Pattern Making

[DSL PWR]

I need to make a pattern for a 10x10x4.5" box. I want to make it out of wood on my cnc mill. What kind of wood should I use? I was thinking of sandwiching a bunch of plywood together and just milling it out. Is this a good idea? I wanted to avoid having any joints in the material on the walls of the box, and if I glue and clamp the plywood together it should be plenty strong... right?

Or would I be better off making a rough box out of 2x6 lumber and cleaning it up?

BTW the box walls are .5" and I have given a 2% shrink allowance for aluminum.

[Geof]

Remember to put on enough draft for it to release from the sand.

I suggest using Baltic Birch ply, the stuff that has more thin laminations than fir ply; it will give a smooth finish without much hand sanding. Then you need to seal it with varnish.

Sometimes it is a good idea to talk to the foundry people because they might suggest putting extra thickness somewhere on the pattern to minimize shrinkage stresses or voids in the casting. Also if you are machining this all over they may suggest extra thickness so you can clean off any surface porosity.

[under-dog]

It would seem to me there are some major issues if you went with the method you described:

Sandwhching a bunch of plywood togehter to make a solid block and millling the cavity out.

1) waste of material
2) time consuming
3) surface finish will be worse than the faces of the plywood especially if you find voids
4)You say you are trying to avoid joints but wouldnt this make two whole side faces and the bottom of the box entirely constructed of joints? Rather than joints just at the corners you will have joints all across the faces at 3/4" intervals. Assuming that you are using 3/4" plywood that is.

[DSL PWR]

Remember to put on enough draft for it to release from the sand.

I suggest using Baltic Birch ply, the stuff that has more thin laminations than fir ply; it will give a smooth finish without much hand sanding. Then you need to seal it with varnish.

Sometimes it is a good idea to talk to the foundry people because they might suggest putting extra thickness somewhere on the pattern to minimize shrinkage stresses or voids in the casting. Also if you are machining this all over they may suggest extra thickness so you can clean off any surface porosity.

I'm putting a 1.5deg draft on all surfaces. I already have some allowance for machining. The foundry is less than helpfull, as I think they want the $1000-$2000 for the pattern. The office people seen to be OK with it, just their pattern shop forman won't even give me their jacket sizes.

For my prototyping how well would it work to just do half of a mold? Since the bottom (top when pouring) is flat, and will be face milled, is there any reason why I can't do an open face pour?

[DSL PWR]

It would seem to me there are some major issues if you went with the method you described:

Sandwhching a bunch of plywood togehter to make a solid block and millling the cavity out.

1) waste of material
2) time consuming
3) surface finish will be worse than the faces of the plywood especially if you find voids
4)You say you are trying to avoid joints but wouldnt this make two whole side faces and the bottom of the box entirely constructed of joints? Rather than joints just at the corners you will have joints all across the faces at 3/4" intervals. Assuming that you are using 3/4" plywood that is.

Have you done any pattern making?

1. Plywood is cheap
2. Put the block in the mill and let it run
3. Small voids are easily filled with glue and saw dust, large voids (or a loose knot) in lumber means you will have to start over
4. I will be putting the orientation of the block such that the seams will be horizontal.

I hope that the plywood will be much more stable than lumber once it has been milled (no warpage etc...)

[Geof]

.....The foundry is less than helpfull,....their pattern shop forman won't even give me their jacket sizes.

For my prototyping how well would it work to just do half of a mold? Since the bottom (top when pouring) is flat, and will be face milled, is there any reason why I can't do an open face pour?

If the foundry is being petty you may be in for some problems. If the casting comes out with cracks or voids it will be "your" fault because the pattern is not correct.

I don't grasp exactly what you mean by half of a mold; do you mean the sides without a bottom?

How many of these will you be doing? It could be that you are better to chew them out of a solid block if it is only one or two.

Also is this box intended to contain liquid or be liquid or gas tight? Aluminum castings are notorious for porosity particularly in thin wall sections.

[JerryFlyGuy]

Could MDF be used? It's even cheaper than plywood.. and can be bought in up to 1.5" thicknesses.. machines beautifully.. would need a top coat of sealant.. varnish.. or whatever..

Not that I've made a pattern from it or have any experiance in the matter.. [yet ]

[Geof]

Could MDF be used? It's even cheaper than plywood.. and can be bought in up to 1.5" thicknesses.. machines beautifully.. would need a top coat of sealant.. varnish.. or whatever..

Not that I've made a pattern from it or have any experiance in the matter.. [yet ]

0.5" walls in MDF will be delicate; one good knock by a careless foundryman and you no longer have a pattern. Also some MDF is graded in density across the thickness; less dense in the middle so your pattern is going to have softer spots.

[JerryFlyGuy]

ahh.. I missed the 1/2" part.. If I was doing a thin wall pattern, personally I'd be doing it in two parts and using locating pins for assembly.. it's getting machined after all.. who cares if the pattern's made of two parts??

I just finished designing a bell housing which was cast and they machined the pattern w/ a thin wall from what looked like a solid block of wood [not ply] it was built up from some smaller chuncks and glued together..The smallest pc of wood might have been 10" x 10" x 10".. in hind sight they wished they'd done it in two pc's [ inside pattern and an outside pattern].. it would have elliminated the possibility of movement in the pattern wall when ramming up the mold.... FWIW..

[DSL PWR]

If the foundry is being petty you may be in for some problems. If the casting comes out with cracks or voids it will be "your" fault because the pattern is not correct.

I don't grasp exactly what you mean by half of a mold; do you mean the sides without a bottom?

How many of these will you be doing? It could be that you are better to chew them out of a solid block if it is only one or two.

Also is this box intended to contain liquid or be liquid or gas tight? Aluminum castings are notorious for porosity particularly in thin wall sections.

I mean sides and bottom, with no top half to the mold. My box would be cast upside down (like you are dumping out the contents) with the base exposed to the top.

I hope to do hundreds (if things go well thousands), so casting should be the way to go. But I want to keep the proto costs down in case it doesn't work out.

The box's function is to hold some mechanical works, and keep out chunks.


I'd settle for some 10x10x.5" square extrusion if I could find such a beast.

[Geof]

.....I'd settle for some 10x10x.5" square extrusion if I could find such a beast.

My guess would be probably nothing for the die cost provided your first order was for around 10,000kg .

Here is another thought..get the prototypes fabricated.

[DSL PWR]

I have a steel proto made out of 1/2" plate steel. The concept works, but it is way too heavy.