Help with deep, blind hole boring

TGraves3 · Jul 12, 2021

All,
I need to put a blind hole 12” deep x 1.35” diameter in 6Al-4V Titanium bar stock. I need the bottom of the hole to be flat. My first thought was use a ~1.25” gun drill to establish the hole, then bring it to size with a boring bar, which would get me the flat bottom as well. The issue I’m having is finding a bar that will reach that deep but requires a small enough minimum hole size to fit. Anyone have any experience with this type of scenario? FYI: I’m a hobbyist with access to the gun drill, but I’m on my own for the bar, so I don’t want to drop a ton of money on it. Any insight would be appreciated. Thanks!

gbent · Jul 12, 2021

If all you need to do is square up the bottom of the hole make a flat bottom drill bit.

TGraves3 · Jul 12, 2021

Noted. Thanks for the insight. Any thoughts on the best way I can bring the hole to size? I thought about buying a used 1”x12” indexable heavy metal bar on eBay, then just adding an extension to it to get the depth I need.

EmGo · Jul 12, 2021

12" deep 1" hole with a flat bottom in Ti ? Can you retire early instead ?

This is gonna be a bitch

(They make flat bottom inserts for spade drills but I dunno, maybe. At least you could pump a bunch of coolant in that way.)

DDoug · Jul 13, 2021

TGraves3 said:
Noted. Thanks for the insight. Any thoughts on the best way I can bring the hole to size? I thought about buying a used 1”x12” indexable heavy metal bar on eBay, then just adding an extension to it to get the depth I need.

What is your tolerance on that "final size" ????

jscpm · Jul 13, 2021

To make the bottom flat, you can make a tool called a "D-bit" which is just a cylinder with a chamfer through the diameter. To bring to size you can use a cylinder hone.

Engine rebuild shops are experts at this stuff because they rebore cylinders all the time and they can probably do it pretty cheaply.

AlfaGTA · Jul 13, 2021

jscpm said:
Engine rebuild shops are experts at this stuff because they rebore cylinders all the time and they can probably do it pretty cheaply.

Don't waste your time! Engine shops almost never hone to a blind hole, They are in general piss poor at doing while keeping the bore straight....They are familiar with honing where the stones can pass outside the end of the bore, not stop at a hard shoulder or step.
We run into this all the time on engine blocks that have blind cylinder bores (Offy, Bugatti, Miller, Alfa etc.)
Also almost always a blind cylinder bore has a relief between the chamber and the top of the bore, so they never hone to a hard stop at the end of the bore....
To go further...Rarely will an engine shop have a 1 3/8" hone mandrel that can reach 12 " in length....its not a size they would need or use.

Can you tell us what machines you are planning to use here...Is this a turning or mill application...
Manual or CNC capibility?
Cheers Ross

TGraves3 · Jul 13, 2021

Tolerance is +/- .002”. It’s a manual lathe. I thought about the hone, but I’m with Alfa, I don’t think I could pull that off. I would be fine with just gun drilling, squaring up the bottom, and calling it good; but, I know the drill will walk. There is a hole in the blind end that needs to be concentric with the bore, hence the need for a boring bar to achieve that. That’s what makes sense in my mind, at least?

ezduzit · Jul 13, 2021

AlfaGTA said:
Don't waste your time! Engine shops almost never hone to a blind hole, They are in general piss poor at doing while keeping the bore straight....They are familiar with honing where the stones can pass outside the end of the bore, not stop at a hard shoulder or step.
We run into this all the time on engine blocks that have blind cylinder bores (Offy, Bugatti, Miller, Alfa etc.)
Also almost always a blind cylinder bore has a relief between the chamber and the top of the bore, so they never hone to a hard stop at the end of the bore....
To go further...Rarely will an engine shop have a 1 3/8" hone mandrel that can reach 12 " in length....its not a size they would need or use.

Can you tell us what machines you are planning to use here...Is this a turning or mill application...
Manual or CNC capibility?
Cheers Ross

Nothing here so far has suggested the need for honing. Should be able to achieve this with a boring bar.

Superbowl · Jul 13, 2021

TGraves3 said:
Tolerance is +/- .002”. It’s a manual lathe. I thought about the hone, but I’m with Alfa, I don’t think I could pull that off. I would be fine with just gun drilling, squaring up the bottom, and calling it good; but, I know the drill will walk. There is a hole in the blind end that needs to be concentric with the bore, hence the need for a boring bar to achieve that. That’s what makes sense in my mind, at least?

If the hole at the blind end has a reasonable size you can put a shaft through, you could make up a line boring jig to get it perfect.

TGraves3 · Jul 13, 2021

Hole in blind hole end is .625”. I didn’t think about a line bore setup… I’ve never done it. But, there’s a first time for everything I guess!

gbent · Jul 13, 2021

What other features are you going to throw at us? Now you tell us its not a blind hole, but a small but unknown diameter hole with a 1.35 diameter counterbore 12" deep.

Garwood · Jul 13, 2021

TGraves3 said:
Hole in blind hole end is .625”.

Read that a couple times and let it sink in.

gbent · Jul 13, 2021

Ok, missed the late 5/8. Gun drill the 5/8 and counterbore the 1.35. Far different, far easier problem than 1.35 blind hole 12 deep.

sfriedberg · Jul 13, 2021

TGraves3, do you just need a shoulder 12" below surface level? If so, consider a through hole/bore with a groove at the right place for a suitably stout retaining ring. I like Smalley spiral rings (or competing products) for things like this, and it's a dirt simple job to make an installation tool. Just get the groove width and depth correct. Engineering data, including load ratings, in the Smalley catalog.

DDoug · Jul 13, 2021

What manual lathe ?

Ray Behner · Jul 14, 2021

Can you drill through entire piece and plug the ass end with a 1.350 x .625 plug.

FJsapper · Jul 14, 2021

I’ve been doing similar work in 17-4 on a manual.

I drill with an extra long amec spade to .050” under final ID. I’ve done a flat bottom by drilling a preliminary hole .75” shy of final depth and then swapping to the flat bottom to finish out.

I bought a .75”x13.5” carbide blank on eBay (you have to dig but you can find them for under $100) and ground a tip on it. When I got tired of touching that up I grafted on an indexable head. At that depth you will be much happier with a carbide shank. I’ve worked at 12xD with that bar going suuuuuper slow and taking little cuts. It helps to hone a super fine edge on the insert and use a little oil.

After the spade work is done I do finish passes with the boring bar until complete. If you are going to thread the wide mouth end then do it in the same setup.

Once you finish that up you would flip and then make your threads as required.

You can do it, im a hobby guy too. Plenty of archived knowledge on here to put it all together. It took me a while to figure everything out.

FJsapper · Jul 14, 2021

Here is the insert i grafted on. You could obviously buy a ready-made solution but in my case that would cost more than my lathe did...DIY it is

Here is a shot down the bore (not a great pic). When you transition from normal spade point to flat bottom you will get some chatter. Just take it slow. You could probably drill the whole thing with the flat bit but i didn't. This one had an internal step towards the rear that had to be brought to a different ID than the front part.

Here is the XL spade drill mounted on the compound. I tried the tailstock and it was a pain in the ass to keep cranking. This is good enough. Eventually i will block mount it directly to the compound vs a tool holder. Just tighten everything up real snug if you go this route. Ask me how i figured that out....

TGraves3 · Jul 14, 2021

It’s a Mori Seiki 850. Thought about going the plug route, but I like the one-piece concept for added strength; no joints to worry about. FJsapper, thank you for insight. I’ve been trying to find a solid carbide blank to just grind my own geometry into, but I can’t find one long enough. Would need at least 16” or so to get the depth I need and have an adequate amount of surface area in the holder. For some reason nobody makes anything longer than 13”… from what I’ve found.

Help with deep, blind hole boring

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