One of these:






Is Harbor Freight 'good enough' for a few NMO holes?


Because I went to ACE Hardware and they had them; $55 and $50 in the two sizes they had.  Talk about sticker shock

invest in a greenlee. they are expensive but will last. they lase even longer if you drill a pilot hole first

FPNI

and they wear out used at high speed. run them slow or they dont last too long

Laird Tech HS34 is the basic industry standard hole saw for commercial installing.

If you are only going to make 3 holes, the HF will work. Otherwise Greenlee!!!

Go with the hole saw. Much less swearing involved.

I actually have a 3/4" hole saw.

But it leaves a rough edge.  I do have a burr removal tool.

But I watched a sirennet YT video Charger install and the guy used a step bit on the roof and it came out squeaky clean.  Burr free on the side that counts (the o-ring side)

Knockout punch is easier on the gear or whatever you are drilling in. Just drill pilot hole, then tighten with wrench or ratchet until it pops. Perfect hole ever time and usually cheaper than step bit

I've seen those but can't see how to use one in the center of a car roof (unless I were Mr. Fantastic )

Take a quick read on the antenna install saw info sheet. The design keeps you from cutting through a head liner the stopping shoulder also has a second purpose. "Shoulder scores paint for a better ground."  Sometimes people suggest a Greenlee punch. Those do leave a nice clean edge but depending on the installation you might not be able to get good underneath access.

Laird Tech HS34.pdf
Laird Tech's antenna mounting instruction .pdf

'good ground'

that could be worth it

I was dropping the headliner anyway so that wasn't an issue.

As the diameter of the hole increases - the feed and speed needs to be reduced.
This is simple Machining 101 stuff.
A good way to remember speeds is that a 3/8 electric drill turns about 1800 RPM's - when drilling in wood - you can turn the bit as much as 4000 RPMs, but when drilling a steel alloy you have to slow it down to about 300 RPM's..   Plastic you can turn it about 1500 rpm's and still make a decent hole.
Although when drilling plastic - if the plastic is hard - like Plexiglass - you should run the drill bit backwards.
The friction from the drill bit will cause the bit to remove just enough plastic to make a hole the same size as the bit.
If you run it in the forward direction and it gets a burr when it gets to the bottom of the hole, the Plexiglass usually fractures and cracks.

When you increase the bit size to one half of one inch - you have to reduce the drill speed to 229 rpms for a steel alloy and 3000 rpm's for wood and 2300 for brass or bronze..

*Surface Feet Per a Minute

RPM = SFM X 3.82 / Drill Diameter

Aluminum and its Alloys 200-300

Steel - Mild (.2 carbon to .3 carbon) 80-110
Steel (.4 carbon to .5 carbon) 70-80
Stainless Steel
Free Machining Grades 30-80
Work Hardening Grades 15-50

Wood 300-400

Feed per drill revolution

Drill range     Light                 Medium               Heavy
1/16 to 1/8    .0005 - .0010    .0010 - .0020     .0020 - .0040
1/8 to 1/4      .0010 - .0030    .0030 - .0050     .0040 - .0050
1/4 to 3/8      .0030 - .0050    .0050 - .0070     .0060 - .0100
3/8 to 1/2      .0040 - .0060    .0050 - .0080     .0080 - .0120
1/2 to 3/4      .0050 - .0070    .0070 - .0100     .0090 - .0140
3/4 to 1         .0070 - .0100    .0090 - .0140     .0140 - .0200

I bought this HF 3-piece set years ago thinking they'd only last a few uses. They've surprised me. Still going strong and I've used them to bore out a lot of thick pieces of steel. (I always use cutting oil)

LINK

This.  Get a regular NMO hole saw, don't screw around with step bits.

A large flat screwdriver will deburr the hole  in a few seconds if that really concerns you.

Taking the headliner out is ridiculous.  I've installed several hundred NMO mounts and have never removed a headliner.

The HF steps work just fine and when your done with this application they will work for quite a few more.










 

how did you run the coax?

Generally the rubber trim at the side of the headliner pulls right off.  The most frequent way is to tape the end of the coax to a short piece of fish tape or equivalent, go in the hole and out the side, then down a pillar.  Do recon ahead of time after pulling the rubber trim off and maybe through a removed dome light or overhead console to make sure you don't drill into a support beam reinforcement or something like that.

This is good advice. However IF you are drilling one hole, AND you drill a pilot hole the Harbor Freight one might last through the job.

Get the Laird one mentioned above. I just got one and drilled my first NMO hole in my truck. One work of advice though. Drill the pilot hole with the bit, then stop before the saw hits the roof. Then carefully place the saw portion of the bit on the roof and start slow. If you do it all in one motion, you'll break teeth off the saw portion of the bit.

HF step bits are damn good for the money.

I often drill a small pilot hole with a smaller bit which ensures the hole is exactly where it's supposed to be.  Also if you have access to the underside and need it in an exact location, say within a support in a trunk, you can drill the small pilot up from the bottom, then drill the hole saw in from the top.

It would work fine... I'm not going to argue its the best tool for the job but it should work fine.

this.

important note: a 3/4" electrical knockout punch such as you find at Home Depot or Lowes DOES NOT MAKE A 3/4" hole -- it makes a larger than 3/4" hole.

summary: make sure the punch you obtain makes a 3/4" hole.

ar-jedi

wood?  plastic?  plexiglass?  "electric drill"?

what does all this nonsense have to do with making a simple 3/4" hole in mild sheet steel, such as a car body panel?

even after i buy a Delorean with stainless body panels, or an MRAP with 1/2" thick AR500 plate, and i have to mount a thru-hole NMO antenna, i'll not consult your post again.

ar-jedi

the usual ARFCOM HRF offer -- if you want to borrow my NMO punch, simply pay shipping both ways (probably $8-10 total) and squirt a little BreakFree on it when you are done.

don't use it to punch through 1/2" thick AR500 plate though...

PM if interested.

ar-jedi

Irwin step bit. Had one since high school and has been used on many projects.

After 2 NMO holes, I invested in a NMO saw ($35 from Hutton's or Tessco) for the other 6 holes,

this.

just gain access to above the headliner through whatever means is easiest.  this could mean through the dome light, or other.  or, take some edge molding off and gently pull one corner of the headliner down, just enough (as described above)

but to reiterate the MOST IMPORTANT step: measure twice, drill once.  DO NOT BLIND DRILL because if you hit a structural member in the roof you are going to hate your life.  there is no easy way out of that.  

just FYI... below are pics from a writeup i did a decade ago over on Tacoma Territory (TTORA).  while probably not exactly applicable to your situation, you do see how the dome light opening is key to the entire operation.  from that hole you can see where the structural members are prior to drilling, and then use that same hole as an aid to reeving the coax from the mount to the side of the vehicle.  

ar-jedi

I was investigating my car a bit further and it should not be difficult to leave the headliner in place, snake the cable from the hole to the side, over the side airbag thingy, down the B-pillar (behind the mouldings), out the bottom moulding, over the carpet, under the seat, to wherever.

Now quick question:  do I need the under part of the NMO mount to be on bare metal?  Because a peek between headliner and roof seems to show a fully painted roof underside.

The antenna ground plane doesn't have to be attached to chassis ground. However when you tighten the mount up it'll scratch the paint up just enough (on the underside) to ground out to the chassis. Most mounts will also ground out through the actual inside of the hole.

the underside is electrostatically primed.  it's tough stuff.
take some emery cloth and on the underside of the roof scuff around the perimeter of the hole to show bare metal; about 1/4" out should do it.
this ensures that the bottom of the NMO mount makes good contact with the sheet metal.
if you have a bit of electrician's anti-oxidant paste you can apply it to the underside before installing the mount.  if you don't, don't worry about it.

ar-jedi

Is it feasible to get to the underside from the top, ie thru the hole?

Otherwise I do have to drop the headliner

Emery cloth or some sandpaper should do the trick if you just stick you finger in through the hole ( a 1" strip or so). The edge will be sharp though so  work accordingly.

^^^ that.

or, hire a very small person.

ar-jedi

I use the antenna saw, works mint.deburrs and clean ground path. Hole saws wobble and are a RCH too big for the base.
The greenlee punch is great too, sunvisor,dome lamp, A and B post trim removal and you can get your hand inbetween the headliner and roof
dedicated sheetmetal cutters are far more accurate than hole saws.

But what point angle do i need?