Torque and RPM requirements at the spindle?

[JMcDonald]

I need to know what would be some decent torque output and speeds at the spindle of typical benchtop mills. Like, after gearing and such, what kind of spindle RPMs, and what kind of torque outputs (or, HP if that is the only way they are measured) are typical for the smaller mills?

Thanks!

[Oldmanandhistoy]

Hi,

These might be some help.

http://www.cnczone.com/forums/showthread.php?t=38087

http://www.cnczone.com/forums/showthread.php?t=38120

Here for conversions.

http://www.sensors.co.uk/home/pages/torquecalc.html

John

[JMcDonald]

Hmm... Really, I just want to know the required or recommended speed ranges for effective cutting of steels and aluminum mostly.

[Geof]

Originally Posted by JMcDonald Hmm... Really, I just want to know the required or recommended speed ranges for effective cutting of steels and aluminum mostly.

Your question is a bit difficult to answer. I am sure there are some people who have wonderful theoretical answers but in practical terms I think you can only go by example.

I have a manual mill with a 2 hp motor. On this machine I have cut leaded steel at 2700 rp,m with a 3/4" diameter cutter taking a depth of cut of about 0.3" and a width also about 0.3" and a feed of...pretty fast. I can't really tell you the feed because it was by hand but it might have been 10 inches per minute.

I also have a Haas MiniMill with a motor rated at around 7 hp and with a 5/8" cutter at 0.32" depth of cut, full width cutter engagement and a feed of 30 ipm the motor runs up to around of 140% full load which is about 10hp.

I am biased because I have worked with really big manual machines and small to medium sized CNC. I think it is not worth considering anything less than 2 hp for a manual machine and 10 hp for a CNC.

[JMcDonald]

Looking at other standard designs it seems 0 - 2500 is pretty common for spindle speeds, especially for smaller machines. The larger Knee mills (the Bridgeport copies like from harbor freight) seem to go up to 6000 RPM. I also just read that thread about the toolmaker making his own mill from scratch, and he is using a router that does 7000-30,000 RPM, but one person did note that they thought 7000 RPM seemed a little fast for mills.

I ask because I am planning on building my own mill (or... router...? It has a gantry for the Y-axis and the tables slides for the x-axis), and I am trying to decide if I should just try to fit an X2 head on there, or try to come up with something different for mostly bulk and rigidity (as far as how the head mounts to my column) reasons.

I am not so concerned about travel speeds, though. I am building it with a 36" X travel and 24" Y travel, with plans of being able to set it up to run all day / night to produce multiples of the same parts, rather than one or two of the same part much quicker. Plus that would require less of my time (less repeated changing of materials / stock, etc) and increase productivity.

I do plan on using a cooling system. Probably a flood-type (rather than mist) system to get the longest cutter life and most cutting efficiency possible.

My understanding is that you can use higher spindle speeds and travel rates, using a 4-flute mill, and still get away with it. Otherwise, you would use slower speeds and travel rates, but hog out deeper cuts and require greater spindle torque and stiffness. I'm not counting on my mill being all that stiff (like, not "several-hundred-pounds-of-iron" stiff, but still pretty stff), so it seems Id want to choose the former option of high spindle and travel speeds with small cuts. So, how high would "high speed" be? Like 5000-10000+ RPM? Or like, 2000-3000 RPM?

And thanks, Geof!


*edit*

I forgot that I did also read that a 2-flute end mill can be used with higher speeds at a lower travel rate. Can this apply?

[Geof]

You do ask awkward questions. High speed in the context of manual machine is probably around 3000 to 4000 rpm. In CNC's high speed can be anything up to 20,000 rpm, or more.

In practical terms I think 5000 to 6000 rpm is probably sensible for a DIY machine. The ability to feed at maybe 100 ipm is also maybe practical. I will mention I have not made a DIY machine I have just used a variety of manual and CNC machines.

The question is what are you machining; wood, plastic, aluminum, steel? The optimum speed and feed differs; also the the power equirement.

Unless you are planning on machining steel or wood I would say aim for 5,000 to 10,000 rp, 2 to 5 hp and a feed rate of up to 200 ipm. This will allow you to handle plastics which need high feed rates or aluminum which needs moderate feed rates and moderate power. For steel you need to go slower with about the same power with wood you need to go faster but again more or less the same power because you need to take the feed rate up above 200 ipm.

[JMcDonald]

I need to do steel and aluminum. Probably 98% of the time. I suppose there is always a chance I will do some stuff with some plastics / fiberglas, but that is definately not the main concern for now.

And sorry for the odd questions... I can be bad about that, heh.


And... I dont know if this information is even useful at all... but I just remembered that my die grinder does ~20k RPM and I use carbide steel bits, and that seems to work fine.

Could steel be milled at all at those 5-10k spindle speeds? Also, can aluminum be milled ok with 2-flute end mills at slower speeds like 2-3k RPM?


*edit*

Well, being that most smaller mills run at max speeds in the 2XXX RPM range, it seems that would be a good place to start. *shrug*

[rustamd]

what kind of frame construction are you thinking of for your machine? How much material do you want to remove/how fast?

For aluminum i would say you would want slightly higher RPMs than 2-3k. What tool diameter are you thinking of using for aluminum?

[JMcDonald]

No idea on cutter diameters really. I will be doing mostly things like flanges and such. Mostly 2d stuff. However I have a few 3d parts that Ill need machines (that are, however, much smaller).

And now that I think of it, I could probably build a much smaller mill for milling the relatively small amount of 3d parts I need, and just buy a plasma cutter (and even do a CNC plasma cutter) since most everything else I need cut will be essentially 2d (various flanges and mounting plates, etc).

Frame construction for the gantry-type build would be concrete-filled rectangle steel tubes for the columns. .75" round rods for the cross rails (or a .5" x 6" flat steel bar). The base would be a concrete foundation with inlaid steel rectangle tubes, that the x-rails are mounted onto, along with the columns. The x-rails would be aluminum rectangle tubes that extend the full 6' so the 3' table could have 3' of supported longitudinal travel.

If I built a smaller mill capable of the 3d parts I need (many of them using a 4th-axis), it would be much easier to get greater rigidity and precision, I think. Ive thought about that design some, but not as much as the gantry-type design.

Here is an Excel drawing I did of the grantry-style mill (it's been changed a few times, but that's the only one I have uploaded). It is mostly proportional, except the z-travel in relation on the head, because I am still not sure how large the spindle assembly would be. The columns are 36" tall total, with I think 2" inset into the base. I don't think I would actually make it that tall. Also, I redesigned the sliding rails to have solid support between them (well, its now designed as a solid flat bar that the head rides on, rather than two round rods).