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Milling Speeds for end mills

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Chris TickTock23/10/2020 10:33:17
622 forum posts
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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

Mick B123/10/2020 10:53:30
2023 forum posts
117 photos

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.

Emgee23/10/2020 11:02:17
2161 forum posts
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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

Martin Connelly23/10/2020 11:07:05
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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

SillyOldDuffer23/10/2020 11:21:25
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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

Dave Halford23/10/2020 12:00:10
1758 forum posts
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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..

Chris TickTock23/10/2020 13:25:09
622 forum posts
46 photos

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

Mick B123/10/2020 15:29:19
2023 forum posts
117 photos
Posted by SillyOldDuffer on 23/10/2020 11:21:25:

Agree with Mick.

...

Dave

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.

blush

 

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

Chris TickTock23/10/2020 16:01:27
622 forum posts
46 photos
Posted by Mick B1 on 23/10/2020 15:29:19:
Posted by SillyOldDuffer on 23/10/2020 11:21:25:

Agree with Mick.

...

Dave

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.

blush

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

Emgee23/10/2020 16:10:24
2161 forum posts
265 photos

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

JasonB23/10/2020 16:24:08
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Posted by Chris TickTock on 23/10/2020 16:01:27:

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

Chris

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

Chris TickTock23/10/2020 16:35:35
622 forum posts
46 photos
Posted by JasonB on 23/10/2020 16:24:08:
Posted by Chris TickTock on 23/10/2020 16:01:27:

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

Chris

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

JasonB23/10/2020 16:51:30
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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

Chris TickTock23/10/2020 17:22:39
622 forum posts
46 photos
Posted by JasonB on 23/10/2020 16:51:30:

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

Roderick Jenkins23/10/2020 18:24:39
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Posted by Chris TickTock on 23/10/2020 17:22:39:

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

Chris

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

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

Nicholas Wheeler 123/10/2020 18:26:13
745 forum posts
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I wonder how many people are actually using their 12mm cutters at 800rpm?

JasonB23/10/2020 18:32:44
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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.

Dave S23/10/2020 19:04:28
236 forum posts
49 photos
Posted by Nicholas Wheeler 1 on 23/10/2020 18:26:13:

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

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

Dave

Martin Connelly23/10/2020 20:31:13
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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

Mick B124/10/2020 15:55:45
2023 forum posts
117 photos

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.

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