homemade mold - guide pins?

[TRX]

I finished a mold this afternoon, except for the guide pins. It's still screwed together with the #8 screws and superglue I used to keep the halves from moving while working on it.

The mold blocks are 6061 aluminum.

Will ordinary hemispherical-head pins work without wearing out the hole in the opposite block, or do I need to use a steel wear insert of some sort?

I'd originally planned to use 1/4-28 set screws, one screw center drilled, the other pointed to match the center drill angle, but I realized there's nothing to keep them tight so they wouldn't back out.

[geargnasher]

Steel will eventually wear the aluminum, but if you use Bullplate on it and are careful closing them, it would take a long time wear the holes out, and you could always redo them if you needed to. It would be a different story on a production mould.

Pics please!

Gear

[stubshaft]

I agree with Gear, you can always bush the pins later if they show wear. But, with careful use that should be much later.

[deltaenterprizes]

Use a shorter set screw to lock the ones you use as guide pins/ I use male and female alignment pins when I make mold blocks. The pins are usually one the first steps and keep the halves together while the cavity is being formed.
Did you lathe bore the mold?

[theperfessor]

I use pressed-in steel alignment pins in my aluminum and iron molds. The female side is drilled with a 60* center drill and the male side has a 60* point that I make with a cutting tool with a straight edge set to the proper angle to form it. I drill the block holes and ream with a 5/16" reamer. The OD of the pins is cut so there is an interference fit between 0.0008" and 0.0015", any less and the pins come loose when the block comes up to the right temp for casting, any more and you risk damaging the blocks. Of course you need a lathe to do this but even with a small hobby lathe it's easy enough to do.

[TRX]

I think I'll go with theperfesser's suggestion, pressed smooth-sided pins with 60 degree angles. Same thing I was thinking of, except without the threads.

gear: pics coming soon!

delta: no, I did it on the drill press with a 27/64 drill; that's the tap hole size for a 1/2-13 thread.

It's a mold for casting lead slugs for balancing crankshafts. The price of tungsten alloy "heavy metal" has gone so high it's now economical to use lead for street or economy balance jobs.

In this case, it's an ideal practice run for a bullet mold, since the dimensions and finish of the cavity aren't particularly important.

[deltaenterprizes]

My pins are similar to the perfessor's but mine are made from 1/4'' drill rod. The first batch were too short (1/4'' long)and did not have enough surface area to stay in place. I put some knurling on them by rolling them with a file on a hard surface. The knurling increases the diameter and provided a tighter fit and stopped the movement.
The next batch of pins will be at least 1/2'' or longer.

I copied the design from Magma's molds. They want $3 per pin and you need 4 per mold, 2 male and 2 female.

I bought some from LEE that are used on the 6 cavity molds but they are odd sizes and the cost or 2 reamers to use them is not worth it.

[theperfessor]

I cut my mold blocks to be 0.700" thick from 3/4 thick stock. I mill one face flat on two blocks to make the inside surfaces of the mold halves, then I put the milled faces together and make sure they're clamped together somehow and line bore the pinholes through both blocks.

I make my pins longer than the block thickness and press them in with a cheap Harbor Freight hydraulic press. I put the female pin in first and press it in from the back until the female end of the pin is flush with the inner face. I have a flat hardened steel plate that I use under the blocks to give totally flat support when pressing in the pins. (It's hard to get accurate pin seating using the cast iron plates that usually come with cheap presses.) The block is hard enough that the pins won't harm it when I seat them.

I prep the male block by pressing in the male pins in partway, with the tip of the pin just about even with the inner face. I mill off the back of the female block to cut the block to the final 0.700" thickness which cuts the pins flush with the outside of the block. I put the female block flat on the hardened plate with the inside surface facing up. I put the male block face down on top of the female block and gently seat the male pins, they will line up pretty easy if you ease them in. Then I mill off the back of the male block to 0.700" thickness.

Having the initial set of the pins when the mold is cut is indicated by making one end of the pin flush with the outside surface can show over time of any adjustments needed to keep the mold working properly.

A couple of tips: I turn the pins to two diameters - the first 0.100" of length of the male or female end of the pins is turned about 0.002" under the hole size, the rest of the body is 0.0008" to 0.0015" over the hole size. The reduced diameter helps guide the pin straight as I press it in and gives displaced material a place to flow. Think of it as the bore-rider part of the pin.

I also put a chamfer on both ends of each pin hole. The one the outside helps start the pin straight in the hole, but I don't make it large enough to go deeper than the finished block thickness. I don't want to leave a ring on the surface around the pin after I mill it off.

The chamfer on the inside faces prevents any material that is rubbed off or upset from seating the pin has a place to go and won't upset the inside faces. It's easy to flatten off the face of the female block but much harder to dress of the male blocks as you have to go around each male pin with something to cut away the upset material. Putting in a good chamfer prevents most of this.

Maybe I'm overthinking this but doing it this way seems to give me good results.

YMMV.

[JIMinPHX]

You can see what I did here - http://castboolits.gunloads.com/showthread.php?t=45741

1/4" ground steel dowel pins work pretty well if you don't bang things around too much. I put the pins in the blocks before machining any of the critical features. That way I know that things will line up when the pins are engaged. Adding the pins later is a bit risky.

If you are real worried about the aluminum wearing, you can use bullet nose liners & pins like Lee does on their 6-cav molds.
http://www1.mscdirect.com/eCommerce/NavigationServlet/Clamping-Workholding-Positioning/Locating-Positioning-Tooling-Components/Locating-Components/Locating-Alignment-Pins/Bullet-Nose-Liners/_/N-77dn3?cm_re=Category-_-BodyLink-_-Bullet+Nose+Liners

http://www1.mscdirect.com/eCommerce/NavigationServlet/Clamping-Workholding-Positioning/Locating-Positioning-Tooling-Components/Locating-Components/Locating-Alignment-Pins/Bullet-Nose-Pins/_/N-77dn1?cm_re=Category-_-BodyLink-_-Bullet+Nose+Pins

I've had good luck with Gibraltar brand components in the past.

[Buckshot]

..............Jim, you really think Lee uses liners and bullet nose inserts? Even for a truckload they'd still be a bit pricey (I'm thinking) for Lee?

http://www.fototime.com/5F9B1C52B08A8C1/standard.jpghttp://www.fototime.com/55D15A4B5F3108F/standard.jpg

This is a slug mould I made to produce 'Proof Slugs' I use for proofing size dies. For the alignment pins and holes I found some seamless 1/4" OD stainless tube. Twenty feet of it AAMOF! It has a 0.195" hole, and whatever size ID wasn't a big deal. Between fractional, letter, number, and metric stock SOMETHING will be close. The tube is chamfered inside with a 60º centerdrill, then the outside edge is also chamfered, then parted off at 3/8" long. The end that's parted off is simply left as-is, and it's driven into the hole in the blocks from that end.

I made a short simple punch to drive in the tubes, with a nose to just fit the ID of the tube and turned the OD to .245" to clear the hole in the blocks. For the male pins, that .195" is close to 5mm @ .1969" (.197" real world :-)). The rod has a 1/4" long medium straight knurl rolled on, the nose turned to .195" and is rounded with a file. It's parted off at 1/2" counting the nose and driven into the mould block. BTW, the block (or blocks if alternating male-female pins) is set on the face of a 150W spotlamp for a half hour, and then the pins are driven in. I don't know how hot it gets but you for sure won't hold it long:wink:

I for sure think 3 alignment pins are one heckuva lot better then 2, and I'd bet do just as well as 4 of'em. You want to do a nice job of matching the pin nose OD to the ID of the insert, but they DO have to have a tad bit of freedom to fit. Since they swing together in some manner of an arc, if they're fitted too snuggly you may be able to get them to snap together but you'll have hell to pay getting them apart!

..............Buckshot

[JIMinPHX]

..............Jim, you really think Lee uses liners and bullet nose inserts? Even for a truckload they'd still be a bit pricey (I'm thinking) for Lee?


On the 6-cav molds - yep. They're small ones, but they are collar & pin inserts.

[Ben]

Rick:

you'll have hell to pay getting them apart!

That reminds me of my old granddaddy ! ! Sounds just like something he'd say.