Tweaking shopbot????

[hammerhead]

is there a way to tweak a shopbot that is running a little big or a little small??? is there anything in the software for this type of accuracy calibration??? basically after you set your machine up and find it is running not true how do you improve it?? thanks

[Ted Hall, ShopBot Tools]

In general, if the Unit Values for your ShopBot are entered correctly, since the gear rack is pretty accurate, your tool should be accurate. However, there are a couple of elaborations that can be made here.

First, the accuracy of the motion of the tool as defined by the Unit Values is what you should get under a no load situation. For example, if you ask your tool to move 10 inches in the X direction, it should move exactly 10 inches (measuring the carriage movement). However, during actual cutting, the cutter is always being pulled to one direction of the cut. Runout in the router and flex in the bit and flex in your ShopBot will cause the bit to deviate slightly. The deviation will be greater the more aggressive the cutting (i.e. the harder the material and faster the feed rate). If you cut a circle going clockwise, because of cutter pull, the cutout will be a little larger than if you cut the same circle going counter clockwise.

A number of different machining and cutting techniques can be used to compensate for this type of error. For example, a finishing pass that brings your part to a final dimension under light cutter loading can reduce this type of error considerably.

So with respect to the tweaking question, while theoretically you might just change the Unit Vale (which is the gearing ratio with respect to motor steps), this is unlikely to address the problem. If you are experiencing real distance problems, first check Units. Then look at the tuning factors which might be creating unwanted flex during cutting. Make sure the Z car is tight in its track (the excentric nuts can be used to tighten it if loose). Check the hold-down wheels on the Y-car for tightness. Make sure the bolts are tight on all the cars. It's also a good idea to keep the router as close as possible in the tool bay to the Y-stepper motor (e.g. slide the Z axis closer to the Y motor).

[Wdyasq@yahoo.com]

HH,

The older ShopBots have a UC command where you can vary the number of steps to a unit. Once you do that calculation and find it correct, if you have a problem you have something loose or binding that needs to be fixed.

On the OLD ShopBots you can get the Y axis out of square. As I have said many times "I find my old converted ShopBot my most accurate woodworking tool."

Ron Brown