using 4th axis on cnc

[dfox1787]

Hi

I have a 4th axis and im wanting to mill some groves in a aluminium pipe.

Im not sure where to start with this. I use V carve for my engraving. What do i use for the 4th axis?

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[dfox1787]

so far i have got his in mach3. Iv figured that vcarve can do wrapping. but my z moves too high and it doesnt look quite right in mach3. any one else have a 4th axis they know how to use?

[dfox1787]

what kind of 4th axis cnc router? Do you have set Z axis zero point

Iv got a cnc 3040. Im trying to engrave wood and iv set my z point. Ill try and get a video posted.

[handlewanker]

Hi....I'm very interested in 4th axis control too as one day I will want to do some work with one.

As the move for an X,Y or Z axis is programmed in G code as a linear move in Metric or Imperial and gets translated into steps to move the stepper motor, what is the move to rotate a job in a 4th axis as it's a rotation with a circumference that increases as the diam increases.

I would think that if you wanted 10 lines engraved around a 10mm diam you still get 10 lines if the diam increases to 50mm but the lines would be further apart.

Are we talking about degrees of movement with a 4th axis as opposed to a linear movement that occurs in a flat plane?

That means with a flat plane the linear move is translated by Mach3 into steps.....what is the G code move in a circular plane?

I'm talking about simple hand written G code rotational moves to control a 4th axis for pure positioning moves, irrespective of the diam.......for example to rotate a 4th axis chuck 90 degrees CW or ACW etc......the diam/circumference variable is of no importance there.
Ian.

[X07gavrp]

I use a lot of 4th axis rotary work on mach3 and to be honest all software I've used the best is desk proto 6 it is specially designed for 4th axis and all but really makes the job easy and it's not expensive for a hobby licence, maybe download the demo and try it

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[X07gavrp]

Also when I use 4th axis I turn on continous rotation to speed up the progress but depends what your Machining also centre of your rotary should be the 0point for z axis

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[chronomill]

I've used Fusion 360 which is free for makers with some success

here is a good video from NYC CNC

[handlewanker]

Hi, yes I realise to do 3D work with a 4th axis does require a CAM program like Fusion, but I'm only interested in a 4th as a work positioning/holding device not a continuous motion device etc.

That is, with a job mounted in a 4 jaw chuck and the work piece milled on various sides alternatively, I need to know how to make the 4th rotate to a specific angle for further milling etc.........like milling a round bar to a square and then drilling holes in each side of the square block etc......hand written G code to make it turn so many degrees as needed.

I know that on a linear plane like an X axis move the distance in mm is converted to steps proportional to the pitch of the screw.

So, for a direct drive 4th axis.....1.8 degrees per step or 200 steps will move the spindle around 360 deg....would that be A1 for 1 step at1.8 degrees...... or something like it.....what is the G code to make it move 1 step?

To get a rotation of 1 degree per step instead of 1.8 deg, you would need a reduction of 200/360 or 5/9........quite a coarse movement but quite ideal for just a 1 degree angular rotation.

That would mean if you pumped 360 steps into a 200 step stepper motor the spindle it was attached to by a belt reduction drive would move 1 degree per step.....what is the G code?
Ian.

[X07gavrp]

Maybe I'm misunderstanding what your after, but to turn 1degree your steps need to be set correctly there is a setting in mach3 in settings witch will automatically work out the correct steps, once that is sorted the g code for me anyway is A and degrees so for example A30 is 30degrees.
Sorry if I've misunderstood your question you probably already know this anyway but for what it's worth.

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[handlewanker]

Hi....thanks...that's clear to me now......I wasn't aware that Mach3 did the sums etc.....have to look into that.

At this stage in my learning curve, I'm walking the hand written G code path, specifically for angular rotation....just needed to know what commands were needed to do the meaningful etc.

The example I posted for analysis was a simple one and the actual 4th I'm working on will probably be something like a 1:100 reduction or something like it with a degree factor for the reduction

I think multiples of the reduction would have to be in proportion to the 200 steps of the stepper to give an exact degree move......I would be working in tenths or hundredths of a degree as opposed to minutes or seconds of arc......45 1/2 degrees means more to me than 45 deg and 30 minutes of arc....whatever......I may be totally wrong on that score.
Ian.

[dharmic]

Big reduction on a 4th is all well and good for increasing holding torque when you're using it as an indexer but be aware that a large reduction ration makes your 4th a lot less useful when you eventually want to look at coordinated, continuous cut paths including the 4th because it reduces the effective top speed of the axis down to a crawl. The 4:1 I have on my machine is kinda the worst of both worlds, I'm never using it as a 2nd spindle but, by the same token, I have to be careful with my tool paths to try and keep cutting forces as close to along or directly into the axis to avoid too much torque on the axis, otherwise it just skips.

It might be worth looking at a worm drive despite the backlash issues to stop this kind of shenanigans, or even add a brake to the 4th to clamp it in place when it's not moving.

[X07gavrp]

Set steps per unit is what you need to set steps correctly for a axis to turn the degrees you want

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[handlewanker]

Hi, Thanks, I've added that info to my 4th axis notes.

Building a 4th with a worm drive has 2 problems.........1st is that you have backlash and 2nd is you have a large reduction and slow rotation, but the inbuilt holding force of a worm drive is a boon to milling on the 4th.

That is my choice for a drive and I've doodled various designs for a constant mesh drive to overcome the backlash problem and have settled on a worm drive with a spring loaded pivoting worm to the worm wheel....it only has to move a MM or two to give the flexibility without being overcome by the force of a cutter ..

As long as you have a certain amount of resilience in the worm/wormwheel interface....with the spring..........you could safely drive the wormwheel metal to metal without getting to a tight spot and stalling the stepper, but at the same time the loose spots are automatically "adjusted" to give constant mesh backlash free rotation, which is what I want.......the drive is slow most of the time so lube can be an oil bath in the bottom of the box, or just grease...... I don't intend to use a 4th as a lathe so rotational speed is not on my wish list.

There comes a problem if you only want a small reduction, and for that design I think a set of gears is needed with a spring loaded jockey gear to give the resilience needed to go metal to metal.

I think if you have one gear in plastic would also be a better solution for low ratio drives.

I'm not a fan of a belt drive reduction for a 4th even though they are very common, but that's just me.

There will always be a better solution in the eyes of the maker, but if you get it right the CNC World will knight you.
Ian.

[dharmic]

Mine is the cheap and nasty one that came with my router. Its frame flexes if I lean on it and, with the NEMA-24 stepper (no idea of specs) holding it I can just turn the chuck. Utter rubbish which turned out to be a lot less useless than I thought as long as I took care in toolpath planning to keep the torque imposed on it to a minimum.

I think, if I upgraded it, I'd be looking at a hypocycloidal gearbox or harmonic drive for the positioning and low speed drive side of things, possibly with a brake and ideally with those driving via a clutch on a VFD controlled sub spindle. We can dream, right?

[handlewanker]

Hmmmm....cheap and nasty maybe, but I think if you just change the stepper to a more powerful one it will work better, but then you're into the more expensive models anyway.

If it's for carving then you won't need much power as the cutter is probably driven by a 2.2KW water cooled high speed spindle at 20k rpm, so the force on the 4th spindle is not all that high, and the fact that it's a belt drive type means they, the sellers, didn't think there would be much load on it anyway.

BTW.......if you want to sell that "cheap and nasty" 4th....and the tailstock,,,,,, PM me a price inc postage (bargain basement etc)....the chuck alone is worth $80 new, but you can keep the chuck if you want it as I have one

We're both in OZ so it shouldn't cost much to send......the cheapest models go for about $250 on EBAY.....with bigger motors and you get up to $400 + but still belt drive.

The topic of an ideal 4th drive has been discussed at length on another thread some time back, but after musing on the advantages of various drives and to make one, I decided the drive part would be easy and better with the worm and worm wheel design, but with a spring loaded worm.

If you over tension the belt on one of those other drives you get too much friction that robs the motor of half it's power..

With the power off you should still be able to turn the motor easily with some tension..

The spring back (backlash) on a 4th axis spindle from the cutter force is something to be avoided, that is why I decided to build.....still on the go.... with the worm drive as it has inbuilt holding power when stationary no matter what motor is fitted......judging by the force..... even that a Nema 17can generate....... the motor with a worm drive is not all that critical.
Ian.

[daniellyall]

dfox have a search in the Vcarve forum there is a post in there done for Mach3 that set your rotation correction, it's something that needs set with Vcarve in Mach3 also under config - toolpath you need to set the axis of rotation.

[A_Camera]

Big reduction on a 4th is all well and good for increasing holding torque when you're using it as an indexer but be aware that a large reduction ration makes your 4th a lot less useful when you eventually want to look at coordinated, continuous cut paths including the 4th because it reduces the effective top speed of the axis down to a crawl. The 4:1 I have on my machine is kinda the worst of both worlds, I'm never using it as a 2nd spindle but, by the same token, I have to be careful with my tool paths to try and keep cutting forces as close to along or directly into the axis to avoid too much torque on the axis, otherwise it just skips.

It might be worth looking at a worm drive despite the backlash issues to stop this kind of shenanigans, or even add a brake to the 4th to clamp it in place when it's not moving.

Another option is to remove the holding torque power save function. I mean, my drivers normally lower the current to about 50% if the axis is not moving, which I find is OK for the X, Y and Z but would not be very good for A, where I would prefer 100% motor current all the time. Perhaps you have already done that, I don't know, but it is an idea to try if your PSU is powerful enough. Expect much more motor heat if you do it, but that should not be a problem.

[dharmic]

Thanks, Iain, but I'm not going to be upgrading any time soon. Even though the 4th is weak as p1ss, now that I know what to expect it's generally doing enough on the occasion that I need it. Eventually upgrading the 4th will be a bigger project aimed probably at a mill rather than the router and looking at a chunky 4th for indexing and co-ordinated 4th axis which offered lathe spindle functionality to be used with the main spindle and an indexer to lock a tool changer. Preferably (because if I'm asking for the sky, why not) that could also support a trunnion and 5th.

First the toolplate for my router, a work in progress. Then playing with a few ideas. Then probably looking to upgrade to a mill instead of a router, then onto the 4th - at which point I'll let you know about the crappy 4th I have now!

In the meantime, search for the cheapest nastiest 4th you can find on eBay, about $350 or so last time I checked, and it will probably be pretty much identical to mine.

A_Camera, thanks didn't know they did that. In my case given how much of its life the spindle sits idle, not requiring braking, I'd be quite happier to leave it as is and not cook it. But if the job comes up that needs help holding things, you've given me a door to peek behind. Cheers!

[handlewanker]

Hi...thanks DH....as I'm into building one it was just a quick fix 'cos I like messing with things.....LOL.

The rotational braking force for a 4th is one aspect I realised was a necessary factor for milling with it, not only on the move but when stationary too, and as the stepper motor only needs to have a small holding force to prevent the worm from turning, it will run very cool when stationary with the power on...........hence the worm and worm wheel preference I want to make.

Doing the sums and I get.........with a reduction of 1:18 for a worm drive and a 200 step motor on the worm, that gives me .1 degree per step, or 10 steps per degree, which is an exact multiple of the 360 degrees rotation of the spindle.

Each rotation of the worm gives me 20 degrees, so 20 X 18 = 360 deg.

To get a common 45 degree rotation would take 2 1/4 turns of the worm........45 X 10 = 450 steps......how Mach3 works that out is the next step to ponder.

I think for positioning purposes .1 deg per step is quite fine for most purposes.

With a carving operation the circumference on a 50mm diam would be 157.1mm, so that is ,436mm per degree.

A 1:18 reduction with ,1 degree resolution on 50mm diam would give lines spaced.044mm apart per step......you can't see a spacing of that width with the naked eye, so I kill 2 birds with one stone.....resolution for carving and precise positioning.
Ian.