Need a Device to Help Find Blind Holes

Hi Forty Maybe you can use the electromagnet as are used as holder for construction_drilling_machines, Maybe you've seen them, the ones you set on metal walls or ceilings and then switch the magnet on for drilling. They disperse a big EM field and you might be able to find that back on the topfloor with a needle compass Helpfull ? T L

Maybe I misread the OP, but if you can, "drill a pilot hole from underneath the ceiling", then you don't have a "blind" hole.

Why not just attach a flashlight to the underside of the hole, shining upward, then go above and find the light. A little smoke may help to find the light, if needed. I'd think you could get close by dead reckoning.

Hi. I meant that I could drill a pilot hole and thats what has been done in the past. I wanted to know if there was a device that would avoid having to do this. My plan was to have someone in the room moving the magnet or similar around the room stopping at each hole location while the person on top of the ceiling will use a detector to find the magnet below and then just drill one hole straight through the ceiling and move onto the next one.

I've entertained the kids before by using two magnets on a table top, one above, one below. Moving the "below" magnet would drag the "above" magnet around with it. So, get close using this method, then if needed, you could use some powdered iron, or iron filings to find the center of the magnet below.

Maybe hang the "above" magnet on a string, like a plumb bob?

Why not create a template for the top of the cieling based on a reciprocal of the lower side. Create a datum point, or maybe two for more accuracy, set it, mark it, and drill it.

Ok I think this is gone slightly off topic. The ceiling is metal with rockwell insulation sandwiched in between. 100mm total thickness. I was hoping there was an instrument that could be used, It doesnt seem like there is. I am not insterested in spending time on it. I can just get the guys to measure out like before. Its not a major concern for me. Its more of a nice to have as opposed to a need to have item.

Thanks for all you're advice on this topic

Metal ceiling complicates it.

Good luck. It's nice to have "guys" isn't it.

How big are these holes? And what tolerance are you trying to achieve? Will +/- half inch be sufficient? It seems like you wouldn't get much closer than that using magnets. But, as was mentioned... the metal ceiling confounds using a magnet.

Either hire beg or steal an angled drill or angle drill attachment to give more room for a longer drill bit. or drill the hole from under and bend and insulate an arc welding rod except for where it will make contact with the metal in the upper part, the inside of the metal panel may be insulated so you will have to make the rod pointed enough to scratch the metal.

Connect the supply and carefully push the rod home.

Obviously this entails some risk of fire if lots of dust is present, and also requires you to have A: a fire extinguisher on standby, and B: an Arc welder present.

Good luck

How about shooting bullets through it?

Another idea: would a right-angle drill fit into the restricted space?

Sorry. I have mislead all of you. I am cutting 6" holes in a clean room ceiling with a jigsaw. The holes are for connecting an AHU to a machine. I wanted to do this work from the outside to make life easier fror the guys that have to do it. It looks like they will have to do it the conventional way!

Use a RH drill with a 6" drill bit in it to drill the CL of the proposed hole, inside a plastic bag taped to the ceiling, (to prevent dust/shavings generated from contaminating the clean room.) drill through both sandwich layers. take a small doughnut shaped magnet, stick a wooden or plastic dowel at least 6" long in the magnet hole and glue it with epoxy. paint the end of the stick fluorescent blaze orange then stick the magnet/dowel assembly through the hole(s). the magnet will hold the stick in place and plug the hole temporarily whilst you remove the contaminated bag/drill etc, then tape a new bag in it's place.

now go on the ceiling/roof, note the location of the stick standing proud of the sandwich. use the stick as the pivot point of a beam compass. take a bit of 1X4, drill two holes 3" apart, stick a pencil in one hole and leave the other open. stick it over the stick and rotate. you've now circumscribed the hole outline. push the stick back through the hole and it will drop into the bag. insert the blade of jigsaw into the hole, and cut final hole outline. cut-out will fall into the bag still stuck to the ceiling. (you used good duct tape and a strong plastic bag right?) and you won't have contaminated the room with a bunch of crud.

If you are having to cut a number of holes a 6" hole saw would give you a good clean cut and be quicker than a jig saw.

Would a laser measuring device/tape help?

2 pieces of string, attached to the room corners (90 degrees apart). locate and mark intersections on string from underside... (marker) transfer to upperside... relocate intersections. faster than measuring tapes.

Chris

A very good system. The strings can be from anywhere the top and inside have in common, including a couple of temporary (pilot) holes. As long as the strings have a straight run, the accuracy will be good.