Stuart? MTB 1M

[JasonBModerator @jasonb]

Having followed a couple of Sanjay’s threads on MEM about his enclose Stuart engines and a recent discussion on here I found this topic had moved up towards the top of my “future projects” list. The final straw was an article in the German MiM magazine about the restoration of an MTB 1B. So I had to get making one.

Sometime 100-115yrs ago Stuart introduced the MTB 1B as a replacement for the No1 MTB and that is what my engine is based on, in particular one from towards the end of the production period. These engines were intended as high speed power plants for model boats and the MTB stands for Model Torpedo Boat which was a popular hull type of the period often using flash steam and being able to reach 4000rpm.

I have made a few changes to the original design mostly due to not starting with castings and the main ones are. Drawn up as usual in Alibre

-Aluminium crankcase rather than iron which was actually an option with the MTB 1A “featherweight”
– Separate cylinder “casting” as the original had it cast integral with the top half of the crankcase, this was mainly to save turning so much metal into swarf.
– Bronze crankshaft bearings due to the aluminim crankcase where as the orininal ran straight in the iron casting.
– Probably not going to have cladding on the cylinder which was “russian iron” as I like the look of the engine without and you do see quite a few surviving examples like this.
– Although mine will be a 1:1 size replica it will be done in metric using whole dimensions not just a direct mathmatical conversion so the 3/4″ bore x 3/4″ stroke is 19mm x 19mm dead. This is also why I have named my version the 1M for metric.

Although the main “casting” on this one will have a fair bit of CNC work I probably won’t do as much video of that like I did for the previous A J Weed engine as it is much the same unless anyone asks.

For a change I started towards the top of this one with what could be called the cylinder head come valve chest. After sawing off and cleaning up a block of cast iron it was held upside down so that the bottom oval shape could be machined, various screw holes added and also the steam passages cut half way through. It was the first time I have done these on the CNC and it certainly reduces the bum clenching as it just works it’s way back and forth in a slowly descending zigzag path producing the 1.5mm wide slots with ease.

Repositioned in the vice the other way up an adaptive tool path was first used to remove the bulk of the waste material

This was followed by scalop and horizontal finishing paths, a bit mor edrilling for the cover screws and repeating the ports to join up with the cut from the underside.

One “casting” in the hand to give an idea of size. Did i mention that when machining cast iron on the CNC your hands don’t get anywhere near as dirty as they do on the manual ones!

The cover was done in a similar way with an adaptive followed by finishing scalop and some clearance holes for screws, the larger hole is for a plug to allow access to oil the valve.

The cylinder head just needed drilling and tapping for the inlet, valve rod and gland which are tacked together here. I will be making suitable slotted head screws but the cap heads are easy to use during the build. These ones are M2.5 rather than the original 3/32″ Whitworth.

[JasonBModerator @jasonb]

As I mentioned in the first post rather than having the cylinders and top of the crankcase as one piece, I decided to split them in two. So after machining a block of iron to finished height and 1mm extra on all sides I put it into the CNC to cut two shallow locating spigots and six M3 tapping size holes. I also added a ctr drilled hole in the middle of each spigot.

Back onto the lathe while it was an easy to hold square sides box and one of those ctr holes was located then the first cylinder drilled out to 16mm and finally bored to 19mm

A quick reposition in the chuck and the other one was bored to match. I lapped the two bores at a later stage.

The outer shape could then be tackled, first an adaptive tool path of the exhaust side

Then a scallop to refine the shape

Flip it over and do the other side, just a photo of the adaptive on this one

Two blocks of aluminium were cleaned up on the manual mill

Then using a couple of tapped holes in the bottom so the material could be bolted to a block that was held in the vice the top profile was machined

I was just able to hold it in the vice so that the internal cavity could be hollowed out.

A quick test assembly to see how it is starting to look, bottom half next.

[Nigel Graham 2Participant @nigelgraham2]

Impressive work – an interesting engine design and a fine display of your manufacturing methods.

[JasonBModerator @jasonb]

Thanks Nigel, it was something a bit different and I have enjoyed doing it, ran for the first time yesterday.

I normally like to see what is going on a bit more so this is kind of the opposite particularly as it does not have the Banjo like the popular Sirrus does, all you see moving is the scotch yoke and that is a bit of a blur at most speeds.

[David George 1Participant @davidgeorge1]

A lovely piece that you have made Jason. This makes me so envious of a CNC mill. It is a fabulously made piece of engineering and I may have to get a CNC mill of my own. What mill do you have? Yesterday I was stamping some dials for my cutter grinder and wished I could engrave them instead as as I get older I fail to get stamps exactly in the correct place and CNC would be nice.

David

[JasonBModerator @jasonb]

I have got a SIEG KX-3 but unfortunately no longer available from ARC. For just engraving steel and light work on aluminium and brass a gantry type machine may do but for anything heavier then something mor esolid and mill based would be best

 

I noticed earlier in the week that Warco have just introduced a CNC mill, looks quite nice.

[DiogenesParticipant @diogenes]

Have you seen the ones that Warco have just introduced?

Thoughts most welcome..

[JasonBModerator @jasonb]

I have only seen what is on their website. I assumed it is made by Weiss but can’t see that on the Weiss website or their Youtube channel although there are some mills with that same screen/panel it only really controls three powerfeeds.

From the spec it looks reasonable, More powerful motor than my KX-3 which I assume is also brushless and a reasonable speed, good travel and the weight suggests it is fairly solid for the size. Less expensive than a KX-3 too. You would need to add a Mach licence at about £150 and find a basic laptop to run it as CNC.

Would be nice to see it taking some more suitable cuts under CNC control than the video shows and also in something a bit more challenging than aluminium (Warco if you are watching send one for review!)

Only downside is they say it will be in Warco Green and I quite like the grey.

[JasonB]

On JasonB Said:

.. …(Warco if you are watching send one for review!)

Only downside is they say it will be in Warco Green and I quite like the grey… ..

‘..send Diogenes one for review!’, surely – you have a Sieg one already.

I love the green.

[bernard towersParticipant @bernardtowers37738]

Can anyone tell me in all honesty what difference the colour makes???

[JasonBModerator @jasonb]

Based on running the KX-3 with and without it’s light grey chip “trough,” it does reflect the lighting back towards the work quite well and reduces shadows. Maybe that is why 99% of the bigger commercial machines and even the like sof Tormach, SIEG and SYIL also tend to be the similar light grey?

[Richard BParticipant @richardb44403]

Jason,

I have a pair of quite similar engines dating back to probably 1925 or earlier – however they are of fabricated construction. I was given them as a young lad back in the early 60’s when a friend of my mothers was clearing her late husbands models of large steam boats. He was a sea captain and latterly a marine superintendent in Buenos Aires.

They are soldered together elements rather than castings and run on ball bearing races and have piston rings of brass.

His ships were all soldered tinplate and amazing quality – in specially built wooden cases – largest was about 6ft.

I really should get at least one running !

Featured in the 1925 Model Engineer and Electrician Magazine.

The exert I have copied is not written by him but he refers to his good friend making engines for his boat.

I have the 1899, 1925 and 1926 bound editions of the magazines which i have kept all these years but doubt anyone will want them after me !

 

 

brass.

 

[JasonBModerator @jasonb]

Thanks for that Richard, an interesting read. I suppose the methods of construction were based on what the builders workshop was likely to contain at the time, maybe a treadle lathe and some files as all the machined parts could have been done on a basic lathe with the rest being built up with whatever methods were available.

The first of the enclosed Stuarts – the original No1 MTB had a similar pivoting lever to operate the slidevalve but rather than the disc arrangement made use of a face cam.

Also interesting is making use of the easily machined discs of the crankshaft webs to also create the flywheel effect.

Bit of MTB history for anyone interested including pictures and details of the No1 MTB

[Howard LewisParticipant @howardlewis46836]

Seeing the bevel gears on the crankshaft, thought that it was a rotary valve, but then spotted the eccentric and realised that it was a slide valve.

Did any of the “Overhead ” valve engines use a rotary valve?

Another method, for anyone who wants to experiment would be a slide valve operated by a pivoted lever and a face cam on the crankshaft./

Just musing

Howard

[Howard Lewis]

On Howard Lewis Said:

 

Another method, for anyone who wants to experiment would be a slide valve operated by a pivoted lever and a face cam on the crankshaft./

 

See my reply above yours and the link to the original No1 MTB with it’s face cam.

Westbury did the Gemini high speed steam engine which had a gear driven rotary valve, Gray Meek did his own designs inspired by the Gemini which use a toothed belt.

[JasonBModerator @jasonb]

The No2 is missing from that link I posted earlier today, but can be found here, it’s the twin screw one.

If anyone wants to see my insides this is a section.

[JasonBModerator @jasonb]

The Lower half of the crankcase was machined in much the same way as the top. Starting out with an Adaptive roughing pass and then a parallel finish path as the Curve was fairly shallow which meant the horizontal stepovers were fairly constant along the circumference of the arc

Then turned the other way up to hollow it out. I nearly came a cropper as I selected the finishing path for the top half but heard the cutter taking heavier cuts than it should and stopped well before it cut right through.

After power tapping the various holes using the tapping feature of the manual mill the two halves were screwed together, drilled and then reamed 10mm. The original has the crank running in the iron castings but I made up some bronze bearings as I did not want to run the crank in this grade of aluminium (6062)

As I drilled and reamed from each end it was reasuring to have a 10mm ground HSS rod rotate freely within the two holes. You can also see a shallow recess in the top surface, this is to locate the lower camshaft support and there is a similar one in the top of the cylinders.

The two camshaft brackets were milled to size and screwed to their respective parts, the shaft position established and the lower one reamed 6mm for a bronze bush. I then fitted the cylinders and with the mill spindle still in the same location ream the top cast iron bracket 4mm as the shaft will run directly in the iron, there is a bit to come off the top.

The last bit of machining was to drill and tap M4 fine for the drain plug and filler spout, as this is angled at 10degrees in two directions, I set it at 10deg in the vice and then tilted the vice at 10deg. Started the hole with a 4mm milling cutter then drilled 3.5mm before tapping.

After a bit of hand work to remove any fine machining marks the two halves were bead blasted to give a nice even satin finish.

[DiogenesParticipant @diogenes]

..not too shabby..😉.. ..just out of interest, I see the top camshaft bracket comes as close as you could get it to the cylinder bore, is that just to make sure it has a good seat..?

[JasonBModerator @jasonb]

Yes it is close, there is just 0.5mm material in the ctr.

Partly to get as deep a pocket as possible and also to make sure the threaded hole is not too close to the thin (2mm) cast iron edge.

I had more material to play with on the bottom half as the hole could go deeper and there is less chance of cracking with Aluminium. Though I wish I had gone in a bit closer thus moving the screw inwards as you can see a bit of teh CSK head. Would not be a problem if I was painting teh bottom half but does show against th ebare aluminium.

[simondavies3Participant @simondavies3]

Jason, would you contemplate making this base by getting a 3D casting made as discussed elsewhere here?
Not for any reasons other than limiting the amount of swarf generation (and ignoring any commercial aspects)

Simon

[JasonBModerator @jasonb]

3D printing would certainly be an option if you have an aversion to swarf or just don’t feel able to cut it from solid on manual machines. The parts could be simplified a bit for manual machining  eg just straight internal sides, etc.

I just roughly joined the two crancase halves together with a couple of sprues and exported a STP file for pricing. The wall thicknesses could be reduced to bring the cost down and the holes omitted, a bit of machining allowance would need to be added.

Price for printing in aluminium is around the £75 mark with £10 postage. vat included at source.

The top half could also be simplified for manual machining particularly if you clad the cylinders or even just made them an oval shape with some dummy screws and a seam line to look like cladding. Price for the cylinders, head and chest cover in stainless 316 was £62 but higher postage at £23.

So you could have all the major “castings 3D printed for £170, similar if all done in stainless

 

So that does not compare too badly with the slightly larger Stuart Sirius kit at £500 plus postage but is a bit more spendy than raw materials from M-Machine that work out at £14 for aluminium and £15 for cast iron plus a bit of postage. Gears plus a few other bits of metal would be £30-40.

 

So £65 total materials cut from solid, just over £200 3D printed including sundries and gears.

[JasonBModerator @jasonb]

With the crankcase complete it was time to start filling it. The crankshaft started life as a piece of 10×30 hot rolled steel which after a pickle to remove the mill scale was reduced to 10×28 and the ends milled square so that three ctr drill holes could be added to each end. Doing each pin in turn I milled out most of the waste then used a parting tool to turn the pin to 0.1mm oversize. I then used a small boring bar with a CCMT 04 insert to machine the sides of the webs, first as shown then mounted upside down and with the lathe running in reverse to do the opposite side

The pin was then taken down to final size of 7mm dia with a parting tool moving side to side while gently feeding in, quick polish with emery cloth on a stick to complete.

Now that the outer two ctr drill holes were no longer needed the waste was sawn away so that the end of the shaft could be taken down to the required 7mm which was checked using the previously made bronze bearing.

Similar for the other end which also needed to be reduced to 4mm to suit the bevel gear.

Pistons were fairly straightforward turning and then milling the pocket for teh conrod, then drilling and reaming at right angles for the wrist pin.

Some 20mm dia 2014 aluminium was milled to a rectangular section to make the conrods from. First job was the shape one end and then drill and tap for the big end bolts

Then the caps were sawn off. Not sure what I did but the cut was a bit nearer the end than I intended. Wrist pins can also bee seen here and the tapped holes for 3mm grub screws to keep them from moving sideways and marking the bore.

Holes were then drilled and reamed

Before final shaping on the CNC

[JasonBModerator @jasonb]

[DiogenesParticipant @diogenes]

Looking good – is that ‘proper graded’ black bar and not just some ol’ 275 found in a skip behind a fabricators?

[JasonBModerator @jasonb]

M-Machine only list it as “Black MS” and don’t give an EN grade so could well be S275 which is about EN14. I’ve used other black bar for crankshafts on much larger hit and miss engines needing 25×50 material over 300mm long.

So far it’s been upto 3000rpm a few times and has held together, did not go further as I have not filled the sump and had no lubricator on the inlet.

Now before everyone tells me I should be using En24T a little story. A few years ago I took on the completion of two hit an dmiss engines, the previous builder well known in traction engine circles made a big point of saying the crankshafts had been cut from EN24T. Well all I can say is they were a pile of P**p. Incredably poor finish where it looked like the tool had been getting pushed away from the work as there were small changed is diameter and they had a nice banana shape too. All I can say it EN24T is strong stuff as it was hard to straighten them, thought I was going to pull the vice off the bench!.

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