Advice on 4th Axis

Here is the machine I want to install a 4th Axis on and the panel showing proposed position for the AM882 Driver.
Attachment 29589Attachment 29590

I make various size cone lamps and want a 4th Axis on the machine to do fluted, twisted lamps ect.
.Attachment 29591Attachment 29592
Diameters of initial work piece are 80, 100, 150mm.
I realise that I will probably have to lower the bed of the machine to accommodate the larger size of work piece.

Any recommendations or suggestions on 4th axis to purchase or advice on fitting, set up greatly appreciated

Funny, just been reading your very good build log. Did you ever manage dovetails successfully on your machine?

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Originally Posted by Andrewg

Funny, just been reading your very good build log. Did you ever manage dovetails successfully on your machine?

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Yes, Nests of tables and bigger tables up to 900mm long.
But you need a hole in the ground or away of jacking your machine up to the ceiling if you want to go longer.
However you could make a wall mounted machine, that would do it.


Attachment 29603

I think I've probably looked at 4th axis more than most on here, but still never got around to making one.
For woodworking, you do have the benefit that you won't need quite as much holding torque as for my intended metalworking use.

In terms of ready made 4th axis, the options are pretty limited.
There are plenty cheap ones, that are simply a stepper motor connected to a basic chuck/spindle setup. Probably ok for light engraving/cutting on smaller diameters, but lacking torque for anything larger.
There are one or two with Harmonic drives, which greatly increase torque compared with the previous belt option, and would likely be good for your purpose, but they don't have enough torque for what I'd intend to do with one.
Then you're into worm drive options. Provide the most torque, but with the sacrifice of speed.

A good option if it would work for you, is something with a brake. It does mean you end up with a glorified indexer, but for lots of uses it is often enough.

For ready made options, http://www.skyfirecnc.com/Parts.asp do harmonic drives, although I have no idea what their current supply times are like. Komatias on here (owner of Emvio Engineering) is probably the best to ask, if he is still a Skyfire dealer...
Belt drive, the InTurn http://www.theinturn.com/ is probably the most advanced option, and if you're considering building your own, there is a big thread over the mach support forum detailing the original design/build.

Other than that, a search of Alibaba is likely to produce lots of Chinese options of varying levels of complexity/quality. There'll likely be plenty cheap/basic options on AliExpress, but the more industrial/less popular items can be found on Alibaba.


Although, before you commit, I'd suggest working out what your torque requirements are likely to be. If you post up your spindle power, some cutters and the speeds you normally use, I can run some figures for you, so you can get an estimate of what 4th axis torque you're likely to need.

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Originally Posted by m_c

Although, before you commit, I'd suggest working out what your torque requirements are likely to be. If you post up your spindle power, some cutters and the speeds you normally use, I can run some figures for you, so you can get an estimate of what 4th axis torque you're likely to need.

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Spindle is this 3kw https://www.ebay.co.uk/itm/EU-CNC-22...QAAOSwfeJgOILv

Cutters: Straight 12.7mm Dia, Speed 18000rpm Feed 40mm/sec , Bull Nose 19mm Dia,Speed 18000rpm, Feed 40mm/sec

Nothing bigger than these I would think

Thanks for info and torque estimates

So scrap of paper calcs.
3KW at 24000RPM, gives 1.19Nm of torque. I just used the second calc down on this page - https://planetcalc.com/1908/ , as I can never remember the full formula! Other online calcs are available, that was just the first one I found.

It's worth a quick mention, that the speed you run the spindle at is pretty irrelevant, as this kind of electric motor is a constant torque source, so the torque remains reasonably constant up to the motors rated speed.
The only thing you do need to allow for, is the VFD may allow the motor current to go above the setting for a short period of time before tripping due to an over current situation, which IIRC is typically 150%, so the spindle may be able to output 150% of that rated torque briefly (torque is directly proportional to current).

For simplicity, we'll just round up to 1.2Nm for now.

Now running a 12.7mm cutter, we have to work out the force at the edge of the cutter.
Force = torque/radius.
radius = 6.35mm, or 0.00635m
1.2/0.00635 = 188.97637795N, but we'll round up to 189.

Now we know the cutter force, you can then use that figure to work out the torque needed for a 4th axis.
Torque = Force x radius.
80mm or 0.08m, torque = 15.12Nm
100mme = 18.9Nm
150mm = 28.35Nm

Now you have those figures, you need to consider a few things.
They are based on the spindle running at full power - This isn't likely.
All cutting forces being at the full cutter radius and perfectly tangential to the rotary axis. Again unless you happen to be taking a very shallow cut and somehow still manage to use full spindle torque, it's not likely.
So in that context the torque figures are likely to be high.

However, you need enough torque to prevent the cutter from grabbing and spinning the work, which is where you need a reasonable safety margin, as due to inertia in the spindle, the torque in such a situation could be very high, but as you'll be working in wood, I would very much doubt that would be as big a problem as a cutter suddenly digging in to a lump of steel/alu which will overcome an axis far more than a cutter digging into a bit wood.

Personally, I'd probably suggest aiming for 20Nm.
If you're making it yourself, 4:1 reduction belt drive should be doable. Combine it with either a big Nema 34 10/12Nm stepper motor for simplicity (and not much speed, but you don't really need speed for this kind of application), or a 750W servo (3000rpm will only be ~2.4Nm, but peak torque will typically be over 7Nm) if you want more speed. You could use a 1000W servo, but it all depends on how much money you want to spend.


Other option I forgot to mention, was Arc do a rotary table with stepper motor - https://www.arceurotrade.co.uk/Catal...-Stepper-Motor
However it would be slow, and using a rotary table for woodwork will likely lead to it eventually filling up and jamming with sawdust. Metalworking stuff really isn't that suited to woodwork, unless it is very well sealed.

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Originally Posted by OMLCNC

Here is the machine I want to install a 4th Axis on and the panel showing proposed position for the AM882 Driver.
Attachment 29589Attachment 29590

I make various size cone lamps and want a 4th Axis on the machine to do fluted, twisted lamps ect.
.Attachment 29591Attachment 29592
Diameters of initial work piece are 80, 100, 150mm.
I realise that I will probably have to lower the bed of the machine to accommodate the larger size of work piece.

Any recommendations or suggestions on 4th axis to purchase or advice on fitting, set up greatly appreciated

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Hi,
I went down the road of building my 4th axis for the cnc with a ER32 collet end that I can also fit a 3 or 4 jaw chuck onto and went one step further and made a stand alone indexer controller so I could take the setup to the milling machine as well.
I have used it for cutting wood,aluminium and brass, I made a brake for it but never had need to fit it as yet I would say have a go and build one.

Attachment 29609

Attachment 29610

Phill

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Originally Posted by phill05

Hi,
I went down the road of building my 4th axis for the cnc with a ER32 collet end that I can also fit a 3 or 4 jaw chuck onto and went one step further and made a stand alone indexer controller so I could take the setup to the milling machine as well.
I have used it for cutting wood,aluminium and brass, I made a brake for it but never had need to fit it as yet I would say have a go and build one.

Attachment 29609

Attachment 29610

Phill

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I like it!

I was mulling over m_c idea of building one when your post poped up.
I could start off with this https://www.ebay.co.uk/itm/EU-CNC-A-...8AAOSwl29gOIir
and up grade or modify.

Then remembered I have old tail stocks and chucks.

My CNC will cut aluminium if I'm careful and I have a metal lathe so can do the shaft.
Just a Nema 34 and driver required, some pulleys and a belt.
I may have a go.