Originally Posted by
Goemon
I researched this topic to death when I was choosing my spindle and I have done a good bit of testing (with cutting steel and aluminum) since I finished my machine. There is a lot of regurgitated innacurate and outdated info out there.
As a general point, you most definately can mill steel with even a basic wood router. There are a number of YouTube videos demonstrating this. E.g.
Obviously there is a difference between milling an occasional steel pocket on a hobby CNC router and large VMCs making steel parts quickly all day every day. I wouldn't buy a Home Depot router for the latter...
As with any other type of spindle, you need to choose tools and set feed speeds and spindle RPM etc at the appropriate level for your machine, spindle limitations and work piece.
My personal experience has been that you run into other limitations like machine stiffness, poor spindle cooling, missed stepper steps and broken tools long before your spindle gets stuck due to a lack of torque. I have yet to see my 24,000 rpm spindle get stuck on any cuts when I use G Wizard, even in steel.
You only have to look at how well the Datron routers cut steel with (ultra) high speed spindles (with smaller tools). They work as fast (with steel parts) as any comparably priced VMC and they do so with very little torque. Their accuracy is top notch too. Their machines are just appropriately stiff and heavy for metal milling and have great acoustics.
It's also a myth that all mills have stronger, larger or better bearings. Obviously a $200,000 VMC spindle will have better bearings than a $300 Chinese eBay spindle. But, if you compare like with like, it's a different story. I have a 7.5hp PDS ADES 90 high speed spindle and an R8 spindle from LMS's high torque bench mill. The bearings on my PDS spindle are considerably stronger and higher quality than the cheap bench mill spindle.
If you look at the various Chinese eBay belt drive milling spindles, they use the same cheap 7007 or even 6007 bearings as their comparably priced 24,000 rpm wood spindles.
If you buy a high speed spindle that is cooled by water, electric fan or compressed air (instead of a shaft fan), you have the same (or a better) speed range as you would with any belt drive milling spindle. The main reason why cheap air cooled eBay spindles can not go below 12,000rpm is poor low speed cooling with shaft fans. My electric fan cooled spindle does fine from 50rpm up to 24,000rpm.
The main advantage with belt drive spindles is that they can use gearing to increase low speed torque. It is / was a good way to take deeper cuts with smaller 1hp or 2hp motors on manual mills (where that matters). The trend has been shifting though. Newer high end VMCs are moving towards high speed direct drive spindles to take advantage of high speed machining with the newer generation of coated carbide tools. Motor sizes have increased accordingly to maintain torque. This is key as a 3hp milling head has no more torque at 12,000 rpm than a 3hp high speed 4 pole spindle at 12,000 rpm.
Anyway, the advice I got here (and I'm glad I listened to) is to buy a spindle that is optimal for the material you will cut most often. Then, focus on building (or buying) a machine that is stiff enough to handle the work at the required level of precision. You can't put a 20hp cat 40 milling head on a cheap aluminum T slot router frame and expect good (or even safe) results.
Even if that weren't the case, one look at the specs of your average Bridgeport milling head will shatter any illusions one might have about mounting one on a diy T-slot build. They weigh 300lb + and they are enormous. Many VMC heads are considerably larger.
I mainly want to cut aluminum and the average RPM suggested for my 1/8", 1/4" and 3/8" carbide 2 flute end mills is between 12,000 rpm and 18,000 rpm. A low speed geared belt drive spindle would be sub-optimal for my needs.