How would I machine a flat-bottomed hole with a boring head?

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clevinski

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Hi, All,

Several people have recommended that I buy a boring head for my mini-mill. I'm trying to visualize how I would use it, and come up with this question, which I'm sure you experienced machinists have resolved thousands of times.

I understand how one would use a boring head in the mill if making a through-hole in a something. But what I don’t understand is how you would make a blind hole, or a hole with a lip at the bottom? If you did this on the lathe, you would, as a last step, “face” the bottom of the hole or lip to make it flat. But that doesn’t seem possible with a boring head.


I could imagine doing this with a rotary table, using an end mill to machine the flat bottom surface, but only if the hole were shallow enough that the end mill could reach it.


How is this operation typically done?
 
There's a lot of variables, and without specifics as to the part your trying to make, size and depth of the hole etc... it's difficult to give a definite answer. With a boring head, it's generally easier to drill the hole first undersized, and then bore it to your finish size. The depth of the hole can pose some challenges too. The deeper the hole is,The more challenging it gets, because rigidity and reach become factors.

Creativity in setups is the name of the game. The more tooling you have the easier it gets to accomplish some of those difficult parts.

Sent from my SPH-L710 using Tapatalk
 
If the step or bottom needs to be finished perpendicular to the bore then you would use a boring and facing head. This type head has the ability to feed out and back while rotating. You can also use them to make grooves and do back facing. They can be quite costly depending on brand, age and mounting.

See here for an example of one of the brands available.
http://www.narexmte.cz/system/enMTE_vcc2C3_01AA.htm
 
I think you would have to drill and bore it out normally, and make/use an HSS flat bottom "form tool" to complete the cut. Just a boring tool with a flat top, long cutting edge.
You would want to leave as little as possible for that last cut to avoid chatter.

That's what I would do!

Bernie
 
Do it on the lathe if possible. Usualy get a more ridgid set up on the lathe.

If that's not possible then I use a boring bar with and angled tool bit.
I have square broaches so I make my own.

The bar on the left and in the center (in the pic) have centered tool bits so are for the lathe. I would build one like them but with an offset bit like the one in pic on the right.(to put cutting edge on center)
CIMG2088.JPGCIMG2089.JPG

CIMG2088.JPG CIMG2089.JPG
 
xalky said:
There's a lot of variables, and without specifics as to the part your trying to make, size and depth of the hole etc... it's difficult to give a definite answer.
Click to expand...

Hi, Xalky,

I understand, but since I'm considering whether it's worth buying one, I don't have a specific project in mind. Thanks for your comments; like most everything else in machining, rigidity is key...

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OldMachinist said:
If the step or bottom needs to be finished perpendicular to the bore then you would use a boring and facing head. This type head has the ability to feed out and back while rotating. You can also use them to make grooves and do back facing. They can be quite costly depending on brand, age and mounting.

See here for an example of one of the brands available.
http://www.narexmte.cz/system/enMTE_vcc2C3_01AA.htm
Click to expand...

Hi, OldMachinist,

Wow... I didn't even know such a thing existed. Seems there's a "proper" tool for everything! :thinking:

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itsme_Bernie said:
I think you would have to drill and bore it out normally, and make/use an HSS flat bottom "form tool" to complete the cut. Just a boring tool with a flat top, long cutting edge.
You would want to leave as little as possible for that last cut to avoid chatter.

That's what I would do!

Bernie
Click to expand...

Hi, Bernie, Rgray,

I can visualize how those would work for a lip, but how would you do a blind hole? Or maybe I don't understand the explanations?
:think1:
 
Hey Charlie!

Well, you know how the top edge of a grazed carbide boring bar is flat and square? If you made that out of HSS, and made that top edge a cutting edge, you could just feed it straight down. If the cut was light enough, tool sharp enough, it should do it. It is effectively a form tool, just a flat form :)


Bernie
 
I do this on the lathe. I use a run of the mill boring bar with a triangular insert, but any boring bar that cuts on its end, and whose width is less that 1/2D should work. In my case the insert is held with the flat of the triangle perpendicular to the bar - i.e., parallel to the bottom surface of the hole. Angle the tool so that the outside point of the triangle makes contact first. Now you can cut the bottom of the hole by moving from center to the edge.
 
rgray said:
Do it on the lathe if possible. Usualy get a more ridgid set up on the lathe.

If that's not possible then I use a boring bar with and angled tool bit.
I have square broaches so I make my own.

The bar on the left and in the center (in the pic) have centered tool bits so are for the lathe. I would build one like them but with an offset bit like the one in pic on the right.(to put cutting edge on center)
View attachment 62927View attachment 62928
Click to expand...

Russ, I recently used a homemade bar similar to yours. It worked fine, but I think yours might have more ridgidity. I like the orientation of the cutter and set screw in the middle bar. Will copy that design. Thanks for the picture! Larry
 
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