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Thread: Dimensional accuracy when cutting plywood

  1. #11
    Join Date
    Jan 2004
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    iBILD Solutions - Southern NJ
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    Quote Originally Posted by Gary Campbell View Post
    On a small machine like the desktop, aggressive cutting will deflect ̶t̶h̶e̶ ̶c̶u̶t̶t̶e̶r̶ the machine, towards the vector when using conventional direction and away from the vector when using climb.
    There I fixed it for you.

    (Gary already knows everything I mention here...it's posted for others)

    Quite often the cutter is to blame for 'deflecting' when parts come out too big/small - and in reaility, carbide doesn't deflect, or bend. Being crystalline and stiff, it will just snap clean off...unlike HSS, which is great for aluminum and other dense materials where you want it to deflect a little (to resist breaking and absorb harmonics). Deflection DOES happen but it happens because of what it's attached to, the structure of the machine...not the cutter on its own. Wait...what? Read on...

    It's also probable that microstepping, with its inherent hyteresis (aka willy-nilly slop) between 1/4, 1/2 and full step torque detents that there's some 'electronic backlash' to contend with here as well. Did you know that when a 1/10 microsteppping drive is hooked up to a stepper that it will deliver only 15% of the advertised nameplate torque when at a microstep position? Stop and think about that a moment...especially as it relates to parts cutting off-spec. (then subtract 50% of the 15% for drives that do current reduction when the motor is stopped and supposed to be holding postion...what's that 7.5% of nameplate torque?)

    The DT, for reasons I can only imagine, doesn't have the most well thought out mechanical reduction at only 2:1 on the screw, which doesn't help with cutting torque. The single screw in the Y doesn't help either...which can cause each end of the gantry to do the watusi. You have to make provisions for these things - meaning, make sure your RPM is high enough, your MS is slow enough (somewhere between 1-2 inches per second) to keep the chipload light enough not to deflect and not to lose steps. Once you learn where the reliable speeds/feeds are, you just learn to run in that range - and can get some nice accurate cuts on these machines. If I were designing the DT, it would have 5:1 ballscrews, higher torque motors and a motor on each side of the gantry...but that's me.

    I know few reading this spent the paltry price of entry for a dial indicator with magnetic mount ($30-ish) - It is one of the most useful tools in the shop once you understand how to use it. For those that do have one, put it on the machine and preload it to the table/bed. Then ever so gently press down on the gantry in the middle. It is scary how much these lightweight tools deflect under the smallest of pressure. See for yourself...PRS, PRT doesn't matter. Some are better than others...only through instrumentation (like an indicator) can you track down the source of slop or over/under sized parts after you've eliminated the obvious.

    Others have made good suggestions about getting accurate cuts - check climb vs conventional, rough cut it with allowance then shave it to final size etc. All very good pointers....but the reason you have to do that in the first place is because the machine either needs maintenance (v-rollers adjusted, rack to pinion lash etc) or you've just reached the limits of the machine chassis/structure itself. In the case of the later, slow things down and lighten the chipload. Adjusting VR to soften abrupt moves also helps.

    Quote Originally Posted by pro70z28 View Post
    I've run into the same problem cutting metal parts. I solved it by cutting slow...
    Case in point...machine deflection. Don't take my word for it though...buy an indicator and see for yourself. ...unless you're one to believe that ignorance is bliss

    -B
    High Definition 3D Laser Scanning Services - Advanced ShopBot CNC Training and Consultation - Vectric Custom Video Training IBILD.com

  2. #12
    Join Date
    Mar 2018
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    Coopersburg, PA
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    I tried some more test cuts last night. When I first posted, I was cutting 3.000" diameter circles and they were measuring between 3.010" to 3.012" depending where on the circumference I measured the diameter. This time I did the usual multi-pass climb cut but with a 10 thousandths offset and then went back and did a final full depth conventional cut to the final dimension. Measuring the diameter at various locations, I was getting 3.000" +/-0.002". Much better than before. I may try adding a second final conventional pass just to see if that makes any difference.

    Thanks for all the tips guys.

    Randy

  3. #13
    Join Date
    Oct 2000
    Location
    Willis Wharf, VA
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    Just to clear up one thing, the Desktop MAX has always had drives on each end of the gantry, and the Desktop added the second motor and drive not too long after the MAX came out.


    Quote Originally Posted by rcbrust View Post
    I tried some more test cuts last night. When I first posted, I was cutting 3.000" diameter circles and they were measuring between 3.010" to 3.012" depending where on the circumference I measured the diameter. This time I did the usual multi-pass climb cut but with a 10 thousandths offset and then went back and did a final full depth conventional cut to the final dimension. Measuring the diameter at various locations, I was getting 3.000" +/-0.002". Much better than before. I may try adding a second final conventional pass just to see if that makes any difference.

    Thanks for all the tips guys.

    Randy

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