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7x12 mini lathe in fetching blue!

Big round parts - direct spindle mounting on a mini-lathe

For a lot of my projects I require large round stock to be machined. These are typically 6 inches in diameter and about 15mm thick. Whilst I've got a 4 inch chuck to deal with big stuff, turning something over 5 inches in diameter on a mini-lathe requires a different approach. Direct spindle mounting allows this. You could use a face plate, but in years of mini-lathe ownership I've never needed to buy a face plate.

If you remove your chuck from the mini-lathe you'll be faced with the spindle. This is a round metal part with holes in to attach the chuck and a raised land in the middle to allow accurate locating of the chuck. The chuck is held on by M6 studs with nuts screwing on between the spindle and the head stock (note: handing 6 year old is a vital metalworking workshop accessory for doing those nuts up in the confined space.

Of course, there is nothing to stop us mounting a part directly on the spindle of the mini-lathe with M6 studs, just like a chuck! All we need are 3 tapped holes in the right place.

That would be the difficult bit! Start by cutting a slice of round stock the correct diameter, or, as I do, rough it out of some 1/2 inch plate using a band saw. Locate the rough centre (doesn't need to be accurate) and mark it with a prick punch. Now adjust your scribing dividers to a 1.300 inches - this is the radius of the bolt circle on my mini-lathe - you might want to check yours. Scribe a circle with the prick punch mark at the centre. It is important to use a prick punch - a regular punch makes too big a hole. Double and triple check this radius with your dial calipers.

At some point on this circle, make another prick punch mark. Now adjust your dividers to 2.25 inches. Using the second prick punch mark, scribe an arc that crosses the circle in two places. To calculate the 2.251 inches, multiply the diameter by sin( 180 / X)) where X is the number of holes, in this case. 2.600 * sin ( 180 / ) = 2.251. I usually error on the side of caution and aim for about 2.25 inches to allow for play in the prick punch holes.

Now measure very carefully between the two intersections of the arc and the circle - it should also be 2.25 inches. If not, start about.

If you have the three holes exactly equally spaced on a circle of the correct diameter, you've done the hard bit. Mark the two intersections with the prick punch, and then enlarge the three prick punch marks with a centre punch.

Use a mill or drill press with a centre drill to start the holes. Do not clamp the work down, just hold it flat on the bed of the mill with your hand - this will allow the centre drill to locate the centre punch marks. Raise the work off the bed of the mill a bit with some parallels and drill out each hole through with a 5mm drill. Again, do not clamp the work down, allow the drill to naturally find the centre of the hole.

Next, tap the holes out with an M6 tap. I use a micro mill to do the tapping to make sure I tap at right angles to the plate.

Now borrow the studs from your chuck, or make up some new ones from some (ideally stainless steel) threaded rod, and screw them into the plate. If all is well, the three studs will slip magically into the bolt holes on the spindle, you can do up the nuts and you are away.

IF you want a central bore in the part, make sure you raise the part off the spindle with something parallel so you do not accidentally machine the spindle.

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Make sure you machine the central bore and the our edge in one operation without removing the part from the lathe other you cannot get them concentric.

To machine the outer edge use a right hand turning tool and a lot of tool post adjusting :)

My series on making telescope worm wheel and gear sets shows a good practical application of this approach.

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