IOMICA Forum

I want to bring my lead bulb down to its final weight.
I plan to drill a 3/8th inch hole.
How deep does the hole need to be to remove 39 grams?

Can someone check and confirm my logic and arithmetic

lead bulb weighs 2427 grams

Target weight is 2388 grams so weight of lead to be removed is 39 grams

A 3/8th inch drill is 0.9525 cm dia

Volume of cylinder is Area *depth ( pi*radius squared * depth)
so the volume of a 1 cm deep hole of 0.9525 dia is 0.71265 cu cm

The weight of lead is 11.34 grams per cu cm.

The weight of lead removed from a 1cm deep hole of 0.9525 cm dia is 8.0814 grams (0.7165 * 11.34)

So the total depth to drill to remove 39 gms is 4.8 cms

Thanks

Hiljoball wrote:I want to bring my lead bulb down to its final weight.
I plan to drill a 3/8th inch hole

Hi John

If you are planning on a perfectly legal keel, you must be serious about a race-ready IOM. Drilling a hole is not recommended in this case, although it is common practice and is an easy approach. It would be better if you could skim material off the top of the bulb. I understand a "surform" is a good tool to use for this (it is a kind of shaping plane with a toothed surface).

If you really do want to drill, then 10 or 20 shallow holes along the *upper surface* of the bulb would be the way to go.

Remember, if drilling, to add back in to the equation the weight of the filler you'll have to put into these holes... If you imagine that the density of the filler is, say, 1 g per cc, then your effective rate of lead removal is not 11.34 g per cc, but 10.34 g per cc. The weight of lead removed from a 1cm deep hole of 0.9525 cm dia is 7.41 grams (0.7165 * 10.34) *after the hole is filled*. So the total depth to drill to remove 39 gms is around 5.26 cms, or 20 holes each 2.63 mm deep. If you are milling the holes, no problem, but otherwise ...

... we still haven't accounted for the 110 degree "V" in the first 3.26 mm or so of your drill bit... Wait, the first 3.26 mm of the bit is greater than the intended depth of 20 shallow holes, which is 2.63 mm! Argh! OK, let's have 10 holes. So... The volume of the bit's cone is pi.height.radius^2: 3.14 * 3.26 * .476^2 = .232 cc, so 10 of these cones lightens the bulb by 2.32 * 10.34 = 24 g. The remainder of the drill depth is a cylinder, so 10 of these must lighten the bulb by the remaining 15 g. Each cylinder must therefore be 1.5 / 0.741 = 0.2 mm deep, so total drill depth is 3.46 mm for each of 10 holes... I think... (smile)

ah - but if youre going to be so precise so early on a friday morning Lester - have you taken the effect of atmospheric pressure into consideration on his scales (joke)

peter

PS - see you tomorrow

I, in my humble opinion, would of thought it very easy to drill the holes a little over depth then fill them. If the lead is then to light its very easy to take out the filler and add some lead mixed with filler. You can then add back weight either at the front or rear of the lead.

Problem solved! or is it?

Hi, I'm at the same stage as John Ball in trying to get my bulb and keel fin to the optimum weight. The only thing is that my bulb is underweight by about 55gm. It was supplied with rather a deep slot, and I think that the only way to 'up' the weight is to melt some lead and pour it into the slot, and maybe add some small strips on top of the bulb in way of the top of the fin slot, and cover them up with a fillet 'faring'
Has anybody any idea of the weight that a good thick coat of car primer on the bulb will be.

Lesters treatise on the conical borings of lightening holes and their arithmetical/mathematical addition/subtraction of 'variable' quantities of lead, brings me to the subject of using a wood drill with its reverse conical ground dressing, you know the one with the 'pip' in the centre.
I think Lester would blow peoples minds with a discussion about this type. (smile)

cheers

Ralph

Thank you to those you replied to my question.

The consensus was that I grind down the lead to get to the desired weight. So that is what I have done. I'm almost there, just a few grams to go.

Thanks