Milling Speeds for end mills

Hi Guys,

Have read contradictory advice over milling speeds for end mills. I should state I have a Sherline Mill

Namely I have read:

1. A bigger end mill can cope with higher speeds removing metal.
2. RPM is approx the recommended 4 X CS (per metal) divided by size of cutter in inches.

Nos. 2 then means increasing speed as cutter size is reduced, Nos. ! implies the opposite. Which is right here?

I acknowledge carbon cutters can be run at higher speeds than HSS but as a generalisation is 1 or 2 my preferred mental guide as to RPM?

Regards

Chris

Edited to fix formatting only.

Edited By SillyOldDuffer on 23/10/2020 11:23:25

No. 2 is correct in that surface speed at the periphery has to reduce as diameter increases.

However, No. 1 can be correct in terms of volume per unit time of metal removal - providing the machine has the torque to achieve it, the mill and workholding have the strength to withstand it, and lubrication is adequate to prevent overheating.

Those last three can require quite good mathematical capability to resolve and may require data that isn't easy to obtain - so they are often only accurately resolved in volume production operations where the manufacturing cost saving would justify the resource cost in time and expertise.

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

This thread is a good starting point for this topic it is reachable through the home page under workshop processes but here is the link for it. Thread 95687 cutting speed tables

Martin C

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

This (A bigger end mill can cope with higher speeds removing metal.) sounds like feed speed as they are stronger and able to take the extra depth of cut..

Thanks Guys,

All makes sense now. Nos 2 is correct. Nos 1 needs qualifying in terms of feed. If you were to feed a 1mm end mill too quickly it would not last long, especially if carbide (yes carbon was a typo). But that same 1mm carbide end mill needs higher speeds than say a 5mm.

Chris

No!

I was wrong!

It's not as I wrote: "...surface speed at the periphery has to reduce as diameter increases."

It's RPM that has to reduce in order to keep surface speed (unit length per unit time) at the periphery within working range.

Some of you will have noticed that. Sorry. Engineers are supposed to know what they're talking about.

Edited By Mick B1 on 23/10/2020 15:30:48

Now I am confused how does this relate to RPM = (4 x CS) / Dia in inches.

I take it CS is peripheral speed for e.g. an end mills outer diameter. Anyone help on explaining Micks point.

Chris

I understood Mick to say in his 1st post rpm would have to be reduced to bring down the surface speed if using a larger cutter, although he didn't state rpm in his post, reducing the rpm reduces the surface speed at the cutter tip.

Emgee

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

Nicely explained..thanks

Chris

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

Steady on Jason 10,000rpm on my Sherline??

Chris

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

I wonder how many people are actually using their 12mm cutters at 800rpm?

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.

IIRC I run mine at 560, as it’s the closest speed on my mill...

Dave

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 agree with Martin C - whether milling or turning I'm very often using speeds very much lower than maximum with HSS tooling, and you can, of course, still obtain very satisfactory finishes. Carbide demands much higher speeds in some situations - I'm less inclined to use it because I'm uncertain of its flexibility in that regard.

I'm sorry if I raised doubts when I corrected my earlier posting - it simply occurred to me that my wording was unclear: if you maintain RPM constant, surface speed at the periphery would obviously increase with diameter; and it's surface speed you have to hold within limits, so to maintain that you must reduce RPM as diameter increases. I don't know how to make it any clearer.