LaymarCrafts Woodturning Hints & Tips

Ball Turning Jig

Although I already had a Ball Cutting Jig with my Multi-Function Baseboard System I had the need for a more compact and an easier to set up Ball Cutting Jig  for the making of one off's.

There were a number of designs available on various Woodturning and Metal Turning Sites and I used some of the features from some of these, whilst utilising available materials and more importantly my limitations in Machining Facilities.

Apart from the necessary Hand Tools, I am limited to a Pillar Drill, whereas a more sophisticated unit could definitely be made with the aid of a Milling Machine and Metal Lathe.

Most modern Lathes have a relatively large swing over the Bed, in my case I could if I really wanted to Turn a Ball up to 400mm diameter [nearly 16"] in reality I only was looking to Turn Balls of 100/125mm diameter [4/5"] and as any Jig will have a High Centre of Gravity the lower you make the Jig then lighter materials of construction can be used whist maintaining stability.

My final design actually gave me the potential to Turn a Ball of 180mm diameter [7"] and I could have added another layer of the 20mm MDF to the base and still had the capability for a 160mm diameter Ball [6"+], of course if your requirement is for a larger capacity then you will need to adjust the dimensions accordingly.

 

The main component parts of the jig are:-

Tool Post from 25mm x 25mm [1" x 1"] Square MS Bar with a 10mm HSS Round Nosed Cutter.

Tool Post Support Spigot from 12.5mm Silver Steel Bar, this allows for height adjustment for various Tool Types and Tool re-grinding.

Tool Post is locked by two M6 Grub Screws locating on a flat ground onto the Spigot.

Main Bar has Support Spigot fixed, again by 2 x M6 Grub Screws + an Optional Spot Weld.

The Main Bar also has 7 x 6.5mm Holes at a pitch of 10mm for the Swivel Bolt [M6] this allows Radius Adjustment of Cutter and therefore will determine diameter of the Ball.

The Support base is constructed from 20mm thk. MDF with a central M12 Screwed Rod. which has a dual roll for both the Pivot Screw and Locking.

Laminated MDF construction Glued and Screwed, Top has a piece of Formica for smooth running of the Jig in use.

M12 Screwed Rod is drilled and tapped M6  in one end to take the Jig Swivel Bolt whilst at the other end locking in place is via a Black Plastic Knob or any suitable alternative, Lever, Nut etc.

The Base Board/Table is constructed from 20mm thick [because that is what I had to hand] MDF, the top piece is 220mm long by 185mm deep [8" x 7"] and this is off set from centre with the overhang at the front.

 

The dimensions for the top piece is based on the swing dimension of the Main Bar taken from the centre of the First Pivot Hole to the opposite end of the Bar [110mm or 4"] this will equal the width of the Top and using the same distance plus half the width over the Lathe Bed [Ways] for the front to back dimension i.e. 150mm 2 = 75mm [3"] + 110mm = 185mm.

If the Lathe Bed [Ways] are 150mm [6"] wide then it must follow that the Centre Line of the Headstock will be 75mm [3"] from the front or rear of the Lathe Bed.

You will have to make the dimensions to suit your particular Lathe by increasing or decreasing the dimensions accordingly.

The 3 off pieces of MDF which make up the Block below the Table are cut the same length i.e. 220mm and the width   equal to the width over the Lathe Bed [150mm]

A fourth piece is cut to the same dimensions for the Clamping Plate, you also need two pieces, again 220mm long and this time they need to be equal to the inside dimension of the Lathe Bed, in my case 50mm [2"] this dimension is critical as any slackness here and you will find it difficult to Centre the Jig and therefore make a perfect Ball.

The fit should be any easy fit but without any lateral play, the base piece is less critical so make the top one first and if it is to sloppy use it on the bottom and try again with the second piece.

Now you need to drill the hole for the M12 ["] Screwed Rod which forms the Pivot Point and the Locking Bolt, the holes are central left to right and in from the back edge by 75mm [or half the distance over the Lathe Bed], the hole needs to be a close fit and therefore a 12mm hole is best.

In two of the lower pieces you also need to recess the holes to accommodate the M12 Nuts which aid the alignment of the Screwed Rod in the assembly and the accuracy of maintaining central alignment.

The M12 Screwed Rod requires to be Drilled and Tapped in one end to take the M6 Pivot Bolt.

You now start the assembly by Gluing and Screwing the MDF Pieces together as shown in the sketch above, using a Set Square to check that the Screwed Rod is 90 in all directions, check after each layer is added and adjust if necessary.

For the Bottom Block or Clamping Plate you Glue and Screw the two pieces together and when set open up the 12mm Hole to say 13mm so that it slides easily on or off the Screwed Rod, I also fixed a metal plate around the Hole to protect the MDF from the rigors put on it by the Clamping Nut.

When the main block is set and cleaned up you can add a piece of Formica to the top as this will give you a smoother and harder wearing surface for the Jig itself to Pivot on, do not forget to drill a hole for the Pivot Bolt before you Glue it in place, this will also act as a reference point when carrying out the final Gluing and Positioning.

The Tool Holder/Swivel Bar as photographed above was constructed using some of the left over 25mm [1"] Square Bar from my Deep Hollowing Bar project.

The Horizontal or Swivel Bar/Base is from a 120mm [4"] length of the 25mm Square Bar and is drilled as shown in the sketch.

The important factor here is accuracy as any holes out of true or square will be amplified at the Cutter Tip.

As I was using an M6 Bolt as my Swivel Point the series of 7 holes, to give me adjustment according to the size of Ball I would be cutting, I drilled them to 6.5mm to limit the amount of play,

Again the hole for locating the Pillar was determined by the diameter of the Pillar I was using, i.e. 13mm hole for the 12.5mm Pillar.

The Pillar is locked in place via 2 M6 Grub Screws.


For the Vertical Bar / Tool Post I again used a 80mm [3"] length of the 25mm Square Bar.

The actual dimensions for both pieces will be determined by the materials you use and the Lathe you have.

The hole for the Pillar was drilled with the Bar in my Lathe Chuck [I have the Axminster with the standard Engineers Jaws] and the 13mm Drill held in the Jacobs Chuck in the Tailstock.

For the 10mm Tool Bit I drilled a 10.5mm hole and in the top of the Post I drilled and tapped an M6 hole for the Tool Locking Screw.

To fix and provide a means of adjusting the Tool Post height, I  used 2 x M6 Grub Screws, which clamp onto a Flat that is ground onto the Pillar, this can be seen in the Photo above.

 



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Last update 06 May 2008
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