"Machine marks" on mould block face

[Jech]

A novice's ponderings on mould surface design of Lee 6-cavity moulds...

When leementing, it's my understanding that common practice suggests taking the underside of the sprue plate and the top of the mould to a piece of lightly oiled fine-grit sandpaper over a table (or some such flat area) to true up the respective surfaces. This combined with properly tightened screws and some Bullplate seasoning makes for baby-butt smooth interaction between the plate and block face. Lead smearing and galling in the aluminum are virtually guaranteed to never occur if done properly. This all sounds great in theory but I couldn't help but wonder how this affects overall/base fillout. If both surfaces are "perfectly" smooth and properly lubricated, wouldn't that make it more difficult for the cavities to vent as the melt pushes the air out?

When I received my first Lee 6-banger, I saw "machine marks" on the bottom of the sprue plate as well as on the top of the mould. Initially I chalked this up to the much-foretold lack of finish in Lee products but in hindsight I think those were put there on purpose. They resembled the alignment grooves found on the mating surfaces of the mould halves but substantially shallower and at opposing directions.

If one used a super-fine sandpaper and put enough elbow grease into it, would it be possible to polish those surfaces "too much" and create problems? Am I on to something inherent to proper mould design here or are the are the mineral spirits getting to me?

[Bullshop]

Some molds come with vent lines on the surface like LBT and some smooth. I do not recall seeing any sprue plates with vent lines.
It is always a good idea to vent the seem at the top of the blocks where the blocks mate. It is simple and takes only a minute to do.
There are likely different ways to do it but the way I do it is to hold something tightly between the blocks to hold them apart so there is a small gap between them.
Then I lightly stroke the top edge with a three corner file so each side of the angled file is taking the sharp edge off the top of each block half. Just a wee bit will do it. Now when done when the blocks are held tightly together you should see a very slight v cut or bevel at the top edge that forms a sort of micro troth that the air can escape through. This way you can have very smooth surface interface between mold block tops and sprue plate bottom and still get good base fill.

[Maven]

Jech, You may want to confirm this with Lee Precision, but the underside of the 6 cavity sprue cutters are convex by design (less drag I think). Thus, you don't want to polish that feature away.

[GRUMPA]

After just getting done with the LEE-Menting process myself I looked really close at the sprue plate. I actually lapped mine flat as well as the top of the mold where the sprue plate rests. Mine was convex (on the 1 I lee-mented) as well as my other lee molds that I have not yet done. But when I looked at the sprue plate itself I noticed not machining marks but rather line graining from something like a belt sander for fast production to remove burrs and such, not very precise mind you but just a fast way to remove material on a productive basis.

I lapped mine on a very flat froma sheet of glass layed on a flat surface. This is probably the easiest to get, and I used 320 grit wet\dry sand paper. The results were fantastic when I cast fresh boolits, and the bases of the boolits were much flatter than before after polishing everything in, and the boolits themselves never dropped better. Just take bullshops advise and after all is said and done should have a better mold.

[evan price]

Use extreme caution with Bullshop's filing procedure. If you take more than the tiniest amount away here you will get boolits with a fin on the base. This has been a PITA in the mould I did this to.

[montana_charlie]

Use extreme caution with Bullshop's filing procedure. If you take more than the tiniest amount away here you will get boolits with a fin on the base. This has been a PITA in the mould I did this to.
Yes, you don't want to 'file a bevel' on the blocks...just 'break the corner'.
A jeweler's file takes a finer cut that the average 3-corner file, and a whetstone requires even more work before you get into trouble.

You can also just lightly do one block, and see if that is enough of a vent.
If more is needed, do the other block.

CM