Pattern Making (Off Topic)

Nothing to do with old vehicles, but I don't know of anywhere else that amateur pattern-makers hang out.

I want to have this cast:

I have been quoted £130 for laser-cut parts to be fabricated or £80 for a casting, and as a casting will be a better solution too, that is the way I want to go.

It clearly needs a core, and I am planning to make it a single-part core. I imagine that the foundry can ram up the assembled core-box from the far (closed) side of the part and then form a radius on the core at the ramming-end by hand?

I think I need to split the main pattern at the indicated plane to be able to extract the core-prints for the top holes.

I am wondering if it would be reasonable to _not_ have core prints on the two large front holes. This would make ramming up the small front section of the pattern easier. This would mean that the front "holes" of the core rest against a flat surface. Is this OK? I can live with a fair bit of flash on the holes, the part is to be machined square on all sides.

Will a big left-side core print and two smaller top ones be enough to support a core? I imagine that the core face that rests against the flat face of the mould would be at the top as cores float in iron?

Andy,

What are the approximate dimensions of this box?

Barry.

What are the approximate dimensions of this box?

12" x 304.8mm x 8"

I have likewise been planning the style of fabricating patterns for making replacement radiator frames and a few other parts for our trucks.

You have here, a pattern with many difficult areas to force and pack the sand into. Rather than split into two sections, a third section would give better access to the curved base area. Another option is to build up the pattern from polystyrene as a one piece unit and allow it to be consumed with the heat of the metal. However if something goes wrong there is no pattern to repeat the casting with.

A visit to foundry staff with the idea will soon indicate which route to take.

Doug

One could certainly make the core for this in one piece but you would need quite a bit of draw to get the centre divideing part of the core box out cleanly. Normally this core box (or mould) would be in two pieces so that all the functional surfaces of the core were exactly defined. The core sand (probably bound with a CO2 setting resin) would be put in and rammed up via the outside faces of a couple of the core prints, and fairly substantial bits of wire / steel rod would be used to reinforce the weak bit in the middle. I would devide the core box on the same line as the main pattern as it makes it easier to make the draw of the core prints fit their registers in the main mould. The core prints that form the holes at the top of your drawing could be joined up outside the casting which would greatly strengthen the core.

As you say, if you cast with the large solid face at the bottom, the core is then best supported as it tries to float. Also any dross (muck) in the molten metal will be easier to void from the much smaller area of the big face with two big holes in it.

However I don't see that it would be significantly easier to not extend the core prints of the big front RH face (top when casting) and it will involve more work to remove the flash from the holes. Also it will reduce the support and location of the core which is important as untill the metal is poured in there is nothing supporting the corner of the core that is at the bottom / back of your drawing. It may be necessary for the moulder to use a small prop to keep everything in place untill pouring. This would be incorperated into the casting and would not show after machining.

David

The core sand (probably bound with a CO2 setting resin) would be put in and rammed up via the outside faces of a couple of the core prints

It occurred to me on the way in to work today that you can ram up the core through the core print holes.

However I don't see that it would be significantly easier to not extend the core prints of the big front RH face (top when casting) and it will involve more work to remove the flash from the holes.

You are right, I was imagining that the mould would be rammed up with the pattern sat on the front face with the holes. But of course it will be sat on the split face at that point.

Another way to make the core is to fill each half of the core box with CO2 sand, rammed in and suitably reinforced with wire etc. have the sand just proud of the joint surface of the box but with no sand at all on the surfaces of the box that will touch. Clamp the two halves together and squirt the CO2 in through a small hole. Leave for a few hours to make sure that the resin has gone off and open. With luck the joint will need minimal fettling but can be filled if needed with purpose made fillers.

Not relavent to this job but it is quite possible to modify the core to create undercuts or details that would not justify the effort of a more complicated core box.

It would be interesting to see the patterns and castings that you finnish up with.

David

If the core box and pattern share a split line, is there any reason that the pattern can't be the core box too?

I am probably going to CNC mill the patterns from high density PU model board. I found it in a skip, but knowing that it is expensive I would prefer to economise.

A very tempting concept: make a plastic / wood version of the finished casting but in two pieces and use it both as the main pattern and the core box. The difficulty is that where the core prints project through the holes in the finished casting, they need to fit exactly into their registers in the main mould. There is then no space for the part of the pattern / core box that generates those shapes (that are outside the actual casting). I think that for a simpler casting it would be practical to do as you suggest and use removeable bits to change the function from pattern to core box but I think it would get a bit complicated here.

Would it not be easier to build the pattern up from bits of MDF / plywood / low density UPVC since it is mostly flat surfaces that will have to be at slight angles to provide draw. Also both the pattern and core box need to be robust enough that they don't flex when the sand is rammed into them.

David

I am probably going to CNC mill the patterns from high density PU model board.

No I am not, my mill is too small and the blocks of PU are too small.

Would it not be easier to build the pattern up from bits of MDF / plywood / low density UPVC since it is mostly flat surfaces that will have to be at slight angles to provide draw.

Yes, plywood is the plan now.

At home I am much better set up for metalwork and machining than woodwork, though. All the woodworking stuff (with which we are rather well equipped) is at my parents' 250 miles away.

Woodworking in my machine-tool shop always makes a nasty mess.

Whilst we are completely off topic and talking about castings, I would like to describe a route which I would now always follow. My wood working skills are limited and my time is even more limited, hence the following approach when I needed a new two into one exhaust manifold for one of my Dennis's.

I drew the manifold first in Solidworks, and then produced the models for the pattern and core box. I then had them 3D printed at a cost of around £25 each.

I tried out the core box making a C02 core. This was perfect. I need two core halves to glue together to make a complete core

I tried out the pattern in some greensand and this pulled out easily too.

The advantage of doing it like this is that you can define the draft angle easily for easy extraction from the sand and achieve absolutely consistent wall thickness. Also things like scaling to allow for contraction is just a mouse click away.

Barry.

I tried out the pattern in some greensand and this pulled out easily too.

The advantage of doing it like this is that you can define the draft angle easily for easy extraction from the sand and achieve absolutely consistent wall thickness. Also things like scaling to allow for contraction is just a mouse click away. .

Something else you can do in CAD and not in real life is put your pattern in your core box to test that everything is in the right place (not trivial when everything has had drafts added)

This is the core box lower. I now need to decide how to make it.

Also, you can analyse the overlap between the parts, and that is what the actual casting will be

(I am not sure about the volume calculation, that sounds like a 112kg casting...)

Interesting to see when I started this, it seems like months that I have been at this, and it is.

These are the patterns, almost ready for the foundry.

https://picasaweb.google.com/lh/photo/gQuPtDB0J_ZiqVVDHaf8vtMTjNZETYmyPJy0liipFm0?feat=directlink

Thinking about Barry's pictures of 3D-printed patterns reminded me of another process that is very applicable to the vehicle restoration game.

We actually have one of these machines at work, but I don't get to use it.

Excess capacity on the machine is sold via an intermediary company. When I enquired about Dennis cylinders they estimated around £10k for two cylinder blocks, starting from a CAD model.

I got the castings back at the weekend:

https://picasaweb.google.com/108164504656404380542/Holbrook#6230479127730875154

I don't actually know what the total cost was, I just realised that I haven't paid for them.