Boring Bar Tip Orientation

[the penguin]

I have 4 different set of carbide insert boring bars, 2 made in the US and one made in Poland and the final one made in Taiwan. Two of the sets use CCMT inserts (21.51 and 32.51), one of the set uses TPGB inserts (221 and 321) and the final set uses TT type inserts (221). All of the sets have flat bottoms, tops and sides, on the rounded shaft so the can be used in the standard QCTP blocks. All of the tip observations are based on using the standard QCTP tool block.

On the CCMT boring bars, both of the sets, the insert has a slight downward tilt (5-7 degrees) in the front, but one set the insert is level, side to side and on the other, there is a similar tilt down on the cutting edge side (again about 5-7 degrees). Which one is right or doesn't it matter.

The set that uses the TPGB inserts, the insert is almost level front to back and side to side.

The set that uses the TT stype insert, the insert like the one for the CCMT is tilted dow in the front (again about 5-7 degrees) and with about half of that tilt (2-3 degrees) toward the cutting edge.

When I use the QCTP blocks that made for boring bars with the pinch sleeves, obviously the insert can be orientate anyway, up or down. I realize that the boring bars have different inserts and are made by different mfgrs, since I almost alway using the boring bar QCTP block, where is the best spot of cutting tip location, or is it dependant on the type of insert used. Any rule of thumb or guide line would help.

This topic came up in conversation today with a fellow home shop guy and neither him or me had any concrete data to base a solid conclusion on.

[PixMan]

Jack,

For starters, could you look at the nomenclature off the boring bars? Most will have something akin to "A08M-SCLCR 2". Boring bars are the most common tool to find with small differences in construction that can make a big difference in performance. Should you see (for example) "A08M-SCLPR 2", there's a difference right there. The former will use a CCGT or CCMT 21.5x or 32.5x insert. The second one will use a CPGT or CPMT insert. That's 7º side clearance angle on the CCxx, but 11º on a CPxx. Getting inserts that fit is the first step because any instability in the insert seating results in poor tool life.

The TPGB can usually be swapped for a more common TPGT or TPMT, though you might have to use a screw with a different included angle on the head (usually 60º.) The "TT" inserts are proprietary or at least non-standard designation and you're likely stuck with the OE maker's inserts.

When choosing inserts for boring, look for very high positive chipbreaker designs to counteract that tilt. If the insert is tilted down by 7º and you have a 10º positive chipbreaker, you're effectively down to 3º. In some cases, you just turned a positive rake insert into a negative rake one just by mounting it onto a boring bar. And you wonder why it chatters. ;)

To your main question: The orientation of the flats on the bars would in ideal clamping situations put the tip of the insert on-center, at midway point of the O.D. of the bar. The inserts need tilt for clearance in the minimum bore diameter that they're rated for doing, as the side clearance angle of the inserts may not be quite enough by itself. The smaller in diameter the bar, the more clearance (or tilt angle) needed.

My QCTP bore bar holding blocks are also the same "pinch clamp" style. What I do to align them is get the bar in the holder, the holder in the tool post, and leave the bar loose. A simple but high quality level placed on the top flat of the bar is all I need. This gets the orientation correct. I then use the "gauge bar" I've got that is the correct length to match the distance from the flat way to the centerline of the lathe to adjust center height. I wish I could afford to have one block to dedicate to each boring bar, but I wouldn't have enough room to store them all. So, I have to go through that procedure...a lot.

Any further clarification need fire shoot away.

Ken

[the penguin]

Ken,

Thank you, What you said make sense.

Both of the set that use the CCMT inserts are labelled as SCLCR / bar size / insert size.


example SCLCR 12-3

3/4" and uses CCMT-3XX inserts


While both of the CCMT sets are US made, neither are what you would consider top of the line, I've been hesitant to replace them as they do work, but maybe it time.




Again thank you.