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[TowerTalk] Drill bits

To: <>
Subject: [TowerTalk] Drill bits
From: "Jim Thomson" <>
Reply-to: "Tower and HF antenna construction topics." <>
Date: Wed, 24 Mar 2010 01:38:06 -0700
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Date: Wed, 24 Mar 2010 04:11:35 +0000
From: Guy Molinari <>
Subject: [TowerTalk] Drill bits
To: <>
Message-ID: <BAY129-W14493CAE03D374CEF078CDE8250@phx.gbl>
Content-Type: text/plain; charset="Windows-1252"

Hi all,

    I don't know why this is so hard.   I'm trying to drill out some hole on 
some 3/8" steel plates that I'm going to have galvanized.   I bought a 13/32" 
drill bit and it worked great for about 10 holes or so.   The problem is that I 
have about 140 more to go.   Any recommendations on a good quality drill bit?   



Guy, N7ZG

##  13/32 = .40625"     3/8= .375"     .40625- .375 =  .03125"      That's a 
big difference in diameters... even with  galvanizing.   I think the 
galvanizing is only  aprx .004"'  thick.    If these holes are for 3/8"  bolts, 
perhaps you should be using  a  25/64"  drill
bit??      I have tried them all... and was not impressed with the titanium 
bits.   The packaging on the cobalt bits warns of not overheating cobalt bits.  
   I worked on loan at our tool crib at work one summer, abt  30 yrs ago.  The 
Dormer brand, and also the Butterfield brand HSS drill bits   [ both made in 
the UK]    didn't fare too well.     I ended up lately, using Zirconium bits.  
They appear to last a long time, and also have  THREE sides to the shank, so 
they never slip in a chuck. 

I don't have my book handy either, but for 13/32 holes in 3/8"  thick mild  
steel plate,,,, it has to be slow... I think around  300 rpm or less.    How 
hard is the plate ??  As noted, you need lots of cutting fluid.  Overheat the 
bit.. and it's toast, it will have lost it's temper.... permanently.  

Plan  B  is have em all punched.     I bought one of these  small roper whitney 
#5 jr  compound hand punchs  for 1/8"  thick and less...  6061-T6.... but they 
only go up to 9/32"       I have a requirement to punch a lot of  3/8"  holes 
in 3/8" thick  6061-T6 plate....  so bought a  bigger roper whitney punch.... 
think it was a model  #16 or #17..... with a 3.5"  throat.   Think my new punch 
is 7-9 tons.      It  will punch holes up to 9/16".    When punching holes in 
6061-T6.... only 60%  of the tonnage is required  VS  mild steel  of the same 

If your local machine shop wants a small fortune to punch  140 x holes..... I 
would be inclined to buy/beg/borrow a  larger  roper whitney punch.    My new 
one has a 25" steel forged handle.. and is bench mounted.   It's a   cam action 
arrangement.  The secret to using the punch is to bring the work piece    up 
into the center punch mark.. and not the other way around..... but this is not 
always possible.     I'm sold on punchs.  We used electric-hydraulic punch's at 
work yrs ago, and punched literally hundreds of 3/8" holes in 1/4...3/8" thick 
mild steel.   If ur just drilling 20 x holes or less... a drill bit will  
suffice... but with 140 x holes.. a punch will make short work of it.  Drill 
presses are slow and cumbersome as is.  Material  has to be precisely aligned, 
everything clamped down, check, and make sure nothing shifted a fraction of a 
mm.   Then add lubricant, drill, then clean up the mess.. undo 3 x clamps,  and 
repeat  for next hole.   Bottom line is... I can pu
 nch 15 x holes  faster that I can drill  just one.   The punched holes are 
always dead on, and clean holes. 

The punch must have the  correct diam  die  for the thickness  of steel to be 
punched though.   The thicker the steel, the dies have to be oversized even 
more... or they will bind.   Oversized dies  will not work on thin stuff... you 
will get ..'roll over'  on the edges.    I just  came back late today from the 
metal store.. with a trunk  full  of  6061-T6  3/8"  plate.. for boom to mast 
plates... and also a bunch of  4"  channel 6061-T6.  [ 40m brackets, since the 
F-12 flat plates ones all bent slightly, 340N]     The new punch will make 
short work on the new plates.   I have also used punched holes.. as pilot 
holes, that are in turn drilled out to a bigger diam.    This is in cases, 
where I didn't have the correct size large  punch + die... or the larger punch  
was beyond the tonnage capacity of my punching tool. 

I wish I had bought one years ago.  

Later... Jim   VE7RF   


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