[Splint]

Hi,

I'm looking to upgrade my spindle to a Jianken JGL series. They have three models in that range, 1.5kW, 2.2kW and 2.5kW which are rated to 24,000, 30,000 and 40,000 RPM respectively. I'll be machining aluminium mainly so 2.2kW is probably more than enough for me. The question is more relating to the benefits of the higher RPMs. Is the surface finish between the three speeds significant? I intend machining components which need to gave a water tight o-ring seal, I imagine they would all be well capable of that but aesthetics also come into play, so I wan't to keep chatter to a minimum. Anyone able to comment on this?

Splint

[joeavaerage]

Hi,
depending on the grade of aluminum to a certain extent then the rotational speed must be such that the surface speed of the cutting tool
is 200-500 m/min for aluminum. Each material has an optimum surface speed.

Thus with a small diameter tool you spin it fast to reach the target surface speed and for a larger diameter tool you spin it more slowly.
Having a lower surface speed than recommended won't hurt but going to much higher invites overheating of the tool and Built-Up-Edge (BUE)
wrecking the tool and the job.

Lets do a few sample calculations:

Assume a target surface speed of 250 m/min, suitable for 6061 say.

With a 6mm endmill:
circumference= PI x diameter
= 3.141 X 6
=18.8mm
= 0.0188m
rpm =surface speed / circumference
= 250 /0.0188
=13,300 rpm

With a 20mm endmill:
circumference=3.141 X 20
=0.062m
rpm = 250 / 0.062
=4032 rpm.

With a 1mm endmill:
circumference= 3.141 X 1
=0.0031m
rpm=250 / 0.0031
=80,600 rpm.

You can see that larger tools don't require high rotational speed but when you get to small and very small endmills
then high rpms are desirable. Note that most of us can't afford 80,000 rpm spindles......that's air bearing territory or at least very
expensive ceramic bearings.....way too costly for me.

If you intend on using small (1mm) endmills then by all means get a high speed spindle. For more normal size endmills then really high speed spindles
are a waste of money.

Just as a matter of interest with a coated carbide endmill cutting mild steel you want a surface speed about 100m/min, ie a lot slower than aluminum.
For stainless you need to get down to 50-60 m/min and titanium and superalloys 20-30 m/min. Thus for tough metals like steels and stainless you want
high torque slow speed spindles.

For aluminum, brass, copper higher speed spindles are OK but you don't need to go overboard.

If you want to try H(igh) S(eed) M(achining) toolpaths then you will need 30,000-40,000 rpm but be prepared to pay $200-$300 for each CBN tool.
I would not recommend going there until you have lots of miles under your belt....its a really good way to spend lots of money and achieve very little
otherwise.

I like HSM Advisor:
https://hsmadvisor.com/
You can download a free trial.

Craig

[Splint]

Thanks Craig, great response.

[Bob La Londe]

IF, you run flood coolant and use very small cutters you can feed MUCH faster at higher rpms. I am currently running 24,000 rpm spindles on 3 machines. For engraving and some types of small detail work the feed rates seem almost glacially slow. I use 1/16 inch ball mills everyday, and its not uncommon for me to use .026 inch ball mills for slotting grooves in aluminum for inserts. BUT if you go with faster spindles you run into issues with larger tools that may work better at lower RPMs.

I have never gotten a clear answer as to whether or not the low torque at lower rpm is mitigated by using a 4 pole motor as opposed to the more common 2 pole spindle motors.

[joeavaerage]

Hi Bob,

I have never gotten a clear answer as to whether or not the low torque at lower rpm is mitigated by using a 4 pole motor as opposed to the more common 2 pole spindle motors.

If you increase the poles from two to four you end up with twice the torque but half the speed for the same frequency input.
Consequently most high torque servos and/or spindles are 4, 6 or 8 pole motors.

A 4 pole motor could still do 24000 rpm if your VFD can manage 800Hz when most VFDs top out at 400Hz.

Craig

[raychar1234]

Many diy guys who like me want to have their medium size milling machine or router can do high speed engraving as
well as slight cutting on steel material. Even a rigid machine construction is built, it is not easy for them to do such things as those
industrial cnc machine. Because the latter one has a high speed spindle and does a direct coupling to high speed servo motor. For diy one,
those of them are of high price and heavy it weight. To resolve it, using a servo motor which is normally has speed 3000rpm and has a belt
coup ling to a high speed spindle is a method. But, still a large gear reduction ratio of 5:1 to 6:1that will get a 15000~18000rpm spindle
revolution is needed. It is overwhelming for timing belt and normal spindle bearing as they will not sustain it.
Lastly, most of them will resort to use high power and speed vfd driver type spindle and using high speed and light doc for steel cutting.
Again, from the above spec. of said cutting speed of steel, it is not a good approach...

[joeavaerage]

Hi,

Lastly, most of them will resort to use high power and speed vfd driver type spindle and using high speed and light doc for steel cutting.
Again, from the above spec. of said cutting speed of steel, it is not a good approach...

I agree. I bought a 750W 24000rpm spindle mainly for engraving with small diameter tools which it does well. I aslo hoped
that it would have enough power to do steel. It did not work out like that.

I have used the high speed spindle to cut steel but with 3mm endmills and smaller only. Even then I had to use very low DOC or risk stalling
the spindle whereon the mill would overdrive the endmill and break it. It works with 3mm endmills, just not very well.

What is required for steel is slower speeds but high torque.

The high speed spindles you are considering would do very poorly in steel.

Cutting aluminum, brass and copper is however within reach of these high speed spindles but not steel, stainless steels, titanium or superalloys.

I now have two spindles, the 24000 rpm one for aluminum and PCB engraving and a 3500 rpm, 6.2Nm, 1.8kW AC servo drive spindle for steel and stainless.
I swap them over as required.

Craig

[raychar1234]

Yes, this is an option. But, in seldom case if you want do high spindle speed small size cutter cutting on steel for some details works after rough cutting on the same steel workpiece. Changing spindle motor and spindle are needed, then the home original may be deviated and not accurate enough. Furthermore, if there is the planning to have ATC, two systems will have completely different types and make all things redundant and cumbersome...
BTW, for your 1.8KW servo drive motor spindle. Which brand is it? Do it has motor outer case fins and cooling fan for cooling in case of a long period cutting time and heavy cutting are needed so ..to exhaust the heat.

[joeavaerage]

Hi,

BTW, for your 1.8KW servo drive motor spindle. Which brand is it? Do it has motor outer case fins and cooling fan for cooling in case of a long period cutting time and heavy cutting are needed so ..to exhaust the heat.

It is an Allen Bradley and matching drive. I bought it second hand here in New Zealand for $700NZD, about $500USD or about $600USD delivered.
I think it came from a carpet weaving factory.

It does not have a fan built in. Servos tend not to as they are required to produce rated torque at low speeds, less than 10 rpm and they have to turn in either
direction. None of the conditions would allow a fan blade attached to the main shaft do much good.

My servo is just the standard ridged rectangular motor frame. Its never overheated even when I'm working it as hard as I can.

This is the first 'modern' AC servo I have ever owned/used and I am very impressed....I wont be going back to any brushed servos or induction motors anytime
soon!

Craig