Running machines at Max RPM

All,
I have not been on the zone in several years. I have been through a couple different jobs and have most recently started working a new job. Most everyone here believes that you should not run CNC machines at their max RPM for fear of burning up the spindle. They tend to limit the machines to 80% of what the manufacturer says the machines capable is. I believe this is complete nonsense as it makes no sense on many fronts. First and foremost the machine was designed to run at that RPM and if it would damage it to run at that RPM the manufacturer should have set the max to something lower. Secondly when I buy a machine that goes 10,000 rpm I have an expectation that the machine will run at that RPM without issue. Lastly, by arbitrarily limiting machines to 80% of their potential we are costing the company money and leaving up to 20% on the table.

I was just curious on people thoughts on running machines up to their max spindle speed. I realize many people have strong thoughts on this but what data or official documentation is their to support this either way?

in general most cnc's will run at max rpm for years without issue . Where guys tend to run into problems is with putting faster spindle motors without changing the bearings on converted manual machines .

With that said , after dealing with 2 new fadals which blew their spindles within 6 months , we were told to drop to 80% because they weren't meant to run constantly at top rpm (which I called bs ). That was also 25-30+ yrs ago . I've worked on numerous brands of high and lower end mills and running at max rpm is not an issue

You ask a very interesting question. I can't offer any hard data, but a friend of mine worked in an aerospace shop and that seemed to be their experience. Machines with 10K RPM spindles seem to require spindle rebuild about every 6 months when operated at 10K on 2 shifts, but they would live for a year or more at 8K rpm. My Haas has a 6K RPM spindle and we run it at 6K all the time and have for about 3 years with no problems.

It kind of seems that 10K RPM is the upper limit without getting into special bearings or what ever they do to make higher speed spindles. On the other hand, if you ran your car at 100% RPM all the time the engine wouldn't last long.

I guess there is a balance point between maintenance and production. But given the cost of production, I kind of have to agree with you that the extra maintenance cost might be more than offset by the increased production.

When it comes to machinery a lot of things are an advertising gimmick.
Just because it can, doesn't necessarily mean it's up to the job.
Basically 'mine will go faster than theirs, so you should buy it!'. It's all sales pitching.

Exactly the same thing with our £1/2 million stitching line in print finishing. (Heidelberg branded ST450). Muller Martini made a 14,000 cycle per hour Prima Plus.
So Heidelberg tweaked their software to go from 12k to 14k So they could (try to) compete.
If you ran it pst 12.8k it literally started eating itself!!!!.
We actually have both lines side by side.

Bearings were a nightmare because the stupid thing went from zero to full pelt in seconds and effectively scared them to death.
Once I mentioned / convinced they should tweak the servo accel/decel peramiters for a soft start (it had a lot of servos) we never had another bearing let go.

This speed competing malarkey between manufacturers in any industry is a load of bollox.
Machines have their limits and these limits are being pushed to the limits to get sales.

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Originally Posted by Xenomorph

All,
I have not been on the zone in several years. I have been through a couple different jobs and have most recently started working a new job. Most everyone here believes that you should not run CNC machines at their max RPM for fear of burning up the spindle. They tend to limit the machines to 80% of what the manufacturer says the machines capable is. I believe this is complete nonsense as it makes no sense on many fronts. First and foremost the machine was designed to run at that RPM and if it would damage it to run at that RPM the manufacturer should have set the max to something lower. Secondly when I buy a machine that goes 10,000 rpm I have an expectation that the machine will run at that RPM without issue. Lastly, by arbitrarily limiting machines to 80% of their potential we are costing the company money and leaving up to 20% on the table.

I was just curious on people thoughts on running machines up to their max spindle speed. I realize many people have strong thoughts on this but what data or official documentation is their to support this either way?

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Part of what you are saying is correct, the machines can run at there max, there is a trade off by doing this, everything will wear out faster, so by running at 20% lower will add a longer life to the spindle, and the machine in general, some shops use a much lower number than 20%, is this a correct way to run a machine, if you are paying the bills for spindle repairs, and slower speeds gives a longer life, with the same production output, then I would go with the slower speeds.

It's quite ok to use it occasionally at the max speed, if the job being done can run 20% faster.

Running a spindle faster does not always get the job done any quicker, so this would be a waste adding more wear for no cost or part benefit.

I have one machine I have never run it very fast, it's 10 years old and still like new, another machine that is the same machine, it has run max it's whole life and it has had ( 2 ) spindles ( 1 ) Ballscrew and many other parts, the first machine has had ( 1 ) proximity switch, it resale value will be high, the machine that is the same and same age would sell for scrap value if I'm lucky.
It has paid for itself many times over, so has the other machine, which will still be good for another 10 years.

Do a test for a day and see how many more parts you can make by running the machine at it's max, how much tool wear you get, and then compare it to doing the same job, at the slower speeds, this will give you a part cost value, this will tell if it is worth running the machine faster or not.

It seems like a question that's too complicated for a simple answer.

You have spindles which are specifically designed for high speed / high rpm low torque / small tool, ceramic bearings, liquid cooling etc. These differ from 24,000 rpm 40 taper spindles with large high torque seperate motors that run larger tools in harder metals.

There's also the "sweet spot" for the tool size and material. Running a 3/16" coated carbide end mill at 24,000rpm in aluminum is not the same as running a 1" end mill at 24,000rpm in steel.

Running significantly outside the sweet spot causes a lot of extra vibration which can't be good for spindle life. The level of impact will also be effected by machine damping and runout.

I notice on my machine that cutting at 24,000rpm generates a lot more heat than running the same tool at 16,000. So differences on coolant matter too.

Then there's feed speeds. Cutting at higher rpm but taking lighter cuts can take axial load off the bearings. Increasing rpm and feed and depth of cut to rush a job is a different story.

So... um... In summary... who knows...

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First and foremost the machine was designed to run at that RPM

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hy :) not machine, but spindle; same spindle can be shared by diff machines; a single machine may have a range of different spindles to choose from (high power, wide range, high speed, high quality for die-mold), including the offer to custom build a specific one; in the end, each one has a different diagram, and the differencies between spindle diagrams can not be reduced only to top speed :)

for example, available top speeds may be 6k 12k 20k 25k 35k, but as those increase, max torque and max admisible toolholder + tool mass will decrease

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I believe this is complete nonsense as it makes no sense on many fronts.

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yes, but also it makes sense on other fronts, and those other fronts may be more important; this balance is specific to each application, and/or shop profile:
... heavy parts require torque
... light parts require speed

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Lastly, by arbitrarily limiting machines to 80% of their potential we are costing the company money and leaving up to 20% on the table.

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there are many things to take into consideration; obviously, you are leaving 20% on the table, but this 20% is from a specific cost, whose importance, into overall cost may vary between a low and a high fraction, and those variations may be so uneven, that simply may not be relevant to talk about an average value; also, sometimes, going only for 5% higher may lead to less productivity

through data, one can see such discret variations, in real time, thus having a more relevant feedback in comparison to someone that only sees an yearly statistic that says that on some machine, there is a limitation of 80%

some shops, that really target consistent accuracy, will never use same machine for roughing and finishing

if you need constant high rpm, then you don't need a spindle, but a windmill, somewhere, where the wind is always the same / kindly :)