Confused with Headstock RPM Settings

[oskar]

Reading on the web about RPM settings for a mill is getting very confusing for a beginner but somehow I was set for a SFM range of 250 to 400 for aluminum which gives me a RPM setting range of 4000-6400 with a 1/4” 4 flute endmill. Before we go any further please note that I have a Taig Mini mill, all axes are driven manually and it’s used as a hobby machine.

Today I came up on the video below and this guy settled at the end the RPM to the lowest range of 500RPM. (He is using the Taig 6 steps pulleys and is using the smaller step on the motor pulley to the largest step on the headstock pulley). He has the same setup as mine but he drives the axes with a CNC

So now I wonder what best RPM is for my mini mill. It was suggested on this forum under another thread the RPM setting for aluminum to be around 4000. Perhaps the thinking then was that I had a good size CNC mill and not a MINI mill?

Nicolas

 

[Mitch Alsup]

Best RPM is dependent on the material {iron, steel, aluminum, copper, brass, bronze, titanium,wood,...}
and is dependent on the cutter type (end mill, face mill, dovetail mill, ...)
and is dependent on cutter edge material (hardened iron, HSS, carbide, CBN, DLC, diamond)
and finally it is dependent on the stiffness and vibration absorbing qualities of the machine.

It is less expensive in buying cutters to start slow, and work up the speed and feed range.

 

[oskar]

The tools I use are all HSS endmills, 1/4" or 3/16" OD and I only cut aluminum. My Taig mini mill is stiff enough and has good vibration absorbing qualities for small jobs. I will never use a 1/2" OD endmill on my mill as the guy on the video did. I think thats too big for such a small mill.

Nicolas

 

[P. Waller]

Spindle RPM is determined by the diameter of the tool and the cutting speed required, in inch units this is expressed in Surface Feet per Minute.
For instance commercially available aluminum alloy stock is readily milled at a conservative SFM of 300 with high speed steel tools.
A 1/4" diameter end mill has a circumference of .785"
.785" / 12 = .065 feet
In order to arrive at a spindle speed for a 1/4" tool at 300 feet per minute
300 / .065 = 4600 RPM,
The feed rate is a different animal depending on the tool and the direction it is moving through the cut.

 

[mikey]

Nic, you may be cutting only aluminum with small end mills right now but someday that may change. You need to know how to calculate the required rpm for the cutter you're using and the material you're cutting. One common formula most of us use is:

RPM = cutting speed (for the material for the type of cutter used, in surface feet per minute or SFM) X 3.82 / diameter ( of the cutter).

Cutting speed is obtained from a table like this one: https://littlemachineshop.com/reference/cuttingspeeds.php#Milling. Note that there are separate tables for turning, milling, etc. Find the cutting speed for the material you are working with under the column for the type of tooling you're using. That speed will be in SFM. Plug that into the formula.

The cutter diameter is whatever you're using - 1/4" for a 1/4" end mill, etc. If you're using a flycutter set at 3" OD then use 3", got it?

Now solve for RPM and you'll have a good starting point for speed. Keep in mind that this is not written in stone. If the speed seems to high for your mill or operation then slow it down. Note that speed will often be higher than your machine can run if you're cutting aluminum with small diameter end mills unless you have a high speed spindle. Also remember that speed is only one factor. You have to sort out the proper speed and depths of cut for the end mill you're using.

To sort out the depth of cut, it depends on whether you're making a slotting or profiling cut. The commonest newbie mistake is trying to cut everything with the tip of the end mill; this just wears it out sooner. To guide you, I'm attaching some guides from Niagara Cutter - see below.

If you look at the "T6 feeds and speeds" chart you will see that you can do either slotting or profiling cuts. The specific cutting speed depends on which operation you're doing and the type of cutter you're using; take your pick. Note also that they refer to axial and radial depths of cut and this is based on a fraction of the cutter diameter. For example, if you choose to use 1/2 the cutter diameter of an uncoated HSS end mill as a depth of cut for a profiling cut, the cutting speed would be 400 sfm. You take this and plug it into that RPM formula above and that will give you the speed to use for that cut. For your 1/4" end mill, this would be a 1/8" radial depth of cut and you can go as deep as 3/8" (EDIT: you can use up to 3/8" of the length of your end mill's cutting flute length) while taking that 1/8" deep cut.

Note also that you can calculate feeds but this doesn't help unless you have CNC or NC or power feed. If feeding manually, feed so that you feel a slight resistance to the cut and you'll do fine. Don't forget coolant.

Anyway, have a look and see if you can sort this out. If you have questions, post them up. Hope this helps.

 

[Bob Korves]

Reading on the web about RPM settings for a mill is getting very confusing for a beginner but somehow I was set for a SFM range of 250 to 400 for aluminum which gives me a RPM setting range of 4000-6400 with a 1/4” 4 flute endmill. Before we go any further please note that I have a Taig Mini mill, all axes are driven manually and it’s used as a hobby machine.

Today I came up on the video below and this guy settled at the end the RPM to the lowest RPM range of 500. (He is using the Taig 6 steps pulleys and is using the smaller step on the motor pulley to the largest step on the headstock pulley). He has the same setup as mine but he drives the axes with a CNC

So now I wonder what best RPM is for my mini mill. It was suggested on this forum under another thread the RPM setting for aluminum to be around 4000. Perhaps the thinking then was that I had a good size CNC mill and not a MINI mill?

Nicolas

Speed stated in SFPM (Surface Feet Per Minute) is about getting a decent finish and getting work done at a highly efficient speed while not unduly damaging the cutter. That number is most important in a commercial shop. They want to get work done and out the door quickly, to save labor costs and to maximize production. They want the cutting edges going through the work as fast as is reasonably possible. So do we, but often costs and other factors are more important to us.

Speed stated in RPM (Revolutions Per Minute) is about spindle speed, nothing else. We are always limited by the range of speeds possible with our machines, both on the slow end and on the fast end.

With a turning cutting tool like a milling machine has, diameter of cutter in inches times Pi (3.1416), times RPM, divided by 12, equals SFPM.

There are many things that come into play when choosing a spindle speed. SFPM is one of the most useful parameters for deciding how fast we want to run the tool. We want to be below the maximum SFPM the tool is capable of (manufacturers' specs are given for ideal conditions, sharp tool, rigid machine, solid work setup, heavy coolant flow, and more.)

Also, the machine always has it's limitations. The spindle will have a minimum and maximum speed it can attain, and often will be limited to only a few steps of different speeds (RPMs.) So, we must choose a useful SFPM, lower than the maximum, and try it out and see how it works. From the results we get, we can move the speed up or down as necessary to achieve the goal from my first paragraph (getting a decent finish and getting work done at a highly efficient speed while not unduly damaging the cutter.)

The same rules apply no matter what kind of machine you are using.

That covers speeds. Equally important is feeds, which we will cover when you ask for it...

 

[benmychree]

A good idea is to find a slide rule type of cutting speed/feed calculator, they were giveaways by the cutting tool companies, look on e bay; they save a lot of time by (not) doing calculations. My machine shop teacher told us to do the calcs. in our heads, a estimate of cutting RPM was done by taking the cutting speed in FPM (100 for steel), times 4, divided by the diameter of the moving part; this is close enough to the formula given above to work OK.

 

[oskar]

[QUOTE="....... The commonest newbie mistake is trying to cut everything with the tip of the end mill; this just wears it out sooner........[/QUOTE]

Mike the above describes exactly me, LOL but I try to improve. Thanks for the handy info which I have to read and digest and hopefully I will have no more related issues afterwards.

Nicolas

 

[mikey]

I threw an edit in my last post to clarify something.

By the way, all of us start out cutting with the end of our end mills but we eventually learn to use the thing properly. It takes time to learn about feeds, speeds, chip loads and stuff so take your time and enjoy the ride.

 

[magicniner]

Try a single flute fast spiral carbide endmill with a small light machine, a bit of air to clear chips and the occasional drop of WD40 or 17/30 Paraffin/30 weight oil.
Another advantage to the single flute EM is that it's easily sharpened off-hand on a bench grinder.