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Author Topic: NMO Hole Saw?  (Read 955 times)
K1CJS
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Posts: 6034




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« on: August 10, 2004, 11:48:53 AM »

Greenlee also makes a "D" shaped 3/4 punch, and a "Double D" punch, too.  The D shaped one makes a hole with one flat side, the double D leaves two flat sides, just the shape of the NMO mount base.  If you want to buy a punch, that is.  It helps to have the mount held in place when tightening it up, and the D or double D punches holes that do just that.  I installed quite a few NMO mounts and bought the double D for just that reason.

Also, a little silicone grease on the o-ring used to "seal" the mount will help to keep the o-ring from bunching up and possibly leaking.
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KZ1X
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« Reply #1 on: August 10, 2004, 11:23:35 AM »

Some folks have discussed the headliner ... I never remove mine.  Instead, I take off the dome light assembly and fine a good spot just a bit fore or aft of this location, but which I can still reach simply by 'flexing' the material slightly ... works every time, saves a LOT of effort!!

1 - Any issues with the hole punches on (very) slighly curved surfaces? My experience is that they tend to try to flatten the surface out. I am not sure how big an issue this is on typical roof curve. My truck is a a Ford F-350.

>> The chassis punch works fine as long as the arc is shallow; it helps to put the anvil on the convex side if you can

2 - Seems like Greenlee make two types of punches (?) one for electrical, one for radio?

>> yup, has to do with the inside vs. outside diameter and also the 'coarseness' of the punch ... the radio punch cuts a hole at the INSIDE diameter as listed; my 3/4" radio chassis punch maks a VERY exact 3/4" hole

3 - Where can I get the Motorola hole saw or punch?

>> I use the Antenex version, which has an easily-removable/replaceable fine cutting blade ... the Motorola unit is beefier though ... Dave Unkles KC2LT sells lots of this kind of stuff 603-421-0015 ... I have no commercial affiliation with his company ... you can generally also get goodies like this from Gene K2KJI at KJI Electronics in NJ ... same disclaimer ... but I have known both guys well over 20 years, feel free to drop my name
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KB1LKR
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Posts: 1898




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« Reply #2 on: August 10, 2004, 09:56:17 AM »

The ones for electricians are nominal sizes make holes to fit 1/2" 3/4" 1" etc. TRADE SIZE conduit and & EMT fittings, I'm not sure of the actual dimensions but would guess they might be *about* 1/4" over nominal.

I assume the NMO and other roof mounts use a 0.750" hole -- an ACTUAL 3/4"

Probably best to buy from MSC, McMaster, Grainger, or other general industrial supplier vs. from an electrical wholesaler to avoid mixup, and double check first!

As for the headliner, isn't it going to be dropped anyway to run the coax?
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K4CMD
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Posts: 99




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« Reply #3 on: August 10, 2004, 08:32:31 AM »

How the hell did the response I just made get placed at the top of the list, and my call get made ORIGINATOR of the discussion???!!!
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K4CMD
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Posts: 99




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« Reply #4 on: August 10, 2004, 08:31:14 AM »

Wusses! WUSSES! ALL OF YOU!!!

I've had a 3/4-inch sheet-metal hole saw, purchased for about five bucks from a hardware store, since I first started putting mobile rigs in cars in 1985 (when I was 20). To date I've installed permanent-mounted NMO (Larsen) mounts on seven cars and a pickup truck, and helped my brother drill holes in three more cars. Never had a problem, and always had a professional installation. Just take your time (this includes measuring TWICE and drilling ONCE) and let the weight of the drill do the work. As long as you pay attention to what you're doing and go lightly once it starts to break through, you'll never hit the headliner.

Or do like I usually do and go through the trunk lid! (But I have done my share of roofs too.)

BTW, I've traded in seven cars with 3/4-inch holes in them (most of the time I just leave the NMO mount on the vehicle, uncapped) and I've never, in 19 years, had any dealership ask me about them.

73 and happy drilling!
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KZ1X
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Posts: 3228




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« Reply #5 on: August 10, 2004, 11:24:46 AM »

Some folks have discussed the headliner ... I never remove mine.  Instead, I take off the dome light assembly and fine a good spot just a bit fore or aft of this location, but which I can still reach simply by 'flexing' the material slightly ... works every time, saves a LOT of effort!!

1 - Any issues with the hole punches on (very) slighly curved surfaces? My experience is that they tend to try to flatten the surface out. I am not sure how big an issue this is on typical roof curve. My truck is a a Ford F-350.

>> The chassis punch works fine as long as the arc is shallow; it helps to put the anvil on the convex side if you can

2 - Seems like Greenlee make two types of punches (?) one for electrical, one for radio?

>> yup, has to do with the inside vs. outside diameter and also the 'coarseness' of the punch ... the radio punch cuts a hole at the INSIDE diameter as listed; my 3/4" radio chassis punch maks a VERY exact 3/4" hole

3 - Where can I get the Motorola hole saw or punch?

>> I use the Antenex version, which has an easily-removable/replaceable fine cutting blade ... the Motorola unit is beefier though ... Dave Unkles KC2LT sells lots of this kind of stuff 603-421-0015 ... I have no commercial affiliation with his company ... you can generally also get goodies like this from Gene K2KJI at KJI Electronics in NJ ... same disclaimer ... but I have known both guys well over 20 years, feel free to drop my name
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K6UL
Member

Posts: 2




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« Reply #6 on: August 09, 2004, 09:44:52 PM »

Does a box store (home depot or lowes) 3/4" hole saw work ok for drilling an NMO modile mount?  I have never put one in, and need to do two installations.  The NMO 'specific' hole saws seem a bit pricey by comparison.  I am wondering if anyone has experience with lower cost alternatives.
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AA4PB
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Posts: 12847




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« Reply #7 on: August 09, 2004, 09:52:43 PM »

A hole saw would seem pretty risky to the headliner. I use a 3/4-inch Greenlee chassis punch. I drill a small pilot hole and then a larger hole to fit the threaded bolt for the punch. I have a wood spacer to keep the drill from penitrating very far thru the metal. Put the punch side of the chassis punch on the inside and the die on the outside so you can tighten the bolt with a wrench from the outside. Place a thin piece of cardboard between the die and the roof in order to keep the die from scratching the paint.

Before you drill the first hole, feel around the inside between the headliner and the roof to make sure the mount will clear any metal roof bracing supports.
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KC8YVE
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Posts: 51


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« Reply #8 on: August 09, 2004, 10:33:18 PM »

I agree with AA4PB hole saws can make a mess of headliners, not that I ever Did that Wink its embarassing trying to get that headliner out of that 3/4" hole when it gets all wraped up on the saw!

I use a special hole saw on police cars and fire trucks that limit the depth of the cut.

Greenlee punches work well.

Tracy.
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KZ1X
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Posts: 3228




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« Reply #9 on: August 09, 2004, 10:38:56 PM »

Good advice here from all, as is usual ... A conventional arbor-type hole saw, like you'd find in a hardware store, is a no-no.  Only the specific Motorola / Antenex NMO hole saw, or the 'RADIO' style 3/4" chassis punch (NOT the 3/4" electrician's punch) are going to yield acceptable results.
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WB1GOT
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Posts: 5




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« Reply #10 on: August 09, 2004, 10:40:31 PM »

I really recommend the 3/4 hole saw sold by Motorola or, second best, gaining access and using a Greenlee punch.

The Motorola hole saw also cleanly scrapes a small ring of paint just outside the hole to insure that the antenna is properly grounded.

If you decide to punch a hole you should really insure that the mount makes positive metal-to-metal contact lest stray RF flowing back down the coax shield do a tune on the computers in your car.

Bill - WB1GOT
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N0MUD
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« Reply #11 on: August 09, 2004, 11:02:13 PM »

Nobody mentioned the fact of dropping the headliner down at least halfway.  I have a Ford F150 and the headliner came down halfway with no probs and then I used  a holesaw to drill out the hole.  Worked perfectly and the headliner went back up just like it was made to and no fuss no mess.  Antenna coax nmo fit perfectly.

73's Mike N0mud
Colorado Springs, CO
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K0IPG
Member

Posts: 252




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« Reply #12 on: August 09, 2004, 11:54:55 PM »

Mike's got a good point - a lot of time seems to be spent trying to avoid the headliner. With my installation, the headliner came down about 1/2-way, which, needless to say, makes the hole-drilling process much easier! See how hard it is to get your headliner down - that might save you a lot of trouble.
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K6UL
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Posts: 2




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« Reply #13 on: August 09, 2004, 11:55:01 PM »

Thanks all for the great advice.  Your tips have promted a few more questions:

1 - Any issues with the hole punches on (very) slighly curved surfaces?  My experience is that they tend to try to flatten the surface out.  I am not sure how big an issue this is on typical roof curve.  My truck is a a Ford F-350.

2 - Seems like Greenlee make two types of punches (?) one for electrical, one for radio?

3 - Where can I get the Motorola hole saw or punch?
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W2WRX
Member

Posts: 27




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« Reply #14 on: August 10, 2004, 02:09:22 AM »

You can also use a Uni-Bit drill,they sell this at your local Hardware store.One bit comes in from 1/4"-3/4" or 1/4"- 1 1/2",there are about 5 different Bits,what they are,is that the bits are in steps of 1/4"-to what ever size you need to complete your task of job.Here's a site to check out or go to your Gooogle searh. W2WRX !!!!!!!!

http://www.northerntool.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=4006970&storeId=6970&langId=-1&productId=200137924&cm_ven=TL&cm_pla=DF&cm_ite=powertool
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