Thread: CNC 2.0 - Knee mill

First step was to make the bearing blocks and motor mounts. No mean feat, considering the bore tolerances specified by Nachi are measured in microns! No point spending ungodly amounts of money on precision bearings to throw them into sloppy housings, or worse, run too much interference and close up the 0.001mm of internal clearance in the bearing.
The bores came out at 46.996 and 49.997mm, as best as i can measure )






The 2 angular contact bearings go in the first block in DB configuration and are secured by the retaining ring. I bought matched pairs so no shimming is required to set the preload. (Each bearing comes with a 'birth certificate', cool)
The second block is the motor mount, which aligns to the bearing block via the large spigot. There is a total of 26 fasteners in this assembly, including the 4 alignment pins.
The quill and knee units just mount the motors to existing fixtures where the handles used to be. Of course everything is mounted on spigots to maintain alignment.

Next step was the Y axis ballnut mount. Another tricky one. I wanted to cheat slightly, and pickup off the existing leadnut mount to maintain alignment to the ways. However the existing mount was poorly placed and limited the travel. In the end i used the old one as a mounting jig, using its machined faces to clamp my new block onto while i drilled the bolt and pin holes into the saddle of the machine..
I wanted to cut the ballnut bore and the mounting face in one operation. That was a long day of interrupted, unbalanced turning! Had to cut 20mm off the face, so the work was gonna be in there for a while. I decided to actually start with the stock way longer than required in order to help balance it out. I cut the rest off afterwards in the bandsaw, boring a hole in the corner of such a large piece of steel is not ideal in the lathe.



Finished...



All mounted up and as far as i can tell, perfectly aligned to the ways. There is absolutely zero clearance to the machine, hence all the funky reliefs on the top of the block. There really wasnt room for such a big ballnut in that position, but it all just fits, luckily.




The X axis was much easier. I just bored out an existing mount for the original autofeed system and drilled/tapped it. This was lucky as all the alignment work was done for me )



No ebay-spec Chinese ballscrews this time )

Time to extend the travel. I like to cut multiple parts at a time, and have a few large things i want to make so i wanted as much as possible. My cylinder heads are also 610mm long, and 200mm wide. So my minimum X travel needed to be 810 and i wanted 300 in the Y.
When i bought the machine it had 630x200, not enough by a long shot. However its over-engineered construction meant i could stretch that somewhat and still have lots of rigidity.
I now have 830 x 300, happy days )

The Y axis was an easy fix by moving the ballnut mount.

The X axis was much more involved. In fact the size of my motor mounts had already cut down the existing travel. I ended up taking the saddle to my friend's workshop and making some clearance on his new knee mill. This was also a good chance to put it through the hot tank and get all the old gear oil out from the mechanical autofeed system.


You can just see where i squared out the end and removed some of the mounting boss for the old autofeed to clear the motor.



Aaannndd now we have uber-travel. The cutter goes from the very front of the table to the very back, and each end of the T slot area )

Limit switches are hall effect, and largely hidden. These were super simple to calibrate, attach multimeter and move magnet triggers til it works where you want. Hopefully these will be more repeatable than mechanical switches and triggers.
I think these are pretty cool, especially the quill.

This was the existing quill stop, modified to take the hall sensor


Triggers (magnetic ball bearings)



X axis, this one hides under the table...



Seems i didnt take any photos of the Y axis or the knee, but you get the idea....

Hi Andrew,

Only one word "Awesome" !!!

Originally Posted by BaronJ

Hi Andrew,

Only one word "Awesome" !!!

I second that!

Ben.

I'll come in third - amazing work - the level of engineering and precision is remarkable.

Thanks for the comments guys. Im pretty excited to get it going and see what it can do.
Progress has slowed as im snowed under with work presently. I started making the couplers last weekend but still have a bit to go on those. They are going to be a bit special as well )

Finally got a bit of time to finish off the couplers. Quite a lot of thought (and work!) gone into these. They had to have zero lash, and be able to 'quick disassemble' meaning i dont want to have to undo the clamping bolts to pull the motor off. Problem being that any system that slips together has to have some clearance for assembly (lash)
I also wanted them to act as a 'mechanical fuse' so that if i have a big crash on the machine something will give before it breaks a casting or drives itself through a wall!!

So after much pondering i came up with these:


The motor side of the coupler has shear pins installed (tapped for easy removal) which lock in to the legs on the screw side when you drop the tapered keys in. Once its all sitting together you just nip up the bolt and the tapers fill up the assembly clearance, voila -no lash!


Installed


And the windows in my motor mounts are so you can easy access the screw to loosen off the keys

Hi Andrew,

Looking really good.

A question, I can see how you achieve zero backlash and think that is a neat arrangement, but I don't quite understand how the shear pin will work.

If the pin breaks how is the drive decoupled ?

Hi baron the pins will break off at the motor side, right at the base. When this happens the broken parts should be retained by the tapered keys, leaving the motor side of the coupler as basically a large sleeve over the output shaft, with no means of mechanically driving the screw. Once the pins are gone there is nothing sticking out to actually transmit drive to the 'legs' on the other half of the coupler. The motor will just free spin until i E-stop but the screw and table will come to rest immediately.

The pins are tapped so when they break off, the part still in the coupler can be removed with a simple pulling tool. At this point i turn up some new pins, press them in, put the motor back on and be thankful i didnt snap a ~$1k ballscrew or a casting on the machine.

The other reason to drill and tap the pins is to weaken them, and the internal thread creates a stress riser which again aids in allowing them to break off. If i need to increase the 'amperage' of my mechanical fuses i simply reduce the thread size inside the pins, which increases the wall thickness and therefore the breaking point.


Hope that makes sense )

Hi Andrew,

Ahh ! That makes perfect sense. I hadn't realised that the hollow threaded pins were the ones designed to shear.
I'd got it in my head that that it was the bolt securing the segments that was intended to break. Rather like a pin through a co-axial shaft.
I like the idea of using the thread size to control the shear strength.

I'm watching this thread with great interest.

Thanks.

More progress. Finally organised a day on a proper big lathe to do my ballscrew end machining (what prick of a job) so things could finally start going together.


Angular contact bearings greased and installed. DB or double back configuration for maximum load carrying capacity



Delrin seal at the back, the whole front section is sealed by the motor and inspection cover so no dramas on that end


Knocked up a quick rear bearing support, the large center hole will get a cable gland which becomes the exit point from the electrical enclosure. I bored the bearing recess by interpolating a circle on my little cnc and was quite impressed with the fit (i did tease it out to final size in a couple of small steps)


Waiting for motors...



A view showing the X axis screw.


The stepper motors are HUGE )




All my super anal designing and high precision machining paid off, the screws just dropped in with perfect alignment. I can move them all the way to each end and it is exceptionally smooth, no hint of binding or even stiffening up. No shimming or adjusting required )



That's the bulk of the mechanical work done. I have to quickly mount the X axis linear scale, then i can move onto wiring it up.
Of course there are other small things to make or finish but these are non essential to the machine actually running.

Well, this made its first chips under CNC power today. Big ones too )

Progress pics and cutting videos tomorrow.