Plain-basing a gas check mold.

[PBSmith]

I've read posts here by Ben and others about plain-basing a mold using a drill press and a twist drill. Tried it myself on a cheap aluminum mold with mixed results, holding the mold in a drill press vise bolted securely to the table of the press, slowest possible speed. I took great pains to center the mold under the chuck, but nonetheless the results were not always as precise as I hoped for.

Now I'd like to try this with other molds (aluminum as well as iron) but would like to see better results.

These are .30 caliber molds. A 5/16" drill is perfect for the finish diameter of 0.3125".

Heard about a 90-degree point drill in 5/16" that might work more smoothly than a common twist drill. Seeing how I have several molds to plain-base, the cost of the drill isn't bad. Am I correct in believing the 90-degree point drill will make the job easier? I realize I'll still have to center the mold carefully under the chuck and bolt everything down.

Alternatively, I see a straight fluted 5/16" reamer is not much more expensive that a 90-degree drill. However I don't know if simply holding or clamping the mold halves around the reamer is going to work, even at slow hand twisting of the reamer.

Look forward to your thoughts on this project. Open to other methods such as emery cloth on a round-shanked tool or dowel or drift punch of suitable diameter. Needless to say, I do not have use of a complete machine shop. Something tells me that shipping and special custom work by a machine shop or mold maker is going to cost close to what I paid for one good mold.

Look forward to your thoughts on this project.

PBSmith

[country gent]

I would go with the hand reamer and a stop collar, set the stop collar to depth on the reamer clamp the blocks together and slowly ream to depth using plenty of oil.

The steel ( normally a mehanite) moulds by hand will go slow but can be done.
They might require a light lapping to cleanup and get a good finish

[country gent]

A reamer with an odd number of flutes 5, 7, 9 wont grab as bad. 2, 4, 6 will have flutes straight across from each other wanting to grab on the seam

[dverna]

A reamer with an odd number of flutes 5, 7, 9 wont grab as bad. 2, 4, 6 will have flutes straight across from each other wanting to grab on the seam

You always give great advice.

[john.k]

Ive done all my cast iron moulds ...Using a lathe,its simple to chuck the square block ,centre the cavity and skim off the gas check ........I havent done any Lee moulds yet ,due to removal of the handles............To use a reamer ,first you must have a reamer in the correct diameter .........I think a reamer in a drill press would be a qucik way to ruin a mold.

[deltaenterprizes]

Ive done all my cast iron moulds ...Using a lathe,its simple to chuck the square block ,centre the cavity and skim off the gas check ........I havent done any Lee moulds yet ,due to removal of the handles............To use a reamer ,first you must have a reamer in the correct diameter .........I think a reamer in a drill press would be a qucik way to ruin a mold.

Amen!
Precision work on precision equipment needs precision tooling!
A mold was made with precision tooling on precision equipment and is a precision item.

[Maven]

What delta enterprises said! However, I have a Lee .30cal. "soup can" mould that Ben converted to plain base via a drill press and it is perfect and accurate. Not to be argumentative, but many GC designs shoot quite accurately without the gas check, albeit at lower velocities. That may save you some time and investment, etc.

[MT Gianni]

I have done several with a drill press that I know is mostly straight and a good machinist vise. Set up is three times as long as the drilling.