First real part off the mill...

So I've finally got a part to run to completion! It's not the x-axis part I was making last weekend. I decided to give up on that for two reasons (1) it is a large piece and (2) if anything the printed x axis is the least weak part of the system. I have a bunch of aluminum cutoffs I scavenged ( ~ 50mm x 150mm x 5mm) so I made a smaller part. It's a spinning weapon from one of my kids' combat robots, scaled down by 50%.

This lovely weapon is then attached to a 2822 brushless outrunner and spins at about 7k RPM. It slices, it dices, it kicks other bots out of the arena!

Here's the first attempt. First g-code to run to completion! There are four, 1mm high tabs to hold the part to its stock to prevent flyoff. You can see there is a thin layer of aluminum between two of the tabs, clearly the stock is not flat or the head not perfectly trammed. 

The part tore out easily. I used a decomissioned pair of wire cutters to trim the flashing then a rough diamond file. Careful eye can see a fair bit of backlash in the larger holes and a dimple (due to backlash) on the bottom curve.

So back to the machine. Fiddled around with a number of things and found two things I thought were culprits. The first was play in the y-axis - I fiddled around with my printed housing for the motor and the bolts, and they were a bit loose. I did this on purpose at first because when I tightened everything down the axis became very hard to turn, likely because the print is slightly misaligned which shifts the housing around the shaft out of center. I took the housing apart and drilled out the bolt holes a few mm wider so that the print could slide around laterally. This helped quite a bit - I could lock down the print and the tension only increased a little bit over being loosely coupled. Secondly I tightened a few of the set screws in the saddle - again, to the point where it felt a little harder to turn, but not completely locked down. I also slowed down the Z axis a bit - some testing I was doing mid-week showed slips in the Z over time. I wanted to make sure I wasn't driving down the end mill too quickly and either moving the part or sucking the part up.

This one turned out much better. tolerance on the outer holes is ~0.15mm. The interior hole is about 0.05mm average narrower than the CAD. Which may make sense. In Fusion 360 I have the tolerance set to 0.1mm, and when you are adaptively milling out a hole you are essentially milling concentric circles. If you discretize a circle you get a polygon with a lot of sides, which is inscribed the circle, so perhaps this outcome is a-o-k. I need to study a little more to make sure my intuition is correct.

Anyways, super happy to have a part come off the mill. Repeatability in the Z axis is a bit of a concern and rebuilding the Z axis in metal is likely the #1 priority. 

Mill tear-down

So Robothon was yesterday. My metal combat bots were both 0-2 but neither was demolished; in fact they both still run. The next competition is at the end of January at the Northwest Model Hobby Expo, so I have a solid 3 months and some change to finish the conversion. 

Today I stripped down the table of my mini mill, because the y-axis has always been particularly sloppy ever since I got it. And I figure I should start with a clean, dialed-in machine. After cleaning chips and wiping down the machine I took off the x-axis by spinning the table all the way to the right and then taking off the handle and key. By jamming the table left to right a few times I was able to free the outer bearing and remove the table. The last bearing I took off by placing a crescent wrench against the bearing and then tapping it with a rubber mallet. Now the Y axis is exposed and the culprit became immediately apparent: the nut on the leadscrew is free floating in a slot in the y-axis table and is supposed to be pinned in place by a screw which was loose. Screwing it down and tightening the handle, the slop was instantly gone. Great, but I proceeded to finish stripping it down to clean and oil. Once the table was off I discovered the column was not properly aligned:

I loosened the outer two bolts and took a half-turn off of the center bolt and was able to rotate the column to visual alignment - once I have the table rebuilt I'll use an indicator on a magnetic base to confirm proper alignment. 

I cleaned off all of the packing grease I could find and rubbed everything down with an oily cloth and reassembled the y-axis. I tightened the screw which pins the leadscrew nut and tightened the nut on the handle to a comfortable tension and there was no visible play in the bed - of course once fully assembled I'll verify with an indicator. 

Tomorrow night I'll assemble the x-axis and work on confirming things are nice, smooth and aligned.