Information on making patterns

[JCSteam]

A slight diversion from the usual topic of mine, but i hope youll forgive me. A friend of mine is looking for a flywheel/fan for a Stuart air compressor. He's had no luck finding one to press..... I believe a copy or something similar could be made from an Ali casting. So I'm looking for info on making a pattern.

The picture posted below I've done some scaling on, dimensions i have are as follows.
OD=5"
Width of tread=0.774"
depth of tread= 0.221"
Hub OD=1.144"
Width of hub=0.774"
Width of blade=0.717"
Thickness of blade=0.100"

Can anyone confirm or correct these sizes please? I assume that the castings would have been made to imperial sizes originally, so for example

the hub OD, of 1.144", it would have been 1.9/64" which is 1.1406", or 1.5/32" which is 1.563".

The width of tread, 0.774" would be 25/32" which is 0.781"

Just looked up shrinkage for Ali, and it's 6%, (5% for brass if anyone's interested) so the overall dimensions of the pattern would have to be 6% bigger, and then the machined faces (outside OD, and tread width depth, hub width) another 1% minimum ontop to give enough material to machine away to finished size.

(Ps gosh my heads getting a workout tonight, it needed it, blow away the cobwebs�&#65533

I was planning on making a pattern from plasticard, one because that's the material im used to working in, it can be laminated to any thickness and turned easily on a lathe at reasonable slow speed. It can also be sanded to final finish and extra material added to the pattern if one is a little heavy handed with the sandpaper.

Can anyone confirm the sizes posted above, provide an original machined casting to measure, or let me know if plastic patterns can work, I've seen 3D printed patterns work well??????

 

[bmac2]

I don’t think I have done as much reading on a topic as when I started getting into casting. Metallurgy, chemistry, furnace and burner design, pattern making, gating and don’t forget the math.
The flywheel should definitely doable my only concern would be the .100” blade thickness.
How would you be casting it? It could be done in sand (green sand, petrobond) just remember to add enough draft to the pattern.
You mention 3D printing and I’ve been reading a lot lately on lost PLA casting where people are getting some pretty good results.
[ame]https://www.youtube.com/watch?v=1VNy6QuakxY[/ame]
Another alternative would be to build it up. Make the rim, hub and blades and have it tig welded. Then go the final machining.

 

[fpravenscroft]

give me some time and i will measure the one i have and let you know asap i have visitors coming for the next two weeks will try on Tuesday
regards
peter

 

[ThomasSK]

If plastic card works for a pattern or not depends on the type of casting you are doing. The pattern size is imho a bit big for investment casting, and die casting probably isn't what you should try for your first casts. Since the pattern will be double sided, greensand casting is faily easy to do and should give you decent results.
As long as the pattern is made strong enough to hold up to the ramming forces, it should work out fine.

Remember to add draft the correct way (been there, done that) and split the pattern down the middle so its easy to ram up. A pouring spout at the outside, and a riser in the middle should be required, if you have problems with shrinkage, then a riser on the oposite side of the pattern from the spout should help.

You may want to consider casting the flywheel in a zinc-alu alloy to give it some more mass. Just make sure that there is no tin or lead in that alloy if you make it up from scrap.

myfordboy on youtube have some great videos showing how to make patterns, ramming the molds and pouring the metal. And with the additional benefit that he does not excerice his mouth all the time.

Thomas.

 

[JCSteam]

give me some time and i will measure the one i have and let you know asap i have visitors coming for the next two weeks will try on Tuesday
regards
peter

Thank you, I appreciate your help.

Found out that the flywheel is actually 4.5" not 5" as I had originally been told. So my scaled measurements above are all wrong. As this was what id used to calculate the size.

 

[JCSteam]

Thank you Thomas, I will be handing over the actual casting to someone that's done it before, (a lot, for a number if years). So as long as the pattern is suitable it should be ok, I am wondering now though what you've said about the ramming forces, and I'm questioning myself now as to wether the blades would break away. The glue is a solvent and melts the plastic together, like a plastic weld. But it can break if it's not in contact properly over the length of the joint.

 

[fpravenscroft]

hi
i have been able to measure the flywheel for you it is 4 1/2" inches in diameter by 3/4 deep the boss is 1" in diameter the outside wall is 1/4" thick the blades are roughly .8475 across
hope this helps
regards
peter

 

[abby]

Use a backing board ! this is a plaster half mould which will support the pattern whilst it is being rammed , most aluminium foundries use "air-set" sand which does not require much ramming anyway.
I have done investment castings in brass of 5 1/2" dia for steam engine fly wheels.
Check out my web-site http://www.unionsteammodels.co.uk
You might also consider having your pattern 3D printed , bear in mind that you don't need the whole thing printed.
You could turn the outer rim and hub from wood or whatever and just have the vanes assembly printed.
This would give better accuracy than most other methods.
When having 3D patterns printed ( I have hundreds made) wherever possible make them hollow , you are charged for the actual volume of the part , make it hollow with an exit hole and you can save more than 50% on some designs.
Dan.

 

[MRA]

Wouldn't the parting line be pretty weird for those spokes / vanes, to preserve the draft in sand? I have a belt buckle I'm doing at the moment as a present for someone. My (wooden) pattern is curved, with detail on the back, and isn't going to part easily. I'm wondering if I can (and, by extension, you could) bury the thing in the drag as usual, then turn it over and mess about with a little trowel to dig it out a bit and sort the draft out, before adding parting powder and ramming the cope. That way it might come apart in one piece (well, two, but you know.)

 

[JCSteam]

Thanks for your input all good points to consider.......mmm more reading needed. The only thing I can think of is to aline the pattern with dowel pegs, around the circumrence of the outer rim, and shape the outer rim, hub and spokes larger toward the centre line of the pattern.

 

[ThomasSK]

For a flywheel, the easiest way of making them may be using a split pattern, just split it down the middle, and ramming the mold should be easy. Attached a picture of one of my flywheel patterns to illustrate. A couple brass pins keeps the registration between the cope and drag half of the pattern. The important part is that there is some draft, the metal don't care about the draft, its only there to make it possible to remove the pattern.

For the sloping part, a pice of cardboard or such could keep the sand out of the undercuts while ramming the drag. Just make it so that it becomes a V, so that there is no sharp corners when ramming the cope, then remove the cardboard pices before ramming the drag.

You could be able to use the same principle for the belt buckle, or you could do a false drag or molding board that helps you ram. A molding board could be as simple as a a pice of plywood with some cutout so that most of it lays flat against the board. False drag is a drag where you dig it down as you suggest.

Thomas.

 

[JCSteam]

Thank you Thomas, looking at the Stuart one, this looks like it is most probably how Stuart did their casting for the flywheel/fan. You can see a taper which runs from outside edges to center line of the casting. Interesting.

 

[Charles Lamont]

Wouldn't the parting line be pretty weird for those spokes / vanes, to preserve the draft in sand? I have a belt buckle I'm doing at the moment as a present for someone. My (wooden) pattern is curved, with detail on the back, and isn't going to part easily. I'm wondering if I can (and, by extension, you could) bury the thing in the drag as usual, then turn it over and mess about with a little trowel to dig it out a bit and sort the draft out, before adding parting powder and ramming the cope. That way it might come apart in one piece (well, two, but you know.)

A good way to deal with this would be to use a pattern that is not split and an 'oddside'. In this case it the oddside would be in the form of a plate, with a circular recess to locate the rim of the wheel pattern to half its depth, and projecting radial curved lumps to fill up the spaces under the spokes, and a bit more, to go up to different parting lines for the leading and trailing edges of the fan blade/spoke. The shape under the blade does not need to fit exactly, so long as it does so at the edges. It could be just two walls. However, the shape would also give consideration to the desired parting line on the inside of the casting rim and the outside of the hub, where it has to cross from the leading edge of one blade to the trailing edge of the next. This could be diagonal, or stepped adjacent to each blade.

The oddside, and the pattern on it, are placed on a board to ram up the drag, then turned and only the odd-side removed for ramming up the cope. The resulting mould cavity sits within the drag rather than half-and-half between the two boxes, but this does not matter. An additional benefit is that the draft around the edges of the base plate of the oddside forms a plug on the cope that helps to locate the two boxes accurately.

 

[MRA]

Thomas - thanks. I'd been thinking about a moulding board with some holes in it to get the clasp projections out the way and the (curved) back of the buckle nearer to the parting line. And digging out = false drag - good to know it's got a name, which implies a degree of legitimacy!

And Charles - that's really neat, I can picture it in my head. It would be a bit of a sod to make, but the only way, really. Actually, a set of 'sloping slices of cake' might not be so hard to turn out and stick onto a board.

 

[Wizard69]

When you say blades im assuming you mean fan blade spokes as opposed to plain spokes. If the original casting is iron or a die casting, you may have trouble duplicating it in aluminum gravity pour.

The problem would be freezing before the metal fills the vanes. And if not that stresses due to differential cooling between the rim and the vanes leading to cracking or failure. You may be able to address this with far mor material in the vane cross sections.

Another consideration is making the flywheel out of bar stock. For a one off this makes a lot of sense as the time involved would be far shorter. If the vanes are indeed fan blades the machining could be complicated especially if done manually. There is an option here though that is simple, that is to machine a conventional flywheel and then attach sheet metal vanes for air movement.

I know this is a casting thread but i had to point out that it may be far quicker to machine a solution. I suppose you could also cast a conventional flywheel and the attach sheet metal fan blades to it for a far simpler casting. I just see making a decent mold pattern for a flywheel with integrated fan blades as being very involved and time consuming. A bit of precision is required too to get a decent balance.

 

[JCSteam]

Thank you Wizard, there is actually two that need making.

I can see your point on getting a balanced flywheel. The vains are for a fan the flywheel is for an air compressor, and cools the compressor whilst in action. I did think that it may be an easy (ish) job however I'm realising that it is not as easy as I first thought, for the reasons that are pointed out above, issues with the metal reaching all the areas of the cavity of the mould before it begins to cool. I'm thinking that they'll have to be multiple areas for the pour, and several risers to let the air escape. Certainly a lot more to think about.

 

[Cogsy]

Multiple pour points isn't generally a good idea. Just as stopping and starting when pouring, this introduces oxidised layers into the cast material. The last thing you want is a 'layered' part with fault lines in it, especially when it's going to be rotating at a decent speed.

 

[Wizard69]

Realize that most of my experience comes from working in a Zinc die cast shop for a few years. In general the concepts are similar.

First you wouldn't want multiple pour points for reasons already pointed out above. The only way that might work is if you had more than one person pouring at a time. You would want to avoid freeze lines in the part.

This is where the problem with thin cross sections comes into play. You can have the metal freeze before the section fills completely with liquid metal. If the cavity with frozen metal fills from the other side you can get a casting defect that you might not notice immediately.

On top of all of that differential cooling of the metal between the rim and the vanes can put a lot if stress on the vanes. This creates another strength issue.

Now given all of the above i can see where making a pattern would be a nice challenge. It certainly isn't a place to make your first pattern though.

For a repair i don't have any other suggestions other than to go the cheapest route possible. If this was a model with visible flywheels you would obviously want to put more thought into it. It could take a lot of trial and error with the mold to get a presentation quality casting. So while it makes sense for a model I'm not convinced it is worth it for a repair.

Thank you Wizard, there is actually two that need making.

I can see your point on getting a balanced flywheel. The vains are for a fan the flywheel is for an air compressor, and cools the compressor whilst in action. I did think that it may be an easy (ish) job however I'm realising that it is not as easy as I first thought, for the reasons that are pointed out above, issues with the metal reaching all the areas of the cavity of the mould before it begins to cool. I'm thinking that they'll have to be multiple areas for the pour, and several risers to let the air escape. Certainly a lot more to think about.

 

[JCSteam]

Sorry for the delay in getting back to you all, been preocupied with a new job. Thanks for the info. Unfortunately it isn't a repair job, if it were I'd look to drill and peg the games, or casting. Filling gaps with JB weld and filling sanding flat.

The flywheel is absent on two models, so I was looking to possibly have two cast, but first is to make a pattern for my friend to help me cast. I can see the complexities of the pattern making process, I will have a read up on some more material before tackling this flywheel.

Again thanks to those that have offered up info and advice. It's much appreciated

 

[john_reese]

Thanks for your input all good points to consider.......mmm more reading needed. The only thing I can think of is to aline the pattern with dowel pegs, around the circumrence of the outer rim, and shape the outer rim, hub and spokes larger toward the centre line of the pattern.

The spokes appear to be angled. If so the parting line would be at the edge of the spoke so there would be no benefit to thickening the middle.

One post mentions using a solid pattern and cutting sand down to the parting line. That is sound advice. It is probably easier than making a split pattern with angled spokes.