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First Run on CNC machine
« on: December 31, 2017, 11:03:36 AM »
Finally got my CNC machine running cutting, I have only got a 400watt spindle motor and using a 2 flute 6mm cutter running a Face profile G-Code program 4inch X 4 inch using Mach3, the motor didn't like the 1200mm/min feed which g_code was set too, so slowed down to 150mm/Min with a 1mm depth of cut a bit of vibration but think a 2 flute is not enough4 would be better and also motor as I suspected is no where near powerful enough.

I also want to cut Aluminium and was looking at a larger spindle motor with an ER20 collett so I can go up to 13mm max shank, the only one I can see is a 2.2kw motor, about £300 on ebay.

Can someone advise me best way to go on Motors.

Regards and Happy New Year

John
Re: First Run on CNC machine
« Reply #1 on: December 31, 2017, 11:05:29 AM »
Finally got my CNC machine running cutting, I have only got a 400watt spindle motor and using a 2 flute 6mm cutter running a Face profile G-Code program 4inch X 4 inch using Mach3, the motor didn't like the 1200mm/min feed which g_code was set too, so slowed down to 150mm/Min with a 1mm depth of cut a bit of vibration but think a 2 flute is not enough4 would be better and also motor as I suspected is no where near powerful enough.

I also want to cut Aluminium and was looking at a larger spindle motor with an ER20 collett so I can go up to 13mm max shank, the only one I can see is a 2.2kw motor, about £300 on ebay.

Can someone advise me best way to go on Motors.

Regards and Happy New Year

John
Re: First Run on CNC machine
« Reply #2 on: December 31, 2017, 03:08:47 PM »
Hi,
if you wish to cut metals, steels in particular you'll need a spindle which spins fairly slow but with a great deal of torque.

A good surface speed for mild steel is about 150m/min. With a 13mm tool that means an rpm of 3670 rpm. The same tool in aluminium you could spin
at three times that, say 10000 rpm.

High speed spindles are generally poor at low speeds and have low torque. They're great for engraving, cutting wood and do pretty well in aluminium but
they don't do so well in steel and even worse in stainlees steel.

I have a 750W 24000 rpm spindle and its great for a lot of things but hopeless in steel. I have cut steel with it, a 3mm tool spinning at 9000 rpm, the lowest
effective speed without overheating, but the cut depth and feed rate had to be so slow because the spindle is so inclined to stall given its low torque.
I have since made a spindle with a 1.8kW servo and drive and its max is only 3500 rpm but it has a continuos rated torque of 6Nm, it makes mincemeat of steel!

Download HSMAdvisor for a free trial and look at the recommended speeds and feeds for different materials and then look to the required spindle power
and torque. Very useful software tool, so much so I bought it.

Craig
My wife left with my best friend...
     and I miss him!

Offline Davek0974

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Re: First Run on CNC machine
« Reply #3 on: December 31, 2017, 04:26:22 PM »
+1 for HSMAdvisor, I bought it too ;)

And yes, steel/ferrous metals are high torque/low speed - alloys and plastics etc are low torque/high speed.

I have a 24000rpm spindle and could use higher, a 2mm carbide bit in aluminium wants around 30,000rpm
Bridgeport Mill, Mach3 V062, CSMIO-IP/A controller, AC Servo Drives
Re: First Run on CNC machine
« Reply #4 on: December 31, 2017, 10:07:46 PM »
John,
You don’t say anything about your machine other than spindle power and if it came with a 400w high speed spindle it is very likely it is no where stiff enough for a larger spindle motor or larger diameter cutters.  You need to match cutter diameter, engagement, rpms, and power for the best performance. A software tool like FSWizard is really valuable in doing this. You should try for a tool that will run at full motor rpms. Cutting the speed in half means you only have 200 watts to the cutter. There will be an optimum tool diameter for each material and for some materials at the maximum speed there will be no good choice.

I wrote in another thread about machine stiffness and how to measure it. You should give that a try before buying a bigger spindle.
Re: First Run on CNC machine
« Reply #5 on: January 01, 2018, 08:36:03 AM »
Yeh i realised after posting..i was using MDF and spindle is only running at about 2700rpm as i only put a 12 volt power supply ned to up that to 48v to get 10000rpm.
It didnt like 6.4mm cut depth so was taking 1mm cuts..will have a look as you advise but would like to cut MDF and Aluminium
Regards john
Re: First Run on CNC machine
« Reply #6 on: January 01, 2018, 03:59:59 PM »
Hi,
power is proportional to the square of the voltage, if you are running your spindle at 1/4 rated voltage expect 1/16 rated power, 25W.


Craig
My wife left with my best friend...
     and I miss him!
Re: First Run on CNC machine
« Reply #7 on: January 01, 2018, 06:53:25 PM »
Craig,
For DC motors speed is proportional to voltage and torque is proportional to motor current.  So for a common DC controller with I^2R compensation the torque and therefore the current will remain constant. So power=current x voltage will be 1/4 at 1/4 voltage.
Re: First Run on CNC machine
« Reply #8 on: January 01, 2018, 07:43:37 PM »
Hi,
well reasoned and I stand corrected it doesn't change the fact that OP is trying to spin a 6mm tool with a cut depth of 6mm and a feedrate of 1200mm/min
with a 100W spindle, thats la-la-land stuff.

Even the much slower rate that he actually achieved 1mm depth at 150mm/min occasioned excess vibration which is not to be wondered at at 2700rpm.
I would be aiming for a surface speed of 500m/min in MDF, with a 6mm tool thatworks out to 26500 rpm. I would allow a 1-2% of diameter per tooth per rev
as a feedrate, with a two flute tool that is 3-6m/min. I could well imagine that a several kW spindle would be required to do it which then begs the question
whether the machine could contain the cutting forces generated, as a previous poster commented it seem unlikely if the machine is equipped with a 400W
spindle from new.

If you wish to cut any material at high speed and reasonable cut depths a powerful spindle is mandatory which in turn demands a rigid machine. Thats plain
physics.

Craig
My wife left with my best friend...
     and I miss him!
Re: First Run on CNC machine
« Reply #9 on: February 12, 2018, 06:24:52 AM »
Hi,
My late reply way I had flue over January.

The CNC is home built, revamped from an earlier build, I will use as it is for the moment but If I install a 2.2kw motor I intend to beef up the "Y" axis gantry with a "U" shaped  12mm Aluminium form around the  underside to give added strength, and the uprights will construct from a 5mm and 25mm and 5mm sandwich to make a box section, at the moment it is pretty rigid, also the "Z" axis assembly will be thicker Aluminium and I will put a 300mm  leadscrew and 16mm supported rails instead of the 8mm at the moment, I don’t want to run as if it was a Bridgeport but just as a hobbyist.
I enclosed a couple of Autocad sketches of my revamp, the base is wood but will be 2” thick and bolted to a frame for added strength.
The main construction is Aluminium.
When I cut the MDF profile I was getting some vibration, mainly when got to end of stroke and stepped into the next cut.
Found out later I hadn’t secured the Y Axis Nut to the Lead Screw OOPPSS.
I want to install a 2.2kw Square motor with VFD supply, what is the main difference between the Cylinder Type Spindle Motors and the square type is it just cost.
Regards and many thanks the comments have been very helpful.

John