Novamill Gets a 4th axis!

[flyingmike]

It was always part of my plan to have a 4th axis but with all the retro fit and mods it got put on a back burner. But finally I have got around to it

I had a look around at all the usual rotary tables and couldn’t really get away from the backlash issue in the worm gear arrangement. I thought about putting together my own but decided the best way to go was with a harmonic drive. Having used these in work I knew they had minimal backlash and massive reductions although maybe not so good under large loads. The downside of these is the price, the ones from Germany where hundreds and even thousands depending on the spec.

Then I spotted this on ebay! A 4th axis advertised for a cnc router and it uses a harmonic drive all for just over a couple of hundred quid
It arrived after a couple of weeks and was pretty rough to look at just in unfinished aluminium with a shocking Taiwan 3 jaw chuck. I cleaned it up, changed the way the bearings were preloaded and skimmed some of the surfaces and with a lick a paint it looks the part. I also bought a 4 jaw from Chronos. Having adjusted the preload and just giving it a general tidy up it runs a lot smoother that when it first arrived. I Think in its supplied condition it would have been a bit rough and the load on the motor just from the mechanism would have been huge.
I haven’t cut metal with it yet but its positional accuracy looks good (as good as I can tell with a wixey digi angle gauge)

The tailstock was supplied in a similar state, just unfinished ali, with a few knocks and dinks in. I lobbed off the rear mounting lug and machined a slot in the rear face to house a nut.

All in all I`m quite pleased with it now. I don’t think it would be that great for hard work but for my light cuts on model heli parts I`m sure it will be good. Time will tell...Just need to think of something to make on it now

[angel-tech]

looks good Mike.

[Denford Admin]

I agree, looks great - please let us know how much of a cut it'll take.

[JohnHaine]

Mike, was that a 1-off from ebay or are they regularly available please?

John.

[Denford Admin]

I found this on ebay, which looks similar:
https://www.ebay.co.uk/itm/CNC-Router-Ro ... 3a6f68119d

[JohnHaine]

Thanks! I had a hunt but didn't find that one, wasn't using the right search terms.

John.

[flyingmike]

Thats the one! I bought the tailstock from the same guy too.

I priced up the harmonic drive on its own from china and it is about £200 so I dont think the whole unit is that bad a price. (if its the same make of harmonic drive)

I think the design needs a better bearing arrangement as there is no real way of setting the preload but just to tighten it up a little more so you can go from too loose to too tight. There is no spacer between the bearings so its just a case of how much you press the end one in.

The chuck was rubbish too.

Apart from that looks good! . I`ll cut some metal on it this weekend. I have been busy with a starturn conversion

[Denford Admin]

If you get chance, would you take some closer photos of the harmonic gear please..it's an interesting solution for the price but I'm worried about quality a bit
We've thought of a couple of other possible uses for it:
Light duty indexing toolpost for something like the sherline/microturns ?
or a tool carousel indexer for the likes of the Triac ATC --no need for belts or pulleys and should be very repeatable.

[flyingmike]

Morning! Unfortunately I didnt take any photos of the drive when I had it appart but the arrangement was quite simple, maybe a little too simple for what a harmonic drive needs for good running.
I once worked on an actuator concept and we had more alignment features and some additional bearings than whats used in this 4th axis but I guess this was designed with a slightly less budget than aircraft components have

I have observed some testing for space applications and it appears the efficiency of the drive does fall to about 40% over time and one thought was that the drive pushes the grease away as the flex spline walks around the outer ring gear. This would be less important if the lubrication could be re-applied (not so easy in space!)

The place in china I got the quote from is:

https://www.hanzh.com/English/gongsi.htm

If I did it again I would make it myself using one of these drives for about £200 but at the time I wasnt aware I could get the harmonic drive so cheap from china (cheap compared to one from Germany or Japan) I dont think the ebay guy is using them (I asked but he never replied) as there would be no margin left for all the machined parts.

Let me know if you need a design engineer haha

[flyingmike]

I have had a play and the results were good. I did a simple skim of the face and the suface and got a runout on the face of 0.01 or thereabouts and 0.05 on the outer surface, which is more or less what I measured on the 4th axis chuck backplate.

I then tried a spiral cut using a 6mm ball nose cutter. I cut a depth of 2mm first, then 0.5mm then finished off with 0.2mm I thought the results from that looked pretty good.

I would probably use the tailstock on everything if I could as I did see a little deflection when I tried a heavy cut. With the unit being quite small and the shaft bearings quite close together I wouldnt expect anything less, seen the same with small rotary tables e.t.c

The stepper motor got warm but nothing more than normal and the repeatability seemed good so the harmonic drive seems to be doing a good job without slipping. I would probably limit the depth of cut to 2mm and in my application I would only take very light cuts in steel (not that I use steel much) The feed rate is a bit of a tricky one as the surface speed will depend on the diameter of the workpiece but I think I was cutting about 60mm/min

[JohnHaine]

Mike, it's an interesting idea to make one using a harmonic drive. I've contacted the company but of course they ask which drive I would want! Did you specify a type number please?

I wondered if there would be a few people interested in making one of these, to make it worthwhile buying a few drives and doing the design work. Anyone interested?

John.

[flyingmike]

Hi John,

I have my doubts that the ebay man is using this supplier but from their data sheet I think the closest is the XB1-40-100-C, and the unit price was Euro 235 for a one off. Which was a surprise as most prices coming from china are in dollars and 235 dollars might have been a winner!

I did this research as there doesnt appear to be anything else on the hobby market like this. If I spent £200 on the harmonic drive plus another £200 on bearings, a half decent chuck, stepper motor e.t.c then add the effort to make the thing I think I would need to sell it for about £700-800 to make it worth while which isnt going to happen, even selling it at cost would be hard work! so I have left that idea for now

But a very nice and rewarding project for in the workshop.

If you want some help on the design side then let me know....

[JohnHaine]

Thanks Mike! I gether this supplier has resellers in France and Spain which I guess is why he is quoting euros. Food for though...when I built my drive box I included a 4th axis driver which is all ready to go but then started thinking about backlash etc so this might be a possibility.

John.

[flyingmike]

I did the same thing

If you do go for the ebay one I would suggest asking the guy to quote on a unit without a chuck and without a stepper motor (so you can fit a double stack one). And maybe even not assembled, as you will end up taking it to bits anyway

[JohnHaine]

That's a good idea.

[RNR107]

Hi Mike,
Nice work! Looks like part of the machine!
What CAM package do you use to give you the GCode for your 4th axis ?
I know Mach3 support 4th axes and rotary axes, but what CAM give you the GCode?

Also, I have seen a few 4th axis system from China using a belt drive system, is that no good?

https://www.ebay.co.uk/itm/CNC-Router-Ro ... 3cbd993ac5

Ta,
Laurent

[flyingmike]

Hi Laurent,

All my 4th axis work has been done with gcode so far in mach3. I believe deskproto can accommodate a 4th axis but I havnt tried it. The more I practise with gcode the more I prefer it for simple jobs compared to using a 3D programming package.

The 4th axis in your link is from the same guy that sells the harmonic drive version.

From memory this was my thinking when I first thought of a 4th axis for my mill so I think the numbers are roughly right......

If I was designing a 4th axis I would try and get a movement that matched the other axis of the mill. On the X or Y axis on the novamill the leadscrew has 5mm pitch and the pulleys give a reduction of 2.5 (30/12) so that means that a 1mm movement of the table requires 100steps (to keep things simple here lets assume no microstepping, which I do use on mine, 8 I think)

Ok, so for the 4th axis I had to remember that the larger the diameter of the workpiece the greater the surface movement I would get for every increment in angle, remember similar triangles and all that. (This is why clever lathes change the speed as they turn a part down)

But we have to start somewhere so I assumed that my workpiece was 60mm in diameter. To get a surface movement of 1mm this equates to an angular change of ~1.9 degrees on the 4th axis.
I want to keep the 100steps/mm that I figured out before so 100steps = 180 degrees (1.8degree/step) therefore I need a reduction of ~94 from my gearbox.

We have no leadscrew reduction in our 4th axis and this is a big reduction for gears and would require a lot of them to keep the package small and backlash at a minimum, the same with a belt drive. You would need a few belts to get down this low and the ratio of belts is limited due to slippage e.t.c. A typical worm drive rotary table reduction is about 70. Therefore the harmonic drive was the best option at 100:1

On the link you sent me that reduction is 6:1 so to get 1mm movement on our workpiece the motor will do ~11.5 steps a massive difference. This means the resolution is lower, the precision is lower and the affect of losing a step is huge. Not sure if I have ever lost one, just read about it. I Can’t see any on the workshop floor

But it all depends on the job you have in mind....

[RNR107]

Hi Mike,
I am more interested in making a 4th axis as a project rather than really make something with it...
That's why the belt system is more interesting for me. I don't see how I could build a harmonic drive!

Thank you for the response!

Laurent

[flyingmike]

Then maybe a worm gear mechanism would make a more interesting project and with some clever preloading you could take the backlash out.

Mike

[angel-tech]

i would go down the worm route.