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The material on this website is provided on a strictly "as is" basis. Use at your own risk. |
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3 Axis CNC router plans (PDF format, public domain) |
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3 Axis CNC router plans (table component) Can also serve as a solid stand-alone workbench |
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An Arduino sketch to drive a stepper motor using an SMCC board (full, half or wave drive). |
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I have had some questions about potential deflection of the 20mm shaft over the "Y" axis 900mm free span. My calculations for this are as follows: Anticipated load on "Y" axis: 10kg. Anticipated load on the "X" axis: 8Kg. Freespan for "Y" axis = 900mm, freespan for "X" axis = 500mm. Calculated deflections are: "Y" axis - 900mm freespan, 10kg load, anticipated deflection: 0.044mm (0.002") Total deflection 0.05mm - well within my design criteria of <0.1mm. For 20Kg "Y" axis load, deflection will be around 0.08mm or 0.003" , for 30Kg 0.132mm or 0.005". I don't recommend going beyond about 23Kg though.... The maximum load on the "Y" axis to ensure adherence to the original design parameters is approximately 20Kg. Note that this includes dynamic loading so if you make it heavy, take it easy and use sharp tools, or reduce the "Y" axis freespan...... However, if you go for "supported shafts" to maintain the 900mm, remember that open linear bearings are sensitive to load angles and you may therefore not get the results with them that you were expecting..... STEPPER MOTORS On the question of which stepper motors to use, that depends on a number of factors: 1) Loading (weight of the super-structure) Some examples (my calculations, these are intended to be a very rough guide, not guaranteed....) Weight assumed to be 20 kg, transmission efficiency assumed to be 50%. 2.54mm/revolution = 1468 g/cm (20 oz/in) - 2 Watts @ 100rpm Note that these are the amount of torque required to move 20Kg @ 50% efficiency continuously. They do not allow for additional forces due to material cutting. As a rule of thumb (and if you can afford it) it might be wise to get steppers that are double the required torque rating (i.e. if you determine that you need a 80 oz/in motor for your design, I would try using a 160oz/in, but that's just a personal opinion. You will also find that the quality of your controller will have a significant impact; i.e. a cheap controller will demand a more expensive (powerful) stepper, whilst an expensive controller will most usually result in the need for a lower spec stepper. Opt for a good quality (midrange) controller and a good quality (and well specified) stepper and you won't regret it.... Update: Some simple extrapolations for you. Half your weight, half your torque requirements (i.e. 10Kg weight, 13mm / stepper revolution travel results in 3755 g/cm (52 oz/in), 4 Watts @ 100rpm. For the same @ 200 rpm demand will be 8 Watts etc. For 75% transmission efficiency (expensive to achieve...) divide by 75 multiply by 50 ((result/efficiency)*original assumption)e. For the latest example that would result in 2504 g/cm (35oz) and 5 Watts @200 Rpm). Most torque extrapolations (with the notable exception of power consumption and for this "rough rule of thumb" excersize) are reasonably linear. If you design your stepper to move a 20kg load and you double its torque specifications you should logically be able to expect it to be able to exert that level of pressure on your cutting tool, but I'm afraid that I'm not familiar enough with the relevant calculations to be sure. |
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