Box Joints on a Snap Flask

I use snap flasks for making molds.
My flasks are generally all custom made one-off items that basically fit as tight as possible around the pattern, since I use bound sand.
A little space is allowed for things such as gates, runners, sprue, risers, etc.

I have been gluing two opposing corners of each flask side, and the other two corners are secured with wood screws.
To open the flask, I removed the screws and moved the sides apart to release the mold.

Some folks use a hinge on one corner and a latch on the opposite diagonal corner, but hinges are not cheap, nor are latches, and I don't want to keep buying hardware as I make more flasks.

I have been toying with the idea of using box joints on the flasks on two opposing corners, and gluing the other two corners.
With box joints, I can drill a hole through the joint and install a pin.
To open the flask, all I would have to do is pull out one pin.

I don't like making hardware for flasks, and so I don't use any.
I just drill a hole through the two ends of the flask vertically, and use rods that protrude from the cope into the drag.
Quick and easy, and I don't have to fabricate metal hardware, or install the hardware on the flask, or try to get the pin holes aligned exactly.
Another benefit of using pins versus hardware is that I can turn the flask up on any outside surface and stand it on end, which is often done when you are opening the mold and removing the pattern.

Previously I have been able to make a custom wood flask in about 10 minutes, and I did not want an elaborate setup to make super precision box joints, but rather something I could set up and build quickly, and something that would cut box joints quickly with consistent accuracy.

This is what I came up with.
I bought a dado blade set today and installed it on the table saw tonight.
I had to make a new throat plate, but I like a wood one where I can raise the blade through it and get a perfect hole with no gaps.

The cuts are spaced using one 1/2" spacer, and three 1" spacers.
I made a sample piece, and was able to cut the ends of two 2x4's quickly just by swapping in the appropriate spacer(s) on the side adjacent to the fence.
I drilled a hole for the hinge, and the joint was quite accurate, and hinged open easily.

The cuts into the wood are are circular in shape, not square, but that does not affect the joint as far as using it for a flask, and does not affect how it hinges.

I am very pleased with my setup and how fast it can make box joints.
Now I need to make a few flasks for various things I need to cast in the near future (bearing caps, windmill gears, etc).

 

Nice! That looks like it's working well. You get the blade to the same height each time and you'll get consistent cuts.

If you were to build a sled you can easily cut the fingers all the way through by holding the work vertically and safely push it through but what you're doing gives you a tight inside joint with enough relief to allow it to hinge.

 

I considered building a rig to make the cut straight (ie: the piece held vertically), but then I would have to sand the corner to allow the joint to hinge open, and just by chance I tried it this way, and it automatically had the required relief.
I accidentally got a working solution; not the first time that has happened, but I am glad it did.
I will take success any way I can.
Less is more as they say, and this is a really simple way to do it, and it works well.
As far as I am concerned, low tech is high tech if it works well.

.

 

Looks great. I think you have the joints figured out. My flasks are always temporary, cause I usually burn them up. I should use snap flasks.

 

I was burning up flasks, and decided there must be a better way.
I changed to bound sand, and I cement the cope to the drag, and also use a few weights on top since otherwise sometimes the cement joint will fail.

If I were using greensand, I would use a steel slip jacket around the mold after the flask was removed, which is what is done in industry.

Edit:
The taper that can be found on some commercial flask walls is created by the draft angle on the walls of the snap flasks that they use, and the taper that ends up on the outside of the bare sand mold allows the slip jacket to be easily installed over the entire mold, even if the slip jacket is one-piece (which they are one-piece).
The slip jacket has the same taper on it (I assume).

.

 

I may be causing some drifting from the focus of this thread, but Pat’s mention of box joints got me thinking about describing a sled jig I have used for box joints. However I thought I’d try to find s video already made. Here is one that does a much much better job than I could hope to do and taught me a couple new tricks.



Denis

 

Another guy with $10,ooo worth of tools to make a $20 box.

You can never have too many tools, just too little space and/or money. With me it's both.

 

That is a great video for making box joints for boxes.

Note that if you want to hinge the box open at a corner (ie: like when making a snap flask), the inner edge of the wood would need to be sanded round, else you would break the hinge joint trying to open it.

Also note that he does not try to center the the cuts across the board, and so you can end up with a very thin finger on one side, which is not a problem if you are going to glue it all together, but would be a problem with a flask since the thin finger will break off, since it will not be glued.

I wanted a method that automatically would center the fingers across 1.5", 2", 2.5", 3" and 3.5" tall flask halves (these are the standard flask half heights I use), and a method was was really hard to screw up when I am working late at night and an exhausted (a frequent occurrence for me).
So as long as my fence is 3" clear from the edge of my blade, and that does not have to be perfectly exact, then it is just a matter of using the right spacer or spacers adjacent to the fence, and with 1/2" fingers, it is immediately visually obvious whether you have the right spacer(s) or not.

And for flask hinging, the relief is already in the cut if you make the cut with the wood flat on the saw table.
Not the most elegant solution, but it is a highly functional and fairly accurate system that is fast to use, and difficult to screw up no matter what the height of the flask side you are using (as long as you use the standard sizes I mention above).

And last night I just manually drilled the hole for the hinge pin, and did not use a drill press, and the joint still works perfectly with no gaps when closed.

And I manually drill holes for the two different pins (rods) on either side of the flask (so the cope and drag cannot be accidentally reversed, generally 1/4" and 3/8" rods with an "L" bent on the top), and the fact that the holes may not be exactly vertical also does not effect the function of the flask in any way.
My pins are perhaps 12" long, and it is just a matter of inserting them fully in the cope part of the flask, then putting the ends of the pins against the holes in the drag, and then aligning the pins with the holes in the drag and sliding the cope down on top of the drag.
It works perfectly even with not very accurate hand-drilled pin holes through the wood.

No metal hardware to make, install and align, and no metal hardware to get knocked out of alignment.

Its so simple and fast to make, and works so well.
My motto is "Simple, fast, easy, highly functional".
As Bob Puhakka said "Non sunt multiplicanda entia sine necessitate".
.

 

In fairness to to the box joint video maker’s rather ingenious (in my opinion) method of making box joints and his unusually clear explanation of his method:

1) If you need the joint to open up a bit with the pin in place, just set the blade a 32nd high. That will provide plenty of clearance for it to open 10 degrees or more. And no appreciable greensand leakage will occur through that gap. Or, sand a partial radius or just a slight angle on the end of the pins.

2) It is pretty cool that he shows an amazingly simple and foolproof method of ensuring that the pins will register perfectly regardless of of the board width (flask height) chosen. If the maker needs to avoid a thin finger, all he has to do is choose an even multiple of finger width for board width. So, for 1/2” fingers, use 1, 1.5, 2, 2.5 etc width boards—-no thin fingers.

3). Once the jig is made there is no need to juggle stacked spacing boards. With the sled you can whip out a 3-finger or 10-finger board in short order.

4) If a person settles on, say, a 1/2” joint, once the jig is tweaked the first time to cut 1/2” fingers, it will hang on the wall ready to go at a moment’s notice.

5) Expensive tools needed? Not really. He uses a 25 dollar caliper (a convenience I’d hate to be without) but you could go by feel. He uses a table saw—yes. Hard to do by hand. The sled is shop made. The blade height gage is not needed.

Denis

 

I have watched several box joint videos, and that guy's is by far the best, and he has some really slick tricks too.

No doubt I can get a more accurate joint using a sled, but I don't need an accurate joint, and feeding the stock in horizontally by hand is faster than vertically clamping what could be an awkwardly long piece of wood.

I would like to make accurate box joints for boxes, and so if I get time (doubtful this year), then I am going to make a sled using his method.
I would really like to make cigar boxes with box joints, but I think a gang saw would make quicker work of that small of a joint.

I do use the digital verier calipers, and those are worth their weight in gold, but if you want true accuracy in lumber thickness, such as in planing operations, use a micrometer, because the calipers are not generally as accurate as a micrometer.

All good points you make though.

Edit:
Having open cracks at the joints would most likely not work well with the fine bound sand that I use because I think it would work into those cracks, harden, and then every crack would have to be cleaned every time I used the flask.

.

 

I agree Pat your method is spot on for what you're doing. I've used a sled for years and like it, but the joint you're cutting flat is great.

However I believe calipers are best for getting an accurate reading on wood. It's pretty easy to dent wood with a micrometer, and the expansion and contraction of wood makes the increased accuracy insignificant. I stick with micrometers for metal ft.

 

Pat, by all means make the joint in the way easiest for you.

FWIW There is no need to clamp work when using a sled. The exception being round stock. If you watch the video you will see the maker just positions and holds with his hands. I have made thousands of cuts that way.

If you run into any trouble with bound sand working it’s way into joints and causing sticking, a spritz of silicone spray will prevent sticking. That is what I apply to my 18” prism pattern to make the core. It causes very easy release

https://www.amazon.com/CRC-03300-Silicone-Release-Aerosol/dp/B0013IZSDM



I find a lot of uses for it in the shop/Foundry.

And, though I agree that my Mitutoyo mics can measure metals more accurately than my Mit calipers, I can’t hold .001 tolerances in wood anyway. So the calipers provide as much accuracy as I can use. They are good to a thou. And if I were making box joints for a flask, .005 would be plenty close.

In my day-to-day for flasks, if ever chose to make a box joint, I would probably choose a very coarse box joint with maybe just 2 fingers on each piece and would probably just use a sled and cut to a line rather than fiddle with a jig. Sure would be nice to have a few sets of metal snap flasks.

Denis

 

That is a very good idea.
I am going to try that.

In the hardwood lumber business, if you send out $100,000.00 worth of lumber that is 0.001" too thin, then they send it back, and you have to buy it, so you use a micrometer.
If you plane it too thick, and the customer is understanding, they will let you re-plane it to the correct thickness.
For me, a vernier caliper is fine, but the first steam engine I tried to make I used a vernier caliper, and that is when I found out that the $20.00 versions are not that accurate.
I can remember very well saying "gosh, I measured these pieces exactly with these new calipers, and nothing fits". A good caliper is accurate but also expensive, and don't drop or abuse it. Another problem I remember having is that the zero point would drift on inexpensive calipers sometimes, and I learned to close them and make sure they read zero frequently.

I made some nice 3D patterns for aluminum snap flasks, and then got into the bound sand mold thing, and so I never cast them.

Edit:
Someone here (the other site maybe?) had a nice set of flask patterns, and they were circulating around between members. Here is a photo. I keep forgetting this guy's name but I recognize his furnace setup.




Here are the patterns that were circulating.
Not exactly how I would make them, but very nice patterns for sure.






And the unfinished 3D models I made.
I would not make flask sides like these either.
These were made before I really had made a lot of molds, and so I would make some changes to this design.

 

The metal snap flasks you showed look very nice. So much to do, so little time!

Denis

 

Its almost enough to make me go back and use greensand, but I am spoiled on bound sand, and alas I will never go back, so almost every flask I make is custom sized to minimize sand, and of a quick and dirty design.

That cast flask looks downright elegant though.
I think I am going to 3D print some miniatures and cast them, perhaps 4" x 4".
David could use them for his steel melter machine.

.

 

I'm sort of going the other direction size-wise. My next set of 3-box flasks will be 52" X 12" or so. Wood with simple rabbet joints.

Denis

 

I made these snap flasks so I could bake the linseed oil bonded cores for the Indian heads. They were sized to be able to fit in the toaster oven and only needed two patterns. I just machined the hinge side for a single tongue and held it closed with a wingnut on a stud.

 

That is nice clean design.
And a clamp or vice-grip could be used instead of the wing nut/bolt for quick release.

I am thinking there must be a better way to do what that big machine does.

 

It had to fit in the oven to bake. For your airset you aren't limited to overall size.