Bars for 2″ boring head

[Gareth Jones 11Participant @garethjones11]

Happy New Year to all.

Do any round shanked boring bars actually have their cutting edge on centre?

I’ve searched but not found a conclusive answer. My cheapo 50mm boring head came with a selection of 12mm brazed carbide bars but all have the cutting edge way too high.

I know there isn’t technically a centre height for a boring head, so for clarity I mean a diameter, parallel to the ways of the head, passing through the centre on the tool mounting holes. Having the cutting edge in this line is the only way for diameter adjustment to be sensible.

I know I could rotate the tool in its hole, to get the tip on centre but this would give VERY negative rake.

The indexable bars I have for the lathe are also above centre, though less so.

All I can think of is using a smaller diameter boring bar in an eccentric sleeve to bring it down to centre.

I know this is far from a new problem so someone must know the answer.

Cheers

Gareth

[JasonBModerator @jasonb]

You can get insert bars specifically for boring heads. I’ve got a set of three from Glanze which are 1/2″ but there are 12mm ones about. They have a flat on the shank for the grub screw to bear against and the insert is inline with the flat so not angled like you get on a lathe boring bar.

Set of 3 1/2″ Dia Indexable Boring Bars

Will have to check if the tip is on ctr, not found it to be an issue in 18yrs of use if it is not on ctr. But I have a feeling the insert sits above ctr line so you get better clearance on the common 7deg edges of the insert.

[Gareth Jones 11Participant @garethjones11]

Thanks Jason.

I saw that set and looked for similar with 12mm shanks. There are some on eBay without clamping flats but I could add the flats myself.

I’ve ordered real cheapo 12mm CCMT bars from AliExpress to experiment with.

I suspect I’m overthinking it and obsessing about the cutting edge being bang on centre, when it’s not really necessary.

I’ll mess about and see how much of an issue (if any) the discrepancy is.

Cheers

[bernard towersParticipant @bernardtowers37738]

Gareth, the ccmt bars work well on my boring head especially with the polished tips

[Phil SParticipant @phils66830]

Gareth

My understanding is you set the tip on the “diameter” so the scale on the head reads correctly. The cutting is done on the end of the bar not the side. Thus the top rake is provided by the slope axially rather than radially, as per image.

[Gareth Jones 11Participant @garethjones11]

I see what you mean but the tool should behave exactly as it would in a lathe, with the same tip geometry, including relative to the point of contact with the work.

When you mount a boring bar on the lathe tool post, you don’t rotate the tool along it’s axis, to get the tip on centre. You shim or adjust it vertically. The tool geometry stays the same either way but the actual cutting angle is very different.

Again, I’m sure I’m overthinking this. Once my cheapo CCMT bars arrive, I’ll have a play and all will be well.

[BazyleParticipant @bazyle]

Phil’s post above reminds us to think more carefully about the actual geometry of the cut. Get a similar confusion with flycutter tools effectively the opposite of this as they cut sideways not down.
You also need to think of the direction the cutting forces will bend the tool.

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