Need advice re drilling from engineers

[PhilipA]

I have today had made up a longer towball mount for my camper trailer to allow opening the D2 rear door to clear my newly made winch mount to close the hard floor camper.
I bought a 330MM towbar mount but the only type available has about a 150MM "drop" compared to the original 30MM drop which is just about perfect. So I cut apart my 30mm drop receiver and had a trailer manufacturer certified welder weld a blank bit of 50MM square tube onto the original head.
I redrilled the pin hole in the 330 MM to 290MM ( vs 220 before) which allowed the door to Just clear the mount with an old 11/16 stepped bit that I have had for 20 years which I did in stages , as the old bit is pretty blunt and it took a lot of time and brute force and chattering to do it.

So I went and bought a new 11/16 drill which is not easy as Bunnings and Masters skip from 16 to 18MM and don't have 11/16. It is a Chinese Atlas.

Now my question is whether it is better for the drill bit to do the hole in one go or to do it in stages like 5MM, 10MM, 13mm and 11/16. I have Bosch cutting oil. The drill is a cheapo 600watt ALDI bench drill.
Any advice appreciated as I wonder whether doing it in stages tends to blunt the outside portion of the drill.
Regards Philip A

[justinc]

Hi Phillip,

If using cutting oil, a sensible rotational speed and pressure (feed) to suit the bit diameter in use then stages are always better. Only a few are required, the ability for the bit to cut more at the face than the shoulder is preferrable.

JC

[V8Ian]

Drill a 10 mm guide hole.

[Blknight.aus]

Measure the center of the bit where the two cutting faces meet, your minimum pilot hole should be no smaller than that, the maximum should be no more than about 2/3 of the drill bits diamter or you will increase the chance of chattering or chipping the outside of the cutting edges off.

[Bigbjorn]

The pilot drill should be larger in diameter than the chisel point of the next drill. If you only have a feeble bench drill with insufficient power to drive the 11/16" drill in one pass then I would drill in three steps. 11/16" is a standard size. I can't understand why the size is hard to find. Cutting speed for HSS tools in mild steel is 9000/diameter in mm.

[Rick1970]

What he ^^ said......


1/4 - 5/16 pilot, then 11/16. Moderate speed, coolant and keep moderate pressure on the bit.

[Andrew D]

Philip

Unless the entire modification has an Engineers Certificate you will be accepting full liability if it happens to fail.

Having a certified welder complete a portion of the work does not impart liability to the welder. The welder can certify the weld that has been done, but not the (your) intended purpose of the entire arrangement.

Regards
Andrew

[lr110qld]

Much better to use a rotabroach than twist drills for that kind of work.

[PhilipA]

Thanks to everyone who responded.
Regards Philip A

[Tombie]

Phillip, extending your tow hitch by that much will have greatly reduced its rated towing capacity / ball weight...

Best get it checked mate..

[Aaron IIA]

I don't like the idea of extending the hitch. Much better to make a new one the length you want.
Aaron

[PhilipA]

My camp'o'matic is less than 750 kg loaded with about 70 kg ball weight. I don' t think it will worry the hitch as it is for all intent a new hitch.

I made the hitch with less drop to minimize the length increase which will end up at about 60mm from the stock 220 mm. The 330 mm hitch I bought still has AFAIR 2250kg rating, so 280mm is a non issue. I am trying to minimize the length increase by raising the pivot point which brings the door away from the winch mount which is high, so every mm you can raise the hitch means a mm more clearance.

The other consideration is I can flip the small drop hitch if necessary but not the one I bought as the door fouls on the mc hitch. My research found that there is only one drop available in longer hitches. That is why I did it to try and minimize the length increase.

I would defy anyone to be able to be able to tell that the hitch is not as supplied by a hitch manufacturer. The new shape is even stronger than the one I bought as the weld is more in compression than the vertical weld ,as it is on a 45degree angle and also longer. The hitch at 220 mm had a 3500 kg rating.
I guess I am a bit of a perfectionist but I see enough value in being able to open the door fully with the camper trailer hitched say at lunchtime , without raising the lot with the jockey wheel to offset any loss of departure angle. It is all a balancing act as I couldn't mount the winch lower or further back as the handle has to clear the front tool box.
Regards Philip A

[PhilipA]

To Tombie. I don't see any need to check as the hitch is made of stock hitch components and welded by a trailer company who weld towball mounts to trailers every day. That is why I had them do it as there are few places around who have the BIG welder , experience, and qualified welder to do something like that.
Also, as above the camper weighs less than 750 kg with small ball weight .
In any case in a worst case scenario I have very beefy cross over chains with 1tonne rated shackles but I am very sure that it won't happen. I don't take chances.
I have to laugh about standards a bit as the long 330mm Chinese Supercheap receiver came in a nice box with specs on the outside, stating the pin hole is 16.5mm. Funny that my 17.2 mm 11/16 bit fitted straight through.

I have seen caravans with 200 kg ball weights and equalizer mounts on 330mm hitches, which makes me grimace.
Regards Philip A

[POD]

Sounds to me like you have taken all precautions, particularly if you keep your extended hitch exclusively for use with the lightweight camper trailer. You didn't mention the wall thickness of the 50mm SHS, I would be using the heaviest available, which I think is 5mm.
The issue with extending the hitch insert is not so much in the strength of the insert itself, but the increased moment applied to the attachment points at the chassis. Safety chains are usually attached to the towbar assembly, so would be small comfort if the bolts mounting the towbar to the chassis failed due to increase in leverage acting on them.
The original owner of my 130 had fitted a hitch insert about 600mm long because of the big rear overhang, absolutely ridiculous. I'm in the process of extending the drawbar on my camper trailer as a much more sensible solution.

[Bigbjorn]

Much better to use a rotabroach than twist drills for that kind of work.

A decent drilling machine and rigid set-up will drive a small drill like an 11/16" through mild steel in one pass without a pilot hole. 500 rpm is about the right speed. If your machine doesn't have a coolant system, use an oil can and some Dexron, or EP diff oil diluted with laundry detergent and a bit of water. If deep hole drilling ( 2 or 3 times diameter) you need to be able to flush chips out of the hole. The diff oil mixture is good for stainless machining.

[Tombie]

All good Phillip, wasn't having a go - just pointing out may be placing additional leverage at vehicles rated towing capacity.

Based on your use - no problem!

Enjoy!