[TowerTalk] drilling chrome-moly mast

[Grant Saviers]

I agree with Jim.

There is a problem not mentioned with the "pin the mast" strategy.  That 
is the hole and pin need to be a "close" to "line" fit.  Otherwise the 
mast will move back and forth in the actual sizes of the 4 holes.  That 
leads to wear and high shock loads on the mast and antennas.  Eventually 
the holes (or bolt or both) wear enough to allow a large excursion and 
more shock.  When I fit a taper pin with a special reamer to pin a shaft 
to a collar it is a tight fit that locks both parts in place with the 
jam taper of the pin.  Not used so much anymore, but it is the classic 
way to prevent this problem.

Any hand drilling of alloy steel tube likely won't be round or anywhere 
near desired diameter.  Even done in a vise on a mill, if drilling thru, 
it is likely the hole on the far side of the tube will wander from true 
position.

IMO, if your mast clamp slips, install a better one.

Re clamps, I'll chime in also.

plain U-bolts only are the worse - if slightly off size, only two points 
contact, one at the flat plate and one at the top of the bolt.  Can be 
made to work using lots of them of the right size. Prone to distorting 
the tube so clamping force is limited for thin wall tubes.

muffler clamps are one better IF the right size - more contact area in 
the saddle, but the stamped saddle is prone to splaying out when 
tightened, see Cushcraft A3/4 designs

really good are the DX Engineering cast saddles of correct fit - lots of 
contact area in the saddle.  The double V ones are not as good with 4 
points of contact but are better than muffler clamps when the exact size 
isn't available, less distortion of the tube. My go to clamps for 
everywhere on antennas and masts.

near perfection are full top an bottom saddles with thru bolts - 
probably 300 degrees plus area contact, see JK Antenna designs, and the 
SteppIR DB36 I am assembling right now.  clamps tighten with very little 
tube distortion.  JK ones are CNC machined and SteppIR has invested in 
extrusions they slice into saddles - nice.  I make them when needed, eg 
driven element insulators.

perfection are the sleeve clamps, fit to the exact diameter of the tube 
- several folks make these as mentioned. Nearly impossible to distort 
the tube.

Grant KZ1W



On 8/29/2018 4:07 AM, Jim Thomson wrote:
> Date: Tue, 28 Aug 2018 15:29:03 -0400
> From: Warren Munro <wmunro1 at hawaii.rr.com>
> To: towertalk at contesting.com
> Subject: [TowerTalk] drilling chrome-moly mast
>
> <All
> <This may have been mentioned before, but my machinist brother-in-law advises drilling starting with a quite small bit and then use new gradually larger bits as you step up in hole size. This will cut down on the annealing, and will help eliminate the "bite" at the <end of the final hole.
>
> <73
>
> <Warren    KH6WM
>
> ##  IMO,  trying to drill a .375 hole  into a  2.00  OD  X  .375 inch thick  heat treated  4130 chromolly mast.....  while up at the top of a tower would be a night mare.   And trying to free hand it with a hand drill ??   Good luck with that.
> The original idea was to... pin the mast, so it wont slip.   IMO, it would be much easier and simpler to just use the correct  rotor clamp in the 1st place, like the K7LXC  or  K7NV units.  They dont slip period...end of problem..at least at the rotor
> end of the mast.   Where the boom meets the mast... that assy is easily  done  with  2-3-4  of the  DXE  super mast clamps.  They come in 2 inch ID,  2.5  inch  ID..and also 3.0 inch ID.   I used the F12 concept, which uses 2 al plates, one is mounted to mast
> using  several of the DXE  mast clamps.   The horizontal al plate is mounted to the boom with either  DXE  SS U bolts  +  solid  al saddles,  OR a pair of DXE  mast clamps.  Each al plate is  .375 inch thick.   Plates are mated  with 4 x .375 inch bolts.
> The  5th bolt is  typ  .5 inch.  I drill and tap the  5th hole into the  vert plate.   Vert plate is mounted to mast before hand.  Horz plate is mounted to boom.   Boom brought up and mated to vert plate simply by sliding hole  #5  over mating  stud  #5..
> done deal.  Then align the boom, such that the  4 x remaining holes line up.  Stuff  4 x remaining bolts in..and u are done.   F12  called that assy the ..easy on mount.     I just took the same concept, and beefed it up a bunch.  And no, the vert plate
> will not  spin on the mast, it cant.   Those  DXE  super mast clamps are things of wonder..with a massive a mount of clamping  force on each clamp.    80m rotary dipole gets a  8 x 14  vert al plate..and a 8 x 17 horz plate.  2 x DXE super mast clamps used on vert plate...and  6 x dxe ss u bolts with solid saddles used on Horz plate.  The 40m  assy gets a 8 x 17 vert plate..and a 8 x 14  horz plate.    40m  vert plate gets  4 x DXE super mast clamp..and horz plate gets  2 x DXE  super mast clamps.   Nothing can slip
> on the mast, no need to pin the mast.  K7NV  small PP  with its 2 inch mast clamping assy.   All TQ balanced, so minimal tq required to rotate..and hold it in a wind.
>
> Jim   VE7RF
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>