NMO mount drilling

avdrummerboy · Jun 17, 2015

Hey all, had a question, has anyone used an actual 3/4" drill bit for drilling NMO mounts, or just hole saws, I've seen the bits around and wondered if they would work just as well as a hole saw. My last install I used a regular old hole saw from any standard set you get at home depot or lowes and had no issues, just curious if a bit would be a little easier to use.

SteveC0625 · Jun 17, 2015

avdrummerboy said:
Hey all, had a question, has anyone used an actual 3/4" drill bit for drilling NMO mounts, or just hole saws, I've seen the bits around and wondered if they would work just as well as a hole saw. My last install I used a regular old hole saw from any standard set you get at home depot or lowes and had no issues, just curious if a bit would be a little easier to use.

A 3/4 bit will chew the hole and leave chips still attached to the metal. It can also jam and bend the sheet metal.

I use either a fine tooth, thin wall 3/4 hole saw (mine is a DeWalt) or drill a 3/8 hole and then use a chassis punch. Both work well but the chassis punch method creates a lot less shavings to clean up and gives a much neater edge to the finished hole.

eng5 · Jun 17, 2015

I've used a Unibit, which I thought was cleaner and easier than a hole saw. Some people don't like them because it's easy to go too far and make the hole too big. I tape off the bit above 3/4" and go easy. I imagine a standard style 3/4" bit would work about the same. I doubt a spade bit would work.

I've also used a chassis punch, which was clean and easy, but use a punch that makes a 3/4" hole, not a hole for a 3/4" conduit!

Lately I've been using 3/8" NMO mounts. Smaller hole, more support, and you can use a regular old drill bit. You need to have access to the underside of the roof, though, since they come up through the hole from the bottom where a 3/4" NMO goes in from the top.

k3cfc · Jun 17, 2015

NMO Mount

avdrummerboy said:
Hey all, had a question, has anyone used an actual 3/4" drill bit for drilling NMO mounts, or just hole saws, I've seen the bits around and wondered if they would work just as well as a hole saw. My last install I used a regular old hole saw from any standard set you get at home depot or lowes and had no issues, just curious if a bit would be a little easier to use.

I would buy one of these. works first time everytime. a little pricy but then you have what works.

Laird HS34 - 3/4 inch Hole Saw - ShopWiscomm

Voyager · Jun 17, 2015

And also one that is less likely to punch down through the interior parts. Definitely +1 on the correct tool - the hole saw.

ke6gcv · Jun 17, 2015

k3cfc said:
I would buy one of these. works first time everytime. a little pricy but then you have what works.

Laird HS34 - 3/4 inch Hole Saw - ShopWiscomm

I'd go with the Laird or Larsen (LARSEN HOLE SAW HS1 | LARSEN 3/4IN. HOLE SAW KIT). You'll have less of a chance of punching through the headliner or causing other damage using a step bit.

W8RMH · Jun 17, 2015

The Antenna Farm has the Laird HS34 for $36.95.

KG4INW · Jun 18, 2015

We use Ripley 3/4" antenna hole saws and they work very well too: https://www.tessco.com/products/displayProductInfo.do?sku=14023

n0nhp · Jun 18, 2015

A standard high-speed bit will leave a tri-lobe hole in sheet metal. Starting a bit as large as 3/4" on sheet metal is almost impossible.
Step bits that go to 3/4' are usually pretty long and as others have pointed out will damage structure/headliners below the intended hole.
Be careful buying a hole saw at a big-box. Most of those are designed for carpentry and drywall. Again with a longer pilot bit than is optimal for automotive use.
If you can get to both sides, a chassis punch for a very clean hole. If you are planning on doing installs as part of your job (I did for years) the time to remove a headliner will kill your profit margin, not to mention the cost of replacing one when it gets a dirty hand print or rip.
An antenna hole saw is the only way to go, and with a bit of practice fishing an antenna cable without touching the headliner will save you time and money.
If you are doing a one off installation and the $40.00 or so for a single use tool is a problem. Stop by the install bay of the local radio shop, offer the installer the price of a six-pack and show him where you want a hole.

Bruce

DJ11DLN · Jun 18, 2015

A Forster-type bit, which cuts from the OD, will work, and will make a nice clean hole just like a fine-pitch metal hole saw, but a lot more chips too. These are usually associated with woodworking but metal types are available if you look for them. They have about a 0.25" pilot bit in the center to start them and get the actual cutting flights engaged. And if you have a real steady hand, you can scribe the hole, use a smaller bit, and then finish it with a Dremel tool or die grinder. I've done that in the past but I do not recommend it.

prcguy · Jun 18, 2015

Any cheap metal cutting hole saw will work just fine but the ones sold specifically for NMO mounts usually have a ridge that prevents the entire saw from poking down into the headliner.

My method for the last 40yrs is to not take down the headliner, only remove molding around the top of a door enough to expose the underside of the roof and identify any bracing or obstacles and to pass the cable through. I always stick a small maybe 1ft square temporary piece of sheet metal or thin plastic sheet between the roof and headliner to prevent the saw from touching the headliner and it will also catch any shavings.

When the hole is through, pull out the sheet metal or plastic shield with shavings and snake the coax across to a door or window post and down inside that to the radio. Sometimes you need something to help snake the cable across the inside of the headliner and an old thin antenna whip works good for that.

You can install an antenna in most cars or trucks in about 10min this way or maybe 15min tops including installing the connector.
prcguy

prcguy · Jun 18, 2015

And another thing, be careful centering the antenna side to side on the roof. I've seen people measure across the roof with a tape measure to find the center then the antenna looks off center when finished. When looking at a vehicle you eye can be drawn to the rear view mirror or molding around the windows as the optical center and a perfectly centered antenna may not look right compared to other features on the car.

In addition to measuring, which I seldom do, I always stick something like a bolt on the roof pointing up then walk around the vehicle to see if it actually looks centered before drilling. You may find the sweet spot to the eye is not exactly centered according to a tape measure.
prcguy

mrweather · Jun 19, 2015

I've used an Antenex hole saw to do almost a dozen antenna NMO mounts on the various cars I've owned over the years and never had any issues.

ke6gcv · Jun 19, 2015

One of the clients I installed for had a template that squared off the center line of the roof. Quite handy, in fact, if you're working on multiples of the same make/model vehicle. Otherwise, eyeing the rear view mirror is your best option to make your line from.

Another option to take into consideration depends on the vehicle, too. Some SUVs have ridge lines that run from front to back of the roof. Find the center "trench" and draw your center mark from there.

cmdrwill · Jun 19, 2015

"Center trench" looks like the center is between the second and third "trench"

ofd8001 · Jun 19, 2015

Forstner bits and hole saw bits are designed for wood working, with the forstner bit leaving a clean bottom for things like European hinges.

While you could use a hole saw bit if you have just a few antennnas to install, for "production level" work, its best to get an antenna specific hole saw. After a few holes with the wood type bit, the teeth get worn and loss of control is easier. Pluse you may have sloppier holes.

I'd never, ever use a 3/4" twist drill bit on a hand held drill. Too many opportunties to injure yourself or your work.

FFPM571 · Jun 20, 2015

n0nhp said:
A standard high-speed bit will leave a tri-lobe hole in sheet metal. Starting a bit as large as 3/4" on sheet metal is almost impossible.
Step bits that go to 3/4' are usually pretty long and as others have pointed out will damage structure/headliners below the intended hole.
Be careful buying a hole saw at a big-box. Most of those are designed for carpentry and drywall. Again with a longer pilot bit than is optimal for automotive use.
If you can get to both sides, a chassis punch for a very clean hole. If you are planning on doing installs as part of your job (I did for years) the time to remove a headliner will kill your profit margin, not to mention the cost of replacing one when it gets a dirty hand print or rip.
An antenna hole saw is the only way to go, and with a bit of practice fishing an antenna cable without touching the headliner will save you time and money.
If you are doing a one off installation and the $40.00 or so for a single use tool is a problem. Stop by the install bay of the local radio shop, offer the installer the price of a six-pack and show him where you want a hole.

Bruce

The days of just fishing it through the headliner is over. With modern cars who have side airbags and some SUV's that have overhead HVAC ducting and controls you have to drop the headliner no matter what. You have to run the coax following the factory looms to avoid air bag deployment zones Profit margin or not there is a proper and safe way to do things. If your installers were getting handprints and ripping them then they need to learn how to do it the right way.

Project25_MASTR · Jun 20, 2015

FFPM571 said:
The days of just fishing it through the headliner is over. With modern cars who have side airbags and some SUV's that have overhead HVAC ducting and controls you have to drop the headliner no matter what. You have to run the coax following the factory looms to avoid air bag deployment zones Profit margin or not there is a proper and safe way to do things. If your installers were getting handprints and ripping them then they need to learn how to do it the right way.

Completely depends on vehicle. As stated, some SUV's have AC ducting above the headliner, however pickups do not and if they have rear vents generally have them mounted in the rear portion of the center console.

Personal favorite approach method for pickups is to either a) pull a dome light and look around and drill within site of what you can see from the dome light access b) pull the 3rd brake light and drill there. Either way simply requires removing some trim pieces to make sure you route the coax behind the curtain airbags.

SUV's I do whatever is easiest. Dropping an overhead console or entertainment system (do they still put those in new SUVs?), dropping the portion of the headliner behind the passenger seating (usually no air ducting in what is considered the cargo area), pulling the overhead speakers (2012+ Ram HD, new body style XJ Cherokee) to look around. Same routing around side curtains applies so just beware.

I have a Laird hole saw and a 3/4" Green Lee punch. I've used the step bit in instances where I pull a DVD player or overhead console down and drill straight up but I've gotten away from that. If its something where I don't have to drop the headliner, Ill use the hole saw. If its a location where the punch is convent or easy to use (near the edges of the vehicle) I'll use that.

ke6gcv · Jun 21, 2015

cmdrwill said:
"Center trench" looks like the center is between the second and third "trench"

Yeah... realized the arrow isn't in the best spot to show my suggestion. Oops!

ke6gcv · Jun 21, 2015

FFPM571 said:
The days of just fishing it through the headliner is over. With modern cars who have side airbags and some SUV's that have overhead HVAC ducting and controls you have to drop the headliner no matter what. You have to run the coax following the factory looms to avoid air bag deployment zones Profit margin or not there is a proper and safe way to do things. If your installers were getting handprints and ripping them then they need to learn how to do it the right way.

Must be careful of those airbags! Anymore, you have to drop the headliner on a passenger-type vehicle so you can visually see the roof. Get a (used) PPV (Patrol Purpose Vehicle). The chances of airbags being in the headliner is minimal. I'm not saying they're 100% eliminated on a PPV, but you have better chances of them not being there. Still recommend dropping the headliner, though.

NMO mount drilling

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