Location of porting hole in Piston tuba

[LibraryMark]

Hi all -

I've got a VMI 3302 (4P1R) BBb that I wanted a a shop to vent the valves among some other work. I got a quote today from them and it was $40/piston. Too rich for me right now so I told them to skip that.

Now I uses to be a reasonably adept machinist in a (very) past life, and have a decently equipped hobbyist shop. I'm pretty sure I could pull off the operation of venting the valves myself. The issues is how do you locate the hole and how big is it? I get that you can't go too deep and you can't leave any burrs.

Thanks -

Mark

[Rick Denney]

Disclaimer/warning: These are your valves. If any little thing goes wrong, you'll have a much bigger problem than forty bucks. It's a mistake you will have to own.

The port is a hole in the piston wall in the center of an opening into the valve branch. When the piston is up, the opening vents the valve branch out the bottom vent opening in the piston. When down, it vents nothing.

The size isn't that important. I've seen them anywhere from maybe 1/16" (1.5mm) to as large as maybe 3/16" (5mm). I suspect smaller is better in terms of risk.

I would not use a twist drill, which will walk and can also grab the piston and throw it across the room. Twist drills cut aggressively--when they break through they can suck the piece into the bit which will for sure break into a port knuckle in the piston. I would probably use a small center drill and let the center cone do just a bit of countersinking to get rid of the burr. Some time spent on building a reliable fixture wouldn't be wasted. And if I wanted a round hole, I would probably clamp a drill in a pin vise and cut the hole manually--for that I'd consider using a twist drill. Stay away from the usual pre-drilling punch. Instead, file a small flat on the piston--no bigger than the intended countersink--to avoid bit walk. This is just my own mechanical common sense speaking--I've had valves vented as part of other work done by professionals for the most part.

The last time I did this myself--30 years ago--I use a Dremel cut-off wheel and made a short lengthwise slot instead of a hole, simply because it was a process I was confident I could control. Worked (and still works) fine.

Rick "who wouldn't dare use a power drill for this" Denney

[bort2.0]

I'd gladly pay $40 per valve to NOT do this myself.

[bloke]

Rick's tips are (extra-)good/valid.

If your drill press has a stop, it might be good to set it on something like 2mm (??)

(I would not want to be drilling into the cross port tube of a piston of (even) a double-belled India sousaphone of real pure brass.

If not, at least put a piece of tape on the drill bit (really close to the tip).

Horn fixers have bent/trashed pistons...We're too lazy to throw stuff away.

Go fetch one and practice on it.

I WILL admit to using a spring-loaded punch, but that's AFTER I rough-up the little area that's going to disappear, and after I put a felt-tip pen dot where the center of the hole needs to be. I have several of those punches. I only use the old small weak-ass one for doing this. I want to make sure that the dent made by the punch (if larger than I anticipated) will still completely disappear, once the hole is drilled.

I drill pretty darn generously-large holes. Tubas are big, their bore sizes are big, slides are pulled quickly, and it's nice to be able to release (six, several more...?) cubic inches of compression or expansion semi-instantly. (I've had to drill multiples of those tiny rotor casing vent holes - vertically oriented obviously, particularly when a spring-return device was involved.) With piston walls, the larger the hole, the less pretty they are - due to drilling on the apex of an arc. (ie. Dril a round hole into a curved surface, flatten out the curve, and it ain't round...It's [techno-geometric jargon ahead:] goofy-lookin'.)

BUT HAS ANYONE ANSWERED YOUR FRIGGIN' QUESTION....!?!?!?

- With the piston in the up position AND THE CORRECT THICKNESS BUMPER WASHERS UNDER THE VALVE CAP AND UNDER THE FINGER BUTTON, pay attention to the two arc'ed (oval, but - sure - appear round) portions of the piston wall that are exposed to the two tubing knuckles leading from a particular valve slide into that casing.

You'll notice (possibly even some witness marks) that (almost always) the lower one is a completely solid area (with no porting exposed).
You'll want to drill as close to the CENTER of that area as possible...(but not with the piston still in the instrument).

You can use witness marks, or you can find that spot by measuring the throw of the piston (correct amount of vertical movement) and laying your calipers down on the piston sideways (to avoid scuffs) and finding the spot. The exposed portion of the valve stem (with correct valve washers) is the same as the throw, and the distance from the top of one port to the top of the one just below it (assuming you sort the ports out, and understand their functions) also indicates the throw.

SOMEONE ELSE SHOULD PROBABLY EXPLAIN IT BETTER, BECAUSE THIS EXPLANATION OF MINE SUCKS.

[LibraryMark]

Folks - I do think that I would be able to create the hole. Thanks for all the tips in that regard.

More important to me is how to find with some accuracy where the hole goes. I know it goes in the place where the piston closes off the branch. Is there a trick to finding that spot?

[bloke]

Folks - I do think that I would be able to create the hole. Thanks for all the tips in that regard.

More important to me is how to find with some accuracy where the hole goes. I know it goes in the place where the piston closes off the branch. Is there a trick to finding that spot?

LOL...I was typing and taking pictures when I realized that no one had answered your question.
and (duh!) you also obviously realized that no one had answered it.

[LibraryMark]

Folks - I do think that I would be able to create the hole. Thanks for all the tips in that regard.

More important to me is how to find with some accuracy where the hole goes. I know it goes in the place where the piston closes off the branch. Is there a trick to finding that spot?

LOL...I was typing and taking pictures when I realized that no one had answered your question.
and (duh!) you also obviously realized that no one had answered it.

Yup - that's it! Thanks to all, and especially Mr. Bloke.

[UncleBeer]

Whenever possible, I like to mark the target on the piston with sharpie, then check with a borescope. Worth the extra effort for the peace of mind.

[LibraryMark]

Whenever possible, I like to mark the target on the piston with sharpie, then check with a borescope. Worth the extra effort for the peace of mind.

You know, I was thinking of something like that. Good idea!

[bloke]

There's rarely a straight-in shot to view pistons from slide circuits. To get that picture, I grabbed my flugabone off the wall. I measure and mark where to drill, and - if I'm sick or something, and my mind might be a bit cloudy - I find something else to do, such as cleaning up a mess or stacking firewood.

It's often crowded around the valve block, but - once I do make the mark were the hole is to be drilled - I try to hold the piston beside its casing at the up position elevation and look laterally across from that lower port to my mark to see if my mark appears to make sense.

[UncleBeer]

There's rarely a straight-in shot to view pistons from slide circuits.

True, but many borescopes can go around bends, and most come with angled mirror attachments as well. Great for checking porting, etc.

borescope.jpg (14.62 KiB) Viewed 582 times

[bloke]

I dig...but - having bought one...and then having bought a skinnier one - I find that I don't ever get them out, because there are (not always, certainly ) just enough sharp 90 degree bends that - after having gotten the thing out of the box - I just say to myself "whatever", and go back to measuring with the calipers.

bloke "perhaps too easily annoyed, too eager to get on with it, and too eager to get done and on to the next person's repair gig...and yes, I measure thrice. "