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Old 29-01-2012, 21:33   #1
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Drilling New Holes

I am now at the point of replacing the Electro Marine depth transducer and speed thru-hulls. I bought the ST-40 bi-data unit. The thru-hulls are larger.

What are my steps to install the new ST--40 thru-hulls? She is on the hard. Please start at the point of pulling the old transducers - that is step one. What are the next and following steps? At the "drill holes" step what is the best way to center over the existing holes?
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Old 29-01-2012, 22:35   #2
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Re: Drilling New Holes

I've never tried this but a smart man once told me that the way to enlarge a hole like that, you needed two hole saws. One hole saw the size of the existing hole and one the size of the hole you need. You mount both saws on the same mandrel with the small saw inside the large saw. When you drill, the small saw acts as a guide keeping the larger saw centered around the existing hole. Has anybody actually tried this?
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Old 29-01-2012, 22:59   #3
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Re: Drilling New Holes

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I've never tried this but a smart man once told me that the way to enlarge a hole like that, you needed two hole saws. One hole saw the size of the existing hole and one the size of the hole you need. You mount both saws on the same mandrel with the small saw inside the large saw. When you drill, the small saw acts as a guide keeping the larger saw centered around the existing hole. Has anybody actually tried this?
I haven't tried it but it is logical and sounds like it will work, just like a normal hole-cutter with pilot drill, only this pilot is bigger.

Could also use a standard hole cutter with pilot mandrel turned to the correct diameter.
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Old 29-01-2012, 23:17   #4
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Just ask somebody to hold a pieceof wood firmly from inside while you start drilling from outside. Once the hole is started you can remove the wood.
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Old 29-01-2012, 23:19   #5
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Re: Drilling New Holes

Why is it important to centre the new hole perfectly over the old one? As long as it overlaps, and is close to being in the same place, it should be fine, or am I missing something? Do transducers need to be located to the nearest mm to work properly?
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Old 29-01-2012, 23:26   #6
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Re: Drilling New Holes

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Why is it important to centre the new hole perfectly over the old one? As long as it overlaps, and is close to being in the same place, it should be fine, or am I missing something? Do transducers need to be located to the nearest mm to work properly?
When cutting a large hole you always need a pilot to ensure a clean clean circular cut.
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Old 29-01-2012, 23:57   #7
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Re: Drilling New Holes

Cut a piece of ply square, round any shape at all and bash it from the other side into the existing hole, glue it in over night if you wish , i usually rely on a tight fit, centre the hole-saw your drill on it and start cutting.
Once you have a groove in the hull your ok i use broom sticks, jigsawed pieces anything at all.Oh and a bit of metho in water will keep the itchys down ......Cheers Frank
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Old 30-01-2012, 00:31   #8
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Re: Drilling New Holes

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When cutting a large hole you always need a pilot to ensure a clean clean circular cut.
That I get, but why does the new hole have to be concentric to the old hole? As long as the old hole is completely covered, the new hole should still be round, whether exactly concentric or not.
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Old 30-01-2012, 00:45   #9
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Re: Drilling New Holes

It doesn't need to be concentric to old unless you are only just opening the hole up a little bit. It's usually easier to whack the bung in and cut the new hole using the old as a guide to centre. Sure you could favour one side to effectively move the new larger hole over some.... Cheers Frank
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Old 30-01-2012, 03:16   #10
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Re: Drilling New Holes

I've used the technique where I had the smaller sized hole saw mounted with the larger one and it worked very well.
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Old 30-01-2012, 04:59   #11
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Re: Drilling New Holes

I've never had that type only the individual Starret style with an arbor i see that yours would work well.
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Old 30-01-2012, 06:16   #12
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Re: Drilling New Holes

You really need something stable to drill a hole a size larger. I would use a hot glue gun and glue a piece of 1/4" plywood over the hole, either inside or outside, and use that as a solid medium for the pilot drill to bite into, and guide the cup sawwhere you need it. I would also extend the pilot bit in the whole saw as far out as it would go to give you etra leverage to hold the drill in place. After it is drilled, a chisel will pop the wood off, and the glue will be easy to remove. Worst case if glued to the bottom, is you might loose a couple of small spots of bottom paint.
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Old 30-01-2012, 06:29   #13
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Re: Drilling New Holes

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That I get, but why does the new hole have to be concentric to the old hole? As long as the old hole is completely covered, the new hole should still be round, whether exactly concentric or not.
Doesn't need to be concentric Saucy, but you should use a pilot to make sure you don't skid off the place you wish to drill, if you skid it could result in gelcoat damage, or even skidding into fingers etc. If you use the existing hole as a pilot them the new hole will be concentric. Just the easiest way to do it really.
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Old 30-01-2012, 06:49   #14
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Take a suitably sized (hole diameter +5% max) wooden bung and plug the hole from outside being careful not to crack the hull when hammering it into place - this will give you the material you need for the central pilot hole/drill of your hole-cutter.
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Old 30-01-2012, 06:53   #15
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Re: Drilling New Holes

Plenty ways to skin a rabbit me&boo... Hope you get the job done easily.
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