Tuning Tips

From Shopbot Wiki

Keeping Your ShopBot Cutting Accurately:

Numerous small problems of tune and adjustment can add together and, over time, can contribute to "tolerance build up" and start showing up in the quality of your cutting or machining. Here’s a list of the things to keep an eye on:

  • Your step resolution is around .001" (PRT a little bigger; PRTalphas smaller) and there’s not much you can do about that. Keep in mind that this number represents the absolute best that your tool could do for accuracy, and it means that every step it takes is a digital move of this length. The digital nature of stepping can show up on diagonals and curves much like a digital diagonal on a computer screen.
  • Check the collet - it should be tight, clean and in good shape. High mileage collets belong in the trash. Good quality spring collets advertise a life of better than 700 hours. As the collet goes bad, it no longer holds the bit correctly, and it will cause the router to vibrate from asymmetry.
  • Check the bit - Dull and out of balance bits are the most frequent cause of bad cuts from chatter and uneven movement. Improperly sized bits can also increase deflection. The type of bit you choose can also have a good or bad effect on edge quality. Your tool has a lot of power, but you don’t want heavy forces put on the bit because it will flex the bit and your tool and put a serious strain on the cutting process.
  • Check the run-out (side to side) and end play (up and down) in your router bearings with a dial indicator. You will find most new off the shelf routers with a range of .004”-.006” in both directions. Unless you purchase a spindle, which has considerably less run-out, you will need to live with this run-out. Be aware that as the bearings wear, this run-out will get worse. When the run-out gets bad, replace worn bearings with new ones.
  • Make sure your part is held down firmly. $100,000 CNC’s that hold very high tolerances still have a ¼” error when their part slips ¼”. Constantly evaluate how you are holding your parts. Consider a special jig or vacuum box. A vacuum table will not only hold the part horizontally, but will prevent it from vibrating up and down.
  • Check the Z axis: Grab the Z-axis at the base of the router (please unplug) and apply pressure in all directions with the stepper motors powered up. If there's play, go through the Z-axis maintenance steps.
  • Next, do the wiggle test for a loose set screw in the X- and Y-axis cars. If a pinion moves with the carriage and the shaft remains still, then tighten your shaft set screws.
  • Rack and Pinion Angle – It is very important that the pinion gears fit into the rack at the correct angle. The motor should be perfectly perpendicular to the rack and approximately horizontal when sighted down the track.
  • Spring Tension: Too much tension on the motor springs can add resistance to the stepper motor and take away some of the motors power. Not enough tension can create backlash between the rack and pinion gear or allow the gear to hop out under load.
    • To adjust the spring tension, pull the pinion gear up firmly into the rack and take the slack out of the turnbuckle, then rotate the turnbuckle 4 full turns.
    • Lubricate the Rack with bearing grease.
  • Hold down bearings: All four hold down bearings on the Y axis should be tight enough to just turn in with your fingers. This means they won't spin easily, but not be so tight that you can't spin them at all. They should also be adjusted to be slightly preferentially touching the bottom side of the bearing – and thus pulling down a bit. This will prevent the Y-carriage from rotating and lifting up.
  • If you are using a single Z-axis, make sure you mount the router as close to the Y-motor as you can. This will shorten the lever arm between the drive point (the pinion) and the cutting point (the router bit) and thus provide the most rigid support.

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