Machne for Turning large wooden spheres

My Son has asked me for help in designing/constructing a machine for turning large wooden spheres.

Currently he does these by hand with an arbortech carver and pattern jigs. These are on a fairly small scale, the ones he wants to produce could be up to 2 metres in diameter.

For an insight into what we are wanting to achieve, take a look at the video on this site

The machine we are proposing would not be truck mounted as that one is but still be able to be dismantled to move to site as needed on a trailer.

Options for driving the main spindle would be 3 phase motor when located at the workshop and a diesel engine when off site.

With a blank weighing in between 1 - 2 tonnes, it needs to have a substantial spindle / bearings and will turn at very low speed, <100 rpm

The design of the cutting spindle has already been done, We are now contemplating the design of the main frame and spindle.

Any ideas / suggestions welcomed.

You need one of these.

Murray

Edited By Muzzer on 25/06/2015 14:10:28

Err. Begs the question 'Why?!!?'

OK, I realise it's because people will buy it.

Suggestions:

1. Spindle bearing as close to the faceplate as possible.
2. 1-2" thick faceplate
3. Shaft probably best if a thick walled tube as large as your lathe can take to machine for the bearing. Aim for 8" dia x 1/2" wall or bigger. If solid, go for 4" minimum. If using a tube, you might be able to get away with wheels running on the surface of the tube as the shaft 'bearing'.
4. Frame to take it all: as massive as you can find.
5. Drive... I'd be terrified to be near something like that at 100 rpm. for 6ft dia, I'd feel jumpy with 25 rpm. Final drive could be a motorcycle chain & sprocket (1000cc bike, not a 50cc!). The sprocket could be a home made very large diameter being driven by a normal sized bike sprocket.

Would love to see it when finished.

Regards,

Richard

Jason,

yep, that's the thought train we had, do you have any more details/pictures of the machine you built?

Graeme

Something along the lines of this lathe ball turner but a lot larger box or girder section frame with the router inplace of the cutting tool, have the work pivot on a turntable as it wil take less support than a horizontal spindle with the frame at right angles to the axis

I'll stick with making small ones

Edited By JasonB on 25/06/2015 15:21:56

Reminds me of when I helped organise a David Nash exhibition in an 'earlier life'. I had a fun day visiting nature reserves and choosing trees with the man himself, even got to write a bit for the programme alongside the great Oliver Rackham!

**LINK**

Neil

Aren't you going to have trouble finding 2 metre + diameter trees?

Have a word with that mad Canadian. They have big trees over there, although 2m is certainly pushing it these days. They chopped down all the big ones....

In short, No. We've got several Oaks already felled and stored on the farm, and there are more to come in the next few months. Other species are admittedly hard to come by in this size. We decided on 2 metres as the largest size we could reasonably handle. Smaller sizes will also be accommodated on the machine.

Hi Graeme

I think I would be inclined to go a different route

Consider moving a high speed rotating tool head say a heavy duty industrial quality router through an arc as you very slowly rotate the work piece. That does away with the rather serious safety issues of a large mass rotating at 100rpm. or even 25rpm. Also bits of tree trunk are hard to mount securely.

There are several ways to describe an arc mechanically the easy way is a cranked arm and pivot. You will need an accurate pivot not that hard to do using self centring bearings like this one you will need two spaced apart **LINK**

The arm can have both bearings at one end (harder to make stiff and accurate enough) however easier to position on large work
or
form a semicircle with a bearing at each end Many lathe ball turning attachments do this. I can imagine it may be difficult to position.

For finishing off replace the router with a custom made 3 pulley belt sander using the slack side to conform with the radius of the work not the pulley ends the centre pulley is the driver. This is the way many tube polishing machines work. obviously you will need light tension and the right abrasive or it will dig in.

There are some rather nasty looking "Hogging" cutters available for angle grinders. Maybe for the initial cuts one of these might be a bit faster if speed Is an issue, it would be another attachment for the arm in place of the router

Oh and if you really want a good finish look through this video You will see a method that will work for you if you make a polishing cup, The video unfortunately has a lot of chatter Hmm But the meaty bits are there. and they use the rotating arm I am talking about. They do make a rather special ball.

**LINK**

Using this setup the power and drive requirements are somewhat less demanding.

For off grid use you would need a portable generator sufficient to drive the router and the slow feed. easy to find.

Oh yes you will need a custom trailer kitted out with a jib crane.

Regards
John

Edited By John McNamara on 26/06/2015 09:33:44

John

I am not sure if their claim that this silicon sphere (in your link below) is the roundest object on the planet. A few years ago NASA launched their 'Gravity B' satelite experiment to prove or disprove one of Einstein's theories. The sphere (spheres?) on that were about 50mm diameter and had an Earth related roundness of about 6 feet. That is about 7 times better than the Ikg standard.

NASA have a web page describing the whole construction but I have mislaid my link. (I'm sure Michael G will come up with it!)

Ian P

https://www.youtube.com/watch?v=ZMByI4s-D-Y

PS maybe the 1Kg is now the roundest ON the planet because the better ones are in orbit!

+1 for John's suggestion including the idea of an angle grinder with an Arbortech cutter end-on initally followed up by a turboplane blade face-on for finishing cuts.

John, not a different route at all, if you watch the video I linked to you will see that what you suggest is exactly what we are proposing to do, the machining spindle has already been prototyped and tested in a smaller scale it is just the main frame for supporting and rotating the blank that I was looking for inspiration on, As I said the rotational speed will be <100 rpm. 10 - 20 being a more realistic figure.

I envisage initial mounting of the blank being between a bolted faceplate arrangement similar to that portrayed in the video and a live centre fixed to the outer end of the blank whilst the initial roughing out is done

Look up wood turning in Honolulu, I remember seeing a video of someone making massive wooden tubs from there.

Clive

.

Ian,

[ Nothing personal ... you just represent a 'tipping point' ]

I regret that you will be disappointed.

... I'm getting rather fed-up of that 'back-handed compliment'.

MichaelG.

Hi Graeme

I did not watch the video, When brainstorming a design It is best to start with a clean sheet of paper.

I just watched it and noted how the work was attached to the mounting plate/chuck using Angle brackets and screws.

In industry when large squat pieces have to be machined the vertical lathe is often used. Have you considered this arrangement? there is a lot less counter levered weight, the work is sitting on the table.

You could make a table from plasma cut say 20mm plate 1000 diameter, Under that you could place a spacer say 8mm plate 850 diameter Then a sprocket made from 12mm plate 900mm diameter. All liberally Bolted together.

The 900mm sprocket has a circumference of about 2827mm If you used a 75mm sprocket you would get about 12:1 reduction that will be well on the way to providing you with a decent reduction drive.

The accuracy will be fine for this type of work laser cutting is accurate to about (point) .1 mm Drawing a large sprocket in cad is not difficult.

The table could have a centre ball bearing pair for location axially with support around the periphery to take the load on radially disposed ball bearings. Ideally crowned for the same reason a car back axle needs a differential, the inner and outer edge are traveling on two different diameters therefore at a different speeds. the crown having a narrow contact area greatly reduces the drag and friction. In the past I have used the inserts used in plumber blocks as a cheap source of crowned bearings. A means of preloading the table against the outer support bearings via the centre axle would be needed. Tot that hard. In the end you would have a very stiff work support. Something the arrangement in the video appears to be lacking made evident by the irregular tooling marks on the work.

The drive power will be much reduced. If the spindle is truly vertical the drive will not have to lift an off centre load as it rotates. several tons of wood.

Because the tool mounted on its arm is describing an arc vertically in this case it will need to be counterbalanced, not a big issue but it will need to be done. In the video the operator appears to be directly controlling the tool. Maybe a geared arrangement with automatic feed would be better?

Regards
John

Edited By John McNamara on 27/06/2015 06:33:39