High speed spindles on conventional mills?

Hi,

I saw recently clough42 put a router-style high-speed spindle on a conventional (but diy CNCed) mill, which surprised me.

I should probably understand the engineering dynamics involved but can someone enlighten me on the general physics envelope of high rigidity conventional mills with say speeds up to 2K rpm and heavy tooling compared to low-rigidity router-style with speeds to 25K rpm and small tooling.

Is 'router-style' a way of getting horsepower into the work with low rigidity or what? Why don't all mills have high speed spindles? etc etc etc.

TIA, Alan

In my view a high speed router with high feed rates is good for cutting wood, and if sturdy enough will cut aluminium at a lower feedrate. Steel is not generally suitable although small amounts with smaller cutters (say 4mm) is often possible.

A low speed, high rigidity mill is great for cutting steel, and will also cut aluminium. But the main spindle speed is quite low so the feedrates end up being low for aluminium. If you want to cut aluminium faster on a mill you can add a high speed spindle. But then you can be limited by travel, especially in Y, so the larger format router can be a better option for aluminium.

Perhaps simplifying it: You can 'cut' wood, aluminum and steel on both a router with high speed spindle, and a milling machine with low speed spindle. But the mill will end up being slow for the softer materials, and the router will end up struggling with the harder materials due to lack of rigidity and lack of low speed torque.

On "conventional" mills aka modern cnc tools, spindle speed is an important consideration.

For those doing mainly alu, 15-24k spindles are sought after.
This is a difficult area in many ways, for 34 kW spindles. 40 Hp.

Basically, the higher the rpm, on modern machines, the higher the mrr and productivity, generally, Alu.
A high-speed spindle at 24.000 rpm can be double the productivity of a 10k spindle.

But is does not double the production or profits/hr of the machine, nor anywhere near it.
Second-level constraints mean a lot of the fast machining value is lost.

High speed spindles like NSK or Nakanishi are commonly employed on high value jobs with modern cnc mills.
They tend to do high value jobs to extreme accuracy, at very low "push" forces.

High speed spindles are essential* when you're working with small cutters, and soft materials.

A conventional mill is generally aimed at being able to machine steel, so you need pretty low spindle speeds, and suitable torque to handle large diameter cutters at low speeds, which doesn't work well with small diameter cutters and materials like aluminium.

If you want to see the benefits/limitations of spindles, just run some figures through a speeds and feeds calculator.


*I say essential, but you can use small cutters at slow speed, it's just that it takes quite a while to the limited chip load, and subsequent restriction on feed speeds.

Several factors come into play here I think. For me, the main reason I would fit a high-speed spindle onto a Mill is so could use modern high-feed adaptive tool paths which can drastically increase MRR.

Next would be the use of small tooling, sub 3mm, where 24K rpm is considered the minimum.

However, the quality and power of the High-speed spindle make a big difference. For a long time, I've fit what would be considered the default 2.2Kw ER20 24K water-cooled spindle coming out of China to just about every Router/MiniMill serious hobby level machine I've ever built and they do an OK job for the money they cost but I wouldn't fit one to a milling machine.
However, I've recently come across a Chinese supplier of Spindles whose spindles are in another class in every way and can supply electro spindles up to 47Kw, but even their 2.2Kw ER20 spindles are out of this world and will quite happily cut steel as well as aluminum with Sub 12mm tooling, these I would fit onto a milling machine and could see them being far more useful than the main spindle for 12mm or less tooling as the feeds and MRR would be so much higher, esp with Adaptive tool-paths.

Interesting Jazz. I note that Tormach do a 1.5hp 24K spindle and suggest it is for aluminium, brass etc. I guess that makes sense if you mostly cut those only.

Alan

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Originally Posted by alanambrose

Interesting Jazz. I note that Tormach do a 1.5hp 24K spindle and suggest it is for aluminium, brass etc. I guess that makes sense if you mostly cut those only.

Alan

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Tormach's spindles are silly overpriced and underpowered IMO, as are the machines in general.!

For small tooling, less than 3mm, I wouldn't fit less than 40K. We are just about to build a machine designed for cutting aluminium moulds with 40k 2.2Kw spindle which the largest tool will probably be 8mm (other than a surfacing bit) and the smallest will be 0.1mm and I consider this the minimum RPM.

It does depend on what you want to cut and how quickly. If your mostly cutting steel and just wanting to hog it out then it's a waste of money and you have the correct tool already.
If your wanting to cut Alu, brass, composites, plastics with modern tool-paths then fitting the right high-speed spindle could easily transform a low to a mid-price range milling machine into a much more useful tool for a serious user.