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  1. #1
    Join Date
    Sep 2006
    Posts
    607

    Drawbar force (MT3 & R8)

    I'm just checking if my calculations are correct (I'm just taking the logical aproach here).

    I want to know how much force you need to hold a tool in a MT3 collet, and so I did some calculations using some numbers on how much force was needed on a R8 collet.

    The R8 collet has a ratio of 1:7
    Some dimensions of the R8
    D1=31,75
    D2=24,13
    L=26,67

    31,75-24,13=7,62
    7,62/2=3,81
    3,81/26,67=0,142857
    0,142857=1/7

    The MT3 collet has a ratio of 7/279
    Some dimension of the MT3
    D1=23,825
    D2=19,7612
    L=80,9752

    23,825-19,7612=4,0638
    4,0638/2=2,0319
    2,0319/80,9752=0,02509
    0,02509=7/279

    The R8 need about 1500lbs of force to clamp the tool securly in the spindle. This gives:
    1500=680Kg
    680/0,142857(Ratio of R8)=4760Kg

    To get this kind of force on a MT3:
    4760*0,02509(ratio of MT3)=119Kg

    I know there are some factors that are not taken into this calculation, but it gives me some clue as to how much force I need to apply to the MT3 drawbar.

    I have seen the number 1000lbs for the R8 too in a topic, and this gives about 80Kg on the MT3.

    With only 80Kg of force needed, I could use a 200oz-in stepper connected to a 1mm pitch screw to push the drawbar down (pushed upwards by spring washers) as it would give 200*28=5600, 5600*25,4=142240g / 142Kg. If I made the stepper with a 1:4 ratio, I could use a 60oz/in stepper (want to keep the size of this thing down).

    So are these calculations right?

  2. #2
    Join Date
    Feb 2006
    Posts
    251
    Did you have any luck figuring out your actual numbers for the project? I am interested in a R8 drawbar tensioner but have no idea how much force is needed. I have seen numbers from 1,000 - 4,000 Lbs. I am going to try a torque wrench on the drawbar and then calculate the rotational force into linear force, but that probably won't be the most accurate either.


    Quote Originally Posted by The Blight View Post
    I'm just checking if my calculations are correct (I'm just taking the logical aproach here).

    I want to know how much force you need to hold a tool in a MT3 collet, and so I did some calculations using some numbers on how much force was needed on a R8 collet.

    The R8 collet has a ratio of 1:7
    Some dimensions of the R8
    D1=31,75
    D2=24,13
    L=26,67

    31,75-24,13=7,62
    7,62/2=3,81
    3,81/26,67=0,142857
    0,142857=1/7

    The MT3 collet has a ratio of 7/279
    Some dimension of the MT3
    D1=23,825
    D2=19,7612
    L=80,9752

    23,825-19,7612=4,0638
    4,0638/2=2,0319
    2,0319/80,9752=0,02509
    0,02509=7/279

    The R8 need about 1500lbs of force to clamp the tool securly in the spindle. This gives:
    1500=680Kg
    680/0,142857(Ratio of R8)=4760Kg

    To get this kind of force on a MT3:
    4760*0,02509(ratio of MT3)=119Kg

    I know there are some factors that are not taken into this calculation, but it gives me some clue as to how much force I need to apply to the MT3 drawbar.

    I have seen the number 1000lbs for the R8 too in a topic, and this gives about 80Kg on the MT3.

    With only 80Kg of force needed, I could use a 200oz-in stepper connected to a 1mm pitch screw to push the drawbar down (pushed upwards by spring washers) as it would give 200*28=5600, 5600*25,4=142240g / 142Kg. If I made the stepper with a 1:4 ratio, I could use a 60oz/in stepper (want to keep the size of this thing down).

    So are these calculations right?
    BlueFin CNC LLC
    Southern Oregon

  3. #3
    Join Date
    Sep 2006
    Posts
    607
    I should have put in some more information when doing the calculations. The calculations are in no way accurate enough as it doesn't take into account the amount of force required to overcome the stiffness in the collet itself.

    Approaching this problem the way you described is not a bad way to do it. You could also find Hoss topics on his X2 build. He has some information on what spring washers to use, and how much force it takes to hold a tool in place.

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