Basic Pattern Making In The Hobby Foundry
Success or otherwise in your hobby foundry will depend to a large degree on your skills & ability to create simple patterns, i.e. Patterns that are easy to mould, lift or separate from the sand mould after a gentle rap.
A pattern that has incorrect draft on vertical surfaces, badly finished corner fillets, or a pattern that has not been finished to an ultra smooth finish will be difficult to work with.
Pattern making is an art in itself, pattern making apprenticeships take around three or four years to complete, before you are given your "Trade Ticket".
The average hobby foundry worker most likely won't have the time, or the patience to get involved with the art of pattern making to that extent.
But the basic skills are worth the effort to learn, because without them you wont get far with your hobby foundrywork, unless you PAY to have all of your patterns made for you, but that would take all of the fun out of it, and also cost you a sizeable amount of money.
Construction costs of professional pattern making can vary from a few hundred dollars for simple patterns, up to several thousand dollars for complicated pattern designs.
I happen to a know a pattern maker who designs and builds patterns & core boxes to make alloy & cast iron cylinder heads, it is not unusual for pattern & core costs to be around $20,000.00 AU before you even think about melting & pouring any metal.
If you are a legacy of the old tech school system of the sixties & seventies, there is a good chance that you still remember how to skilfully use woodwork hand tools.
Some of the finest foundry patterns were made using basic hand tools. More than likely you still have a chisel set tucked away in a drawer, or a spoke shave, wood plane & handsaw, plus a host of other tools that could be used to make excellent patterns in the home hobby shop.
And if they happen to be a bit rusty, then get them out again and bring them back to life, re-grind the cutting edges and hone them with an oil stone.
Remember what your trade teacher always told you; "sharp tools give the best results."
Do the same with the wood plane and any other tools, such as a small set of carving chisels you could use to carve intricate shapes in wood.
You'll need some good pattern timber or lumber as it's called in the states... doesn't matter, it's all wood isn't it. There are many types of timber suitable for pattern making, but, you'll probably be limited to what's available in your area or region. Quality pattern timber is expensive to buy, so ask for off cuts at the local timber merchant, which you may get for a considerable discount
You'll be looking for a soft timber that doesn't splinter, has a straight grain, is easy to work or carve, and finishes to an ultra smooth finish.
One of the easiest timbers to use is jelutong, this timber comes from the Philippines, I don't know whether it is from plantation timber, or old growth forests, but it is great to turn on a lathe, or shape and carve with sharp hand tools.
Some of the other pattern timbers in use are cherry wood, mahogany, maple, white pine, and many others.
Quite often a master pattern can be made from timber, and then a replica mould is made using RTV 585 silicone, this is a quick method to remake replica production patterns.The replica patterns can then be mounted onto a match plate along with the runners & gates, this method will enable you to mould & cast multiple parts at the same time.
With a little practise you will soon know enough to make reasonable quality patterns that create good sand moulds.
There is one important pattern making tool that will make you wince when you purchase, but it is an important tool to have if you want to make accurate patterns where shrinkage rates are concerned.
The tool is the "Pattern Makers Rule", this is a ruler about 500mm (20")long. Made by Rabone Of England. No B5. And the graduations are marked as: 1/30-1/40-1/60-1/80,
The graduations represent the amount of shrinkage allowance for different types of metals. The pattern makers ruler provides a built in shrinkage percentage, which means you don't have to calculate the final measurement or size of your pattern, you simply take your measurement from your shrink rule, and transfer the measurement to the pattern being made.
The system is quite clever in the way it's all been calculated.
For a quick example, the 1/30 scale measurement seems to give the correct result with patterns used for cast aluminium items.
Machining allowance on specific parts of a given pattern may also need to be considered, generally your own judgement can be used to judge that.
Pattern making can be quite a challenge for the hobby foundry worker, but once you learn the basics and develop your skills, with a little practice your patterns will get better and so will the overall casting quality.
If you intend on making lots of patterns, the investment in a shrink rule is well worth the money.
And while you're at it buy a good book on pattern making, it deserves a study all on it's own, but it forms an integral part of foundry work, and because you're the boss of your own workshop, you have to learn to wear the hat of the pattern maker & that of the sand moulder & founder.
A pattern that has incorrect draft on vertical surfaces, badly finished corner fillets, or a pattern that has not been finished to an ultra smooth finish will be difficult to work with.
Pattern making is an art in itself, pattern making apprenticeships take around three or four years to complete, before you are given your "Trade Ticket".
The average hobby foundry worker most likely won't have the time, or the patience to get involved with the art of pattern making to that extent.
But the basic skills are worth the effort to learn, because without them you wont get far with your hobby foundrywork, unless you PAY to have all of your patterns made for you, but that would take all of the fun out of it, and also cost you a sizeable amount of money.
Construction costs of professional pattern making can vary from a few hundred dollars for simple patterns, up to several thousand dollars for complicated pattern designs.
I happen to a know a pattern maker who designs and builds patterns & core boxes to make alloy & cast iron cylinder heads, it is not unusual for pattern & core costs to be around $20,000.00 AU before you even think about melting & pouring any metal.
If you are a legacy of the old tech school system of the sixties & seventies, there is a good chance that you still remember how to skilfully use woodwork hand tools.
Some of the finest foundry patterns were made using basic hand tools. More than likely you still have a chisel set tucked away in a drawer, or a spoke shave, wood plane & handsaw, plus a host of other tools that could be used to make excellent patterns in the home hobby shop.
And if they happen to be a bit rusty, then get them out again and bring them back to life, re-grind the cutting edges and hone them with an oil stone.
Remember what your trade teacher always told you; "sharp tools give the best results."
Do the same with the wood plane and any other tools, such as a small set of carving chisels you could use to carve intricate shapes in wood.
You'll need some good pattern timber or lumber as it's called in the states... doesn't matter, it's all wood isn't it. There are many types of timber suitable for pattern making, but, you'll probably be limited to what's available in your area or region. Quality pattern timber is expensive to buy, so ask for off cuts at the local timber merchant, which you may get for a considerable discount
You'll be looking for a soft timber that doesn't splinter, has a straight grain, is easy to work or carve, and finishes to an ultra smooth finish.
One of the easiest timbers to use is jelutong, this timber comes from the Philippines, I don't know whether it is from plantation timber, or old growth forests, but it is great to turn on a lathe, or shape and carve with sharp hand tools.
Some of the other pattern timbers in use are cherry wood, mahogany, maple, white pine, and many others.
Quite often a master pattern can be made from timber, and then a replica mould is made using RTV 585 silicone, this is a quick method to remake replica production patterns.The replica patterns can then be mounted onto a match plate along with the runners & gates, this method will enable you to mould & cast multiple parts at the same time.
With a little practise you will soon know enough to make reasonable quality patterns that create good sand moulds.
There is one important pattern making tool that will make you wince when you purchase, but it is an important tool to have if you want to make accurate patterns where shrinkage rates are concerned.
The tool is the "Pattern Makers Rule", this is a ruler about 500mm (20")long. Made by Rabone Of England. No B5. And the graduations are marked as: 1/30-1/40-1/60-1/80,
The graduations represent the amount of shrinkage allowance for different types of metals. The pattern makers ruler provides a built in shrinkage percentage, which means you don't have to calculate the final measurement or size of your pattern, you simply take your measurement from your shrink rule, and transfer the measurement to the pattern being made.
The system is quite clever in the way it's all been calculated.
For a quick example, the 1/30 scale measurement seems to give the correct result with patterns used for cast aluminium items.
Machining allowance on specific parts of a given pattern may also need to be considered, generally your own judgement can be used to judge that.
Pattern making can be quite a challenge for the hobby foundry worker, but once you learn the basics and develop your skills, with a little practice your patterns will get better and so will the overall casting quality.
If you intend on making lots of patterns, the investment in a shrink rule is well worth the money.
And while you're at it buy a good book on pattern making, it deserves a study all on it's own, but it forms an integral part of foundry work, and because you're the boss of your own workshop, you have to learn to wear the hat of the pattern maker & that of the sand moulder & founder.