An oddball problem,,,,,

[contender1]

Ok, I have spent a few hours scouring this place for an answer to a question about a mold problem I'm having. No luck,, so I'm asking the question.

I have a Miha brass, 4 cavity mold. The sprue plate is giving me problems. I'm getting longer bases on my bullets, due to the sprue plate being slightly off the mold. I can hold my mold up to a light & see a gap between the mold & plate. It's higher at the far end, and tight at the screw end. The front cavity is the worst, while the back one is fine.
Now,, my first thought was I had a spring washer issue.
Nope,, installed a new washer, same problem. I've swapped around all the parts, including a different sprue plate, and I still get the same gap.
Here is the weird part.
I can remove or loosen the screw to where the spring washer is not under any pressure, and the sprue plate sit perfectly flat.
It's ONLY when I screw it down does the opposite end pull up. And it does it with different sprue plates.
I can hold my mold, screw it down, and then with my finger, squeeze the plate at the front & it will easily go flat, where it's supposed to be.

So,, my question.

Has anybody else experienced this, and if so, how did you fix it?

[tward]

Perhaps a warped sprue plate? I had a similar problem with a NOE mold, laid it on a flat tile with a 400 wet and dry sheet stuck to it and worked it in a figure eight pattern til flat. Hope this helps, Tim

[jcren]

Sounds like a worn "dip" in the mold where the sprue plate pivots on its screw. Take off the plate and check the mold surface with a good straight-edge.

[fast ronnie]

I'm kind of thinking like jcren. Maybe the top of the mold isn't flat, or could be the sprue plate has bent or warped next to the screw. Just my 2 cents.

[contender1]

jcren, you may be on to something. I will check that.
I didn't think about that, as the sprue plate has a larger hole than the screw, and a steel collar. Maybe the collar has turned enough to where it has worn the brass down in that spot.
I will check & see things & report back!

If it IS the problem,,, what's a good fix?

[runfiverun]

that's not all that un common.
loosen the screw just enough for the plate to sit flat on the mold.
now tighten the set-screw.

leave it alone.
cast with it.
leave it alone.

[dragon813gt]

Perhaps a warped sprue plate? I had a similar problem with a NOE mold, laid it on a flat tile with a 400 wet and dry sheet stuck to it and worked it in a figure eight pattern til flat. Hope this helps, Tim

I highly doubt it w/ the ones Miha uses. Now NOE ones are different. I stone every one to remove burrs and insure they are flat. Miha's require no work at all. Like R5R said, loosen the screw until it lies flat and cast away.

[contender1]

Well, jcren was correct. I pulled it all apart again, and under a 10x magnifier,, I can see the groove created by the collar for the sprue plate.
This one is just gotten bad enough to where we may look at a custom repair.
But,,, I will be trying to see if I can do what dragon813gt has suggested.

MANY thanks to all who offered thoughts & help!

[mozeppa]

a bent screw ...even ever so slightly will lower or raise the opposite end of the sprue plate a bit.

has it been dropped to the floor?

[contender1]

The screw is fine. Re-read the part where I swapped out ALL the hardware,,, including the sprue plate. No warped plate,, no bent screw, no warped collar,,, etc.
And no,,, the mold has never been dropped.
I did find brass wear where the collar for the sprue plate moves & all.

[JWT]

Pictures?

[country gent]

Does this mould use a screw or pin for the sprue plate stop? an unsquare or worn edge on the sprue plate where it hits the stop pin, or a stop screw with a worn head can cause the plate to rise also. You stated when sprue pivot screw was loose it sat flat and correctly to the blocks. This indicates plate and blocks are probably flat and true. Is the screw pivot screw free in its hole and not binding on it? Look for wear on the screw heads mating surface ot the sprue plate. You might try lightly polishing the top of the blocks on a very fine piece of sandpaper ( 800 grit) wet with a light oil to show any hich spots. You can try to lightly polish the under side of the sprue plate in the same way. Check both the plate and tops of blocks with a straight edge like a good 6" metal ruller and make sure both are flat. If a good precission square is available check tops of blocks to mating surfaces of mould faces. Just because tey are flat dosnt mean they are square. Also if a shoulder bolt is used for pivot screw look where it seats to blocks for uneven wear as this will put it out of square also.

[jcren]

I don't have brass molds but I have "resurfaced" aluminum molds on diamond knife sharpening blocks. Adhesive sand paper on a piece of glass works as well. Of course a friendly neighborhood mill owner could take of .010 or so in a few minutes with little or no appreciable change to the cast.

[contender1]

country gent,,, this is a Miha brass mold. It uses a screw, with a spring washer under the head to contact the sprue plate. The sprue plate has a larger diameter hole to allow for a steel collar insert. The steel collar is taller than the thickness of the sprue plate, and that is where the spring washer does it's work. There are no marks on the sprue plate, but the brass under the steel collar is grooved,, where the collar has dug into the brass. There is also a set screw in the side of the mold, which is used to lock the screw to prevent over tightening of it.
My theory is that I over tightened the screw,,, not realizing the set screw was loose. It was,,, and I didn't realize it.
I do have a precision square & access to a machine shop. Once jcren posted his idea,,, I felt a bit like a dummy for not seeing the obvious issue.

Now,, my plan is this;
Since the only worn & damaged area is under the collar & screw,,, we are going to use a stainless steel washer, to fix a place for the collar to pivot on, w/o damage to the brass anymore. We'll measure the thickness of the washer, then mill off that identical thickness of the brass in a similar diameter of the washer. Place the washer in the spot, with a little adhesive, allow to set, then hit that area with a surface grinder to true it all up. If we remove any material at all from the top of the mold,,, it will be uniform & complete.
Once the area is true, I'll re-install the hardware & see how it does.

[runfiverun]

then set the plate flat on the mold and tighten the set screw.

then leave it alone
and cast with it
and leave it alone

I like the idea of an inset SS washer.

[country gent]

I would do a light press fit on the washer as adhesives may not hold under the moulds temp when casting. Vut the poocket in the mould on location and to size turn the washer to dia + .0005 -+.001 and press into place. lso check closely to make sure top and bottm of blocks are parrallel. You may have to set it up in a vise and indicate top in to flat to blend in the washer to blocks.

[jcren]

Purple lock-tite is good to automotive exhausts temps, have used it for nusance headset bolts. Great idea on the wear pad!

[Hickory]

then set the plate flat on the mold and tighten the set screw.

then leave it alone
and cast with it
and leave it alone

I like the idea of an inset SS washer.

I've had two moulds that gave me this problem and this has been the fix. Good advice.

[contender1]

"then set the plate flat on the mold and tighten the set screw.

then leave it alone
and cast with it
and leave it alone"

I actually tried this yesterday. However,,, due to the damage,,, the sprue plate still didn't set down good enough to seal. I had overflow on the top of the mold. Milling & adding a washer is going to be the answer.

Any of my other brass molds WILL get this treatment,,, & hopefully, no more will suffer the same fate.