Boring a flat-bottomed hole in a lathe?

[kizmit99]

Hi All - I'm looking for a little advice on the proper procedure for boring on the lathe.
I have a piece of 2.5" dia round stock (2" long, steel) mounted in a 3-jaw chuck on my G0602 lathe. I need to bore a hole 1.5" in dia, 1" deep with a (relatively) flat bottom. I've center drilled, drilled in 1" with about a 1/4" bit, then with a 1/2" bit. I have a small set of boring bars, like these:


My question is probably pretty basic, but I haven't stumbled across the answer yet - I'm also finding it difficult to actually put the question into words...

Should I be working the hole larger by removing metal from the 'face' of the bar (out to 1.5" dia, then return to center, extend tool further into the hole and work 'out' again, repeat), or should I be enlarging the diameter of the existing hole all the way to the bottom of the hole (retract the tool, slightly increase diameter, plunge again, repeat)? I could see either approach working, just curious which would be the "right" way to do it.

Thanks!

 

[Beone]

Why do you need a hole in your lathe?

ok I'll be good. I use the boring bar to widen the hole full or mostly full depth. A set so silver and demming drills to 1 inch might be worth your while. HF has a set for $29 that works fine for me. Much faster than boring the whole thing
dave

 

[pdentrem]

I would start widening the hole but not hitting the bottom until I was close to finish size, unless the depth is not really important. On the last pass or so I would now start from the center of the hole and feed out until I just touch the outer wall and then retract the boring bar. Some people would not touch the outside wall as they prefer to have it finish before finishing the bottom of the hole. I tend to sweep the bottom and as I retract the bit I do the last clean up / finishing pass on the wall. Up to you.

The boring bars will do the job or you can use a end mill held the same way as the boring bars. The first picture shows a boring in use but for threading in this case. The right hand end can be used for what you wanted to do as the bit is held at 45 degrees to the work.
Pierre

 

[mikey]

Hi Kizmit. If I understand you correctly, you are asking: 1) if you should attempt to set a depth of cut, enter the center hole and feed out to 1.5" and then reset the DOC and repeat, or 2) set the cross slide DOC and feed in to 1", then reset the DOC and repeat until the bore reaches an ID of 1.5", right? Personally, I would go with #2.

Those look like the ubiquitous Chinese sets that go for $10.00 for a set of 9 bars. If so, they are typically a zero-lead bar, meaning the brazed carbide tip is flat to the tip of the bar. This orientation is fine for through holes but not really good for boring to a flat bottom - lots of chatter. Moreover, the hardness of the shanks on these tools varies in hardness, from none to some but in no case is it consistent in my experience. This makes holding tolerances tougher. However, they will actually bore better than you might think and can take a fairly decent cut in steel. Finish is usually not great but they can hog. Like any carbide tipped tool you have to be careful not to dwell on entry into the bore - start feeding and do not stop or hesitate on entry into the bore; if you do then you can chip that tip and I speak from experience.

I would use the shortest, largest shank bar you have and be sure the boring bar holder is as solid as you can make it. Since you are boring to a flat bottom you probably should not angle the bar; set it parallel to the lathe axis. Use cutting fluid if you have it.

Like any bar, you need to know how the tool cuts. You need to correlate the change in ID to your DOC. By that, I mean set a DOC and see what the resultant ID turns out to be. Do this for a roughing DOC and finishing DOC, and they will be different; you need to know how the tools cuts so you can plan your approach to final ID. You will do this for any bar you use but it is especially applicable with these cheaper bars because you just can't tell how much they will flex.

When you can afford it consider buying some indexable tipped carbide bars. I would suggest looking for one with a positive lead and, on smaller bars below 5/16", go for a positive rake insert. An example of such a bar is the CCBI series from Circle Machine. You will not believe how much easier it is to bore accurately with such a bar.

 

[kizmit99]

Hi Kizmit. If I understand you correctly, you are asking: 1) if you should attempt to set a depth of cut, enter the center hole and feed out to 1.5" and then reset the DOC and repeat, or 2) set the cross slide DOC and feed in to 1", then reset the DOC and repeat until the bore reaches an ID of 1.5", right?

Yep - you got it. Sounds like the consensus is approach #2 - which I should have guessed, because my instincts were to go with #1 (Probably because when I've used the lathe to bore in the past it's always been a large diameter, but shallow hole...)

Dave - thanks for the pointer to the HF bits. An obvious time saver that never even crossed my mind.

 

[bfd]

drill as big as possible. leave maybe .015" total. with a boring bar (the ones you show will work ) flatten the bottom to depth. if possible don't worry about the final depth, if there enough stock you can cut the face to get the final depth right on. so bore a little deeper if there stock. in order to get a good flat bottom you will need turn the boring bat at more of an angle then normal so you can cut to center in the bottom of the hole. then finish the bore to size andtake a light final pass across the bottom. if it doesn't have to be so flat use an endmill center cutting to it the bottom first to depth and then use the boring bar to finish up the bottom. ps use a stop or indicator to get the bottom fairly flat to stop the boring bar at the same place every time this will make it a lot easier to flatten the bottom. bill if this is not understandable then start a conversation and I will clarify bill