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Member postings for Dr_GMJN

Here is a list of all the postings Dr_GMJN has made in our forums. Click on a thread name to jump to the thread.

Thread: Turning (approximating) a Domed Surface
04/10/2021 22:17:49

Just to wrap the thread up, I used the co-ordinate method. I used the side of the insert touched on the side of the blank cap, and the tip touched on the PCD face to get datums for everything:

Roughed the domes out:

Then using co-ordinates, approximated the profile (nice example of chatter here - solved by using back gear). Also I made a slight error with a CAD measurement which showed up as the wide band. Didn’t make a deal of difference in the end though:

Then using a combination of a dremel, files and abrasive paper:

Smoothed to shape:

Parted off using the home made rear post from earlier this year (worked great):

Then made a spacer ring and reversed in the chuck to turn and mill the inside details:

Took five attempts to get two good ones, mainly due to finger trouble, but eventually I got a decent result. As far as I can measure - with height gauge, callipers and more importantly, eyeballs - they are pretty much identical.

The domes are primed; they’ll be painted dark blue eventually.

Thanks everyone for the advice. Job done with your help - again.

Edited By Dr_GMJN on 04/10/2021 22:20:01

Thread: Stuart Twin Victoria (Princess Royal) Mill Engine
04/10/2021 21:16:24
Posted by JasonB on 04/10/2021 18:51:06:

They look the partsmiley

Yep - really pleased with how they look, and fun to make as well. Thanks for the advice on the other thread.

04/10/2021 18:46:04

The back cylinder caps are domed, and this was a challenge. I took the advice I understood from the other thread (on machining domes believe it or not), and drew out the profile in CAD, then machined from a bar of cast iron:

I set the tool parallel to the bed:

And using more CAD:

Roughed the domes out:

Then using co-ordinates, approximated the profile:

Then using a combination of a dremel, files and abrasive paper:

Smoothed to shape:

Parted off using the home made rear post from earlier this year (worked great):

Then made a spacer ring and reversed in the chuck to turn and mill the inside details:

Took a few goes, but eventually I got a decent result:

The domes are primed; they’ll be painted blue eventually. I checked the height offsets with the height gauge, and they were identical to within half a thou believe it or not.

Thread: Turning (approximating) a Domed Surface
29/09/2021 17:01:21

Ah, I misunderstood - tool rubbing on its flank, on the ‘I/d’ of the step?

Do DCMT inserts have a standard flank angle (to stop rubbing) that you’ve modelled?


29/09/2021 13:26:16

Thanks Jason. I might turn the tip as you say, and might even draw out a starting geometry that I can rough machine out, to make sure the edges don’t rub. I guess you can see where the cutting edges end by looking carefully at the tip?

28/09/2021 22:11:25

Thanks Jason, I drew something out on CAD earlier, I think it will be OK. Just have to work out the co-ordinates. I think 25 increments of 0.020” on the cross slide should be good, using a 0.8 tip radius tool. Might need back-gear for this one:

Couple of questions:

1) The minimum section ends up at about 2.6mm. I say this should be OK to take the cylinder pressure. The original minimum is about 3mm, but I’d expect a large safety factor within that?

2) I want to keep distance x to 7.5mm. Reason is that the front covers are that, and that dimension keeps the bolt PCD in the middle of the faced area. The angle of the tool tip means I can’t touch on the O/D and wind inwards by a set amount before going into the end. And I want that profile generated by the tip going in to form the edge of the dome. Any suggestions? Maybe start at the centre and work outwards - but that means determining the exact centre using the tip I guess.

28/09/2021 14:20:42

I didn't buy castings for these end caps, I got some cast iron bar, and will make them from that.

I'm going to go for an arc, not an ellipse, because I think the apex looks more defined like that (rather than a bit flat if I use an ellipse).

I'm also going to make the apex of the dome project about 1 mm from the bolt face rather than having it flush (as per instructions), again to make the form a bit more defined. If it looks wrong I'll just try again with different settings.

27/09/2021 21:18:31

Thanks everyone - there’s a lot of interesting info here. I like the guide rod idea, but having looked at Jason’s sketch I wonder if an elliptical arc would look better.

Ill sketch some geometry out and the decide between co-ordinates or guide rod.

In fact the guide rod looks too cool an idea to ignore - I’m going to try it anyway on some scrap.

Thanks again all.

27/09/2021 18:41:31

Thanks all.

Jason - I’m not sure what the co-ordinated would do.Do you mean drive, say, a 0.8 radius tool to a series of locations, and blend the remaining peaks? I’d imagine I’d need dozens of positions to do that?

Or do you move the tool to subsequent locations by moving first in x, then in y to give steps, and blend the steps?

It should probably be obvious but I can’t visualise the process.

27/09/2021 16:30:39


I need to turn a dome shape for a pair of cylinder caps. Similar to a typical smokebox door kind of thing.

I've heard of using co-ordinates on the cross and top slide, presumably in conjunction with turning the top-slide to different tangent angles. Can anyone tell me the procedure, and also what's the best way of smoothing the facets generated? I want them to be identical.

Material is cast iron. Radius of the dome is about 50mm, and the diameter of the domed disc is about 25mm


Thread: Stuart Twin Victoria (Princess Royal) Mill Engine
26/09/2021 22:40:20

In other news, went to the Kelham Island museum this morning (it’s only 30 minutes from home). Parked next to a double flywheel similar to the one on the Twin Victoria. Never seen one before, it looks like it was part of some electrical machine judging by the circumferential details:

A few other engines:

And things in glass cases (always good):

And of course the 12,000hp River Don engine:

Bought myself a poster for the workshop:

Unfortunately it wasn’t running today due to a boiler issue. I’m always amazed it can go from full forward speed to full reverse speed in 2 seconds, without shutting the throttles. It’s one of those things where you can feel the power just by standing near to it when it’s working. Same awesomeness as a Typhoon display, or being on the pit wall at Silverstone when the old 3.5 litre V10 F1 cars were current.

Other random stuff:

Sheffield Simplex:

Cavity magnetron (with Sheffield made magnet!):

Anyway, a great museum, and it’s free (of course made a donation though).

26/09/2021 22:16:59

The castings where right on the edge of being big enough for the stated dimensions, in fact there are still a couple of pinholes on one of the gasket faces. The brass extrusion I bought was never going to be big enough to make the glands:

Got plenty spare…

I’ve got some brass bar, so I’ll try to make them from that:

Lessons learned: I should have bored the packing holes rather than drilled them - my smallest boring bar will just fit. Oh well, I think they will be ok.

Next job is to make a mounting block for them for the R/T vice, and get them milled as per the valve blocks. I might put some JB Weld around the bosses - they look very close to being undersized despite everything being pretty central.

26/09/2021 22:15:06

I had a go at the cylinder covers this afternoon. I’ve read the article method loads of times, but still couldn’t make head or tail of the descriptions: It says machine the plain cover spigot first, to a good fit to the cylinder, the reason being that it centralises the packing…but there isn’t any packing on that cover.

It then says make the spigot on the gland cover ends (the ones with the packing) a slack fit. It also says to machine the spigot diameter after facing, to ensure concentricity. Surely concentricity doesn’t depend on the face being true, plus if it’s slack, why is it important?

I was left wondering why in those cases bother with either spigot at all since one has nothing to centre, and the other is a slack fit anyway.

In the end I just went with what seemed logical. I assume when the article was written, the covers didn’t come with cast tails on the back, so I used those to mount in the chuck. I first faced the gland bosses, centre drilled them as best I could on the mill after marking with the verniers, and set up true in the 4 jaw. Then turned the bolt faces and re-faced the gland bosses to the correct depths:

Then drilled the piston rod and packing holes:

The 10V piston hole was reamed, but it says not to bother for this one. Which seems logical since it’s the piston and cross head that seem to define the rod position. Adding a third fit seems pointless, especially when the packing will centralise the rod. Anyway, that got me here:

Then sawed off the tails:

Made a mandrel as per the article:

Set up with a dti (both were within 0.002” O/D and 0.001” on the face, so I thought that was ok:

And turned the inside spigots and the flanges to thickness. I made the spigots a good fit to the cylinders. I can slacken them off if I need to I suppose:

Then finished the spigot faces in the 3 jaw:

I guess there was better way to do it than this, but it seemed to work ok:

26/09/2021 21:30:23

Thanks both.

Ramon - I think I’ll see how it goes before committing to making inserts. I don’t think it’s that far out, but I guess if it turns out to be a problem, it’s removing cast iron rather than adding it. The cylinders are OK, but I think they still need a bit of finishing in the bores, mainly to get rid of the slight discolouration half way down one of them, and also to make sure the JB Weld filler in the screw holes is truly flush.

Steve - thanks for the info about the oilers. I think on mine they will just be simple brass cups with lids, so I can drop oil in occasionally, rather than the reservoir type you describe.

24/09/2021 17:42:19

Thanks Ramon, yes this is the stuff:

So about 0.005” protrusion on diameter ( if I can figure out the nominal thickness)?

And 0.003-4” piston undersized to bore diameter. Presumably the bore needs finishing to be very smooth / polished?

Also, I’ve been marking out the cylinder valve faces for milling to size. All looks fine apart from on one cylinder (on the left below):

The cylinder longitudinal midpoint is out w.r.t. the central valve port, by about 0.5mm. Presumably I can adjust the mid point of the slide valve to compensate with no ill effects?

The valve pads will be machined to the upper and lower scribed lines, giving equal flat areas on all 4 ends.


23/09/2021 17:23:24

Thanks Ramon. I’ll figure something out. The crank oiler with the bent feed pipe looks tricky…

Moving on a bit - you mentioned PTFE piston rings a while ago (I think), for running on air. Can you give more detail of sizings and any piston redesign that might be needed?

I think it was PTFE…it was definitely some change to standard, but I don’t think the details were discussed.


22/09/2021 22:03:37
Posted by JasonB on 30/08/2021 20:04:25:

I've never found the need to do the separate CSK screws to hold the chest in place, I always just use the studs that run right through and retain the cover. Easy enough to slip a couple of larger nuts over two studs to act as spacers so the chest can be held in place when the eccentric rod length is being set then no need to be able to hold the chest without cover after that.

If you still want to go with them then I don't see the need for them being 4BA, get some M2.5 or even M3 CSK socket screws and use those.

Next job is drilling the valve chests. As you say, I can't figure out the need for CSK screws in the corners. It would seem to be better from a simplicity and load distribution point of view to just take all apart from the mid-side 7BA holes right through the block and drill and tap the valve face to take studs. The mid hole on the inlet side can't have a through-stud because it would partially obscure the inlet hole. So I'd end up with 8 through holes in the chest, and the two mid positions threaded for short studs (just for the sake of symmetry).

When you said use a large nut to temporarily fit the cover, was the nut just to compensate for the thickness of the cover? I was going to use threaded 7BA rod so wouldn't need a spacer. Is it better to make studs only threaded at their ends (as per the 10V valve chest studs)?


Thread: 5 Rotary Table/Tailstock/Chuck Kit Info/Questions
21/09/2021 12:07:56
Posted by Ron Laden on 21/09/2021 07:49:33:

I went for the small set of finger collets, 6, 10, 12 and 16mm although I have used all of them it's the 6 and 10mm that's gets used the most as they take the majority of my regularly used cutters.

I suppose one could argue that with just a small protrusion and a direct fit in the mills R8 taper they must be stiffer than a tool mounted in a chuck with a 40mm extension from the taper. However having said that the R8 collet chuck is quite a rigid bit of tooling and I suspect that in practice you would never see any difference between it and the finger collets.

Something that Jason has mentioned on occasion and which I have found to be true is when using the smaller cutters they get a bit lost under the spindle when mounted in a finger collet. With a small protusion and been so close to the spindle it can be difficult to see the cutter, especially on flat work down close to the mill table. You have to bend down to get the tool at eye level should you need to see/watch it or mount the work higher if possible, just something to be aware of.


Thanks Ron, yes I saw something about workpiece visibility with smaller cutters.

I'm mainly using 4, 6, 8, 10 & 12mm end mills. I think 4 mm might be a bit small to see under, so might go for 6, 8, 10 & 12.

20/09/2021 21:59:18

Ron - thanks for the idea of using finger collets. Must admit I’d not thought of using them at all. I’m wondering now whether to buy a set from Arc. Even when not using the R/T, wouldn’t they give an overall stiffer system for milling, ie a shorter column extension (as you point out), and removal of an interface between the collet chuck shaft and the machine taper? I bought the R8 ER25 holder when I got the SX2P only because I already had a set of ER25 collets.

Seems like you’ve got about half the backlash I have. I reckon if I used a ball thrust race I could get rid of quite a bit of backlash because I think quite a bit is resulting from the worm shaft end-float that I can’t fully eliminate due to the skew on the locking collar.

Howard - thanks for the tips on tailstock alignment. My son wants to try it out in that orientation on a part he’s planning on making, so we will need to align it for that.

19/09/2021 22:42:27


One other thing, the height of the R/T plus chuck(or vice) plus a part adds up. It’s quite a stack under the SX2P:

My z-travel is slightly compromised by my DRO sensor, but even so, things would be getting pretty close to extreme amounts of travel with parts much bigger than the ones I was dealing with. Not a show stopper, but possibly worth considering depending on your work.

Oh,  and I also milled the main casting to make the lock-down nut mating surface in the vertical pocket (visible above) horizontal. This is noted in other reviews. I also skimmed the surfaces of the other lugs just as a precaution. Very quick and easy to do in the end.

Edited By Dr_GMJN on 19/09/2021 22:51:07

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