Pattern Making for the Mechanically Challenged

I received the printed coin patterns from DavidF last week and made an attempt to cast a coin today.
The patterns look really great:


The D coin came slightly damaged. I'll try wood filler or something to see if I can smooth it out. Today though I focused on the left coin: the P&A.



I'm somewhat proud of the gating system, and it worked great. I think I set it up properly, but please let me know if there's something I did wrong.



You see the problem though - the two halves got misaligned. I would like to add locating pins to these, unless that's a bad idea for some reason. What % infill are the patterns, David?

Besides that though they came out nice, minus a few defects. I suspect the pouring temperature wasn't hot enough.



Why did I bother cutting it out if it was misaligned, you ask? I didn't notice until I was halfway through grinding the edges

 

Looks good! Some velcro stuck on the backs of each half might work, just a thought.
But still nice work!
Patrick

 

Scientist,

Do you think alignment may be hard to maintain in the future? Or was this just a simple oversight?

Your gating and riser system worked well. Is a riser needed for such a thin bronze casting? I suspect it would not be needed for iron.

Your coins are going to be very cool.

Denis

 

While this works, it's not really the correct method for casting these. You turn out a ton of them in wax and investment cast them on a sprue tree. I saw the guys in the alley in Turkey years ago making squadron coins. They probably die cast them now. Always wondered how they did the paintwork. More colors, more money.

Military guys know what happens when you drop a coin in a bar. You can see the look on the poor saps face that didnt have theirs on them from a mile away. At 50cents a beer, it's not bad, but in Denmark, you'll drop your pay check buying the entire bar a round.

 

Sure, but there's two problems there: 1) I don't need 100 copies of a custom-designed coin for a couple's anniversary, and 2) I don't have the experience or materials to do wax or investment casting. I'd certainly like to try investment casting at some point, though. That strategy would be great for the coin on the right, where I would actually like a bunch of them.

Melterskelter, I'm not sure on alignment. It must have shifted when I was ramming it up, so maybe I'll get lucky next time? I could maybe rough up the backs of the halves so they don't slide as easily. I think I'll try that as a "less invasive" first measure.
The riser probably isn't necessary. You can see it didn't shrink at all. I had it mainly as an air outlet and a check so I know when the mold is full. I'm certainly up for suggestions on ways to improve with my current setup.

 

To ensure alignment both rotationally and coincidentally,
I would consider (I know ugggh) just drilling two small holes through the male half and 3/4 of the way through the female side carefully choosing location while they are secured in alignment. I think I would use 1/8" dia pins for this part. Then cyano glue a 1/8" metal pin in each hole with the pin protruding just enough to engage the female side of the pair. I'd probably have the male pin just below the level of the surface on the male side and fill that depression with Bondo and smooth it off nicely. It is important to not have the aspect ratio of the two pins length/width more than 1.5 . If you use long skinny pins, the pins will tend to bind in the holes and the halves won't separate easily.

Another alignment method would be to use small pancake neodymium magnets. This would not solve rotation alignment unless you got the special split-half polarized magnets. The correct term for that polarization is diametric polarization.

I would be less attracted (so to speak) to tapes and velcro as they occupy space between the halves and separate them.

Denis

 

Wow, that resin must be really weak.

One other strategy that does not require cutting down a part line would be first glue the two halves together in proper rotational alignment. Thento use a follow board that is drilled to the diameter of the coin and equal to 1/2 it’s thickness. Place the follow board around the coin and ram the drag. Flip, Remove follow board and ram the cope.

Denis

 

Also, thinking back to when I was casting windmill gears from printed resin patterns, I requested that bosses of solid resin be included in the pattern for use in placing pins. That provided toughened areas of the pattern that would take and hold pins. So, that might also be a consideration for future pattern printing.

Denis

 

I guess I was thinking of the follow board with my milling machine hat on. I’d just mill to an appropriate thickness a 4” square piece of thin Baltic birch plywood or aluminum and bore a hole in it just larger than the coin. That would take ten to 15 minutes and would make quick, repeatable, and precise work of establishing a parting line. Using it over and over would quickly pay off in time saved.

There is also the question of drawing the pattern from the sand. I’d probably use masking tape using a 3” piece with a bit of each end stuck to the pattern and the remaining tape pinched together to form an upside down T. The arms of the T would be stuck to the coin. Might use two such tape handles on a coin if needed.

Denis

 

Looping back to the problem of shift that occurred: My guess is you placed the 2nd piece well centered on the first, but it did, indeed, shift after you riddled on some sand and poured on some backing sand and then started ramming. This can happen easily and shifting of pattern pieces can be minimized by first riddling on some sand and then, using your fingers, tapping down that sand around the pattern piece. You don’t have to do a lot of tapping—- in this case, maybe a 4 to six bumps with your five finger tips in close proximity to the pattern. That will sort lock down the pattern so it no free to easily displace laterally. Then add backing sand and use some discretion with early rammer blows gradually increasing as the sand starts to become cohesive. Maybe you already did all that. Sorry then for rattling on.

Denis

 

I like your all-in-one idea.

Let’s do a little more cat skinning: A criticism of the follow board or the all-in-one backer/follow board would be cutting the needed hole on-size requires at least a bit adjustable in cutting diameter. And cutting a flat-bottom hole to depth could be challenging even if you have an adjustable cutter. (But not if you have a combination boring and facing head like mine)

If you lack such an adjustable cutter and drill press, make the combo backing and follow board using Bondo. First cut a 3/4 or 1 inch hole in the center of the backing board. Next apply silicone release spray and generous paste wax (belt and suspenders) to the pattern and center it over the hole. Trace and cut out a hole in a plastic yogurt container top just a little oversized for the pattern. Put a few spacer bits of wood equal to half the pattern thickness near but not touching the pattern. Butter the plastic piece with Bondo and press it down over the pattern causing Bondo to ooze up around and in contact with the pattern. Wait 3 or 4 minutes until the Bondo gets to a leathery stage and remove the plastic piece and pick off stray bits of Bondo around the pattern. Wait until the Bondo is semi hard and use your finger to push through the hole and pop out the pattern. Clean up the excess Bondo and now you have an all-in-one.

I wonder how many other ways there are to get the job done.

Denis

 

No problem at all, I love information. This is, in fact, exactly what I did the other night. The only additional thing I did was rough up the backs of each half with coarse sandpaper, to increase friction. I haven't had time to work on it much, but the alignment looks great this time.

 

So you used the tapping the sand method the first time or the second time or both? Your second go looks like the alignment is spot on. I too have found that, with some care, I can match pattern halves and tap the sand into place with good results. I must say, in general, I trust a mechanical alignment system more than my skill and attention using the tap-tap method. ;-)

Thanks for posting.

Denis