Basic Metal mill question

Hello all, very simple question What is a ballpark metal mill spindle speed? Say for alluminum or steel. On the order of 1000RPMs, less? 20,000RPM's? Wood routers spin
close to 20,000 RPM's, so would you need faster speeds for metal or am I way off base. Just wondering, thanks for any help. Lucas
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What diameter of the mill?
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    --Speeds are best calculated in surface feet per minute and not in rpm, but most milling machines top out around 4,500rpm.
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Spindle speed is determined by the "Cutting speed" of the material being machined, and the diameter of your cutter, all of which are defined in terms of surface feet per minute (sfm). In other words, the speed at which a cutting edge passes through the material being machined.
Mild steel is generally defined as having a CS (or sfm) of 100. Hence if you have a 1 inch dia. cutter, the equation is (for practical purposes) RPM = (CS x 4) / dia. of the cutter. ie. (100 x4)/ 1 = 400 rpm.
Next step, different materials (steel versus alum.) have different cutting speeds, all of which can be found easily by looking in machinery handbook or other similar sources.
Please note above is pretty simplistic. The type cutter being used (ie. HSS or Carbide), type of coolant, rigidity of setup , etc. all come into the "equation". In other words, the figures you find published are 'starting points' , and you need to adjust according to your own environment.
Good luck!
Mild steel is generally considered as having a cutting speed (CS) of 100.

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    [ ... ]

    [ ... ]

    And if you are puzzled as to where the '4' came from, it is the combination of 12 inches per foot (to convert the SFM to SIM (Surface Inches per Minute), and the Pi (3.14159......) used to convert the diameter of the cutter to the circumference. Pi is rounded to 3, as being close enough for this, so you are left with the single constant of 4. You can calculate it more precisely, but it is only a starting point approximation anyway, so the easier calculation helps speed things up.
    The speeds of a router, used with normal diameters of endmills would be *way* to fast, and burn up the cutters rather quickly. Assuming the original 1" diameter cutter from above, and 20,000 RPM from the router, you would come up with something on the order of 5236 SFM (using the true value of Pi), which would be almost instant melting for a HSS cutter -- assuming that you had enough horsepower to maintain the speed in steel.
    Going down to a 0.125" diameter endmill (small enough to be easy to break) and keeping mild steel as your material, you finally get up to about 4000 RPM.
    Most aluminum alloys -- with proper cutting lubricants, can handle a lot higher SFM speeds -- but without the lubricants, some alloys -- especially soft pure aluminum -- tend to gum up the cutters rather badly.

    And this can be also affected by whether the metal is hardened or annealed. Note that he started with *mild* steel.

    And those figures are planned for a production line, in which shorter cutter life is traded off against higher production rate. At some point, the cost of the cutter replacement is balanced by the increased production rate. This very seldom applies in hobby metalwork, so you should run on the low side of the calculated speeds to keep your costs down a bit -- unless you have a lot of money to spare, and are quite impatient. :-)
    Good Luck,         DoN.
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1000-2000 for most stuff. Your question has about 10,000 different answers and ultimately the tools engagement with the metal is measured in surface feet per minute so tiny drills require very high speeds and a 2" diameter drill would be rotated slower but 1-2K will get alot of stuff done
Bill H
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Mr. Bla wrote:

Thanks for all the help and I understand there are many different situations, so maybe I should clarify. I guess what I am curious about is the motor (and maybe I should re-post this) but if I were to spec out a motor for a mill what characteristics should it have for say light-to-medium duty applications? HP max speed etc. Im looking at a 1HP 1725 RPM 90VDC. Is that too weak or what?
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lmcgill wrote:

Perhaps you would get a better feel for this subject if you browsed a bunch of specifications for existing machines. Sales fliers and catalogs will often have this data. A few places to start:
http://www.use-enco.com http://www.mscdirect.com http://www.grizzly.com
--Glenn Lyford
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The induction motor on my old Bridgeport J-head mill is 1800 RPM 1 HP 3-phase 220-volt 60-Hz. The mill itself has 8 speeds from 80 to 2720 RPM by means of gear reduction and belt/pulley selection. I use it routinely on steel, aluminum, stainless, brass and plastics. The lower (and higher-torque) speeds are handy for drilling steel with twist drills up to 1" dia among other things. My most frequent use is with a 1/2" end mill running at 1115 RPM. Probably isn't "correct', but that's what I do in steel, brass and aluminum.
I am not a professional machinist. Production rate is not an issue for me. I just like to make stuff.
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    [ ... ]

    O.K. For most milling machines, 1HP is a *bit* on the low side, but not too much so. Normally, you pick a motor with a speed which is reasonable and use pulleys and gears to increase or decrease the speed.
    It is interisting that your DC motor has a speed which is fairly common for AC motors used for the task.
    My Bridgeport Series I has a J2 head (which is variable speed pulleys for a continuous range of speeds) and is a 2HP motor, because the variable-speed pulleys waste a bit of horsepower. The same machine with a step pulley head (you have to loosen the belt and move it to other pulley steps) is 1-1/2 HP. Both of these machines have aside from the pulley changes to change spindle speed, a "back gear" which drops the speed by a significant amount (perhaps 6:1 or so), so the range of the variable speed pulley is significantly increased, and the torque delivered is also significantly increased.
    The earlier heads on the round-ram Bridgeports were 1HP, IIRC. Those (IIRC) had only step pulleys, and not the back gears -- but they also used a spindle which could not handle the torque needed by larger diameter endmills.
    Note that you get a significant benefit from using pulleys or gears to change speeds over just slowing the spindle down. That benefit is that as you slow the speed, you increase the torque which can be delivered to the spindle -- and you usually *need* more torque at the slower speeds.
    Just about any motor loses horsepower as its speed is decreased, though your DC motor may be better at that than most AC motors.
    Good Luck,         DoN.
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snipped-for-privacy@uiuc.edu wrote in

Loaded question. You are looking for SFM (SMM), not spindle speed. The SFM depends on the material and the tooling.
Take for instance, running a PCD tipped 1/2" EM in aluminum, the "recommended" SMM for aluminum with PCD tooling is 930m/min (3000+ FPM). So, put that into the good 'ol RPM calc from surface speed (RPM= (4 *SFM)/dia) which means roughly 24000 RPM. But, take a carbide 1/2" EM in say 4140 PH the recommended SFM is about 100. So put that in our formula..and you get about 800 RPM.
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snipped-for-privacy@uiuc.edu wrote:

875 to 1150 RPM.
John Martin
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snipped-for-privacy@uiuc.edu wrote:

Big ballpark.
I'll bite. Somwhere between a couple hundred rpm and 40 thousand, depending on the mill and the tool in use.
You really need to provide a better question to get a better answer.
Cheers Trevor Jones
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