4" chuck backplate

I have been putting this off, for fear of my own limitations, however I have now turned the register. It was also the first time using carbide insert tooling which was also causing undue trepidation. Here is the result.

I am pretty pleased with how its turned out, sorry, but the surface finish is smooth, not perhaps to HSS finish but perfectly satisfactory for the application and the register is at 90 deg, to the right depth and whilst possibly half a thou under, the chuck is a snug fit. For the record it was turned at 800 rpm with a 10thou DOC, I was going carefully but I reckon you could double that confortably. Apart from the smoke I don't know whether cutting oil is beneficial, but the swarf was coming off in long unbroken twists suitably blued by the heat, I certainly didn't appreciate just how hot. It obviously requires drilling and once on the machine I'll check the runout, but whilst contemplating the next step I was struggling to see in my mind how all the holes were to be accommodated, until I worked out that 13.5 divided by 2 is 6.75 not 7.75. Bit of a basic error, I blame age. Thus far so good, it can't last.

Regards

Martin

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, have to agree with Paul.

Regards Nick.

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.

Thanks for the thoughts chaps, although I fear I may have inadvertently misled you, for which I apologise. What you can't see on the other side of the backplate is the recess for the spindle register, so the backplate was mounted on the lathe when I turned the chuck register. The need to check the runout stems from a whole lot of other conditions that arose from when I first obtained the lathe. Todays job is to mark out and drill the chuck mounting holes, after which I should be good to go.

regards

Martin

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.

NDIY

Sufficient and not too much are what I did for the (depth) protrusion, I agree that the radial accuracy is most important, which is what I meant by maybe half a thou light, I'll see when it's finished.

Regards

Martin

Edited By Martin Shaw 1 on 24/03/2019 12:02:31

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

Perhaps not too fast.

Rod

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

I could I suppose have looked up Tubal Cain before I started which would have been sensible, he recommends for BMS at 4" dia 135rpm, with a 50% -75% increase for carbide, so say a max of 250rpm, which does make my 800rpm somewhat high. As Dave has pointed out speed has a major impact on surface finish, my limited attempts have shown this to be so, and when I was doing it there seemed to be no difficulty in removing metal, nor were there nasty noises. It did generate a fair amount of heat and at that speed I suspect the insert life might be shorter than it need be, I'll put that down to learning. Has anyone any thoughts about cutting oil, my reading on the subject suggests either flood cooling or dry. I have another backplate to do in due course so I will try that at a lower speed.

Regards

Martin

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

Your light 10thou cut won't have helped much either, knock the speed down a bit and go 2-4 times as deep if lathe will allow it and feed quite briskly.

Have a look at my post at the bottom of this page that shows finish improves with DOC and feed, though speeds about the same

Edited By JasonB on 24/03/2019 16:43:49

Dave/Jason

Some very useful info from both of you, thanks. The lathe is an SC3 so I feel I need to be a bit prudent with DOC, and while I suppose that 20thou would work, anymore might be pushing the rigidity of the toolpost assy. I think that Dave's point about v small increments possible with HSS is valid and there seems to be a case for using carbide initially and finishing with HSS if finish or tolerance is critical. Regarding oil I was coating the surface to be cut, rather than dabbing on as the cut progressed so I don't think I would shock it, certainly I felt the cut was "better", but there is obviously some furthe experimentation reqd on my part.

As a follow up to the job itself, using the PCD function on the mill DRO positioned the holes exactly right and even the counterboring went sweetly, any way the chuck fits nicely. I measured the spindle register radial runout which is 0.5 thou. With the wholly unscientifc use of a drill bit in the chuck, the roundest round thing I can easily lay my hands on, I'm getting 2 thou runout, which is perfectly adequate for wee steam engines, and my lack of ability to improve it means I'll have to accept it eitherway. Whether the runout is occurring in the backplate or the chuck itself I don't know, nor really care. It's perfectly usable and best of all the out of balance vibration has disappeared. Result Martin 1 China 0, for a change.

Regards

Martin