Coulter's Smithing Home
Here's an axis prototype for a hole drilling robot that will take up to 8x11 board blanks.
This is the smaller axis. The basic idea here is to use a stepping motor so that no position feedback is needed. That's money–saving. To make this work, the basic carriage has to be pretty good, but we also want to economize on bearings. Those linear bearings go for nearly $40. I have here an example disk head positioning device that just uses regular ball bearings riding on round rod that works well, and we may just go to that design, as this one isn't perfect. To the left is the first try, which is really not perfect. Doug tried hollowing out the acme screw, then pinning the motor shaft in there. Fine if there is zero error on everything, but a mil or so off center makes the screw gyrate and throw off the accuracy. This project is in "early times" at present. Once we get happy with the basic mechanical design, we will do a PIC microcontroller for it that can eat serial output from a PC to do the real drilling.
Interesting that a tap to make ACME nuts should cost less than one manufactured nut, eh? You can guess which we bought. We are using two nuts per shaft, one captive to the square pipe stock, one spring loaded to the first, to take up all backlash. That part seems to work pretty well. Main problem so far is that the ACME screw itself seems a little bent and is very hard stuff to really straighten to high accuracy.
The motor does 1.8° per step, or 200 steps per turn. With the ten turns per inch acme screw, this will give .0005 inch resolution. The accuracy of course won't be that good, but all we have to be able to do is hit hole locations plus or minus a few mils for this to work for us.
Yes, we know that using wood for the base is stupid. For the real thing we'll use something that has the same thermal coefficient as steel, like well, steel. No point wasting stock while fooling around, though. We will use wood for the platen, as that will get holed by the drill. We are planning to use a Dremel drill press or equal (maybe make something) with a solenoid to pull it down to make holes. Bit changes will be manual. Hey, we're trying to get a capability that is only available for some tens of thousands of dollars for a few bucks here. We won't mind changing a bit once in awhile.
The mech parts shown here are from McMaster-Carr, the motor, test controller, and wal wart are from Marlin P Jones. More data later, once we know ourselves what's going to be used in this.
Coulter's Smithing Home
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