Drilling cast iron for new keelbolts

[ianc1200]

I would like to drill for new keelbolts. The boat has a hog, false oak keel of about 6" deep, then a cast iron keel about 7" deep. Has anybody successfully done this? Using HSS drill bits at slow speed and with lubrication I'm guessing only getting about 3/4" in 45 minutes.

To give context, this is a 70 year old motorboat. The original, or perhaps I suspect for various reasons, previously replaced, bolts are rusted and almost non-existent above the hog. They are rusted in, and are strangely through the original steamed timber (see previous thread re steaming and laminating). Seems to make sense to cap them, then drill for new between the new laminated timbers. The bolts are 16mm diameter, and we've tried drilling a 8mm pilot hole first, with little noticeable progress.

 

[Tranona]

never done it, but imagine hard slow work. However why not use studs or bolts from inside? then you only have to go maybe 2" and tap a thread

 

[penberth3]

I'm no expert, but ISTR CI needs drill bits with a different cutting angle to the normal off-the-shelf. Also got vague memories that CI doesn't need a drilling lubricant. Are you hand drilling at 3/4" or have you got some sort of drill stand?

Any reason not to drill out and re-tap the existing holes?

 

[Daydream believer]

Cast iron is a relatively soft material. I would not drill a pilot hole so big as 8mm as it helps the final bit to dig in. 6mm would be my preference. You only need a pilot hole for the centre of the main drills clearance. Then i would be looking for a powerful drill running at about 75 RPM held in a drill stand which would have its base levelled & clamped to the surface of the hull. I suggest long screws on to wood packings which are also screwed down. You do not want to hold the drill in your hand as you will be waving it about like a flag staff & the drill will not cut a neat hole & will just blunt itself.
Using this set up one can then apply a steady even pressure on the drill bit. I am not sure that one needs much lubricant as cast iron contains graphite. That being said it will be a low grade cast iron so you may have to keep stopping to allow the bit to cool.
Get a decent set of HSS taps (Not carbon) from Tracy tools (google them) & a drill bit of correct diameter ( Osborne is a good make for drills) & use a tapping lubricant from Rocol. I am not sure what one as my tin is about 50 years old.You need a decent tap holder to avoid broken taps.( google it)
If you are not proficient at sharpening your own drill bits then get a couple. Ideally you need to sharpen it with 4 facets.
Bear in mind that a bolt gets enough grip when it is in a nut which for a 3/4 is about 15mm. So you need to drill about 5mm to get past any damaged hole from tapered drilling then 15mm of good tapping then another 10mm for the end of the taps. So a hole 30mm deep would be enough for a stud to get a fair grip. It really depends on how many bolts you are putting in. If only a few then a bit deeper to avoid a tear out around the stud.

Wear gloves, the dust gets in the skin & you will take ages to clean your hands. I hate turning CI.

Finally you may find that for low speed a masonry drill can be hired from a decent hire shop, or concrete cutting firm, along with the stand. It will have plenty of power & run slow enough. I have used my SDS plus drill, with a jacobs chuck adaptor, before now, for larger holes. Easy to hold & slower than many 240V drills

 

[DownWest]

Just doing this on a cast iron shallow keel with with a plate hinged in the centre.
Cut off the original bolts (very rusty!) with a little proud. Made up a jig that fitted over the stubs, with a hole to drill the centre at 8mm down, so I could drill out the the rest to the 12mm original thread, then re-thread to 14mm. So, done a couple , but not easy. Six more to go.

Missed DB's post, good info.

 

[Daydream believer]

.deleted comment too late.

 

[ianc1200]

Many thanks for all the replies. I should have said this is for keelbolts which go through the keel with a head/nut under the keel - they are not threaded into the keel. We had a frustrating morning but I met one of the retired guys from the marina at the Yacht Club and he's going to have a look at what we are doing wrong next week. He says should be relatively easy when set up correctly. He says the sharpness of the HHS drill bits & angle are crucial, as is using cutting oil not three in one as we have been doing. Before I met him I tried drilling horizontally into the cast iron - one HHS bit was noticeably better than the others. We had cut off one bolt head, drilled a small hole and tried punching it out with also no success, but again noticeably easier to drill down through this bolt. Thanks DD Believer for your comments - interesting 75rpm & a smaller pilot hole - I'll investigate if the retired guy's methods don't work.

This is the iron keel under the false keel.




This the horizontal test hole we tried



The hog where we are drilling down.

 

[Tranona]

Just because the current bolts are all the way through does not mean you can't use threaded studs for the new. Presumably you are not removing the keel, just making it secure.

 

[burgundyben]

Looks a bear.

I took a set out of a Huntsman, 8" long, 5/16ths, I made a hole saw from tube, then installed 10mm.

Good advice abounds, I think once you work it out the others will be easy peasy.