Shop Stuff: October 2011 Archives

Y-Axis update

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The Y-Axis overhaul is starting to take shape. Last night we squared up the supports, attached the rails and started mounting the Z-axis cart.

The supports are 2 x 3/4" plywood with a 3/4" plywood brace dado'ed in. After struggling to square the bridge to the supports, we noticed the brace was not square and it was causing the bridge to warp when the bolts were tightened. The fix was easy enough - a bandsaw and some slow steady pressure took about 1/16" of material off and gave us a nice square and straight edge.


The rails are 1/8" x 1&1/2" aluminum strapping. Using the table saw we cut slots into the 2x3/4" thick mdf bridge. The saw blade was not quite wide enough so 2 or 3 passes were needed to get the strapping to fit. In the end the slot was ever so slightly too narrow and the strapping, while in, is not as straight/true as we would like. None the less, the cart runs very smoothly along the rails.


Borrowing an idea / technique from the table, we chose the lower rail as the 'square' one, and the top as a guide. The cart is pressed onto the "square" rail by a spring loaded bearing pushing against the guide rail.


CNC update - Z-axis

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Quick update. The driver for my Z-Axis blew when I fed it a very dirty +24V. I decided to try and build my own chopper. The theory is quite simple - the current in the coils of stepper takes a bit of time to rise, find a way to monitor the current and when it gets to a set point turn the power off until the current falls below a certain point. Lather. Rinse. Repeat.

I took my PIC stepper software which had a NOP loop and an Interrupt handler and modified the NOP loop to monitor a voltage sense pin. The voltage sense pin is fed from a transistor logic inverter that is connected to a pair of 2ohm 3W resisters.

PIC Stepper Chopper
It kind of works. There's weird feedback from the Step (IRQ) pin back into the 555 test circuit, the current limiting doesn't quite seem to hold very well, and I'm trying to implement a feature where the current backs off when not stepping that doesn't seem to work very well. Not sure how much my time is worth given I could replace the driver for $15 or beef it up for $35.

Oh well. In the meantime I have made some progress on the physical construction. The cart has been attached to the Z-Axis. I made the stand offs from 1/2" steel pipe. The two bolts at the top allow for rotational adjustment should the tool not line up perfectly vertically.

Z-Axis cart

Adjustment bolts

Also added T-nuts to the actual tool mounting board so tools can be attached.

Mounting bolts

The cart will travel on 3 rails. All 3 will be 1/8" flat aluminum cut into an 8" by 1.5" by 40+" MDF board. 2 rails on the front for weight and one on the back to suck it in.