CNC Lathe Scratch Build

[Joseph Noci 1Participant @josephnoci1]

Simon,

Thanks for the kind words! This project was a mental exercise started some 6 or 7 years ago and all the targets were formed in the mind way back with not much real thought applied to the work involved! And that is becoming a little overwhelming right now..But I am really keen to see the 'end' of it, or rather to see it working, so perseverance has it! From your MEW articles I found the reference to the Onshape info and did register there and take a look, thank you.

I suppose I will go the Hirth coupler route – as usual decisions are also driven by what materials I can get. I can get almost any thickness 'steel' plate cut to 'size' – Living about 50km from a Marine repair and servicing Harbour there is a lot of ship hull steel plate! Cut to size is relative – within 5mm or so, cut with rough plasma, leaving lousy cutting tool tip unfriendly surfaces, etc..No idea what the grade of steel is – the shaper does not approve of it. A challenge in every way here!

I want to try steer clear of hydraulics and pneumatics if possible – life is complicated enough with this thing, but who knows.

 

T.B.

I am impressed by that ATC! Very nice and very well finished indeed. If I may be so bold – have you any detailed drawing or models of the internals that you would be willing to share ? The one thing I have not understood well enough yet is exactly how the tool plate and its shaft slides forward in the ATC block, to clear the Hirth teeth – In the many ATC DIY effort's I have seen on internet, that bit of detail is missing..I would assume the shaft rotates/slides in bushes rather than bearings? That implies that the shaft to bush fit must be very good to ensure rigidity, or does the Hirth coupling provide all the business end rigidity? How does the shaft slide in your design and what drives it? Your (3d printed?) covers hide many secrets!

Your Hirth Coupler design as a bolt on has potential of being done on the shaper, so that will help!

Regarding tool / chuck clearances – When modelling my ideas I placed a standard lathe tool in the tool plate, its tip at chuck center height and about 3mm past stock center line, ie, such as a parting tool might move to when cutting just past center, and then located the boring tools/drill on the circumference of a circle around the lathe spindle axis, so that a 16mm boring bar would clear the 4inch chuck with 4 mm gap. Where that circle intersects a circle centered on the ATC spindle axis, the tools are placed. That ensures those tools will always clear the chuck OUTER periphery, so no crunches, and no shortened tools – well, unless the tools are longer than the chuck and smack the headstock

Very interested to see how you did your implementation of the ATC innards..

Thanks both..

Joe

 

 

T.B. – Sorry, an Edit – 

You kindly offered to assist via a PM, which I will take you up on – just a thought though – seems there are a few folk 'dabbling' in this, and maybe some technical discourse on design within the forum may be of interest to all – if it becomes to heavy for the forum pages we can PM further? 

 

 

Edited By Joseph Noci 1 on 21/04/2021 09:40:41

[T.BParticipant @t-b-2]

Hi again

It was actually a joint project between me and a friend , he did most of the design stuff and I did the machining.

I will ask him if he has any drawings I can send to you but in the mean time I will do my best to show / explain the workings. I did in fact show him this thread a while ago as he often talks about wanting to build a machine along similar lines to what you are doing.

You are correct the spindle slides in bronze bushes , these where made to a close sliding fit but again in retrospect I think it would have been easier making them looser with the spindle less constrained as the hirth coupling when pulled into the locking position does all the location work.

Inside the ATC body is a pneumatic cartridge that contains a stack of belleville washers and a piston.

when at rest the belleville's are pulling the hirth coupling closed with some considerable force , the piston when actuated compresses the washers and pushes the spindle forwards , this leaves the hirth coupling disengaged and importantly the spindle is then totally free to rotate so that it can be driven round with a little nema 17 motor.

Behind the rear cover you can see a disc that is keyed to the main spindle but free to slide axially , this plate has a small hole , which along with a sensor is used for homing / referencing tool position on start up.

The little green stickers are covering up additional holes as to start with we where going to add more sensors and use a sort of Gray scale to give tool position feedback after a tool change.

In the end we decided this was making things overly complicated as the rest of the lathe is on open loop steppers anyway ie: with no position feedback.

I did however add a pressure sensor later on to the air line feeding the ATC as forgetting to turn my compressor on was proving to be the most likely fault scenario, if the pressure drops too low to open the hirth coupling the lathe pauses its cycle and throws out an error message.

I should have more photos showing the workings of the pneumatic cartridge but I cant find them at the moment.

hopefully this link will work , showing the ATC in the testing phase

: **LINK**

[Joseph Noci 1Participant @josephnoci1]

Very nice indeed! The video is excellent. It would be great if you have details of the insides of that air cylinder.

And how does the piston interface with the shaft – I presume it does not rotate with the shaft as that would make the piston/cylinder seals difficult. In fact, seals in general there must be a challenge as the shaft passes through the entire cylinder and piston.

I recall writing a few hundred words earlier I did not want to go the hydraulic of pneumatic route, but your setup is to neat to ignore! I guess this is now my challenge.

very neat!

Joe

[T.BParticipant @t-b-2]

No the piston does not rotate , although if it wasn't for the force of the belleville's pushing against it and the drag of the seals it could.

the centre bore of the piston is sealed to the shaft with a lip or ring seal , I can't remember the proper name but it was only a couple of pounds from the local pneumatic suppliers.

A lot of the hissing sound in the video is the cheap ebay solenoid leaking that was used for testing , I did pressure test the whole cartridge at one point and it held the belleville's in compression for about 20 minutes by which time I got bored !

I've got to go now as I'm due my 1st covid vaccination in an hour or so but i'll try and speak to my mate tonight re some air cylinder details

[Joseph Noci 1Participant @josephnoci1]

Thank You. Looking forward to any info!

Joe

[T.BParticipant @t-b-2]

Ok , hopefully these will help make things clearer

Some details to the rear , namely the homing disc etc changed and the drawings never got updated

but the piston cartridge details and layout are correct and as we made them.

 

Why does this forum software seem to randomly place pictures all over the place seemingly no matter what I do !!

Edited By T.B on 22/04/2021 01:25:46

[simondavies3Participant @simondavies3]

@T.B., very nice indeed, pity I didn't talk to you guys before I built mine!

[Joseph Noci 1Participant @josephnoci1]

T.B – Thanks very much for those 3D sections. I have blown the one up, and filled in some detail as I understand it. If I may pick your brain a little now, to understand better the operation please…

This is just an enlarged greyscale version for view placement:

Then on top of the greyscale, some detail and colour hatching:

Then without the greyscale background, but in extended position, with some descriptors:

And then my questions!

Is the pneumatic cylinder fixed to the housing or can it rotate somewhat? – Fixed somehow at interface surfaces 1/2 and 4/5 ?

When the main shaft moves forward, does the Drive Pulley slide on the shaft? With a keyway?

Is item 3 the air seal between the shaft/piston/retainer ( my naming) against air leaks?

I presume the 'Retainer' moves with the shaft, ie, is fixed to the shaft ( Threaded?) –

I also presume operation as follows-

Piston moves forward with air pressure as labelled. Piston flatten bellville washers and pressure is applied to shaft surface at B – pushing shaft forward.

Losing air pressure causes bellville washers to press piston away, placing pressure on retainer at surface A, pushing shaft backwards.

What is the thin 'blue' item between the drive pulley and the housing?

What is the orange 'bush' surrounding the RED shaft bush at the pulley end of the shaft? – it also has a rebate between it and the RED bush, at the pulley side? Does that orange bush thread into the housing and is then used to press up against the cylinder ( blue) to take up any side to side play of the pneumatic cylinder assy?

Please forgive the many questions..

Looks like a very nice design indeed!

Joe

[Ross 77Participant @ross77]

Hi Joe

What an amazing project this is turning into. the quality is something else. I have been watching this thread for a while as I want to build a multi axis machine as well at some point.

I helped design the ATC with T.B so hopefully I offer some answers to your questions.

Firstly I'm not a mechanical engineer so it is probably way over designed and there are better ways it could be done. this is also the first working prototype and we have planned to make improvements.

The key part of the design is the integral air cylinder that compresses the bellville springs to open the Hirth coupling. When it is energised it dosn't put any axial load on the shaft, which means it is free to rotate and can be driven by a small nema 17 stepper.

Most of the ones I have seen cause the shaft to be loaded and needs massive geared steppers to turn them. I'm happy to go through all design points but may take a while.

To answer your questions:

The cylinder was fixed to the main body but we found that it caused the piston to jam up, it is currently just a friction and allows for an minor misalignment.

Yes the end pulley is stationary and the shaft has a key way to slide on. this detail is missing from the drawing as T.B solved this problem and I never got around to updating the model. There is a thrust bearing between the body and the pulley and spring loaded end stop to keep it aligned to the motor pulley.

Yes item 3 is the air seal

Yes the retainer is threaded to the shaft and applies the pre-load to the springs, there is then a lock nut behind that to lock it up.

The operation is as you have identified, except there is a spacer that connects face B to the piston (this was part of the R&D to get the distance right) ideally face B is extended to meet the piston

I think the thin blue line is the encoder disc.

The orange bit is a steel bush. The main body was Lined bored to ensure it was accurate and concentric on both sides but this meant it had to be at the larger diameter of the from bush. the steel bush isnt really needed but reduced the size of the rear bronze bush. It serves no function other than a cheap spacer.

I think that covers most of the points. I'll get some better drawings of the various component groups if that would make it easier.

Any more questions then fire away

[Joseph Noci 1Participant @josephnoci1]

I am very grateful to both of you for being so forthcoming with drawings and design information. This is really a gem, an excellent design. When I initially looked at the 3D images I thought the design very complex and too much work to duplicate, but the mere outlining of all the parts in CAD showed just how simple and neat it is!

I think I have enough data here to be able to model something to good detail. I will certainly post all of it here for your joint criticism!

Thanks very much!

Joe

[Ross 77Participant @ross77]

No problem and thanks for the positive feedback. Unfortunately I do tend to over complicate my designs. T.B always seems to pull them back to some sort of reality and make them buildable.

It will be interesting to see what you come up with and I don't think I will be criticising your work any time soon.

Im quite interested in your 5c spindle design for one of my projects. I wish I had your skills to make all those parts. I see you are having trouble with the seals. Is the whole bearing/shaft assembly full of oil? or is it a drip feed?

[T.BParticipant @t-b-2]

Hi

just to add , the air cylinder is connected to the body at faces 4/5 , as Ross says we did have some problems with the piston binding , but if I remember correctly this turned out to be something causing misallignment in the rear bush and once this was rectified we where able to make this connection.

It works whether connected or not but if not connected the air cylinder could move axially when energised enough to make an annoying knocking noise as it hit the inner face of the main body

[Joseph Noci 1Participant @josephnoci1]

Posted by Ross 77 on 23/04/2021 00:43:40:

No problem and thanks for the positive feedback. Unfortunately I do tend to over complicate my designs. T.B always seems to pull them back to some sort of reality and make them buildable.

It will be interesting to see what you come up with and I don't think I will be criticising your work any time soon.

Im quite interested in your 5c spindle design for one of my projects. I wish I had your skills to make all those parts. I see you are having trouble with the seals. Is the whole bearing/shaft assembly full of oil? or is it a drip feed?

I don't think its overcomplicated – perusing in some detail shows little room for simplification…I suppose the design could be flipped – place the stepper drive pulley inside the housing and the air cylinder on the right , outside – this may permit use of some variant of commercially available air cylinder? But that brings all sorts of other complications which brings you back to the same amount of effort anyway, in addition to the unit growing in width, etc!

Ross, the 5C spindle is a story on its own – I did cover its birth early on in my series of posts, but possible did not highlight the compromises I endured because of it!

First, its size was again limited by material availability – the spindle end up being machined out of a piece of Caterpillar side shaft…which was rather short…In addition the headstock was made from a chunk of aluminium on hand..which was rather narrow…So the space available for bearings, oil flingers, labyrinth seals, etc, was very limited. These combined bits were what drove the dimensions for the rest of the lathe in its entirety – sort of the tail wagging the dog..

There is no room in the headstock for a conventional oil bath/drain hole at bearing type design. I bored rebates for the bearings and then through for the spindle shaft, and made a trough or sorts feeding the oil drain oil to outside ( for oil changes). The idea was to have the bottom 10mm of the bearing in an oil 'bath' – this level being way below the external labyrinth seals, and hope for the best – turned out for the worst! Works well at high rpm, but below 200-300 rpm it leaks – the flingers are not effective there.

In the end the short spindle is a big problem for boring bar clearance in the ATC tool plate, so we have to compromise further – add a 4" 5C chuck instead, which means a new spindle, etc, etc…

As posted previously –

1 + 6 (blue) Labyrinth seals.End seals.

2+5 Oil Flingers

7+8 Drive pulley preload nut and conical pulley lock

9 encoder pulley

10 5c collet closer

If you want detailed drawings I am happy to oblige – but perhaps more to show how NOT to do it..

Joe

[Joseph Noci 1Participant @josephnoci1]

Posted by T.B on 20/04/2021 23:32:12:

T.B / Ross,

Why am I battling to model the Hirth coupler on CAD??!! It looks so simple! I have dug through all the Hirth sites I could find on internet, ifinding some pretty tools where you input the parameters ( no of teeth, angle of teeth, etc) and it generates a pretty 3D model, but have not found any sensible Mathematic or geometric explanation!

Have you some pointers for me?

Joe

[Dave Smith 14Participant @davesmith14]

Josef

Have you seen this paper on hirth couplings. If the link does not work send me a PM and I will send it to you.

Hirth Coupling Paper

Dave

[Joseph Noci 1Participant @josephnoci1]

Hello Dave,

I don't find a specific paper at that URL – it appears to just be a general search on Hirth using Bing? I have seen about all those items that appear on that search page while searching previously using google.

Most of the 'slightly' mathematical or more technical descriptions that I have found also describe the more conventional coupler, ie, where the teeth are fully formed triangles ( with tops slightly flattened for clearance).

I have not found any design detail on the type as in the images above from T.B / Ross.

Joe

Edit – there is a paper in there..I have seen that one – by a group of Italians – University Bologna – but lacks design info – focuses mainly of tooth shapes versus load bearing capability..

Edited By Joseph Noci 1 on 25/04/2021 07:38:09

[Dave Smith 14Participant @davesmith14]

Josef

Yes it was the Italian paper.

Dave

[T.BParticipant @t-b-2]

Hi Joe

Thats definately a question for Ross as opposed to me , he did the drawings for the ones pictured.

I'll give him a nudge as I don't think he regually reads this forum.

[John HaineParticipant @johnhaine32865]

Hi Joe, I know that you've been focusing on the ATC but did you make any progress on the tool touch sensing?

[Ross 77Participant @ross77]

Hi Joe

As you say there isn't much out there for the design of these things. I did find a design guide for curvic couplings

Curvic Coupling Design – Nov/Dec 1986 Gear Technology

In the end we decided to make it as big as possible to ensure it was really rigid. The angle of the teeth is important as it affects the loading on the Bellevilles. 45deg is easiest but the separating force is the full cutting force so we used a steeper contact angle of 60deg, which reduced the load on the belleviles and ultimately the strength of the puenmatic cylinder to open it.

One point to note here is that the taller you make the teeth the further the piston has to push to open it. The Belleviles are pre-loaded in the closed position and the force to reach the open state can be considerable. we were going to look at reducing the tooth height to bring down the opening pressure.

As usual its all a balancing act

I cant find the calculations just now but I will keep looking. From memory we used a 800N cutting force, 2000N preload which needed a 90-100psi opening pressure.

[Ross 77Participant @ross77]

For the drawing side I just modeled it in the same order that I would machine it.

Main ring first with the tooth profile

Then on the top face I drew the 16 sectors to make the tooth cut outs. (its over sized to make sure it cuts the whole model)

Then used the extrusion command to cut out the teeth. The tooth width is measured from the centre height so I offset the plane 1.75mm and then did a 2 sided extrusion, one positive 30deg and one negative 30deg. I did fudge the model slightly with unequal extrusion heights so it would only contact the side faces and not the tops and bottom. As the machining was done manually the drawing wasn't updated once we had the finished part.

If you want I can send you my 3d file of the coupling. I use Fusion 360 but it can output most file types. Hope that helps and is not to late.

[Joseph Noci 1Participant @josephnoci1]

Thanks Ross.

I had approached it from the periphery rather that the flat face – drew the side view of the disc peripheral 'teeth' and wrapped it around the disc, but extruding it was a mess! your way worked a treat. here is a quick hack – as you say, now to juggle with teeth height and angles, etc. The teeth in this hack are very tall – would require 8mm movement to clear, but proved the extrusion technique.

The loads in my case are perhaps a little less since my stock size is limited to that of the 5C collet max opening.

Since loss of air pressure stops the job regardless, would it not be easier to reverse the role of the cylinder? Let the air pressure close the Hirth, and a lower effort spring push the Hirth open when the air pressure is released? I am contemplating a double acting cylinder to do both functions but maybe that's a road to hell…

Thanks again Ross – you have been most helpful!

 

Joe

 

 

Edit – added text

Edited By Joseph Noci 1 on 26/04/2021 09:35:53

[Joseph Noci 1Participant @josephnoci1]

Posted by John Haine on 25/04/2021 13:49:15:

Hi Joe, I know that you've been focusing on the ATC but did you make any progress on the tool touch sensing?

John, to be honest, no..

I did play around with a number of ideas, but there is still so much on the lathe's mechanical side to resolve ( and get on with!) that I was not fully applied in the experiments!

One idea which showed good promise I do wish to pursue – I took apart an old hand held Piezo gas igniter stick thingy – it had an obelisk of piezo in it – about 4mm square by 25mm long with a silvered cap at each long end. A scope attached showed a 8 to 15 volt spike, with the slab on a wooden surface and very gently tapped in the center of the slab using a tooth pick. I think this fitted into a contact probe for side and end touch detection has good promise!

Too many things on the go…Also busy upgrading the CNC mill, which I need to make the ATC tool plate, etc..!

Joe

[blowlampParticipant @blowlamp]

Wouldn't a detent arrangement be easier to make?

What would make me think twice about using a Hirth coupling is how many faces need to be in simultaneous contact when indexed and how feasible it would be to machine the coupling to the required accuracy.

Because of its design it seems to me that the Hirth coupling is self-centring, so if used with a close fitting bearing there could also be issues with binding whereby the coupling can not fully seat because it's restrained radially by the bearing.

Martin.

[Ross 77Participant @ross77]

Joe

I think 8mm of travel would cause you problems with the bellevilles by raising the opening force. we had 4mm tooth height and 1mm clearance so total travel of 5mm. that was generating a lot of force to reach the opening distance. Theres a good calculator on the mubea site that you can playaround with spring combinations to see the effects of different spring sizes and combinations.

Disc Spring Calculator Tool | Mubea Disc Springs (mubea-discsprings.com)

2000N may seem a lot but one thing to bear in mind with the holding force calculation is that it is deflection and not component failure that you are checking for. We did a mock up and loaded the Hirth coupling to test defection before the full build took place.

"Since loss of air pressure stops the job regardless, would it not be easier to reverse the role of the cylinder? Let the air pressure close the Hirth, and a lower effort spring push the Hirth open when the air pressure is released? I am contemplating a double acting cylinder to do both functions but maybe that's a road to hell…"

Yes it would be much easier but I don't think that would work as well because air is still compressible so it wouldn't be as rigid as the bellevilles. It would also put a lot of wear and tear on the seals, holding pressure for long periods of time.

From a safety point of view if it failed mid cycle it is likely to be a major failure as the spindle wouldn't stop fast enough.

Martin

The coupling is quite easy to machine on a rotary table if you do full passes, then it is only 8 changes. T.B mentioned early that any error isnt really a problem as it is always repeatable.

"Because of its design it seems to me that the Hirth coupling is self-centring, so if used with a close fitting bearing there could also be issues with binding whereby the coupling can not fully seat because it's restrained radially by the bearing."

To some degree yes but it depends how you install the coupling. T.B can explain best but I'm sure it was all assembled and pinned prior to bolting it down to ensure everything was concentric.

I do think this is one area that could be simplified and a normal ball bearing supports could be used, with a loose sliding fit on the shaft. As all the location is done by the Hirth coupling then all the bearing needs to do is support it when it moves forward and allow it to rotate.

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