How To Turn A Shoulder Or Hollow Out An End

[sanddan]

I've also had a bit of trouble with the second version you show. If you have a through hole a boring bar works great but not so much for a blind hole with a flat bottom. I've used a center cutting end mill to get the flat bottom but without a tailstock DRO scale it was hard to get an accurate depth as the markings on my tailstock aren't very good. On larger diameter holes you can use the boring bar to get the bottom flat as you will then have clearance for the bar to cut from the centerline out to the final diameter.

 

[mikey]

I've also had a bit of trouble with the second version you show. If you have a through hole a boring bar works great but not so much for a blind hole with a flat bottom. I've used a center cutting end mill to get the flat bottom but without a tailstock DRO scale it was hard to get an accurate depth as the markings on my tailstock aren't very good. On larger diameter holes you can use the boring bar to get the bottom flat as you will then have clearance for the bar to cut from the centerline out to the final diameter.

Both of these bores are simple, routine bores. Sandan, if you have trouble with boring a blind bottom I would guess you were using a zero lead boring bar. This type of bar tends to chatter at the bottom of a blind bore. A positive lead bar is far more useful for these jobs because only the tip cuts. Typically, you would use a depth stop to rough out the bore to a thou or two less than final depth. On the final pass, the stop is removed and the cut is taken to final depth and ID. The bar is then fed in toward the centerline to produce a cleanly faced bottom before it is withdrawn.

 

[sanddan]

Not to highjack the thread but what is the difference between a zero lead and positive lead bar? A picture would be worth a thousand words here.

The blind hole I was trying to do recently was for a set of machinist jacks and the hole dia was less than 1". I bored the hole as I wanted a snug fit to the mating part.

 

[mikey]

A zero lead bar is square across the front and tends to drag in a flat bottomed bore. A positive lead bar typically has a 5 degree lead angle so that only the tip of the insert or tip cuts. Here are some examples, with the positive lead bar on the right:



Most solid bars - HSS, brazed, cobalt - tend to have zero lead geometry so they are flat across the tip. Higher end bars like Bokum and Criterion can have zero, positive or negative leads. Inserted tip bars like those above are usually zero or positive lead. The former is better for through bores or when roughing heavy; the latter is better for precision boring and in blind bores.