Sunday, 28 June 2020

Side and rear plates for 4th axis

Not the most exciting part of the build but it has to be done. I have enough material to make the side and rear plates. I'll make a top cover later, when I find (buy?) some suitable material.

First, chop up the 700 x 305mm x 9.5mm plate I've had kicking around for the last 20 years or so. The Evolution mitre saw does the trick, although it's not suited to plunge cutting into the material. Requires a slightly different technique until the slot is started....



...then you are off.



Makes an unholy mess but at least there's no grit involved, unlike with an abrasive cutoff saw.



This plate is at least 20 years old to my certain knowledge. The problem there is that the polythene(?) covering has perished and is a sod to get off. Nothing easy here. Being impermeable, there's no benefit in soaking with IPA or WD40. 



And finally we're off:




Counterboring






Quite a bit of stickout from the vise




Tapping M4 with the tension compression head




Side plates done




Close resemblance!



The rear plate has a hole for the control cable (conduit) as well as the hole for long stock to poke through.




The Renishaw probe was essential here.




Yes, this is a broken 3.3mm drill - on the 3rd last hole of the job. It's the closest to a cockup in the entire process, presumably due to swarf clogging within the hole. I was pecking at 5mm steps with full withdrawal - but that doesn't ensure the swarf doesn't stick, even with flood coolant. Aluminium-specific drills might possibly help but I don't have any. Still, there's enough meat to tap to a 10mm or so depth as it turns out:




Well that was exciting. But at least I can now start slapping it together.

Wednesday, 24 June 2020

Chuck fixings - more chuck body machining

Got the backplate finished (finally). Looks good but as you can see, there are some features missing. My plan was to machine holes in the front of the chuck so I can fasten it to the backplate. There is no room to fit rear bolts. Arc Euro don't sell front fitting chucks at this size.


This is a cast iron body. Being round, I need some way of holding it in the vise such as a pair of vee blocks. I don't have anything suitable but I have these soft jaws that were made and used for making something a while ago. I forget what exactly.


Ideal.


I need to drill through the full depth of the body, then counterbore them out to fit the screw heads. So that body is upside down. And I will do best to CNC the holes and the counterbores.

First, I need to dial in the new Chinesium tip for the Renishaw probe. This requires a sensitive DTI that has a very light spring load, so the probe doesn't move when in contact.


Then set up the body with one of the slots aligned with the Y axis. 


Set the original in the centre of the bore, on the top face:



First, spot drill the 3 holes to reduce the risk of the drill wandering, as it's a 58mm deep hole. 


Here we go...




That worked out OK


Followed by counterbore and chamfer. Then doctor the screws to the correct length and diameter. I only have 3 screws the correct length and 2 of them are button heads. A quick session in the lathe got them to the right diameter.


There



Done. I need to clean and reassemble the chuck but it all goes together as planned.

Next - finally get the servo motor working?

Tuesday, 23 June 2020

Chuck backplate - make it - BOLLOCKS!!!!

Do it
Don't bugger about, fatty. Chopped up the stock, loaded it into the vise, lined up the tools, checked the tool length offsets (reset them), then kicked off. What could possibly go wrong?

Well firstly I broke my first ever Renishaw probe. No, not doing anything meaningful - quite the opposite. I was just coiling up the cable to avoid it getting caught in the moving parts. The cable caught the probe tip and PING. I hardly even noticed it, although I caught sight of something fucking off across the table. Shit. Lucky I bought those Chinesium clones off Aliexpress. So now I'm down to the last 2 (only ever had 3). And now I need to dial the replacement tip in before I can use the probe again.


Then I must have forgotten to save the Zref measurement when I set up the stock. I thought this was saved when the Zref operation was done but it seems not. You live and you learn. I was saying the other day that I'm not 100% clear what happens. That and getting a bit doddery. Yep, the wheels are coming off....


The first move of the first operation is to start the spindle at 3000rpm and plunge down to retract height. Given that the table has risen by 30mm or so, this was never going to end well. Having said that, the end mill survived(!) by being pushed into the toolholder. I stopped the machine before anything significant happened. One of the teeth has a slight chip now but nothing bad enough to require a new tool. Reset the tool and remeasured the tool length etc. Then off we go, this time with less drama. Luckily the machine coords didn't get messed up in the crash, as the only move was a vertical one.


So having pinged one Renishaw probe and almost nadgered a carbide end mill straight out of the trap, I seem to be doing well before I've even got going properly.....


Finally doing some machining:

Well that seemed to go OK. 







Hold up a minute...

But wait a minute. The register is supposed to be 0.3mm oversize in radial stock to leave yet it measures 94.5mm instead of nominal 95mm. And with the stock to leave it should be 95.6mm. I know Adaptive toolpaths are rough but that's taking the piss. Something funny(?) is going on here!



Sure enough, though, when you look at the simulation, it predicts that the part will be undersized after the 2D Adaptive operation. 





CAM looks OK on the face of it: 




Software generally doesn't have a mind of its own, so the explanation is likely to lie between my ears - if only I could find it.


In fact I found I needed to leave at least 0.6mm before it will be big enough to actually leave any stock. It's a good thing I left some extra radial stock for the lathe finishing, otherwise I'd be undersized. Oh, wait a minute...


At least the external diameter is OK. For this one I used the 2D Contour operation with 0.8mm stock to leave, coming out around 126.6mm (cf 125mm final dimension in the model). I should be grateful for that I suppose.




So I didn't see that one coming. Looks like time to cut another piece of stock and get cutting again - once I've figured out WTF is going on.


The explanation:

Seems I must have programmed the CAM and set the geometry before I deleted the chamfer feature. I say that because there's nothing anywhere at that location / diameter that I can pick up now. Clearly I'd managed to pick up the chamfered edge rather than the full diameter. And as it was a 0.5mm chamfer (from memory), I needed to set the stock to leave to a greater value just to avoid cutting undersize.

So I created a circle from the projected profile and used that to define the geometry.


Rinse and repeat:

At least most of the g-code files have been created and proven already. So once I'd recreated the 2D Adaptive toolpath, I was off again. The whole thing took about 40 minutes to set up and complete. The counterboring of the 12 fixing holes dominates the total machining time by far. Can't help thinking it could have been sped up a lot but hey.



That's better - ~0.5mm radial stock to leave (~1.0mm on diameter): 




Set free and cleaned up with belt sander and Noga deburring tool:



Now for the other side: Now machine the register that mates with the harmonic drive: 



Good - nice fit, no cockups:



Next, get the harmonic drive set up nice and true in the 4-jaw. Ended up about 10-15um runout, which is better than the 3-jaw will manage by quite a way. Happy with that:



Now mount the backplate and prepare to machine the register for the chuck in situ. Ideally I will remember to mark the backplate and drive face before dismantling them, so that I can reassemble at the same angular alignment at a later date. May not make much difference but it's good practice.




Nice tight fit. Stays on without fixings...



Chamfered and ready to come off




There. 



Now for the fixings (more machining on the chuck body).

Saturday, 20 June 2020

Chuck backplate - how to make it?

Looks simple enough but it could be tricky getting it done without screwing up the runout or having to manually machine loads of stuff. Any cockup here will fuck the whole thing up. 

The backplate sits between the harmonic drive and the chuck. The chuck bolts to it with three M8 cap heads (I will drill these in the chuck body later). The backplate is fixed to the harmonic drive using a plethora of M5 screws - there are 16 holes in the drive in total but I will only be using about 12 of them, as the chuck fixings can't avoid all those positions. 


I have some nice looking 1/2" flat stock, 150mm wide, so that will be ideal. Not least as I have nothing remotely available as an alternative.

I'll need to finish the backplate on the lathe to get a decent (lack of) runout but it's hardly going to be a simple matter even to get to that point. I'm planning to do most of the work before it gets to the lathe:
  • Machine the "front" features ie the central bore, myriad fixing holes (M5 clear) and their counterbores (M5 cap head), the M8 threads for the chuck fixings, the register for locating the chuck and the outline roughing, to liberate the result from the (rectangular) stock. Finish machine the central bore, so I can pick up on it later. 
  • Then flip it over and rough out the register for the harmonic drive, picking up on the central bore. I could do this on the lathe - I'll see how I'm feeling at this point.
  • Finish turn the harmonic drive register in the lathe, picking up the central bore from the milling operations.
  • Finally, mount the harmonic drive in the 4-jaw chuck, dial it it carefully and machine the backplate register in position. That should do the trick.





Here's the full set of operations. It's not the simplest of tasks:


Positioning within the stock, to leave a bit at either side for the tabs.


This is what it reckons should remain. Held in place with 6 little tabs.


Let's set it up, once I've done my painting duties in the bathroom....

Bantam CNC - sequence of events

 There's a right way to do this: Some of the critical machining work will need to be done on the lathe itself - stuff like modifying the...