Am I asking too much of this tool?

[Shotgun]

I got me one of those boring head kits that came with brazed carbide tip boring bars. It isn't the bottom of the barrel kit, but you do have to lean in there to get down to it. I've been using it to expand a 2.5" to 3.000" in a 1/4" then a 3/4" plate. I mount it with the 3/4" plate down flat on the vice, bore the 1/4" plate, then unbolt and move it out of the way. Then bore the 3/4" plate.

The problem I'm having is that I can take the boring bar out and sharpen it with a diamond hone. I get through the 1/4" plate, and maybe one or two passes on the 3/4", and it will start with terrible chatter. Pull the bar out, and the tip will be dull and chipped. Several minutes on the hone, and I can get it to cut smooth again.

I'm running at about 550 RPM, and taking 50thou off the diameter with each cut. Hand feeding using the knee, so that everything else can be locked down tight. The feed rate is very much by ear. I speed up if it starts to chatter, and slow down to as the motor starts bogging down.

Question: Am I asking too much from this cutter, or is this edge life about what I can expect? And more generally, how long can a carbide tool/bit be expected to live?

 

[Asm109]

Slow it down.

 

[WobblyHand]

What's the material? If just low carbon, I calculate roughly 450 RPM for a 3" hole using carbide. Probably ought to slow it down some. Boring is a slow process. It also seems you are removing a lot for each pass, which stresses the rigidity of everything. (Boring tool length, boring head, etc.) The chatter might be breaking the tip? Carbide is hard but brittle.

 

[MrWhoopee]

Those boring bars require SIGNIFICANT grinding to make them suitable for use in a boring head. As received, the cutting egde is well above centerline of the shank. In order to get the cutting edge on center with the boring head (which must be done), the bar has to be rotated. With the cutting edge now on center, the tool has a negative top rake, which, combined with the flexibility of the bar, makes for very poor cutting. This picture shows what it takes to get to just a neutral top rake. (Viualize a line from the cutting edge through the center of the SHANK of the tool)

 

[Lo-Fi]

Those brazed bars are garbage as supplied, they make it almost impossible to get even neutral, let alone positive top rake, so they rub, chatter like hell and don't cut. The geometry is totally wrong without some serious remedial work on a carbide grinder. Got an insert bar like you'd use on a lathe that'll fit? Or a bar you can fashion to take a small piece of HSS? You'll have a much nicer time!

 

[ErichKeane]

I share the same experience as the others: the brazed carbide boring bars need a ton of grinding to make usable.

I might suggest one of these sets: https://www.amazon.com/OSCARBIDE-In...1400802&sprefix=boring+bar+set,aps,173&sr=8-4

They're a touch expensive at $100 or so, but they are inserts, require no grinding, and are easy to use!

 

[Shotgun]

@ErichKeane , good job spending my money!!

Grinding the bar down to size as pictured by @MrWhoopee using a diamond hone just AIN'T gonna happen. $80 for a set of 4 bars is reasonable, but I just got some new stock in. Do I want to buy a green grinding wheel to bring what I have to the right shape?(yes) Do I want to fork over the $80?(yes) Or, do I want to make my own boring bar?(yes)

Decisions. Decisions.

Are there any good guides/videos for grinding the bars to the correct shape?

 

[Winegrower]

I have to say I don't have that many complaints about those cheap boring bars. I don't have to grind them, just hone a little, and be very careful to rotate them to get the right rake angle, can be a bit positive or negative as needed. For positive rake, an issue is the back of the bar can drag on the cut, have to be aware of that. The main problem with my 2" and 3" boring head sets is that the setscrews retaining the boring bars can dent the bars, raising a bump and making it hard to get in and out. Boring bars that have a built in flat, in my experience, generally have it in the wrong place for use in a boring head.

 

[Jake M]

Are there any good guides/videos for grinding the bars to the correct shape?

Here's a good quick overview of the problem, and a solution. If you've got flats to orient it, you may have to accommodate that, rework those, or... Not sure what you're working with, but the picture is worth a thousand words.

 

[Shotgun]

Got another hole bored this evening.

The first mod was what @MrWhoopee suggested. I relieved the back side so that I could turn it more. Slight improvement, but it still chattered (squealing like a stuck pig), and soon dulled.

I sharpened it again, and this time put it in back gear. I don't like to use back gear, 'cause I've got something out of adjustment so that it makes a hell of a racket. But, I did it. Slowed it down to about 250RPM. Not a bit of chatter. Went right through the 3/4" plate like it wasn't there. Even increased the cut to 75thou off the diameter on each pass. May have been able to take more, but I didn't want to push my luck. Maybe later.