drilling a 20mmx 300mm Hole in cast iron

OK, that would make sense. Seems like gun-drilling related advice is on the right track then.

PS, here's a thought, for what it's worth (maybe not much )

If the job in hand is a model cannon barrel (and here we are speculating but with some basis) might it be best to make it from a length of cored round cast iron bar? It can be purchased from some suppliers with the hole already up the middle. Of course it's rough cast but could be bored out with a boring bar of some sort, even an improvised one in the drill press with a guide and bushing etc. Or it could be drilled with a standard drill on a smaller diameter extension piece, in stages.

Of course if the cannon is to be a muzzle loader, some way of closing t'other end will need to be made up: threaded bung etc?

Unless it was to be a working cannon, who would care if the hole was 6” or 12” if muzzle loaded. But we have no real idea of what is wanted. Just guesses unless the OP enlightens us.

I found it easy to extend a standard drill (9/16" in my case) by turning the shank and using a standard die to cut a 3/8" BSF thread. Then I made 2 silver steel extensions, threaded to suit each end, and each one a bit less than my tailstock quill travel.

When the drill shank disappeared up to the threaded stub, I withdrew the tailstock quill, screwed on the extension and carried on.

This was about 15 years ago, so my memory could be imperfect, but I don't remember it as being particularly difficult.  Obviously you have to pull right out regularly to clear the chips. I think it was a 10" deep hole, on a Myford Speed 10. I've still got the extensions, which are convenient to use as extension tubes for Allen keys.

Edited By Mick B1 on 10/05/2018 17:40:46

On the basis as a one off hole i personally wouldnt entertain a gun drill.

Have over the years extended many straight and MT shanked drills, mills and reamers exactly the same way as i make up my drawbars.
Drills are soft only hard just before the flutes start.

Drill out a recess in the drill opposite fitment is a spigot decent fit. V round the edges and you may get away with silver soldering. On draw bars wont last that long in which case mig weld and run the severe risk of runout.

I would make two extended drills up one smaller like 18mm or 19mm and another the 20mm which could be an extended reamer run at a much slower pace with coolant.
Should be able to pick the two up for about £20.

The only downside i can see is if the machine drilling with hasnt got the torque, 1hp minimum and have trouble at that, certainly after 3" deep.

Gone out to the workshop and found the extended tool:-

There's also a 13mm drill with a threaded shank, so I think I piloted the hole with that and then finished with the 9/16" slot drill above. It was straight enough for a length of 14mm bar to pass down it. I think there was a third extension piece, now consumed on some other project. Unhardened HSS in drill and slot drill shanks turns quite easily and cleanly.

How about regrinding the carbide end of a cheap 20mm masonry drill to suit CI ?
Far cheaper than long and extra long series bits.

https://www.bes.co.uk/20-mm-x-300-mm-sds-drill-bit?ref=gs&gclid=EAIaIQobChMIk6aF84_92gIVDZPtCh0C8QK2EAQYAyABEgKaz_D_BwE
 

Edited By john carruthers on 11/05/2018 08:22:22

Hi

First of all thanks for all your advice. I will look for a piece of cored bar as Hopper suggests and then use the drill extension that Rod and the others have mentioned.

It is a through hole and it is for a cannon barrel. I am probably going to have a go at the type shown below.

It needs a through hole so I can put riffling in it, the breech end will then be blocked up with a screwed on cascabel. The real ones had the cascabel screwed into place also.

Thanks again.

Rich

Whether rifled or not, best not drill a touchhole through to the bore unless you're putting the finished cannon onto a Firearm Certificate. Near where I used to live in Redditch, someone had a model cannon destroyed by the police and I don't know whether they escaped prosecution.

How are you proposing to rifle it? That seems to me the most difficult process to do at home.

Is it an Armstrong gun?

Button rifling the cast iron barrel might be quite fun to try.

Jon

My understanding of button rifling is that it uses a hardened blunt protrusion to form the rifling by compression/deformation rather than cutting. My *guess* is that such a process might well destroy a cast iron bore?

Ahhh, I stand corrected then.

I thought it would cut a bit like a broach and the cast iron graphite would aid lubrication.

Unless a working model - and yes, the UK firearms laws may well be flouted if that were the case - I see no reason why the hole needs to be 12” long.

Only the (visible) muzzle end would need rifling and the ‘breech’ end could be turned out to a much larger dimension, to lighten the whole thing, and the cascabel fitted and pinned permanently in position.

Suitable weakening of the structure in the unseen area could be implemented to ensure it was not ever going to be a viable firearm.

Personally, I cannot see any point in making the internals completely faithful to the scale of the original. Particularly in this day and age! Working 3-D printed replica weapons are bad enough without adding possible further notoriety for the press or others to complain about.

Edited to add:  why not make it 0,729” bore add a centre firing pin and be done with it.  An illegal 12 gauge shot gun rifled to take solid slugs?  Actually, some say 12 gauge could be as big as 0.8” (20.3mm).  Plod would not be happy, for sure!

Edited By not done it yet on 11/05/2018 09:26:26

Not sure I'm in a position to correct anyone. I'd just think, once you've got a master spiral and a drawbar, a sharp square-ended cutting tool for the grooves with low or zero rake would be the safest way to do it.

Because of the risks, I'd probably want to do it straight after the bore and before putting the work in to make the cascabel, and form and polish the external profile. But I'm not talking from direct experience of using CI for this sort of project.

I hope this doesn't become a debate on illegal firearms. It is not going to be a working model as it will not have a vent drilled. My intent is to make a good quality scale model of a cannon on a pivoting carriage and not to make a working illegal firearm. Just because it looks like a cannon doesn't make it any more of a weapon than a piece of blocked up scaffold pipe or a bar with a hole in it. Its going to be a good quality model.

Thanks

Yes. I'm a model engineer - and if something I make doesn't work, I'd want it to be clear that it's because of compliance, not incapability.

If you were gonna put it on ticket, wouldn't you want to do proper little Palliser shells with chilled iron point, bronze driving studs an' ev'rything?

Mick

How do you chill iron

As it is non working I wonder if it might be easier to make in two parts with the chase from smaller dia stock that could then be screwed and loctited into the reinforces. Assuming the whole cannon is going to be 12" long that would make each bit about 6" which would be a lot easier to drill and you could just riffle the chase which would be long enough for anyone poking their finger in the barrel to think it goes all the way.