How to machine a hole on a lathe?

[SlugSlinger]

I started by facing both ends of this 4.75” diameter by 4.375” long aluminum bar. That went well.

The I drilled a hole using progressively larger bits ending with a 1.125” hole. That went ok.

Then I tried to part it in half. Things started degrading quick from here. I broke multiple parting blades and ended up cutting it with the bandsaw and re facing the cut ends.

I need a 3.5” hole in the center. I bought a .75” boring bar in hopes to bore the hole to size. Is this dumb? Is the a better way to make the hole bigger.

Advice from the professionals on here would be much appreciated!

 

[Perplexed]

I would have bored out the center hole before parting it. A parting tool with enough stick out to part through a mostly intact 4.75” diameter cylinder is going to have so much flex that the parting bits will chatter and break. As for boring it out, it’s kinda wasteful but using a boring bar is the way I would go, bringing it up to the proper ID.

And I‘m far from being a professional!

 

[Shadowrider]

Boring bar is the only way to make a truly straight hole as drills will always walk some. You can also take small "bites" with a boring bar.

Was your part off tool on center and aligned? They have to be perfectly in alignment or things don't go well. And keep overhang as small as possible, only stick out what you need to make the cut.

EDIT: @Perplexed nailed it. I didn't realize you were parting before drilling. That's a long ways to part off! Yes, drill and bore first! (before parting off)

 

[O4L]

Boring bar.

(It's been a looooong time since I was a professional.)

 

[SlugSlinger]

I would have bored out the center hole before parting it. A parting tool with enough stick out to part through a mostly intact 4.75” diameter cylinder is going to have so much flex that the parting bits will chatter and break. As for boring it out, it’s kinda wasteful but using a boring bar is the way I would go, bringing it up to the proper ID.

And I‘m far from being a professional!

I’m a suffering the consequences of my lack of patience and learning the hard way. I drilled the hole as large as I could, with the tools I currently have, and thought if I cut it in half at that point it would be easier to bore it. Apparently and confirmed, that is wrong!

 

[SlugSlinger]

Boring bar is the only way to make a truly straight hole as drills will always walk some. You can also take small "bites" with a boring bar.

Was your part off tool on center and aligned? They have to be perfectly in alignment or things don't go well. And keep overhang as small as possible, only stick out what you need to make the cut.

EDIT: @Perplexed nailed it. I didn't realize you were parting before drilling. That's a long ways to part off! Yes, drill and bore first! (before parting off)

I think you nailed why I kept breaking the blades. It was aligned but I think the material ended up off center after breaking the blade.

 

[red dirt shootist]

Boring bar for sure, you don't say how precise you need the ID to be, but still boring bar. Nothing wrong with using a big drill to rough it out, but if you have a secondary operation, like threading the hole, boring bar. I take it you indicated the OD when you chucked it up? And I agree with everyone else, too much stress on the parting tool. I see you're using a 3 jaw, if you have a 4 jaw I'd use that.

 

[Shadowrider]

I’m a suffering the consequences of my lack of patience and learning the hard way. I drilled the hole as large as I could, with the tools I currently have, and thought if I cut it in half at that point it would be easier to bore it. Apparently and confirmed, that is wrong!

Yea, bore it first but just to the length you need the part to be plus the thickness of your part-off tool, or there about. Then part it off with your part off tool going into the bored hole.

You will only have .625" wall thickness from your given dimensions and you are working with aluminum. The chuck will distort it easily. You could very well end up with a round hole while you are machining it and a triangular hole (assuming a 3 jaw chuck) when you unchuck the material. You want to be chucking on solid material if at all possible. Steel wouldn't be quite the issue but you still need to be aware. Chucks are strong...

 

[SlugSlinger]

Yea, bore it first but just to the length you need the part to be plus the thickness of your part-off tool, or there about. Then part it off with your part off tool going into the bored hole.

You will only have .625" wall thickness from your given dimensions and you are working with aluminum. The chuck will distort it easily. You could very well end up with a round hole while you are machining it and a triangular hole (assuming a 3 jaw chuck) when you unchuck the material. You want to be chucking on solid material if at all possible. Steel wouldn't be quite the issue but you still need to be aware. Chucks are strong...

Very good points. Thanks, I may need to rethink this project.

 

[OHJEEZE]

What was you using for lube on your parting tool? Generally all parting tools should be wet with some sort of lube.

If your parting tool is set too low, your part will want to ride up on top of the parting tool.

If I was machining your part, I would finish, id and od before parting.

If I can ovoid using a parting tool, I do!

Over the years I have found wd-40 the best lube for working aluminum. It keeps the aluminum from gumming up!

If I am drilling, tapping or parting aluminum, I will have wd-40!

I would not bother with a 4 jaw, unless I absolutely had to for accuracy sake.

If you can machine your part in the 3 jaw, dont hesitate to do so. If your 3 jaw has good repeatability, no wories!

If you remove your part from your 3 jaw and repeatability sucks and accuracy will suffer for what is required, then you might need a 4 jaw.

Rule of thumb is to finish your part before removal if and when possible whether using 3 or 4 jaw!

You want to have the maximum amount of your part in the jaws of your chuck and do your parting as close to the jaws as alowable.

Sometimes you'll want to use the tailstock and a center to help keep the part from flexing out of center and blasting your parting tool.

Many times it is best to finish the part with a hack saw when it is close instead of finishing with the parting tool!