter has to be rotated faster than a big one.

May be a typo in Chris's Point 2 where he refers to Carbon Cutters running faster than HSS. Carbide Cutters can be run 5 to 20 times faster than HSS, not Carbon. Carbon Steel Cutters are run considerably slower than HSS and require careful attention to cooling because they can't take heat. These days most cutters are made of HSS, the obvious exceptions being Silver Steel, inexpensive taps and dies, and woodworking tools. (Which don't get hot!)

Dave

Just fill in the numbers and do the calculation.

Taking a cutting speed CS for steel as 100feet per minute we get

1/2" cutter

(4 x 100) / 0.5 = 400 / 0.5 = 800 rpm

1/4" cutter

(4 x 1000 / 0.25 = 400 / 0.25 = 1600 rpm

So from that you can see that to keep the materials cutting speed the same the RPM of a smaller cutter will be more than that of a larger cutter

For your 1mm cutter

(4 x 100) / 0.040" = 400 / 0.040 = 10,000rpm

Edited By JasonB on 23/10/2020 16:27:19

As you seem to mostly work in metric you may find it easier to just do a metric calculation such as

CS / ( Pi x Cutter dia in meters) = rpm but use a metric cutting speed

For example a cutting speed for steel in metric would be 30m/min so using metric 12, 6 and 1mm cutters you get

30 / (3.142 x 0.012) = 30 / 0.038 = 796rpm for the 12mm cutter

30 / ( 3.142 x 0.006) = 30 / 0.018 = 1592rpm for the 6mm cutter

30 / 93.142 x 0.0010 = 30 / 0.003 = 10,000rpm for the 1mm cutter

Edited By JasonB on 23/10/2020 16:52:26

I've just changed the pulley on mine to give a maximum 10,000 rpm

Maximum end mill size on the Sherline is realistically 1/4" and for that I would run at your maximum speed all the time except when fly cutting.

Stay well,

Rod

That would depend on what cutting speed they chose to use for steel, it's quite a wide range, coatings or lack of would also determine speed . Type of cut would also comes into it as a full width cut is usually run slower than a side cut.

These recommended speeds are generally maximums for industrial use where time is money. For a tool material such as HSS they are the speeds at which you can expect overheating to occur which will reduce the working life of the cutting edge. It will not hurt the tool to be running below the recommended speed but there may be other issues such as the finish of the cut surface being poorer. For small cutters the biggest risk is that running them slowly requires a very slow feed rate to keep the chip load acceptable to avoid broken bits. Not too hard to achieve with well controlled motor drives or CNC but jerky hand wheel movements can snap a small cutter with ease.

Martin C

I just gave a generic speed for "steel" using HSS to show Chris how the numbers work in the formula, Speed could have been anything from 50 to 150 feet per min ( 15-50m/min) depending on what steel and type of cut, type of cutter, any coating, etc as per my previous comment. If you take a look at the likes of Tubal Cain in Model Engineers Handbook he gives us 2000rpm for a 1/2" cutter in free cutting steel in which case 800rpm is a bit tame

Treat published speeds as a starting point and adjust to suit machine and job in hand and as I tend to find with teh smaller machines they are best run towards the fast side but with reduced DOC and possibly feed to keep the motor in the sweet spot and to compensate for their lower rigidity.

There’s fundamental problems with Tom’s tables in the past I have used them but i became exasperated with my copied tables until I realised the error
why? His speed for a 1/4” 4 tooth endmill is 960 in steel with 1 -3/4” feed , yet for a slot drill it lists 1600 rpm and a feed of 3” min. He has the headings the wrong way round! If you follow this point then the tables are very low in cut per tooth

using the 4*cs/ dia yields 1600 rpm ( cs= 100 fpm) Using a feed per tooth as recommended between 0.0005” and 0.002” this yields a feed rate of

fr= rpm x no of teeth x feed per tooth

= 1600*4* 0.001= 6.4 Inches per min.
Tom’s numbers are at the bottom of this range, and he states the tables came from Clarksons Autolock Chuck data.
with more modern spiral endmills with various coatings and the increasing use of carbide cutters in the home workshop, his tables aRe becoming outdated

Edited By Zan on 06/11/2020 21:25:40