Drilling The Holes...Which Direction?

[phask]

1/2"conduit is 7/8" - made 100's of those holes, both with hole saws ( always Sandvik) as well as chassis punches. I'd need to check for 3/4"

- Knowing your expertise - probably an oversight. Fittings are actually same as 1/2" pipe - just not tapered threads.

Agree that actual NMO saws are better, but a competent operator and GOOD and I stress GOOD hole saw will do fine. Regular hole saws are all that electricians use, though for perfection can't beat a chassis punch. (Still have 5 or 5 Greenlee laying around)

I've seen plenty of fenders and other parts of cars drilled by body shop guys and never seen a NMO one used anywhere.

If using a regular saw - and need to limit depth there are several methods to limit. S Since the OP is doing a trunk lid - no issue.

If it was me I'd actually use a Unibit - have several up to 1 inch and nothings cuts sheet metal cleaner.

I happen to have that exact same hole saw sitting in my garage. I have the official Antennex depth limiting NMO hole saw, too.

I also happen to have a set of digital calipers accurate to 0.0001 of an inch.

So take the previous comments with a grain of salt. There is some excellent advice above, and then there is the "me too" crowd.

I'll 100% agree that using the correct tool for the job is the right way to do it.
I'll also 100% agree that if you are doing a single install, not going into the installer business, you probably don't need to buy the top dollar hole saw.

The truth about hole saws is that they make round holes of a specific size. The tooth spacing on the saw part sort of dictates what sort of material it's going to work best in. Finer teeth cut slower, but work better in metals. Coarser teeth cut faster and are usually good in softer materials.

The "magic" behind the official NMO hole saws is that they have a couple of handy features:
1. Depth limiting. That helps reduce the chances of damaging headliners.
2. Fine tooth blades. Takes a bit longer to drill the hole, but does so in a more controlled fashion.
3. Replaceable saw blades. No need to replace the entire hole saw.
4. Some have a feature that scores the outer part of the hole on the exterior of the vehicle to help with grounding.

So, back to size….
If you have a hole saw with a maximum outer diameter of 3/4's of an inch (0.7500), it's going to make the right size hole. Doesn't matter if you bought it from a company that caters to radio installers or the local hardware store. 0.7500 = 0.7500. The source of the hole saw doesn't magically make it drill larger holes.

The 3/4" "Trade Size" statement above is true, but that isn't what you have. In the electrical industry, they'll sometimes call a hole saw or punch 3/4" trade size when it's designed for 3/4" conduit. It's actually 7/8" because that's the size of the threaded fittings. That can be an issue if you grab the wrong thing, but in this case you didn't.

So, I measured my "official" NMO hole saw, and it was 0.7475 inches in diameter. Just shy of 3/4". I've installed a lot of NMO mounts with this hole saw and I have never had an issue with that 0.0025 difference.
I then measured the DeWalt 3/4 inch hole saw. 0.7450 inches in diameter. A bit smaller than the NMO saw, but likely it would work just fine.
The teeth are a bit more coarse on the DeWalt hole saw compared to the Antennex hole saw.

I would absolutely prefer the Antennex saw, but that's easy to say because I have one sitting there ready to use. It makes the right size hole and won't go too deep.
But, the DeWalt hole saw will work just fine for what you are doing. It's going to make the right size hole. You don't need to run out and spend $40+ on an "official" NMO hole saw for one install or to make a bunch of strangers on the internet happy.
Just take your time and measure twice.

 

[W5SAB]

nice to see someone else suggest an unibit. I just posted that suggestion myself. But, I called them by their other name stepdrill bit.

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[N4GIX]

here is a stepbit

Neiko 10194A Titanium Step Drill Bit, High Speed Steel | 1/4” to 1-3/8” | Total 10 Step Sizes https://www.amazon.com/dp/B000FZ2UOY/ref=cm_sw_r_cp_apa_hEn.ybP58PC6C

best way to drill through sheet metal.

If drilling through a roof, that bit is only "safe" if you are able to drop the headliner low enough to be out of the way.

 

[W5SAB]

If drilling through a roof, that bit is only "safe" if you are able to drop the headliner low enough to be out of the way.

of course. but you got to already deal with the headliner, if you want to install nmo mount hidden behind the headliner

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[clbsquared]

of course. but you got to already deal with the headliner, if you want to install nmo mount hidden behind the headliner

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In most cases you don't. A small fish tape will allow you to leave the head liner intact. I go in through the top of the roof with the mini uhf connector first, attached to the fish tape. Gently pull it through and then angle the NMO mount to slide through the hole. Attach the retainer ring and tighten. I've never had to drop a headliner.

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[W5SAB]

In most cases you don't. A small fish tape will allow you to leave the head liner intact. I go in through the top of the roof with the mini uhf connector first, attached to the fish tape. Gently pull it through and then angle the NMO mount to slide through the hole. Attach the retainer ring and tighten. I've never had to drop a headliner.

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good suggestion but in my case I needed access to nmo mount base to keep it from spinning so that the angled coax connection stayed in the specific direction i wanted. connector was never a issue. it was soldered on after coax routing. nmo kit that i used had nmo mount already attached to unterminated coax.

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[clbsquared]

good suggestion but in my case I needed access to nmo mount base to keep it from spinning so that the angled coax connection stayed in the specific direction i wanted. connector was never a issue. it was soldered on after coax routing. nmo kit that i used had nmo mount already attached to unterminated coax.

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Snap ring pliers with a 90° end inserted into the two holes on the mount will keep it from spinning.

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[N4GIX]

Snap ring pliers with a 90° end inserted into the two holes on the mount will keep it from spinning.

All of the NMO mounts I've installed came with a small tool just for that purpose.

If I was out in the field and had to, needle nose pliers worked just as well in a pinch. :lol:

 

[W5SAB]

All of the NMO mounts I've installed came with a small tool just for that purpose.

If I was out in the field and had to, needle nose pliers worked just as well in a pinch. [emoji38]

good point. never really noticed those holes on the mount, however never had to worry about headliner myself. but nmo mount on top flat part of my honda fit's rear hatchback. this allowed me to avoid putting hole in main auto body, use existing wire path for brake/taillights, and of course no headliner to deal with

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[phask]

You are correct. Seems when they first came out (ages ago) it was a trademarked drill by someone - Irwin or someone. I purchased them by the box for a maintenance crew of 60 or so. Bought a lot of 7/8" for 1/2" conduit, a 1/2" (one step) also made a decent countersink. and a 1/8 or so to 1/2"

Recently lost my olf 1/8 - 1/2 and bough a Tn coated cheapie set via Amazon. No idea how long they will last, but so far so good.

nice to see someone else suggest an unibit. I just posted that suggestion myself. But, I called them by their other name stepdrill bit.

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[Rred]

Not saying this is always appropriate or helpful, but many folks will start a hole saw IN REVERSE, letting the teeth "score" the surface and chew through slowly, instead of running it in forward, where the teeth take more bite and are sometimes more likely to walk or tear at the edges.

You might want to try it both ways on the baking tray and see what works best for you. Also, put some cutting oil (or plain motor oil, etc.) on the teeth. That also helps keep them cooler and they'll stay sharper and cut cleaner.

 

[Station51]

Gentle pressure. Let the bit do the work. "Feather" the bit against the surface. It's almost like hovering over the project.
I've drilled several holes over the years with a cheap Lowe's 3/4 hole bit. Never lost a headliner, never had antennas "wobble" or "walk around" when the NMO mount is tightened properly..
If you put you body into it then of course you will go thru the headliner, maybe thru the floorboard lol!
Take your time......relax.
Good luck!

 

[CanesFan95]

I appreciate everyone's advice. I am going to think hard about getting a so-called "proper" NMO drill bit before doing the real thing. In the meantime, at least I can try out the baking tray. But lemme ask this question. Don't modern cars tend to have a lot of wiring and sensors and crap that you could accidentally damage in a roof install? Even with the "right" tool? And what about air bags?

 

[W5SAB]

I appreciate everyone's advice. I am going to think hard about getting a so-called "proper" NMO drill bit before doing the real thing. In the meantime, at least I can try out the baking tray. But lemme ask this question. Don't modern cars tend to have a lot of wiring and sensors and crap that you could accidentally damage in a roof install? Even with the "right" tool? And what about air bags?

wiring can be a concern. what is the make and model?

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[CanesFan95]

Well, just newer cars in general. I've always wondered about side airbags and how you know how to safely route the coax along the sides without doing something that would mess up any air bags or sensors or whatever.

 

[prcguy]

As mmckenna pointed out in a previous post, a hardware store 3/4" hole saw drills a very accurate 3/4" hole and the bit cannot wobble or walk. This is because they have a 1/4" drill bit that goes through the metal first, then that bit centers the hole saw and keeps it from walking. If you have ever used one on a car roof its simple and you just have to be aware of the headliner and take precautions when necessary.

I think its ludicrous for people to insist on a specialized $70 hole saw for someone who is going to install one NMO mount at home when a $15 saw will do the job just fine. If your not comfortable with that then turn the job over to someone who is. And how would you explain to a customer how a big oil stain got on their headliner? I would never use oil when using a hole saw on a sheet metal car roof, its not necessary and would turn a one minute task into a huge one, stuffing things between the roof and headliner to catch any oil, etc.

On the question of encountering wiring or airbags, you must always pull a few feet of rubber trim and peek under the headliner to see what your up against. There are braces, dome light and wiring to widgets in overhead consoles, etc. It only takes a minute to look and you have to remove some rubber trim to route the coax anyway.

On one of my last installs on a Toyota Tundra I encountered air bags all around the side perimeter of the headliner and ran the coax down a back window pillar where I had a clean run, since the radios were going under the rear seat anyway. You don't want to run any wiring near airbags and create a projectile.

I've discussed RF setting off airbags with a friend that works for Takata and its very unlikely you could set one off from RF leaking or riding on coax running past an airbag or sensor. But no guarntees since the airbag mrfs cannot test for everything, so keep all of your wiring far away from airbags and sensors.
prcguy

Not saying this is always appropriate or helpful, but many folks will start a hole saw IN REVERSE, letting the teeth "score" the surface and chew through slowly, instead of running it in forward, where the teeth take more bite and are sometimes more likely to walk or tear at the edges.

You might want to try it both ways on the baking tray and see what works best for you. Also, put some cutting oil (or plain motor oil, etc.) on the teeth. That also helps keep them cooler and they'll stay sharper and cut cleaner.

 

[mmckenna]

Looking at mine again in the garage. The center pilot bit depth is adjustable. You can adjust it with an allen wrench and set it so it just barely protrudes beyond the hole saw.

Again, I wouldn't hesitate to use the DeWalt hole saw to install an NMO mount. You are not a child, you are aware of the risks and you don't need a bunch of people telling you that horrible things are going to happen if you don't buy the most expensive hole saw.

If you were going into the install/upfit business, or planning on doing this a lot, then sure, get the dedicated NMO hole saw. But for what you are doing, a one off install, you are just fine.

How about some pictures when you are done for the nay-sayers?

 

[kb4mdz]

Well, the NMO drill bits are a lot more expensive. I wonder why. This one looks exactly 3/4" diameter held up to a ruler, so I'm not sure how it could cut a hole bigger than that.

They're expensive because they are the RIGHT thing to use. It's the difference between using a hexagonal die crimp tool for RF connectors like Mini-UHF, N-type, etc, and using a pair of 2 dollar slip-joint pliers. Yes, the 2-dollar pliers will work, but not very well.

Ever try to slice raw meat with a table knife? FWOMPT

 

[buddrousa]

I always remove the trim on the side I use to run the coax down and can see or feel where I drill the hole. In trucks you remove the dome light and you can see the roof line most of the time.

 

[CanesFan95]

The baking tray test results are in. So I went outside with eye protection and plopped the tray upside down on the grass. Here's a few shots with the DeWalt drill bit.