Old type boring bar insert

[old mart]

I bought a job lot of 25mm boring bars and one of them looks to me to be an old model. The insert is double sided, 9mm/5/16" with only one cutting edge per side with a 7 degree clearance. This is unusual for a positive insert, double sided with a chipbreaker along the side which means it is also handed. There are no markings at all along the shank to give a clue, the bar is 282mm long and in very good condition. If I cannot get hold of any inserts, there is a good chance of milling the seating for a DCMT or DNMG in a larger size, assuming there is enough room for the screw.

[LKeithR]

That looks like the acute angle is 80 degrees which would make it a CCMT insert designed to fit a
bar with a basic designation of SCLC. If it's 9mm it should be a standard metric size but the problem
I see is that the insert you've pictured is retained by a screw which is inserted from the bottom of the
bar. Standard CCMT inserts are retained by a screw which is inserted from the top. Best bet would
be to find an insert of the right type and size and see if it can be made to work; otherwise the bar as
it sits is probably useless...

[BCRider]

Looks like a cam style pull into place sort of grip. The diamond lower "slider" might be possible to remake so it works with a suitable CC series. The CC series inserts having enough of a chamfer that the proper shape of pin would pull the insert in and down.

The only thing I wonder though is what pulls the slider down hard against the holder? For these to work the insert needs to be pulled hard to the slider and the slider hard to the seat of the holder so there is no possible play. I'm sure there's something that will do that but it's not obvious.

Can it be adapted to a simple taper screw? I'm sure it can if the holder can be drilled and tapped. But it'll need a hardened shim that is the same thickness as the slider to space the insert up to where it should be.

[old mart]

This is how the insert is held, not a very good design, thats probably why it is not used nowdays. I recon I could mill and tap the bar to take a DCMT 11 without too much bother, the seating would have to dip to match the standard bars with this style of insert.

[BCRider]

Ah, now I see. I thought the screw went down into the hole on the upper side.

So pushing the slider in like it does would lever the top of the insert against the back wall and pry the front cutting edge down against the base.

So yeah, that's not a great design. And it also explains why it's only one cutting edge. If it was mirrored the prying forces to hold the insert down would likely chip the reserve edge.

So I'm thinking.... .Cut off the pin a little short but leave enough that it can be used as a shim for a CCMT of the proper size. And drill and thread for a hold down screw on the upper side just aft of the saddle area. It'll need to be deep since you'll run into the hole for the tang of the slider. Cut off the tang. Then make up one of the hold down bridge style clamps and drill a tapered "center drill" hole back further. Then make up a hold down bridge that has ball end fingers spaced such that it pinches down and levers back on the countersunk seat of the modern insert.

Or, if you got enough other more modern boring bars already then just scrap this one. Or cut off that whole front end and start over?

EDIT- re-reading your last reply and the ending where you're thinking of recutting. Or that...

[JRouche]

This is how the insert is held, not a very good design, thats probably why it is not used nowdays. I recon I could mill and tap the bar to take a DCMT 11 without too much bother, the seating would have to dip to match the standard bars with this style of insert.

I thought for sure you knew it was DMT. It looks wide in the hips, D , C numbers are a lil thin and can not handle a interrupted cut like a D profile can.

Make it a D size holder, I would like to see it.. JR

[LKeithR]

...C numbers are a lil thin and can not handle a interrupted cut like a D profile can...

I think you've got it backwards. A "D" series insert has an included angle of 55 degrees while a "C" style
is only 80 degrees. This puts a little more meat in the corner and makes the insert a little tougher...

[old mart]

I haven't made up my mind yet whether the seating can be re-machined to accommodate a D or a C type insert, a lot depends whether there is room for the screw hole. Boring bars of this size are often cut shorter and this one is still full length. When boring on the museum's Smart & Brown model A, we take small cuts, time is not a factor and the lathe is over 70 years old.