Author Topic: Burrell Single Crank Compound Traction Engines - 4" Scale  (Read 63190 times)

Offline Alex

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #255 on: December 13, 2020, 02:11:24 PM »
Glad to see the update; you do wonderful work. Keep going! JohnS.

Offline Jo

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #256 on: December 13, 2020, 02:16:09 PM »
Thanks for the update Andrew, I am going to miss seeing your progress at Stratfield Saye  :(

Jo
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Offline Admiral_dk

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #257 on: December 13, 2020, 05:42:40 PM »
Really great result with the latest batch of parts  :ThumbsUp:

Quote
By large I mean half a farad or so.

 :o  - oh man - that is almost a battery and not a capacitor ...!... does it make sense at all to use an OpAmp ?

Offline Don1966

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #258 on: December 13, 2020, 07:04:09 PM »
Awesome rsesults Andrew........ :Love:




 :cheers:
Don

Offline Kim

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #259 on: December 14, 2020, 05:43:50 AM »
Great to see an update, Andrew!  Nice work all around.

Boy, seeing those wheels up on your portable bench shure gives me a size reference!  They're huge!

Kim

Offline jadge

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #260 on: December 17, 2020, 08:18:47 PM »
Boy, seeing those wheels up on your portable bench shure gives me a size reference!  They're huge!

I was told they'd be surprisingly heavy, and it's true they are much heavier than the wheels with metal strakes. Just as well I have concrete floors in the bungalow.

Andrew

Offline jadge

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #261 on: December 17, 2020, 08:43:54 PM »
... does it make sense at all to use an OpAmp ?

The sensor has three pins, a reference electrode (RE), working electrode (WE) and a counter electrode (CE). Internally the RE is between the CE and WE. When the target gas meets the sensing element an oxidising or reducing reaction occurs that generates a current into, or out of, the WE. To keep the sensor in balance the voltage on the RE needs to be kept the same as that on the WE. To do this once the reaction occurs the voltage on the CE needs to change in order to force another reaction that generates a current (through the large capacitor and parallel resistor) to equal that generated by the WE. At least I think that's how it works.  :) The amplifier that does this is called a potentiostat. The voltage difference between RE and WE needs to be small, ideally microvolts. So the potentiostat needs very high DC gain, but doesn't need a high bnadwisth as the sensor takes many seconds to react to target gases. So I've got an opamp with capacitive feedback, making the DC gain essentially the same as the open loop gain of the opamp.

Andrew

Offline Admiral_dk

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #262 on: December 17, 2020, 10:32:58 PM »
Quote
To keep the sensor in balance the voltage on the RE needs to be kept the same as that on the WE.

Ok that sentence alone kind of explains why one really has to consider an OpAmp - no matter if it's an off the shelf item or one made from discrete parts. This doesn't help your first mentioned concern, and I wouldn't have been surprised if my first solution (if I had that assignment) at least needed tweaking if not a complete redesign .... Modern simulators are great, but they do not always solve all the problems - so you usually need to build a prototype (but I'm sure I don't need to tell you that)  ;)

Offline jadge

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #263 on: December 20, 2020, 11:03:30 AM »
Simulators are a mixed blessing. I've used ones ranging from free to the Mentor Graphics offerings. They're useful for trying out ideas and playing with component values. I don't quite agree with the Bob Pease attitude, but I always prototype analogue circuits. I normally build the circuits "dead bug" style on single sided blank PCB material. I used to prototype buck converters as they never did quite what they said on the tin. But the newer ICs are much better. I still buy dev kits where needed - just ordered dev kits for the selected ADC and DAC on the gas sensor project. They should allow me to check a few things on the analogue side and then let the software writer get a head start.

Andrew

Offline jadge

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #264 on: April 02, 2021, 10:55:20 PM »
Despite working on schematic and PCB design over the last few months I've been able to make the button valves for my traction engines. These divert high pressure steam from before the regulator to the low pressure valve chest, operating the engine as a single using the LP cylinder. The valve is operated by a spring loaded brass button on the regulator bracket and is used for moving in tight areas where one doesn't want the regulator open. Alternatively it can be used to give quick increase in torque when needed. The design is scaled from drawings of the full size valve. I have tried to stay faithful to the drawings. Here is a sectional view of the CAD model assembled on the cylinder:



The single nut on the cylinder cover is to check clearances on the outlet nut. There are a lot of small parts in bronze, stainless steel and brass. Almost all parts were made on my manual machines. The only CNC milling was the elliptical outline of the flanges. Most threads were screwcut on the lathe. Shown is one assembled valve and the parts for a second:



The body is silver soldered from three parts, held in a simple fixture to ensure alignment of the body and mounting flange. The U-shaped bracket started as a brass block. All holes were drilled/tapped including those needed to form the internal radii. A radius cutter was used to form the external radius at the back. Material was then milled away, mostly by eye and the shape finished by filing and filing buttons where needed. The mounting studs were made on the repetition lathe:



In the background is the regulator rod and gland with studs as per full size with split pins. Attention to detail included the hole and slot in the small end of the valve rod, even though it will never be seen. The hole is 1mm diameter and the slot is 20 thou wide:



For interest this shot shows a sectioned body, complete with internal counterbore, that was a failure due to holes not lining up during drilling and reaming:



I've no idea why the holes went askew as the other two bodies were fine. When making the oilers previously shown I used a HSS boring bar to machine the internal counterbore. This worked well on brass but unfortunately it didn't work on bronze, it just deflected and skated over the surface without forming a counterbore. In order to screwcut the 7/16" 32tpi internal threads on the gland nuts I searched high and low for a preformed threading bar that would fit into the 5/16" starting hole without breaking the bank. I eventually found some solid carbide bars made in Israel but sold by a distributor in Kent. They also sold small boring bars which I used to rebore the internals of the body, working through the 1/4" hole that forms the valve seat. Here are the threading tool and a boring bar:



I broke the first boring bar by being over ambitious, resulting in a sulk and damage to the bank account, assuaged by the application of beer. The second time around I made the cut in several steps rather than trying to do it in one go.

Andrew

Offline Kim

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #265 on: April 03, 2021, 01:43:17 AM »
Wow, Andrew!  That is a LOT of work covered in one short post!

I think I see where the cross-sectioned part fits in, but the picture just before it - with the hole and the slot?  I'm not sure I see where that fits in and what it does.

Maybe if I let it settle for a while I'll follow it all better with a second reading.

Certainly is pretty work!
Kim

Offline jadge

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #266 on: April 03, 2021, 10:46:53 AM »
I'll try to clarify where the parts fit. The sectioned body is simply the valve body, as shown top left in the parts picture, minus the flange and outlet . A sectioned view of the CAD model may help:



The problem with the sectioned body was that the 3/16" hole on the left wasn't lined up with the rest of the body, so the valve rod jammed. Here's a picture of the CAD model of the valve rod, which should explain where the hole and slot are placed:



In the parts picture the features are on the small end of the valve rod, bottom centre, just above the '7' on the rule. The small end diameter is 1/8". I'm not sure what function they provide, but they are clearly on the full size drawings, so I copied them.

Andrew

Offline Kim

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #267 on: April 03, 2021, 05:11:37 PM »
Thanks for the additional explanation, Andrew,
Kim

Offline scc

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #268 on: April 03, 2021, 08:42:33 PM »
Nice to see you are still at it :ThumbsUp:  Good work Andrew.    Terry

Offline jadge

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Re: Burrell Single Crank Compound Traction Engines - 4" Scale
« Reply #269 on: October 09, 2021, 10:26:52 AM »
Over the past year I have been machining the cylinders for my traction engines. First job was to machine the faces square and to size. I used an 80mm face mill and the horizontal mill:



It took a number of different setups, and some repeat machining, to get everything square. The front to back dimension could not be machined to nominal size, so they're 15 thou short. It was important to take the time to get these faces square, as they will be reference surfaces for all subsequent operations. The cylinder machined square:



The bores were then cleaned up to give a reference circle for the real machining:



The bores were not finish machined on the Bridgeport as my boring head is a low quality import, and  the quill travel is not great enough. There are a lot of cavities to machine. The regulator cavity was machined with a 10mm endmill and finished with a 10mm endmill with a corner radius of 1mm. These give a much better finish than square endmills when facing:



In similar fashion the valve chest cavities were cleaned out and taken to size, using a long series (100mm) 1mm corner radius cutter. Despite the long stick out I experienced no chatter problems:



The bores were machined on the horizontal mill. Each bore was centred with a co-axial indicator and the boring done with a Wohlhaupter boring and facing head:



I had all sorts of problems with the boring. The first was chatter due to the long overhang needed to clear the flange on the cylinder. i started with a home made boring bar (from EN8 medium carbon steel) and HSS toolbits. Once chatter had developed the cutter seemed to follow at each pass. I tried all sorts of combinations of speeds, DOCs and feeds without any real success. Oddly the boring head was happy taking a 0.1" DOC and there was no chatter noise (as with a lathe) but the finish was awful, consisting of helical bands. I then bought a commercial 25mm insert boring bar and machined a short length down to 7/8" fit the boring head. Carbide inserts seemed to be slightly better and tended not to follow any previous chatter. I used CCMT inserts, the CCGT sharp inserts were much worse and were the only cutter that produced audible chatter. After a lot faffing about and honing I eventually ended up with bores 10-15 thou oversize and a finish of 4um Ra. On the plus side the bores were round and parallel to a few tenths and in the right place.

The other major problem was to do with the boring head. The head has coarse and fine feed knobs and I couldn't get the fine feed to work reliably. I ended up with a DTI on the head and twiddling the fine feed until I got the increment I needed. After discussion on another forum the feed problem turned out to be due to some missing parts in the coarse feed that prevented it from being locked. I stripped and cleaned the head and made the replacement parts after I'd finished the cylinder bores. More on that in another thread.

The underside of the mounting flange was machined with a home made flycutter:



Roughing cuts were 80 thou deep and 8 thou per rev feed. The feed was done by hand - listen for the end of cut after a clonk, advance the handle two big divisions (0.2mm) and wait for the next clonk. Finishing cuts were done at 4 thou per rev feed. As one might expect the setup is pretty rigid so I had no issues with chatter and the motor note didn't change at all during the cut. The piece of sheet steel behind the cylinder at the top of the angle plate is to tilt the cylinder forward by about half a degree so the bores are not quite parallel to the boiler. This is needed so that the vertical centreline of the bores intersects the axis of the crankshaft and makes any angularity symmetric. The slope on the cylinder is also a feature of the fullsize engine.

The steam ports were cut using a 6mm end mill loctited into a home made silver steel (aka drill rod) extension fitting directly into an R8 collet. After machining the first set of ports using numbers from the drawing and the DRO the ports were a thou or so over width. So the numbers were tweaked and the remaining ports are on size, as checked with gauge blocks:



The ports are to my own design, based on scaling from full size, steam flows, pressure drops through the engine and Reynolds number. Note that in the picture the low pressure exhaust port is longer than the inlet ports. There's nowt that says the ports have to be the same length. Making the exhaust port longer means it breaks into the exhaust hole out to the chimney. That avoids the need to drill, and blank off, a connecting hole in the underside of the cylinder. Ideally I would have  made the inlet ports longer too, but one of them would clash with the exhaust outlet hole. According to my model of the cylinder there should be 0.07" of cast iron between the inlet port and exhaust hole. Fortunately this is confirmed by measurement.

The final job was to machine the exhaust steam port from the HP cylinder to the LP valve chest. After a lot of messing about in CAD I came up with a slanted rectangular hole. This neccessitated using the Quillmaster and right-angle attachment for the first time in 15+ years since I bought it:



The right-angle attachment only takes 3/16" cutters, not so common in the UK. Even then I had to modify a atub drill and endmill to get the precise length that enabled access while still being able to machine to depth.

The next post will deal with machining the liners.

Andrew