Stuart Twin Victoria (Princess Royal) Mill Engine

[JasonBModerator @jasonb]

They look the part

[Dr_GMJNParticipant @dr_gmjn]

Posted by JasonB on 04/10/2021 18:51:06:

They look the part

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

[Dr_GMJNParticipant @dr_gmjn]

It’s been a while since I did anything on this for one reason or another (mainly a broken foot, then Covid, combined with a freezing cold garage over the winter).

I did some work on the first cylinder back in November, but didn’t get around to posting: First marked it out ready for drilling the valve chest holes:

Then put it back on the mandrel for machining:

The cast flats either side of the valve faces were slightly too shallow to match the cylinder cap diameter, so I used JB Weld to add material, which was then milled back. The offset from each cylinder end also defined the edges of the valve face:

Then after finding the centre of the valve face, I offset the hole positions and drilled and tapped the 7BA holes:

Then this evening (6 months later!) I repeated the process on the other cylinder. I think I mentioned previously that there’s a slight asymmetry on this one in terms of a difference in length from valve port centre to cylinder ends. I made it a best fit, and at the end of the day it’s only about 0.6 mm out, so not obvious to the eye when everything is assembled. In fact by the time the gaskets are fitted, and considering selective assembly of the cylinder caps, the overall cylinder assemblies will probably be pretty much identical:

A quick check with the height gauge shows they’re pretty much spot-on identical in terms of valve face height:

And the end cap diameters now match the flats, so there’s no unsightly step:

Again, a matter of 10ths of a mm, but for some reason the castings were just undersized.

Still some minor cosmetic filing and filling to do, but pretty much there:

The bosses on the cylinder caps still need milling in the rotary table, but I think I’ll drill and tap the cylinder cap stud holes next, while the milling vice is set up square.

[DiogenesIIParticipant @diogenesii]

Neat work, glad to see you back..

[Jon LawesParticipant @jonlawes51698]

Really nice work, it's given me an opportunity to go back a while and catch up with what you've done to date. Following with interest.

[JasonBModerator @jasonb]

They are looking good.

[Ramon WilsonParticipant @ramonwilson3]

Nice to see you back on it Doc, and a great result too

Been way, way too distracted this year myself but hope to follow your example later in the year and get to grips with the marine engine.

Regards – Ramon

[roy entwistleParticipant @royentwistle24699]

Good to see you back

Roy

[Dr_GMJNParticipant @dr_gmjn]

Thanks all – good to be back. Hopefully I can keep some momentum up now.

[Dr_GMJNParticipant @dr_gmjn]

Cylinder cap stud holes today (all 40 of them!).

Checked the hole PCDs were right – I wanted the studs mid way between the cap o/ds and the central cast bits. This turned out to be slightly different from the plans. I got the co-ordinates from CAD, double-checked with the formula in my Zeus booklet:

Then, in the vice, checked the face for flatness:

Got the centre of the face with the finder:

Double-checked the co-ordinates using a 7BA washer (looked OK):

Then co-ordinate drilled the holes using the DRO. I went to 3mm deep, making sure I didn’t break through the flanges:

Tapped to 7BA using a plug tap:

Then went round each one by hand with a countersink to remove any burrs:

So a bit more progress:

Next job I think I’ll drill the valve chests.

[Dr_GMJNParticipant @dr_gmjn]

Spent some time today drilling the valve chests. They are handed, and have a top and a bottom, but still look very similar, so easy to make a mistake…

Started by putting them in the mill and checking they’re flat:

Then got the centre with the edge finder:

Then co-ordinate drilled with a centre drill. I skipped this step for the second one because the centre drill seemed to leave a central pip. I later realised I’d had this discussion before, and bought a stub drill (I think that’s what it’s called). I think Covid has a lot to answer for in terms of occasional brain fog:

Through-drilled to 7BA clearance:

I thought the drill might wander on contact, hence why I initially used centre drilling, but in the event it was fine.

Then drilled and tapped one 7BA hole, over where the steam inlet will be:

Finished by hand countersinking all the holes.

The drilled chests:

Test fitted to the cylinders using bolts:

So far so good. The only thing left to do on the chests is to drill the steam inlet holes, and the mount holes for the inlet boxes.

[Dr_GMJNParticipant @dr_gmjn]

Does anyone have a good method of holding the domed end caps flat in the milling vice, with enough support for drilling the hole array? Perhaps on some rebated wood? Thanks.

[JasonBModerator @jasonb]

Take your lathe chuck off the lathe and clamp it to the mill table, might need some packing if the threaded boss of the backplate sticks out the back. Couple of parallels will space it off the chuck face

One I did this morning, there is a small 3-jaw fitted with external jaws under there somewhere, 10-20-40 blocks to raise the cylinder cover up.

Here's one where you can see the chuck better

Edited By JasonB on 12/06/2022 16:40:30

Edited By JasonB on 12/06/2022 16:43:13

[Dr_GMJNParticipant @dr_gmjn]

Thanks Jason – or I could use my rotary table chuck with its mounting plate…

I’d have to clamp to the cap edges, because the register isn’t deep enough. Even if it was, some holes would clash with the jaw faces I think. Not sure how I’d use the edge finder to get the centre in that case though?

[JasonBModerator @jasonb]

That's actually what I'm using, could not remember if you had one with the R/T

Put a bit of something round and of similar size in the chuck that sticks up just above the jaws and ctr on that, then remove and replace with the cover, unless your chuck is wildly out it will be fine. In the first photo I have the jaws so I can still get in with my edge finder but may not work with your smaller dia cover

[Dr_GMJNParticipant @dr_gmjn]

Thanks Jason. I just looked in the tool cabinet and saw the R/T chuck is a self-centering 4-jaw, so I should be able to access the edges for the edge finder. It's scary how I forget things like that. Last used it in September to hold the ball differential parts…I blame Covid; not been 100% since I had it in February. In fact not been right since I had the booster Jab in December, but that's another story!

Edited By Dr_GMJN on 12/06/2022 17:53:32

[Ramon WilsonParticipant @ramonwilson3]

Theres an easy option Doc.

Hold your cylinder vertically in the vise and centre the bore, then move to the coordinate of one hole. Place the cap on it and hold in place whilst you just gently spot that one hole position. Drill though on your drill press then place back on the cylinder, pop a screw through to hold it and spot the remaining holes. As you've done the cylinder holes on coordinates then it's no different – just a simple way to hold the covers

I have to say after a lifetime in engineering I'm not an advocate of using an edge finder to find the centre of a hole. I'm not saying it can't be done just that it isn't correct practice (for the edge finder) A DTI in the spindle is far more the correct way

That said if you know the distance from the valve face to the centre of the bore then you can at least position the edge finder on the centre line one way but it's just not the way I'd go about it – a DTI would be far more accurate and probably quicker too

 

Best – Ramon

Edited By Ramon Wilson on 12/06/2022 18:18:58

[Dr_GMJNParticipant @dr_gmjn]

Thanks Ramon. Yes, it's been pointed out to me before about the use of the edge finder on cylinders/circles (albeit significantly less diplomatically).

I understand why it's not ideal on curved surfaces, but It does seem to work – at least for non-critical things like the cover PCD. I did a sanity check by moving a drll/washer to each position to make sure it wasn't totally wrong before drilling.

I used the DTI to accurately centre my R/T under the mill spindle, and of course it was very accurate, but took a lot of trial and error before I got it right. I guess there's a simple technique (like centering work in a 4-jaw chuck), but I can't recall it now.

[JasonBModerator @jasonb]

I think we all have a preferred ways that work for us. I hardly ever use the dti as the edge finder is my weapon of choice, I do use an electronic one which is said to be more accurate than the mechanical ones. Only tend to use the dti on the CNC as the spring end is more forgiving of a wrong jog . For the same reason I use a mechanical edge finder on the CNC for finding straight edges.

[Ramon WilsonParticipant @ramonwilson3]

Yes each to his own Jason but using an edge finder as such does not show you the degree to which you are central to the hole.

Doc – getting a rotary table close (within a couple of thou) and objects on a face plate for that matter for using a DTI is easy if you make something like these 'centres'

They are mild steel firmly mounted on hardened dowel pins held in a collet and the taper turned. Hold in a drill chuck or collet and bring the spindle down so that the cone gently touches on to the RT hole rim. It's surprisingly easy to move the table to allow it to seat firmly. Zero up then remove and fine tune the position with a DTI

On the lathe they are ideal for holding something with a bore against the faceplate whilst clamping, again final tuning with a DTI.

Hope that's of use

[EmgeeParticipant @emgee]

This is a model engine backplate with 4 hole fixings that has been turned for a very close fit in the crankcase so to make sure the holes are drilled in the correct place I use an electronic edge finder first and then check with the lever gauge to make sure, if care is used with the edge finder adjustment is usually only .02mm or so.

Emgee

[JasonBModerator @jasonb]

Yes the few times I have checked it with a Dti after using the edge finder it was certainly acceptably centred. Also industry these days uses a probe which is a fancy edge finder not Dti.

Dti would certainly be the better bet if you did not have a DRO as it does not require you to compensate for backlash from one side of the hole of round to the other.

[Ramon WilsonParticipant @ramonwilson3]

Lets not spoil Docs thread by deviating to the rights and wrongs of finding a centre eh chaps – my apologies for causing that

Doc – you didn't say – was the tip of using the cylinder as a fixture to hold your covers for spotting of interest? It's certainly a quick way around holding a thin cover with ease

[Dr_GMJNParticipant @dr_gmjn]

Thanks all.

Ramon, yes, it was a good idea, but I'd already removed the cylinder from the vice. I suppose what I could have done was clamped the cap, then drilled through both the cap and the cylinder at tapping drill size, then opened up the cap holes after removal. The slight issue I could see is that since the caps (at least the ones with glands) have spigots that are not a perfect fit in the bores (the instructions say to make them slightly loose), the caps may not be concentric with the bore, and then obviously I might end up with a non-concentric cap. A suppose also re-setting up on the bench drill would have been imperfect, so the errors might well have summed to more than if using the edge finder?

I have since temporarily put the R/T vice on the mill table, and it seems to hold all the caps pretty well (leaving some clearance below for the drill to come through). I am going to set up with the edge finder, but also check with a dti on the O/Ds. I then need to set up the R/T to machine the cylinder gland housings, which I will centre just using the dti.

Cheers.

[Ramon WilsonParticipant @ramonwilson3]

I was thinking of just using the cylinder as a fixture ie resetting it again and do all four caps as a quick way of holding – providing your coordinates are all the same but a loose spigot could, probably would, lead to errors

Having a loose spigot can have a benefit when aligning the final fit but to my mind it's much better to have them a tight hand fit and drill/bore/ream the caps holding on the spigot – preferably held in turned to suit soft jaws or a ring chuck turned to fit or best of all a simple fixture on the face plate. I find using the PTFE packing allows a degree of slackness in the piston/liner fit that accommodates any possible slight error in tolerance build up of the three parts.

Drilling through the tapping size then opening the holes in the cap is a good method – any discrepancy is always taken care of – providing the caps are fitted to their mating ends if so

[Author]

Posts