bullet mould alignment pins...?

[krems]

I'v made a number of bullet moulds for myself with a lathe and milling machine over the years. I noticed that a number of custom makers (Brooks, Paul Jones) use three alignment pins instead of two on their lathe bored single cavity iron moulds. Now i've been thinking about making a mould with 3 alignment pins and plan on using aluminum with matching stainless steel pins / bushing. For those who have done this or something similar what is the secret to getting all three pins / bushings to line up correctly. Getting two pins to line up correctly is an easy process but throw that third one in and things get tricky. There must be a simple way to do this with some precision. Any thoughts!!

Is there any reason for using three alignment pins verses two...

Looking forward to the discussion!...............Krems

[BABore]

I've worked in the automotive welding and check fixture business for almost 30 years. Locating an object with two pins, one being a 4-way locator, and the other being a 2-way locator, is how it's done. Check fixtures commonly use a spring loaded cone, 4-way locator, to eliminate hole variation. Not suitable on a mold and your always presented the same hole. There is no benefit to going to more than two pins unless it's the grief factor. If the mold should be designed whereas the pin shanks and/or the bushing are slightly proud of the mold blocks themselves, then you would benefit from a third location. This would established a 3-point deck surface so the mold would always close against the same three points. This is commonly done on tooling to eliminate surface flatness variation. In this case the third pin would be undersized so as not to interfere with the 4-way and 2-way pins.

[ohland]

This is commonly done on tooling to eliminate surface flatness variation. In this case the third pin would be undersized so as not to interfere with the 4-way and 2-way pins.

I am reporting empirical confirmation of BABore's thought. I have two locating pins in an RCBS dual cavity mould. They are both in the same plane, but one is just a scosh higher, and the mould binds at the front when you try to open it.

My though - a rounded head pin at back, but either a bullet point or a diamond pin up front. If the height of the two pins is too much, then they need to be brought together at a straight angle. Nice if the blocks are in your hand, but they usually are on handles when you cast.

O >

A round pin with a prism or bullet point.

My RCBS mould is hanging up with the front pin, both are rounded head pins, but their heights are different. Both pins are in line, so any difference in height is important
Inner pin: .140"
Outer pin: .153"

The old Ideal single cavity mould has a pin at one top corner, the other at the opposing bottom corner. Locks up tight, almost falls open, casts beautiful bullets.
Damn mould hangs on the outer pin.

:coffeecom

[BABore]

The round nose on the pin is just clearance and to help guide the pin into the bushing or hole. It's the round, cylindrical portion behind the round nose that's doing the locating. Technically, the outer pin, or one farthest from the handle hinge could be out a little farther than the inner pin because of the swing radius. It is not necessary though. As long as the pins cylindrical portion is just in the bushing a few thousandths when the blocks are closed, you are located as best as you can be. Any wear or chamfer on the hole or bushing has to be compensated for by extending the pin farther out of the block. If you think your pin is extended out to far and making it hard to open/close, drive it in a bit and check for any movement when the blocks are held tightly together. You can always drive it back out if needed.

Whether the two alignment pins are in line, in the middle of the block set, or on opposite corners, it doesn't matter other than the distance between centers. Two points make a straight line and rock will occur in either case if something is proud of the block face. The longer distance between centers will reduce movement should pin to bushing fit is a little off, as else being equal.

The biggest grief caused by most pin/bushing fit is when the maker does not spot face the locations or have a decent chamfer around them. Burrs from installation or lead particles build up around them and keep the blocks from closing properly.

[Catshooter]

Wow, BA, that was very clear and consise. You increased my understanding of pins and their function quite a bit. Thanks.


Cat

[krems]

I've been able to get a good fit with two pins as BABore mentioned. Ive been setting the two pins furthest from each other first. I then clamp the blocks together and final ream the third hole. When I go and set the third pin / bushing in I can never set the pin to the same depth as the other two, If I did I could never get the blocks to close completely. I can make the blocks work but as BABore mention the third pin is a looser fit and really serves little use. This tells me that either the pins / bushings are being driven in slightly off center, or when the .0005"+/- interference fit of the pins/bushings is not the same on both block halves causing a potentially .001" off center alignment.

Just for grins I'll check my black powder custom moulds by Brooks and Paul Jones to see if all three pins have the same type of engagement.

Thanks.................Krems