Author Topic: Pattern making  (Read 4269 times)

Offline RayW

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Pattern making
« on: September 13, 2015, 05:03:53 PM »
While waiting for my Otto kit to arrive, I thought I would have a go at making a pattern for a freelance engine based on the Crossley H & J types. I just happen to have a general assembly drawing of a suitable scale for these engines from which I was able to trace the outline of the body.
The pattern is made from four blocks of softwood glued together in pairs, then the two pairs glued together with newspaper in between to enable the finished pattern to be split in two having first been drilled for two steel alignment pins.The curved top and end profiles were hand cut with a fretsaw, then the sloping sides milled to the required angle. The top and bottom were then milled out to the required depth and many hours of filling and sanding later the result is as shown in the photos.
My original intention was to have a go at home casting in aluminium, but I think I will leave it to the professionals. If having a casting made proves prohibitively expensive, I can always use it as an ornament!

Offline fumopuc

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Re: Pattern making
« Reply #1 on: September 13, 2015, 05:12:11 PM »
Hi Ray, but it looks like a promising casting.
Kind Regards
Achim

Offline Jasonb

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Re: Pattern making
« Reply #2 on: September 13, 2015, 05:22:48 PM »
Looks good Ray, what sort of scale are we looking at?

You may have to make the 4 bosses for the hold down bolts as loose pieces as it will make it easier to pull the pattern from the sand, alternatively do away with then and machine a pocket and JB weld in some suitable material, the 1/3rd scale Galloway I did uses the later method.

You may also need to make a core for the gap between the two frames as again its not going to work with just a simple split moulding box.

J

Offline Tjark

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Re: Pattern making
« Reply #3 on: September 13, 2015, 06:31:26 PM »
Nice looking pattern Ray.
May I ask what you have used for the out site coating?
I struggle with the releasing from oil bounded  form sand  off small deep fins.
Have tried coatings as shell lacker and special car paint.

   Tjark.

Offline Jo

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Re: Pattern making
« Reply #4 on: September 13, 2015, 06:32:24 PM »
Hi Tjark, Try blackboard paint  ;).

Jo
Usus est optimum magister

Offline RayW

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Re: Pattern making
« Reply #5 on: September 13, 2015, 07:35:21 PM »
Jason, I am not sure of scale but the flywheels on the drawing measure about 7 5/8" diameter. The bosses for the hold down bolts were loose pieces before I glued them in! I made the pattern without having any real knowledge of the requirements for casting, but suspected that the complex shape might present problems. What I will probably do is take it along to a local foundry and see what they say.
Tjark, the coating is simply a sprayed red oxide primer paint.

Offline Alyn Foundry

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Re: Pattern making
« Reply #6 on: September 15, 2015, 07:00:54 PM »
Dear Ray.

That's a really nice job, lets not be too hasty as regards a " door stop " !!

The underside doesn't have to be hollow, just flat, a little extra mass will give the finished engine some stability. I did this many times with mine.

As Jason pointed out your mounting lugs will have to be loose pieces. This is where it gets a little more complicated, I would suggest some oblique pegs that retain each lug on the surface of the main pattern, when the box halves are parted the moulder can pull the pegs out, draw out the main pattern then fish out each lug in turn. Our Gardner base pattern had four loose pieces just for the main bearings. These were made from steel and drawn out with a magnet on a stick.

For the crank race way, well you're going to need a core box for that with it's associated print. You went to a great deal of work to make that fine inside shape, But that needs to be in a separate box.
All you need is a rectangular piece of wood sticking out from the top of your pattern, the sticking out bit being a little longer than the bit that's going to overhang within the mould cavity after the pattern is removed. A picture is worth a thousand words, I'll try and find a pattern and core box to illustrate in a day or so.

Cheers Graham.

Offline RayW

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Re: Pattern making
« Reply #7 on: September 16, 2015, 03:40:29 PM »
Many thanks for your complimentary remarks Graham. Coming from someone with your experience, I am quite flattered!
I understand the problem with the mounting lugs and wonder if the best option would be as Jasonb suggested, ie to remove them completely then to JB weld them in to pockets later. That would certainly simplify that part of the casting process.
I think I understand about the need for a core for the upper recess but not sure how I make the core itself. Any guidance you can offer would be greatly appreciated.
As regards the base recess, I agree that this is unnecessary and was only put in to save on material. I can just blank it off with thin ply or similar. That will just give a bit more meat on the base for machining.

Offline Jasonb

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Re: Pattern making
« Reply #8 on: September 16, 2015, 03:51:43 PM »
Hopefully Graham will find his photo swhich Im sure is better than 1000 words, there are two things you nee dto do

1. Make a split "box" which can be filled with sand that mimics the shape of teh void between the two sides of the part and also has a recess to form a spigot. This is the core box which a sand core in formed in.

2. Fill in the areas on the two halves of your pattern between the sides and add a "peg" to take the spigot on No1 above, this is the easier of the two. The spigot will leave a "coreprint" in the sand mould which the sand core is placed in.

I'll see if I can find an illustration or sketch it out.

This one of Myfordboys, shows the Peg on the main part and how the core is supported by the peg. His others are worth watching.

« Last Edit: September 16, 2015, 03:56:50 PM by Jasonb »

Offline Jasonb

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Re: Pattern making
« Reply #9 on: September 16, 2015, 04:12:30 PM »
Quickly altered a drawing for a Ruston Hornby that I'm hoping to build to show what your pattern needs to look like.

And second picture is half the corebox that you would need to cast the core in for yours, make a pair.

And last pic is the sand core that will come out of the core box to fill the void between the two side frames and be located by the spigot in the print.
« Last Edit: September 16, 2015, 04:28:49 PM by Jasonb »

Offline Alyn Foundry

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Re: Pattern making
« Reply #10 on: September 16, 2015, 04:33:18 PM »
Dear Ray and Jason.

Attached is a photo and text for coring a pattern, I hope it's enough to get you through ?

I couldn't find the Robinson " X " boxes so I photographed the start I made on the Brayton Readymotor. And yes that is a 10 Pence piece !! It would have been the largest engine to come from Alyn Foundry.

Ray, you will notice that the cores are printed top and bottom this allows good support, you, however will need just a longer single print as your core will be free ended.

There are many ways to " skin a cat " we used to use flat nails to support the water jacket core on the Robinson " X " type because at the temperatures involved they fused into the casting. I do remember, however one customer complained that he couldn't remove the nails with a claw hammer !!    :Lol:

Kind regards, Graham.

Offline RayW

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Re: Pattern making
« Reply #11 on: September 16, 2015, 08:17:45 PM »
Thanks Graham and Jason for all your help and advice. In the video, the first half of the pattern is placed flat side down on a board, the box placed around it, then sand packed over it and rammed down. The box is then turned over and the pattern removed. In the example shown, the flat surface extends to the whole outline of the half pattern, but that is not so in my case.
If my half pattern is placed face down, and sand rammed over it, what is to stop the sand filling the top cavity where the core will eventually go?

To make the core box, I presume that I first have to make a positive cast of the cavity, then from that make a female mould, and from that make the sand core. Is that correct?

Sorry for all the questions, but I am really having problems getting to grips with the process. At this rate, I might still end up using it to keep my pencils in!! :thinking:

Offline Jasonb

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Re: Pattern making
« Reply #12 on: September 16, 2015, 08:32:23 PM »

If my half pattern is placed face down, and sand rammed over it, what is to stop the sand filling the top cavity where the core will eventually go?

That needs to be filled in solid, like the first picture of my green Ruston

To make the core box, I presume that I first have to make a positive cast of the cavity, then from that make a female mould, and from that make the sand core. Is that correct?

It can be done by taking a mould from your existing pattern but generally you just have to get used to working in reverse and would form the corebox straight from wood

Sorry for all the questions, but I am really having problems getting to grips with the process. At this rate, I might still end up using it to keep my pencils in!! :thinking:

Offline Alyn Foundry

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Re: Pattern making
« Reply #13 on: September 16, 2015, 08:54:06 PM »
Thanks Graham and Jason for all your help and advice. In the video, the first half of the pattern is placed flat side down on a board, the box placed around it, then sand packed over it and rammed down. The box is then turned over and the pattern removed. In the example shown, the flat surface extends to the whole outline of the half pattern, but that is not so in my case.
If my half pattern is placed face down, and sand rammed over it, what is to stop the sand filling the top cavity where the core will eventually go?

To make the core box, I presume that I first have to make a positive cast of the cavity, then from that make a female mould, and from that make the sand core. Is that correct?

Sorry for all the questions, but I am really having problems getting to grips with the process. At this rate, I might still end up using it to keep my pencils in!! :thinking:

Dear Ray.

" If my half pattern is placed face down, and sand rammed over it, what is to stop the sand filling the top cavity where the core will eventually go? "

That cavity wont be there, you will have a rectangular slab sticking out from the pattern, that's your core print.

" To make the core box, I presume that I first have to make a positive cast of the cavity, then from that make a female mould, and from that make the sand core. Is that correct? "

Yes, correct. you need the internal shape to be a negative so the core becomes a positive. I know it's difficult to start with but you soon get the hang of thinking in reverse for cores.
You will need to extend your core box so that the core has a print or peg as Jason calls it. This allows for setting the overhang into the mould cavity. I tried to make a drawing in " Paint " but it just didn't look right.

We have been so wet up here in N Wales that it's virtually impossible to do any moulding, otherwise I would do a You Tube for you. Pictures and better still video, speak volumes !!

Kind Regards, Graham. 

Offline RayW

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Re: Pattern making
« Reply #14 on: September 17, 2015, 11:36:28 AM »
Thanks Graham. I think I am getting it at last! If I understand correctly, the core print will be split in two halves to correspond with the two halves of the pattern, but the peg can just be on one half, as long as one face of the peg lines up with the flat face of the half pattern. The half with the peg will be the piece used to fill the gap when the first half pattern is moulded, then it wll be removed and replaced with the core itself, before the other half pattern is added.

So, if I am correct, what I will need to supply to the foundry is the two part pattern, the half core print with the peg, and the core itself. Is that correct?

When casting the core print, there will obviously be a slot in one half, corresponding with the  peg on the pattern, so I assume that I will then need to make a peg of identical size to the one on the pattern to glue in that slot? Presumably it must be of exactly the same length to ensure correct positioning of the core when moulding?

Just a couple more questions - would a plaster core print with wooden peg be acceptable or what other material could I use? Also, should I coat the pattern with something first to aid release of the print?

Thanks again for all your patience.

Offline Jasonb

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Re: Pattern making
« Reply #15 on: September 17, 2015, 01:19:23 PM »
Ray you need to give them two things

1. your moddified pattern, this will have the hole filled in solid and the core print sticking out of each half. When they mould the sand around this the filled in bit obviously forms a void and the print leaves a place to locate the core.

2. You also need to supply a core box this usually a split box into which they will place sand to form the core, needs to be split so they can get the core out. The sand in the core is different and has additives which allow it to be bonded into one hard piece.

Process is they make the core in the corebox and then remove it. Next they ram up one side of your pattern, turn the box over and place the second half of the pattern ontop and ram sand around that. They will then take the two halves apart, remove your pattern halves then place the sand core into place and put the two boxs back together ready to poor. The sand core is held by the print (a sort of spigot) which keeps it away from the edges of the sand so iron can flow down the gap and form the sides of the part

Offline RayW

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Re: Pattern making
« Reply #16 on: September 17, 2015, 02:57:09 PM »
Thanks Jason. Things are getting much clearer now. In the YouTube video that you gave me a link to, I see that the core print (spigot) is just on one half of the pattern. I presume that I can do the same, or do I need the print split along the centre line so that there is half in each half of the pattern?

Whereabouts in Surrey are you? I am near Maidstone in Kent. If you were willing, it might be really helpful if we could meet up some time so that you could talk me through the process.

Offline Alyn Foundry

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Re: Pattern making
« Reply #17 on: September 17, 2015, 04:10:46 PM »
Dear Ray.

I have just posted a video here. I hope it explains better than words ?


Kind regards, Graham.

Offline RayW

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Re: Pattern making
« Reply #18 on: September 17, 2015, 06:01:11 PM »
Graham, what can I say! I have never had a YouTube video made especially for me or been mentioned in one!!
That has been really helpful. I am off on holiday on Saturday for a couple of weeks, so will have a go at it when I get back. When I fill the top cavity in each half of the pattern, does that need to be glued in, or can it be a loose piece?

Many thanks once again for your patience.

Offline Jasonb

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Re: Pattern making
« Reply #19 on: September 17, 2015, 06:51:44 PM »
Probably best glued or screwed in place, you could help the foundary man by painting the print part black which is the usual way things are done, rest can stay in your red.

Offline RayW

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Re: Pattern making
« Reply #20 on: September 17, 2015, 07:55:25 PM »
Thanks Jason.  I will use the two pieces to make the corebox before glueing them into the two halves of the pattern.

I just hope after all this effort that the cost of casting will not be prohibitively expensive!

Thanks again for all your help and guidance.