Top side down boring bar

[Bob Engelhardt]

I have made boring bars a few times. By silver brazing a piece of carbide to a notched bar. There was always a problem with the bottom relief. Requiring iterative grinding. Then I had a idea: why not make the notch on the bottom and braze the carbide on its top - hang it from the bar? I was kinda' skeptical that it would be strong enough, but it was: I was boring sometihing with an interrupted cut and the bar broke. But it wasn't that the brazed joint failed - it was that the carbide fractured.

Not very good looking, but you get the idea:

[Alan Smith]

That's pretty good thinking.

[elf]

I've made several boring tools with similar geometry with machine screws to hold the cutter. They have all worked fine.

[Doozer]

Maybe because you can't sharpen it ? ? ?
Chip evacuation path obstructed ? ? ?
Putting the joint in tension rather than compression ? ? ?

-D

[deltap]

You could use a thinner insert for less grinding.

[Bob Engelhardt]

Maybe because you can't sharpen it ? ? ?
Chip evacuation path obstructed ? ? ?
Putting the joint in tension rather than compression ? ? ?

-D

Maybe

[Bob Engelhardt]

You could use a thinner insert for less grinding.

Well, I was using the carbide bits from brazed tools, not inserts. I didn't have much selection in my collection.

[elf]

I was using HSS which is more forgiving than carbide when it comes to being supported. Try putting the carbide in a slot next time to give it more support.

[CalM]

Maybe because you can't sharpen it ? ? ?
Chip evacuation path obstructed ? ? ?
Putting the joint in tension rather than compression ? ? ?

-D

1 Sharpening looks easy, to me....

2 Built in chip breaker! may require some shaping and blending ... see Number 1 above ;-)

3 Brazing is as strong as "many steels".* area is a factor in this application. (Tensile strength values mean little here)

4, One of my favorite bars is done this way ;-)

[eKretz]

The only problem I see with it is like Doozer said, it would be difficult to use up all the carbide - hard to resharpen. And you probably don't want to do any really heavy roughing with it without the support of steel underneath unless it's really thick. Other than that, should work fine I'd think.

[Jammer Six]

Man who says it can't be done shouldn't interrupt man who is doing it.

[I make chips]

Fun filled pro tip:
The ideal material to braze carbide is called tri-met. A layer of 40-50% silver on both sides of a thin copper center sheet. This allows a touch of movement of the carbide to reduce cracking. Just use a scissors to cut and form what is needed for the insert, flux the hell out of it and do the slow heat routine. I've sold hundreds of brazing systems for carbide brazing and it works very well. You'll usually want a 90 degree bent preform as carbide and the joint likes a 'back wall' on the tool holder to push against for a strong joint.

Here's an example.
https://carbideprocessors.com/braze-...sors-cp50-tri/

**Oyeah, when heating carbide never heat it until you see a blue film on the carbide. If you do it's overheated as you're pounding the cobalt out of it and it's essentially junk. Keep the color of the insert at the same color and temp as the base metal.

[Fasturn]

Old school my friend. I have silver brazed many tools similar to yours. Doozer... its silver braze, you can sweat a new piece of carbide in and out as needed.

Not many newbies can braze or grind tools, so the insert bar is it. Fine if you do alot of conventional work. For the rest, custom tooling is a must. That put a smile on my face Bob. Well done Sir !

[Doozer]

The last thing I silver brazed was a hairline crack
in an iron casting for a British lathe. Nice trick
to have when you need a thin and penetrating
method of repair.
I have brazed carbide lathe inserts to the broken
shanks of solid carbide thrills that they throw out
at work. Makes a nice carbide shank/carbide tip
boring bar.

--D

[BCRider]

I think some of you are missing Bob's point. He's using carbide pieces from some source which are intended for brazed use. And he's trying to make use of what is on hand without rushing to buy more stuff. If he were using regular inserts with holes he could go with some of your suggestions. But that's not the point here.

And for the size of the insert look again at his first picture. This is either a really thick carbide or it is a very small boring bar. If he had cut a slot the whisker of metal left on the lower side would have not given much, if any support. At least not as designed.

Bob, if you have enough of that size of carbide what about a more centered slot which would give support above and below. After brazing grind away some of the upper bar just back and in from the cutting corner and roughly half of the carbide's thickness to give the bar some chip evacuation area and get the cutting edge down to the middle of the bar diameter? Doozer has a point about the chips not having anywhere to go. I wonder if it was that which led to them packing up and loading the bar to the point it snapped.

Another option you might consider is unbrazing the teeth off an old carbide tipped wood saw and mount those in a small notch in the end of the bar. Mind you I have no idea of the carbide grade used for the saws and how it would perform with a steady diet of metal.