Depth of cut problem


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    Default Depth of cut problem

    Hello everyone,

    i have one question regarding depth of cut. I was searching all over internet to find something relevant but I am still getting confussed so i need to clarify this with local experiences guys.

    I am very interested in depth of cut determination. I have a DYI cnc machine with very rigged steel frame and i use to have 1.5kw watercooled 24000RPM chinesse spindle. Last week i was experimenting with feedrates, RPMs and cutting depths. My goal was to reach a maximum cutting depth with 8 and 10mm 2 flute high quality carbide tool. I thoungt the higher the tool diameter the deeper depth of cut can be, but i was very wrong. After testing i destroyed the spindle bearings with 700 mm/min feedrate, 24000RPM and 6mm depth of cut with 10mm 2-flute tool high quality tool (with the 1.5kw spindle) cutting birch plywood. The machine frame stiffness is not the issue i am sure of it.

    I am a novice using cncs, i know how to calculate feedrates etc as i found on the internet but i really dont know how to set the maximum cutting depth. I have ordered a new 2.2kw 24 000RPM Teknomotor professional spindle it should be much more rigged then the last one, with more and high quality bearings, but i really dont want to destroy it. They told me at the shop i ordered it, that its important to increase the spindle power if i want to go deeper and instead of insreacing the tool diameter i should decrease it. They told me also, that customers with 2.2kw spindles are ussualy able to cut 10-18mm plywoods in one or two passes using various tools from 3-6mm diameter. I also saw some videos on youtube with guys cutting 18mm plywoods with one pas on wooden framed cnc machines, with 18000RPM 2.2kw spindles mine is much more stiffer than that so I am just wondering what am I missing. I want to be able to cut 18mm plywood in 1-2 pass, with relevant feedrate using 2flute end-mills around 5mm diameter. Can anybody please give me an advice how to achieve this? Should i buy much more powerfull spindle, lower the tool diameter or what am I missing?

    Thank you very much for your advice.

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    Default Re: Depth of cut problem

    I have been cutting 1/2 inch plywood, 1/2 DOC with a 1/4 single flut Compression bit, 2.2KW spindle at 24k RPM on a CNCRouterparts standard model. Edit: Also this was at 160IPM so about 4000MM/Min

    Solid Carbide Compression Spiral CNC Bits - Toolstoday.com - Industrial Quality CNC Bits

    Been working great.

    Last edited by nlancaster; 09-21-2016 at 03:54 PM.


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    Default Re: Depth of cut problem

    Are you certain that there wasn't a problem with your bearing in the first place?

    I can cut at 1200mm/min using a worn out 6mm 2 flutes compression cutter. DOC of 4mm. I'm using a Makita rt0701c router at roughly 12000RPM. Onsrud recommend 1x the tool diameter for this tool in hard plywood.

    Even with my hobby CNC made of plywood 700mm/min is considered slow; especially at 24000rpm.



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    Default Re: Depth of cut problem

    Hello,

    thanks a lot for the tips. And how about the bit diameter? Shoul i use something around 5-6mm would it be better?

    Also I know, that there was no problem with bearings, i ve crushed it because of the wrong set up. But maybe I am missunderstading the speed meter in mach 3,but the maximum speed i can reach with my motors and 25mm ballscrews is something around 1000mm/min everything over the motors (2,2 Nm (X axis with slaved motors and two screws) cant handle and the screw stops because of lack of torque, if i lower the torque in motor tuning it goes ok, but the maximum speed on the feed rate display of mach 3 is something around 1100mm/min. Should i consider repleacing the motors with some more powerfull for example 8,5 Nm motors?

    But if i crushed the spindle using 10mm tool and 700mm/min feedrate and 6mm DOC, i would be crushed as well with faster feed right?

    Thanks for the answers and sorry for my dumb questions, I am so far famlliar with CNCs construction and tunning, but the milling it self its really new to me.



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    Default Re: Depth of cut problem

    While slightly different in metal machining you often use the depth of cut to control how much power is required from the spindle. That is you start out with a feed rate that fits the spindle RPM and material and adjust the depth of cut so that power required is within your spindle capability.

    Quote Originally Posted by laserdog View Post
    Hello,

    thanks a lot for the tips. And how about the bit diameter? Shoul i use something around 5-6mm would it be better?

    Also I know, that there was no problem with bearings, i ve crushed it because of the wrong set up.
    First off the parade "crushed it" means nothing in this context, what actually happened here? Bearing don't go bad because the spindle is overloaded, you should be able to use the full horsepower that the spindle can deliver.

    But maybe I am missunderstading the speed meter in mach 3,but the maximum speed i can reach with my motors and 25mm ballscrews is something around 1000mm/min everything over the motors (2,2 Nm (X axis with slaved motors and two screws) cant handle and the screw stops because of lack of torque, if i lower the torque in motor tuning it goes ok, but the maximum speed on the feed rate display of mach 3 is something around 1100mm/min. Should i consider repleacing the motors with some more powerfull for example 8,5 Nm motors?
    I'm not sure why this is even part of the discussion. How fast you want your axises to move is largely up to you as long as no laws of physics are violated. You want feed rates and spindle RPMs that are suitable for the material you are working on. If your machine can't hit those rates then you need to change parameters to give you the results you want.
    But if i crushed the spindle using 10mm tool and 700mm/min feedrate and 6mm DOC, i would be crushed as well with faster feed right?
    You really need to tell us what happened, "crushed" does not tell us anything here. I highly doubt that those machining parameters did any harm to your bearings in the spindle. Unless of course what you did was to burn out the windings in the spindle. You are running less than half tool diameter so I have to wonder if by crushed you man a burnt out spindle
    Thanks for the answers and sorry for my dumb questions, I am so far famlliar with CNCs construction and tunning, but the milling it self its really new to me.
    The question I got for you is this, is the spindle a new one? I ask because it could have been defective from the factory. The other silly question here is the spindle rotating in the right direction?



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    Default Re: Depth of cut problem

    Hi,

    thanks for the answer i will try to summ it up:

    1.Can you please tell (if the feedrate and RPM is right) how would i calculate or tell how much power from spindle I need in terms of DOC?
    2. Crushed means that I started a job and during the job the spindle suddenly changed its sound from smooth to throwing rocks into metal box like sound. Also when i put my hand on the spindle axis (the collet) and try to move it, there is around 1mm backlash, which was not there before, so therefor the sound and the backlash i assumed the bearings broke. Also the spindle now vibrates, but its still running like it should be, it can be turned on and off etc.
    3. Sorry for bringing this up, but I am a total novice to CNCs so if i encrease the torque of the motors four times, would it be enough to go from 1100 mm/min to 4000 mm/min feedrate?
    4. The windings are fine i guess, there is just the sound and the backlash and also I was running more that a half of the tool diameter (10mm tool 6mm DOC)
    5. The spindle was used only for few hours of wood milling, but before that it was brand new. And yes its rotating clockwise which is flute blades against the material.



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    Default Re: Depth of cut problem

    Should i consider repleacing the motors with some more powerfull for example 8,5 Nm motors?
    What is the pitch of the screws?
    What drives ares you using, and what voltage power supply?
    Changing motors alone may not help at all.

    But if i crushed the spindle using 10mm tool and 700mm/min feedrate and 6mm DOC, i would be crushed as well with faster feed right?
    I suspect that your spindle failed from poor bearings and the high rpm. Your cutting parameters put very little load on the spindle.

    With a 2.2kw spindle, you should be able to make two passes at 4000-5000mm/min. Your RPM should be kept between 12,000-14,000 at that feedrate. Higher rpm is bad for both your tools and the spindle.

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

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    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

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    [URL]http://www.g-forcecnc.com/jointcam.html[/URL]

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Default Re: Depth of cut problem

    The pitch of the screw is 5 its 25x5 ballscrew. I am using Gecko G540 controller with integrated drivers. Details here: G540 4-Axis Motor Control | Step Motor Controls
    My power supply is 500W 48V 10A. I can propably change from my nema 24 motors with 2.2Nm to Nema 34 with 8,5 Nm but I am not sur the drive can handle it or the power supply.

    Ok so would it be better to order 2.2 Kw spindle only with 18 000 max RPM if i will not be able to use 24 000 RPM at all? The guy at shop told the higher the RPMthe bigger the power is, so I am very confused right now.



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    Default Re: Depth of cut problem

    And please what whould be the right setup for 800 mm/min feedrate 6 mm 2 flute spiral bit and 10mm DOC? At this point with these motors I am not able to go faster then 800-1000 mm/min. I tryed some internet calcutarors for these, but my result values are completely diferent to ones you suggested. Thanks a lot.



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    Default Re: Depth of cut problem

    To put my two cents in, start with determining optimal chipload for your specific bit and material you cut.
    For example, open this Onsrud page and one of Plywood Cutting Data documents.
    You will see that minimum recommended chipload for 1/4" (cca 6 mm) bit is 0.005" (0.127 mm).
    (As your DOC is greater than 1x bit diameter, you can decrease chipload (2xD DOC - 25%, 3xD DOC - 50%))

    Quick math: requested feed rate 800 mm/min / (2 flutes x requested chipload 0.127 mm) = 3.150 rpm.

    If it is too low rpm for your spindle, you can
    - use 1-flute bit,
    - set higher feed rate, or
    - decrease chipload.

    E.g., 1000 mm/min / (1 flute x 0.1) = 10.000 rpm

    Wait for confirmation from someone much more experienced before you even start to think about setting any of parameters above...



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    Default Re: Depth of cut problem

    im sorry this is a little off base but i cannot figure out how to post a thread .. does anyone know how determine the time a sheet will take without running it . i know there is a formula but i cannot remember it . i just added up inches and did it that way but was off ..



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    Default Re: Depth of cut problem

    Quote Originally Posted by qwertysimo View Post
    To put my two cents in, start with determining optimal chipload for your specific bit and material you cut.
    For example, open this Onsrud page and one of Plywood Cutting Data documents.
    You will see that minimum recommended chipload for 1/4" (cca 6 mm) bit is 0.005" (0.127 mm).
    (As your DOC is greater than 1x bit diameter, you can decrease chipload (2xD DOC - 25%, 3xD DOC - 50%))

    Quick math: requested feed rate 800 mm/min / (2 flutes x requested chipload 0.127 mm) = 3.150 rpm.

    If it is too low rpm for your spindle, you can
    - use 1-flute bit,
    - set higher feed rate, or
    - decrease chipload.

    E.g., 1000 mm/min / (1 flute x 0.1) = 10.000 rpm

    Wait for confirmation from someone much more experienced before you even start to think about setting any of parameters above...

    Thanks for the explenation now i finally get it. Only question which comes to my mind is, what is the connection of DOC to spindle power actually? How would I say how much power do i need to go for example as exampled (requested feed rate 800 mm/min / (2 flutes x requested chipload 0.127 mm) = 3.150 rpm. )? I dont want to stress the spindle to much, but in this case would I even use the 2.2kw power or should i use a less power for example 0,73kw, 1,6kw ? (All 3000-18000 RPM) now i see i dont need 24 000 RPM spindle at all.



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    Default Re: Depth of cut problem

    My understanding: Assume the feed rate is constant. The bigger DOC, the more material must bit remove per rotation. If spindle does not have enough power to keep torque, rpm decreases and bit must remove even more material, but when it removes even more material, rpm decreases even more etc... It comes to point where bit is unable to remove given material volume and it breaks.

    At low rpm, high speed spindle might not have enough torque. E.g. my Chinese watercooled 2.2kW "0"- 24.000 rpm spinde should be used in range 8.000 - 24.000 rpm to provide enough torque.

    I dont know formula that outputs needed spindle power based on parameters. When cutting wood, you should listen to the cutting sound and adjust parameters. I am a novice into this, cant help you more.



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