CNC Lathe Scratch Build

Arrange for the weight to run along a roller-track of some sort to keep it from swinging, but still free moving?

Martin.

I'm sure a gas spring would be fine, provided you could, firstly, source, and then afford one (or more). I like simplicity and cheap solutions, so my suggestion would be a tension spring, acting via a cable and pulley(s). The spring's force:extension law would be made linear by arranging the 'pulley(s)' to be of varying radius - think fusee. (It would doubtless look a bit ugly - quite out of place on your masterpiece...).

BTW, it may be an optical illusion, but the axial 'depth' of the 'teeth' of your Hirth coupling seems constant across the radial direction, and the 'tooth' flanks appear plane. Does this work? I thought that the usual arrangement was for everything to converge to a central point, common to the two members (hope this makes sense).

Oh, I've just re-read bits of the thread. You have a compressed air supply. You therefore have a constant force available already - it just needs a container...

If you have a weight you want to move, not too fast, and you want to always maintain control of it, I would put it on a rack like a lathe saddle.

Can also take full control if required for any reason

relatively simple, totally reliable

From lathe saddles to trains going up steep slopes

Edited By Ady1 on 18/07/2021 07:12:07

...and fast cross-slide and turret equipment of course, it's a proven system

It is an optical issue - Here is a drawing showing the cut lines for the teeth - the sides are at a 60deg angle and the two par orange lines show the upper and lower part of the tooth flank - they do not pass through the disc center. The single orange line passing through the disc center is aligned with the midslope of the tooth flank. This thing was complex to make, and even worse to describe..

Joe

Going to read all about air tank.cylinder pressure based counterweights - new to me, and seems that it will be complicated..

Ady1, I fear you have lost me..not sure what you referring to..

Joe

Joe. Thanks for the drawing. I'll have to think about this - my intuitive 3-D visualization ain't what it used to be. It looks like you're using a different geometry from that which I was expecting. The standard Hirth, as I'm sure you know, has teeth whose section remains constant, but whose dimension increases proportional to the distance from the centre. (I too find it difficult to describe...) Everything converges to a central point, which is common to both members when mated, so it's all sort-of conical, so the bottom of the 'teeth' get deeper as the radius increases, and their height (above the plane passing through the aforementioned central point) does too. (OK, descriptive powers have been exceeded...). It was that sloping bottom and top that I couldn't see.

I'll try again... You can cut a Hirth using, say, an horizontal mill with a V-section cutter. The centre of the cuts go through the centre. The plane of the workpiece is tilted so the depth of cut increases from zero at the centre to the desired depth at the periphery. The workpiece is turned with the required shallow conical depression before milling. Both members have the same geometry.

The idea of a pneumatic counterbalance could be very simple: a piston in a cylinder. If the volume of the supply's air tank is much bigger than the cylinder, and the supply is fairly well regulated, you don't really need a separate regulator for the counter-force pneumatics. If you do, you could use an off-the shelf supply regulator, or an adjustable blow-off valve, if you can tolerate air loss. Simple enough to make, having got this far, but I can also see the attraction of an off-the-shelf gas spring... (Remember, the rod should be below the cylinder, to keep the seal wet.)

Apologies if this reads like I'm teaching granny to suck eggs, but I'm aware that these posts potentially have a readership with widely differing levels of knowledge and interests.

Edited By Kiwi Bloke on 18/07/2021 11:52:17

Perhaps Hirth is a misnomer in this application? Certainly this coupler is flat, no cone involved - the convergence to a central point is the common line passing through that tooth flank slope midpoint. Likewise, the center of each tooth, on the disc's periphery, all pass through the disc midpoint, so all is aligned..And as certain, my English fails me hopelessly in my description attempts!

The best way to imagine it is perhaps to take one disc as a given, and press it into a flat slab of plasticine - both mate perfectly, both are flat, self centering, etc. How to describe the given disc...

The piston counterweight concept does look neat and appears quite simple. I have just done a pneumatic cylinder - the one inside the ATC body, and that was a lot of work! Also, as usual, finding seals and such is complicated here. I see some solutions in the Parker / FESTO catalogues, but I am sure they want both my legs for one..

We will see where this goes!

Thanks for the ideas.

Joe

Joe.

Is your concern about backlash, or just taking some weight off the drive system?

Martin.

Martin,

The ballscrew nuts are dual, adjustable for nill/low backlash. The Z axis is a 16mm screw, while the X is 12mm so I guess the aim would be to remove the constant load on the smaller balls in the nut on the X screw. It is probably academic as the lathe is the project, not what it will make..The lathe won't have a busy life so the X screw and balls probably won't wear much in the next 10 years!

However, as the project is so OTT, why not keep going.

Joe

Think gas strut but with an air cylinder instead of the strut. If you find a cylinder which gives the right load with your compressor tank pressure, that's all you need. The force will change a bit depending on actual tank pressure but I doubt that would matter. If you need a pressure regulator to drop the pressure then when the cylinder is compressed the pressure will build up, but the volume of the pipework will limit that. What do the moving bits weigh?

Hello Duncan!

I think I have the idea...The moving bits are the tool changer and then the X axis plate and slide bearings - the Tool changer is approx 6.4kg fully loaded with cutting tools, the X axis plate, etc, is another 1.4Kg, so a total of 8 to 9kg max.

Not a lot, so probably not worth the effort..

Joe

I was referring to this bit Joe

GL

Edited By Ady1 on 18/07/2021 20:18:56

Been having a think, always dangerous. Your post of 14/11 shows the tool-post at the far side of the spindle, so gravity will try to move the tool down the slope into the job. Cutting forces will try to move it away. I know that ball-screws don't have a lot of backlash but it's not zero. If this is thought to be a problem you could either clamp the cross slide to prevent it moving, or over-do the counter-balancer so that the tool-post is always up hill if you see what I mean. Perhaps I'm overthinking it, not unusual.

On to some sums. The tool-post/slide assembly weighs 9kg on a slope of 45 degrees so the force due to gravity along the slope is 9*9.81*cos(45) = 62 Newtons. If I assume your air tank is at 80 psi, that is 5.5 bar so you need a cylinder sqrt((68*4)/(5.5e5*pi)) = 12.5 mm diameter. To allow for the tank pressure dropping before the switch kicks in re-do the sums at whatever that pressure is, which will make the cylinder a bit bigger, and then add another bit to make sure it's always uphill, or if you decide to undercompensate make it a bit smaller

Edited By duncan webster on 19/07/2021 12:23:10

Hello Duncan.

Forgive my late response - I became a bit despondent with the project and gave it a rest - decided to go and play with the Pussy cats in the desert..Some pics..

Ok, back to work...

Duncan, I think I follow your idea - makes sense and I will see down what road that all takes me..

This thing is certainly taking me down many a painfull track!

Still fighting the tool changer - still have not receive the Bellville springs from SA..

So I started think about the live tooling or milling spindle. The ATC is small, so not easy to incorporate conventional live tooling. I would like to have a radial and axial milling capability, but that means swinging the milling spindle around, and doing that servo-ed is not really feasible on the cross slide, space, etc. Manual swiveling is ok, since milling on the C axis is not done often. I considered 2 milling heads, at 90deg - the axial head can remain in place and will ( should..) not crash, but the radial head must be removed at times..all together a pain in the butt!

Then I thought if I made the axial spindle coaxial with the ATC main shaft, it would free up space in front of the ATC and allow the radial spindle to be fitted there somewhere.

And so the hack began...

The 9th toolstation was the ER20 Chuck in the ATC toolplate center ( see previous pics..) - the idea was to remove that, and fit bearings carrying an ER16 chuck, with the chuck drive shaft through the ATC main shaft, out the rear and driven by a BLDC motor at the rear.

The RED parts are the milling spindle and spin together. The Lilac parts are the ATC main shaft and are static, except when changing tools - they then rotate to new tool position, and slide left to clear the HIRTH coupler before rotating. On the right is the BLDC motor Bell housing ( contains the motor magnets) which is pinned to the 8mm spindle drive shaft. It is supported by the Bell Support Bearings ( Lilac). The Orange parts on the right are the BLDC motor Windings and rotor(?) core, on bearings, on the 8mm drive shaft. This orange section is constrained from rotating, by a sliding retainer ( not shown) fixed the ATC housing. The orange section, Red and Lilac sections are slide left to clear the Hirth coupler during a tool change.

The 8mm drive shaft has a free length of 132mm.

I have been trying to model what sort of torsional mode will arise in that section when milling - vibration, resonance, etc, but have given up! Very difficult to find data on milling cutter energy needed for an milling edge to cut say mild steel, given DOC, RPM, etc, etc..and then the resulting twist in the drive shaft, and release of that wind-up when the cutting edge clears, etc..

I cannot go larger than 8mm on that drive shaft - increasing means increasing the through hole in the pneumatic piston for the Bellville spring compression. That means less piston surface area and more air pressure needed. I am trying to stick below 110PSI - if you followed the discussion previously in my posts you will know the reasons behind said pressures..

The milling process is low key - not high rate of metal removal, etc.

The BLDC motor is a 400watt motor- 400watts for 60sec it says.. 250watts cont. The largest milling cutter would be 8mm, probably 6mm. The lowest useful spindle RPM would be 300rpm and the highest around 5000rpm. The BLDC motor is capable of 5000rpm at 45VDC supply.

Am I wasting my time???

Joe

So many diversions on this project..

Waiting for materials to arrive also delays things! The Bellville springs arrived - I installed and did a spring-load test which was fine. So now I wait for the steel shafting to arrive so I can make the need ATC main shaft with the through hole for the live spindle drive shaft.

And while on that subject, I started on the BLDC speed controller for the live spindle drive.

This is based on a small development board from ST - the board is about 70mmx30mmx12mm and can take a 45v supply and deliver up to 16amps to the motor.

Managed, with my Good Wife's help, to get a sensored, FOC control software suite running, and the motor runs so sweetly. A leather gloved hand was used to load the motor in tests...

At 500RPM, with 40VDC supply and 12amps the motor speed slows by 40rpm. Releasing the load ( take my hand away..) and the speed momentarily jumps by 70RPM and settles back at 500rpm - the jumps lasts about 300ms.

At 2000RPM, its a little hairier...40VDC and 14amps, the motor speed drops by 30RPM, and jumps by 45RPM when releasing the load - takes 380ms to settle. The PID is nice and tight.. Tests are very quick and time spaced - 40V/14A is over 1/2KW, so the glove, motor casing etc heats up fast when applying load by friction!

A few additions to the code still - a ON/OF control for the motor, so failure detection ( hall sensor failure, motor stalled, etc) so the LinuxCNC can know to not run the milling cutter into a workpiece while not spinning..

The controller underside:

Heatsink side:

One of three hall sensors fitted - 120Mechanical degrees apart, which works out to one sensor spaced four motor slots apart in the 12slot motor. The motor is 14poles, ie, 7pole pairs, so the 120 mechanical degrees equates to 17.1 electrical degrees. Sensors will be epoxied in place next.

Test setup...The Nucleo processor board lower left is just being used for the programmer part of it, to load the code into the BLDC controller.

The project certainly covers all bases!

Joe

Edited By Joseph Noci 1 on 16/08/2021 21:23:52

Posted by Joseph Noci 1 17/07/2021 20:52:35

The ATC weighs around 7kg. That is quite a lot of weight at a 45deg slope on the X axis. I
am looking for some way to counterweight the X axis. A counter weight and pulley
arrangement won't work as the weight would have to move with the Z axis and so
flail about...I am looking at the possibility of a gas strut mounted in the plane of the
yellow line in the image below - Anyone had any experience with such an implementation?
Or any other ideas?

Hi Joe,

I faced a similar problem with the Zaxis down feed on the Universal grinding machine
that i made ,except the axis was vertical and the weight it had to counterbalance
was about 80 lb ,there was not enough room within the hollow column for a suitable
size weight . A pair of cylinders were made from some smooth bore stainless
tube and just using O rings for seals .the sketch shows the layout ,the 2 cylinders
are connected with some nylon tube ,just used 32 grade oil ,the pressure in the
system is just over 350 psi and has worked well for over 8 years.
One hydraulic cylinder fits inside the column the other is remote in the corner
of the shop out of the way.
As you can see here in the photo the cable supports the the head slide both
grinding spindles and the motor and drive assembly.

This did appear as an article in the Model engineer
hydraulic counterbalance published 4494 30 oct 2014 and 4496 28 nov 2014
they made a complete mess of the article drawings missing some redrawn
and published twice the photo's were just ruined.

The force remains constant over all of the travel as it is just gravity and it
is simple enough to adjust to the load that you require.
The counterbalance weight can be remotely located in any convenient
spot.

John

Hello John.

Now that is neat, and the diagram is worth a thousand words. All the other posters 'hinted' at the concept but though I seem to be 'good' at what I am doing here, sometimes 'concepts' remain simply that in my head! Thanks for the info.

John, I seem to recall we conversed some time ago - maybe on the making/grinding of a shop made dovetail cutter?? Or maybe I am just blowing smoke again..The photo of the grinder seems to have triggered that memory.

Joe