Tips for cutting interior moulding

[Plomong]

Problem: I need to cut an 80 x 100 mm hole in the interior moulding on the centre-line of the boat, below the waterline, in between the log and depth sensor heads (2 separate items, one each side of the centre-line), essentially joining the existing holes made for the thru-hulls.

Difficulties:
1) The area is at the bottom of a 50 cm deep compartment with a small access door on the front for use when inserting and withdrawing the log unit. My portable drill is too large to permit using (carefully!!!) a rotary cutting head in such a confined space.

2) The sensor heads are a tight fit in the existing holes in the interior moulding. The thru-hull inner nuts are of greater diameter than the holes, by a small margin, admitedly, but larger none-the-less. Care will need to be taken when cutting in the vicinity of those nuts.

3) The distance between the lower surface of the interior moulding and the hull is between 15 and 20 mm, space enough to permit inserting a protective metal plate between moulding and hull once an initial section has been removed.

Can anyone suggest how I might go about cutting this hole:
- Suggested tool / procedure to cut an initial hole for inserting protective plate.
- Suggested tool / procedure for enlarging that hole to the full size.
- Suggestions for how to avoid damaging the thru-hulls, especially the nuts.

I am aware that this is akin to keyhole surgery, and will require patience. It will probably take some time to do the job, bit by bit, slowly but surely, to avoid doing more harm than good. It is more important to avoid cutting the hull and / or thru-holes than to get the job done in double-quick time.

Thanks for all your help and ideas.

 

[KRG]

I have cut neat holes in the inner linings using a Dremel type tool, the small cutting wheel easily cuts through fibreglass and is small enough for tight control.

 

[srp]

I reckon a Dremel is best too - you can get little milling type cutters, as well as abrasive discs, which work very well on grp. And if you know a friendly gynecologist as well........

 

[Wansworth]

Make sure you use a good dust mask,dremels create v fine dust.

 

[Talbot]

Dremels are good for the fine stuff, but a roto-zip would give you the oomph to cut through thicker material.

 

[Plomong]

Muchy,

Is that the dark grey disk about 20 mm diameter and 0.4 mm thick? If so, I have it so can give it a try on a sample in the garage.

 

[Plomong]

Mogy,

Thanks for the reminder -- will wear a mask.

 

[Plomong]

Talbot,

What is a roto-zip ?? Have'nt come across that name.

 

[Talbot]

It is a cross between a dremel and a router. The tool turns at a very fast speed >10,000 rpm IIRC and can have a great variety of attachments. It is also robust, and designed for serious work, rather than model making. Ideal tool for cutting bow thruster tunnel.

 

[DinghyMan]

Thier website is RotoZip , Yank of course. I've got a proper RotoZip one plus a B&Q copy which was about 1/3rd the price and came with more tools and accessories than the complete price ~£35 I think.

Get the B&Q one as I think RotoZip are no longer supported in the UK, and it come with a flexible drive & a couple of hundred bits and pieces.

 

[Plomong]

Dinghyman,

Over here it is called "Sierra de Calar". Maybe with a shortened (10 or 15 mm) length of blade it would do the trick. What do you think?

Would'nt it be necessary to protect the hull so that the roto-zip doesn't penetrate where it shouldn't? !!!

 

[KRG]

No, that is more of a grinding wheel, it will do it but it generates heat quickly, (and smoke!) the cutter is metal about 2mm thick with fine teeth like a hacksaw. Some practise wouldn't go a miss though as you need to avoid twisting the tool which can cause the wheel to snag a little. The cutters don't last long but are fairly cheap, about £3 for five IIRC.

Actually, it might be closer 1mm thick.

 

[Plomong]

Dinghyman,

Ignore that last reply --- The "Sierra de calar" has an up-down sawing motion. The roto-zip seems to be rotary, with a routing effect, it would seem.

And the depth of cut can be controlled, an important advantage in this case.

Thanks for the information.

 

[Plomong]

Muchy,

OK, I'll look for that one, about 1 or 2 mm thick, and also see if I can find something like a roto-zip to give it a try, before starting on the real job. As you say, to practice first on some offcuts is a good idea.

Thanks a lot.

 

[DinghyMan]

They do something like 30,000 RPM at top speed and are quite controllable, I found that they are best at high speed with slow movement. I've cut some fancy shapes in 22mm MDF using mine with no problems. Not sure I'd use the flexy lead at top speed though.

I reckon the best way would be to use it edge guided with some battens but sound like you may be tight for room. If possible it may be worth superglueing some small battens in place to use as edge guides or if you are carefull with the choice of cutter and use something like the ceramic tile cutters ( Dremel one) which have a small cutting surface on a smooth shaft you may be able to use the edge of the actual cutter above the rough cutting area on the battens.

You could probably grind the tool down to length if absolutely necessary but as its turning bloody fast that would have to be last resort. (Insert usual Health & Safety disclaimer etc here....) It is such a smooth cutting action that you would not be aware of going through the outside hull as well until you got wet so it would be worth trying to do a couple of small test holes to double check the depth available before doing the whole cut.

 

[Plomong]

Thanks all of you for your help -- I'll post a note on this forum when the job is done, indicating tools and methods used, problems encountered, results, etc.