EMD F7 in SCALE

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BigDumbDinosaur
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EMD F7 in SCALE

Postby BigDumbDinosaur » Fri Dec 02, 2016 5:20 pm

EMD F7 in SCALE
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BODY CONSTRUCTION: Part II

Once I had all materials in hand and had worked out a general plan of attack for building up the body I went to work. As earlier described, the body's skeleton handles the important task of maintaining rigidity and panel alignment. Since the body components other than the nose and number two end bulkhead are steel, welding the parts to the skeleton was practical and would produce a strong assembly.

The first step was to attach the number two end bulkhead panel, which as previously described, is made from Alumilite. This item is secured to the rear of the skeleton with 10-24 stainless steel machine screws that extend through the skeleton and are secured with stainless KEPS nuts. Also, when the vestibule diaphragm faceplate assembly is installed, the screws that secure it pass through the skeleton framework, thus capturing the bulkhead and adding more support.

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Number Two End Bulkhead attachment: Outside

The above picture shows the attachment of the bulkhead from outside the body. Later on, the screw heads were permanently concealed by applying a small amount of PC-7 epoxy and sanding it smooth. Due to the tendency for PC-7 to entrain tiny air bubbles during mixing, causing a bit of surface porosity, a dab of Bondo® is used to achieve the final appearance.

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Number Two End Bulkhead attachment: Inside

The above picture shows the attachment of the bulkhead from inside the body. Once the side sheets are in place the attachment hardware is concealed from view, but accessible from the inside. Theoretically, the bulkhead could be removed, but I decided to use PC-7 to blend it into the side panels, making it decidedly non-removable. :D

It was necessary to attach the bulkhead to the skeleton before anything else, as the bulkhead implicitly sets the fore-and-aft positioning of the roof crown and side sheets.

Next came the attachment of the roof crown sheet. As received from Dave Newell, the roof crown was rolled very close to the correct contour, such that only minor adjustments were needed to get it correctly positioned. Careful positioning of the roof crown was essential, as its position determines the vertical position of the body side sheets.

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Skeleton w/Roof & Number Two Bulkhead Attached

The above picture was taken after the roof crown had been attached to the skeleton. Note the curved wooden blocks used to produce a stable support for the assembly. Working with the body inverted greatly simplified the built-up process.

The next step was to attach the fireman's side side sheet. The top edge of the sheet abuts the bottom edge of the roof crown, with the junction between the two being concealed by the roof line batten and the air intake grill when attached. The procedure was to simply set the side sheet into place, resting on the edge of the roof crown, and then secure everything with C-clamps. I used a rubber mallet to "lightly adjust" the side sheet into alignment with the roof.

build_a_body02_reduced.gif
Skeleton w/Side Sheet Jigged: Fireman's Side

In the above picture, the side sheet is in place, ready for welding. C-clamps are like money: you will never have enough. :D Note the shop rag attached to the end of the bottom edge batten. The rag is there so I don't run into the batten and poke a hole in myself, bend up the batten—or both. :lol:

build_a_body03.gif
Skeleton w/Side Sheet Welded: Fireman's Side: Inside View

In the above picture, the side sheet has been secured. There are many small welds rather than a few larger ones, both to assure that panels don't vibrate and to minimize the distortion that is endemic to sheet metal welding. To make all these welds, I used 0.025" wire in my MIG unit, with the voltage set low enough to produce one wire diameter's worth of penetration. The body panels are all fabricated from 16 gauge steel, which is also the wall thickness of the rectangular tubing used to build the skeleton. So there was a need to limit penetration to prevent localized cosmetic disturbance of the panels.

Incidentally, I use 75/25 shielding gas at 15 SCF/H for most of my steel MIG welding. 75/25 produces a less aggressive arc than straight carbon dioxide, making for smoother and more attractive welds.

More photos follow in the next post.
Science makes it known. Engineering makes it work.

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BigDumbDinosaur
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EMD F7 in SCALE

Postby BigDumbDinosaur » Fri Dec 02, 2016 5:20 pm

EMD F7 in SCALE
————————————————
BODY CONSTRUCTION: Part II cont'd

Here are some more photos and commentary.

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Skeleton w/Side Sheet Welded: Fireman's Side: Outside View

The above is what the fireman's side side sheet looked like after being welded to the skeleton. I should note that prior to welding, I prime all parts to eliminate potential corrosion problems in areas into which paint can't be gotten after welding. The particular primer I used is "ruddy brown" spray primer sold at Ace Hardware.

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Skeleton w/Side Sheet Jigged: Engineer's Side

In the above, the side sheet for the engineer's side is jigged and ready for welding. I no doubt could have used a few more C-clamps. :shock:

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Skeleton w/Side Sheet Welded: Engineer's Side: Outside View

Above is how the body looked after the side sheet for the engineer's side had been welded.


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Inside View of Body Structure
Above is an interior view of the body after welding. The wiring tubes that run through the outboard ends of the skeleton arches are not yet installed.

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Nose Casting Attachment Brackets

In the above, the wiring tubes have been installed into the skeleton arches, and the brackets that will eventually secure the nose casting to the main part of the body have been welded in place. Once the nose has been mounted these brackets will be bonded to the casting with a liberal amount of PC-7 epoxy, producing a structurally sound joint.

With all of the above structural work completed the next step was to attach the nose casting. I'll cover that in the next post.
Science makes it known. Engineering makes it work.

Andrew Pugh
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Re: EMD F7 in SCALE

Postby Andrew Pugh » Thu Jun 15, 2017 11:13 am

I would love to read more regarding the mechanicals and control system when you get around to it BDD. I fully understand if further updates aren't a priority for you, just wanted to let you know I (and I'm sure others) haven't lost interest in your build thread.

AP

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BigDumbDinosaur
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Re: EMD F7 in SCALE

Postby BigDumbDinosaur » Sun Jun 18, 2017 1:22 am

Andrew Pugh wrote:I would love to read more regarding the mechanicals and control system when you get around to it BDD. I fully understand if further updates aren't a priority for you, just wanted to let you know I (and I'm sure others) haven't lost interest in your build thread.

I get back to it sooner or later. Too much to do and not enough time in which to do it.
Science makes it known. Engineering makes it work.

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BigDumbDinosaur
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EMD F7 in SCALE

Postby BigDumbDinosaur » Mon Aug 07, 2017 11:37 pm

EMD F7 in SCALE
————————————————
BODY CONSTRUCTION: Part III

With the steel part of the body fabricated and primed, it was time to attach the nose casting. As PC-7 epoxy will solidly bond to steel as well as to Alumilite, I decided to forego use of any screws to connect the nose to the main part of the body and rely solely upon the epoxy bond.

The first step was to double-check that the nose would correctly fit before mixing up the epoxy. The thing with PC-7 is that once it is fully cured it's incredibly strong and the only way to separate the epoxied parts is to destroy them.

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Test-Fitting Nose to Body

In the above, the nose is fitted up. As before, it was more convenient to work with the body inverted.

With that step done, it was time to mix up epoxy and do some gluing. I used quite a bit of it to secure the nose so I'd have no structural problems. Pictures in a future post will explain why the joint has to be very strong. It wasn't practical for me to take photos while epoxying, as there is a tendency for paste epoxies to get messy, especially when one is using his fingers to push the stuff into cracks and crevices. What I can tell you is I stood the body on its number two end and after applying epoxy to everything, set the nose in place and made sure it was fully seated. A heat lamp was used to accelerate the cure and a lot of acetone was used to de-goo my hands, tools, etc. :shock:

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Body to Nose Bond

In the above, a portion of the bond area is visible after cleanup and some primer.

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Body w/Nose Attached

In the above, you can see what the body looked like one the epoxy had cured and I had done some cleanup. The lower batten that extends forward over the nose is secured with number two drive screws, whose rounded heads look a lot like rivet heads. At this point, I had not blended the joint between the body and nose to make it look like one piece. That was later done with some sanding (quite a bit of it) and Bondo® to take care of minor contour discontinuities.

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Body on Chassis, Opened

In the above, I have mounted the body on the chassis for a general fit check. It's propped open at approximately 25 degrees, which is the angle at which it will be opened when servicing the unit. You can also see that I've fitted the cab door frames to the nose.

body_on_loco01.gif
Body on Chassis, Closed

In the above, the body is closed. When closed, the nose rests on the top of the pilot assembly. As the body is only supported at the extreme front and extreme rear, you can now understand why the rigid skeleton is so important to structural integrity.
Science makes it known. Engineering makes it work.

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NP317
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Re: EMD F7 in SCALE

Postby NP317 » Tue Aug 08, 2017 9:36 am

Most impressive!
Is it for sale???
:lol: :lol:
~RN

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BigDumbDinosaur
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Re: EMD F7 in SCALE

Postby BigDumbDinosaur » Tue Aug 08, 2017 11:43 am

NP317 wrote:Most impressive!

Thanks!

Is it for sale???
:lol: :lol:
~RN

It will be...shortly after I "shuffle off this mortal coil," to borrow a phrase from the Immortal Bard.
Science makes it known. Engineering makes it work.

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BigDumbDinosaur
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EMD F7 in SCALE

Postby BigDumbDinosaur » Tue Aug 08, 2017 5:01 pm

EMD F7 in SCALE
————————————————
BODY CONSTRUCTION: Part IV

One of the many design challenges in this project was how to open the body after it was mounted on the chassis, as well as how to reliably keep it opened. The finished body has a calculated weight of 110 pounds (so far, it weighs 104 pounds, but that is without cooling fans and grilles, paint, glazing, number boards, lighting and wiring). Owing to the manner in which it is constructed and pivoted, I calculated the net force required to lift the body at the nose end would be around 43 pounds. Back when I was a young whippersnapper, 43 pounds was something I could handle with thumb and forefinger. Nowadays, lifting 43 pounds seems to me like more like trying to clean and jerk a 100 kilogram barbell. :D So some sort of aid would be necessary to get the body opened for refueling and other routine servicing of the machinery. Also, it would be nice if it would stay open on its own.

I kicked around a number of ideas that were based upon powered actuators, both electric and pneumatic. The idea of being able to push a button or move a valve and have the body open on its own was appealing. However, I also contemplated the scenario in which the battery is dead or the main reservoir has insufficient air pressure. That led to the decision that the mechanism would be passively mechanical, which mean some sort of spring assist.

My first sketch had some coiled compression springs mounted in the upper part of the chassis, arranged so as the body was lifted they would add force in compression. It would have worked, except springs in the force range that was needed were quite bulky. I spent a fair amount of time in CAD trying to get a fit-up and concluded it wasn't going to happen. That led me to consider gas springs, such as the ones used to prop open the hood and hatchback on an automobile. However, I needed something with more travel than is common with automotive springs, and the spring also had to have a known force. Some on-line searching led me to a company called Industrial Gas Springs (IGS), which produces commercial- and aviation-grade gas springs in a wide variety of sizes, extension lengths, force capabilities and materials.

After swapping a little E-mail with their technical support people I made a preliminary pick of one of their "Volumeline" products, which fell into the force and extension range that I determined would be needed.

The next step was to work out the geometry for the gas springs, which is slightly complicated by the fact that a gas spring's force when fully extended is lower than when fully compressed. The goals of the overall design were to reduce the initial lifting force to no more than 20 pounds yet have sufficient force available to reliably support the body in the fully opened position, which is nominally 25 degrees relative to the horizontal. The force differential would have to be accounted for in the geometric relationship between the springs and the structure. Both the CAD program and my programmable calculator got involved as I produced the below layout worksheet.

body_lifting_worksheet.jpg
Body Lifting Layout Worksheet

The calculated unassisted lifting forces were empirically verified with a borrowed force scale, using a cherry picker to lift the body to the 25 degree open position. The result of this design work was to select a pair of gas springs that individually have an extended force of 121 pounds. Compressed force is approximately 35 percent greater than extended force in the spring type I had selected, producing a compressed force of approximately 164 pounds per spring, which would be the force that would assist with the initial opening of the body. As the body is opened the net lifting force approximately reduces by the cosine of the body angle relative to level, which change roughly coincides with the force reduction experienced by the gas springs as they extend.

IGS will produce Volumeline springs to any desired force within the limits of the particular spring being used. Users of small quantities can order through distribution (Motion Industries, in this case). The springs mount on 13mm ball studs, which I got from McMaster-Carr.

The upper end of the spring is attached to a ball stud that is anchored to the body skeleton. As I had worked out all of the mounting details before building the body I was able to build in the attachment points during the construction of the skeleton.

gas_spring_mtg01.jpg
Gas Spring Ball Stud Bracket

Above is the tapping pad for the gas spring ball stud.

Also needed was a gas spring attachment to the frame. Although I had worked out the relationship between the spring and the frame early on, I didn't finalize a design until after construction was far enough along to allow me to verify my measurements and calculations. Below is a drawing of the frame bracket.

bracket.jpg
Gas Spring Ball Stud Bracket

Here is the finished piece in place on the chassis.

gas_spring_lower_mtg.jpg
Gas Spring Ball Stud Bracket (installed)


Here is the body being held open by the gas springs.

body_open_reduced.jpg
Body Open

The above photo was taken as I was checking fit-up between the gas springs and the body skeleton—clearances are tight.

The next post will have some additional photos.
Science makes it known. Engineering makes it work.

Glenn Brooks
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Re: EMD F7 in SCALE

Postby Glenn Brooks » Tue Aug 08, 2017 5:21 pm

Impressive Indeed!

Glenn
Moderator - Grand Scale Forum

Motive power : 1902 A.S.Campbell 4-4-0 American - 12 5/8" gauge, 1955 Ottaway 4-4-0 American 12" gauge

Ahaha, Retirement: the good life - drifting endlessly on a Sea of projects....

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BigDumbDinosaur
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Re: EMD F7 in SCALE

Postby BigDumbDinosaur » Tue Aug 08, 2017 6:18 pm

Glenn Brooks wrote:Impressive Indeed!

Glenn

Thanks!
Science makes it known. Engineering makes it work.


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