Improvements to CNC MkI machine (July 2007)

Wear and tear

All the punishment I have put my CNC machine through whilst trying to learn CNC and machining are starting to show.   Having just 10mm delrin blocks for drive nuts, backlash was always pretty bad, but it's gotten much worse since all those steel shenanigans :) I also had to realign my Z axis the other day, by eye...with a rubber mallet!

I've always meant to make some proper AB nuts but kept putting it off, as I also wanted to upgrade my leadscrews.  Well the time finally came.  I am sticking with all thread but upping from M8 (1.25mm pitch) to M12 (1.75mm pitch) to improve speed.  I know ballscrews or even ACME/trapezoidal screws would be more precise and faster but the main aim of my MkI machine has always been to experiment with what's possible with hardware store stuff and scrap and also as a learning sandbox for myself.  I wouldn't have attempted half the stuff I have done with it, if I had expensive ballscrews! >:)

End Bearings

While at it, I am also upgrading my end bearings to pairs of 7200B angular contact bearings.   As a bit of a result I got 'upgraded' to 4 sets of matching pairs when the nice man at Arc Euro Trade reported out of stock for 2 months on standards. :D

I made a pair of mounting blocks for the angular contact bearings, designed entirely within CamBam.   I'm trying to use CamBam for all my CAD now, so I get a good idea of what's needed to improve it.  My cutter collection has still not recovered from the steel escapades so cheesy mill bits, coupled with the backlash meant the finish was not that nice but all the parts were useable.

10bull's new nuts!

I’d been ruminating on various AB nut designs for a while, but in the end the final design was thrashed out in a 5 minute skype chat with my good friend MrBean (in between kid feeding and bathing duties).  He kindly offered to make me a couple on his machine and bring them across the following day for our first ‘eyeball’.

The design is pretty simple.  A delrin base flange and nut body is connected to the other half of the nut with 3 M5 bolts.  3 springs sit in pockets between the two halves, pushing them apart.

CNC machine tear down

The first task for the day was to tear down my machine and remove it from the coolant enclosure. This took a good hour.  The coolant sump was looking particularly evil, having a good inch or so of swarf sediment on the bottom and a skin of frog spawn like congealed coolant oil on the surface.

I turned down the ends of two M12 rods to 10mm to fit the new bearings and 8mm on the other end for a 608 skate bearing.  While rebuilding my Y axis leadscrews I took the opportunity to replace the T2.5 printer timing belt and geared down pulleys, with an HTD5 timing belt with 1:1 drive.

Result!

So at the end of a pretty frantic weekend and much gratitude to MrBean, the new leadscrews and timing belt are in place.  My Y axis has gone from a top speed of 200mm/min to 420mm/min. I’m pretty sure this could be improved further by tuning or even replacing the stepper drivers.  500mm/min should be possible with these steppers and leadscrews.  I also plan to put a larger pulley on the motor to try gearing up.

I didn't measure the Y backlash before the upgrade, but you could certainly feel it if you shook the axis.
The backlash measurements for each axis are now...
X: 0.230mm
Y: 0.004mm
Z: 0.340mm
I feel M12 leadscrew upgrades for X and Z in my future!

It was also very satisfying watching the machine 'upgrade itself'.  Tearing down my machine I got to marvel at how on earth it functioned as well as it did, despite all the sloppy hand crafted bits and pieces.  And while watching various machine parts emerge miraculously from chunks of solid metal I’ve still not lost that wonder at the magic of CNC I had when I rattled off my first 2D road runner pen plot.

Here is another shaky video of the new leadscrews in action.