Hi Martin,

You said "once on the machine I'll check the runout". From this I imagine that you made the faceplate in a chuck. It is best to do the chuck register machining when the faceplate is attached to the spindle and runout should be negligable.

Paul.

Edited By Paul Lousick on 23/03/2019 21:40:57

Hi Martin,

Nice work, but I'm not sure that you are machining in the right order.

Your back plate needs fitting to the lathe first. Guessing it fits on a spigot on the spindle – needs to be a good fit to ensure repeatability when changing chucks. The spigot for the chuck is then cut with the back plate fitted to the spindle so that the spigot is dead concentric. The chuck should fit the spigot quite firmly – again, to ensure the chuck will fit back exactly in position if it is separated at any time. Then the chuck can be bolted to the back plate.

There likely needs to be an undercut on any internal corners to ensure perfect seating (unless the matching corner has a good chamfer on it). All parts should be pop marked (or other) to ensure both the chuck and back plate are always replaced in exactly the orientation they were machined.

I may be wrong about this, of course, but better to shout now than you get deeper into the job and find errors.

In addition, there is no 'right depth' for the spigot. Sufficient and not too much are the requirements, not an actual precise depth. if you are within half a thous of some measurement, you are likely too close. Take some more off is my advice – but that may depend on what you call 'depth'. Axially – not important. Radially – very important.

I took depth as axial as the other would be described as diameter.

Way, way too fast. You should be turning that 4" diameter piece at closer to 100rpm than 800.

Well a bit fast – Martin used carbide, which tends to cut better the faster it runs, so not insane. I'd probably have tried 200 – 400, but 800rpm or even faster on 100mm diameter isn't unthinkable if I'm in a hurry. Often you have to up the ante to get a good finish from carbide.

I suspect many disappointing results with carbide are reported by chaps who don't push it anything like hard enough. If Dixon of Dock Green were a cutting tool, he'd be made of HSS. Kindly and experienced, but with bad feet, certainly not at his best chasing young thugs. A policeman made of carbide would be more like Judge Dredd – only to be used with maximum brutality. Go for it Martin!

Dave

Ideally with carbide and steel you're looking for smoking hot swarf flying off in small lumps like shrapnel. Ribbons of swarf are bad because they tend to go under the cutter and spoil the finish – this may be why your photo has a somewhat streaky finish. Also, ribbon swarf forms into balls that can be flung in your face by the chuck. What's needed is an even deeper cut, one that causes the ribbon to break into blue chips.

I'm not recommending it though! Apart from the lathe having to be fast and powerful, and there's serious risk of overdoing it on a hobby machine, the effect of sizzling swarf chips on the operator is downright unpleasant!

Conventional wisdom, which seems reasonable, is that carbide might shatter when heat-shocked, which could happen if you dab coolant with a brush or drip it on. So you either run carbide dry, or flood it. I'm not so sure – most of the carbide I use has been tough stuff, and I have dabbed work to improve finish without causing an explosion!

But I strongly encourage experimentation. With carbide, so much depends on the material, machine, depth-of-cut and the insert I hesitate to be pedantic. Carbide inserts have many advantages, but they work best when cutting conditions are in the zone. Provided it's kept in good nick a big advantage of HSS tooling is getting a decent finish is more tolerant of operator and machine limitations. It's also better, I think, at removing very fine layers of metal, which suits homing in on dimensions.

Dave

Edited By SillyOldDuffer on 24/03/2019 16:17:07