Author Topic: Touch Hole Size? (Read 1158 times)

Little Guy · « **on:** June 21, 2006, 02:23:51 AM »

I have been looking but couldn't find what the Dia. should be for the touch hole. I am going to be using 3mm fuse and have herd that i should drill a hole slightly larger but am not sure how much larger and why to do this.
Here are some immages of my Beercan Mortar. It is 303ss. (hope they come out)

Cat Whisperer · « **Reply #1 on:** June 21, 2006, 03:49:31 AM »

Slightly larger is right.

The principles:

Larger means more pressure lost.

Some clearance is needed to allow fuse to be inserted and to blow free.

Too small doesn't allow for crud build up, may hold remains of (glowing) fuse in place.

I'd try 3.5 to 4mm to start with, you can always go larger later if desired.

Little Guy · « **Reply #2 on:** June 21, 2006, 05:11:25 AM »

Thanks Cat Whisperer

Double D · « **Reply #3 on:** June 21, 2006, 07:12:30 AM »

The only rule of thumb I have ever seen on this is that the fuse hole should be 1 1/2 calibers, that is half again the dimaeter of the fuse.

Cat Whisperer · « **Reply #4 on:** June 21, 2006, 11:34:29 AM »

I hadn't heard that, but it seems quite reasonable.

Double D · « **Reply #5 on:** June 21, 2006, 12:37:41 PM »

I have trying all day to remember where I saw that. I do know that I have posted it before. Fuse hole 1 1/2 caliber. It's like mortars the bore section for the ball is supposed to be 2 calibers. It's from some refernce.

Does anyone have any idea where I got this from?

Rickk · « **Reply #6 on:** June 22, 2006, 04:56:58 AM »

Probably the most commonly available fuse is 1/8 inch, which is .125 or about 3mm.

The wall thickness of your barrel (per the drawing you put in the other thread about FFFg powder is really thick at the fuse hole due to the sub-chamber. The drawing you posted said ".125" for the hole. If you drilled it to that and tried to get .125" fuse thru that much hole it would not go even if the hole was clean. It would only get worst with crud buildup.

You want the hole to be somewhat loose (no resistance). That way you can actually feel when the fuse enters the powder charge by the change in resistance.

The 1 1/2 calibers that Double-D mentioned is about .200 inches for 1/8 inch (3mm) fuse. I made that size hole in my mortar barrel. I have only fired it a few times to date (the base is not finished yet) but the fuse slides in nicely. The vent pick I made is made of 3/16 brass rod and it fits nicely. The vent pick is pointy. I made a second pick with a squared end that is good for scraping crud out of the vent. If you make the hole less than .190 - .200 you will not be able to make tools out of 3/16 brass rod, so you will have to find or make something smaller.

A .22 caliber cleaning brush works good for cleaning the .200 hole at the end of the day It will be tight at first... a worn out brush fits even better. If you shoot a .22, save the worn out brushes.

Little Guy · « **Reply #7 on:** June 22, 2006, 05:34:51 AM »

Thanks for the info.
Rickk: what do you use for BP in your mortar, fG or ffG?

Rickk · « **Reply #8 on:** June 22, 2006, 06:59:54 AM »

1F "skirmish powder" (from "powderinc.con) for blanks and Goex cannon grade for live rounds. My mortar is 2.25 inch bore and has no powder subchamber. You may be able to get my with 1F for live rounds... I don't know for sure. Others with subchamber sizes close to yours would know better.