Boring bar rake.

[Mike Burch]

I am not very experienced with boring on the lathe. I have just acquired (from China) a carbide insert-holding boring bar for my mini-lathe (a Sieg C3, a 7 x 14 in American parlance).

The tool holder for this lathe is so sized that an 8mm high tool has its tip exactly on the centre line of the work.

The shaft of this new boring bar is an 8mm round rod with two flats milled opposite each other, presumably to give the tool-holder and its screws something flat to grip. These flats are around 7mm between faces.

The insert has pronounced negative rake with respect to those flats, so its tip is only about 4mm above the bottom flat rather than the 8mm which would put it on centre.

When I bought the lathe years ago it came with some dedicated brazed carbide tooling, and the boring bar with that lot has the tip pretty well on the centre line. It also has a quite pronounced positive rake—quite the opposite of the new bar.

I realise that if I put the new bar in the tool holder without shimming it up, the curve of the hole will effectively alter the rake of the insert to be somewhat less negative, a change that will vary with the diameter of the bore. Is this the manufacturer's intention?

It's entirely possible that something got lost in "translation", if we can politely so call the Chinglish description on the Ali Express-based vendor's website, and that I have thereby bought a tool intended for some specialist purpose that I cannot guess at. The vendor does hold himself out as a CNC specialist, if that makes any difference.

As it was only NZ$11 I'm not going to lose sleep over the purchase, but if anyone can explain what's going on here, I would be most grateful.

[Doozer]

Yes, I always run boring bars just a bit above center.
I also like to have positive rake presentation to the work.
If you move a bar with a negative rake above center,
I believe you still have a negative angle with how the
tool is presented to the work. Ignore the flats and rotate
the tool in the holder to adjust the presentation. If you
are using a set screw holder, switch to a round grip holder
so the flats do not come into play.

-Doozer

[Mcgyver]

I realise that if I put the new bar in the tool holder without shimming it up, the curve of the hole will effectively alter the rake of the insert to be somewhat less negative, a change that will vary with the diameter of the bore. Is this the manufacturer's intention?

Sort of hard to follow you without a diagram, and I'm sure my response will seem equally cryptic.

I'm wonderinig if we are on the same page as to where the (back) rake and the cutting edge actually is on a boring tool. If you are boring, ie running along the Z axis, the cut edge isn't the edge closes to the round bore, its the edge parallel to the X axis. The rake angle goes from that back along the Z toward the tailstock. If it tilts downward, it positive rake. The edge adjacent to the cylindrical surface is a clearance area.

[MattiJ]

As it was only NZ$11 I'm not going to lose sleep over the purchase, but if anyone can explain what's going on here, I would be most grateful.

You have to shim it up to just a bit above center.
And the "strange" rake angle is a result when you try to cram relatively big insert to tiny hole. You could set the insert (or rotate the boring bar like Doozer offered) to "flat" position without negative rake but then there is not enough relief and it is going to rub in small holes.

Edit: here was good comparision photo how the insert has to be tilted in smaller bars in order to fit inside small holes without rubbing on :

http://www.model-engineer.co.uk/forums/postings.asp?th=113631

If you are boring a large hole with tiny boring bar you could rotate the boring bar to more positive rake angle but normally you just select more stout boring bar. But with small tabletop machine already maxed out on toolpost capacity a 20mm boring bar is not that easy..

[Sparky_NY]

Sometimes its deceiving. The chipbreaker on many inserts extends right to the edge and has a fairly deep groove, that makes the cutting edge present itself as a positive rake even though the insert is setting negative. I have several tools with that type of insert and they work great.

[BCRider]

I'm with Mcgyver and find it difficult to imagine all you've posted and "see" ALL of the issues well. But if I'm reading it right then the boring bar is just exactly what it should be. But equally true is that you need to shim it up by 4mm to get the cutting edge up on the center line. And if the holder or tool post you have does not support that you'll need to make up a new holder or a new tool post to hold the bar.

I'll just add that sometimes you simply "can't get there from here". Some items of tooling simply do not fit together and work like they should. Hence why QCTP's have a wide ranging array of holders of all shapes. And why those of us that got into it before the cheap Aloris clones were available tended to make our own solutions for holding things like boring bars and other special needs.

For example... if your present tool post has limitations that prevent you from shimming it up to the proper position then you might need to look at either buying a QCTP or making your own dedicated tool post to hold the boring bar. This does not need to be as crazy as you might think. Here's a picture of two custom tool posts I made back some 25 years ago. The boring bar holder was done mostly in the lathe to clean up the faces then to pilot drill the boring bar holes so they would be on center. The holes were then finished in the drill press. The size of the material required the spacer shim in my own case. It's not there for anything more than that. I hope from this picture that you can see how it would not be all that hard for you to make up your own boring bar tool post if you don't want to spring for a QCTP for your lathe.



A hint for a boring bar option that I found worked like a treat. In one of the smaller bars you can see what looks like an old end mill. Oddly enough that's exactly what it is. And old end mill with a chipped end. I needed a smaller bar and put the bar and end mill together in only a few minutes of lathe and drill press time to "get 'er done". Thus the cobbly holding screws instead of set screws. But the point is that a 3/8 end mill can serve well as a shorter sort of boring bar that is easily sharpened.

If you are wondering about the other tool post for the knurling tools I made that one up because here again these tools need to be set up "on center" and I could not do that with the old 4 way post. Also the pressure needed for knurling tended to pivot the large 4 way which this more compact and centered tool holder does not do. This one was done in the mill to slot it. But with a bit of ingenuity it too could have been done in the lathe using shims and turning the block over as needed to make the slot wide enough.

So if your boring bar cannot be shimmed up high enough to get the tool bit centered and at the proper geometry I hope this gives you some ideas for an alternative solution.

[MattiJ]

A hint for a boring bar option that I found worked like a treat. In one of the smaller bars you can see what looks like an old end mill. Oddly enough that's exactly what it is. And old end mill with a chipped end.

Add a cheapo ER collet set and straight shank ER collet chuck and you get even more odd but very rigid and versatile boring bar holder.

Cheapo asian ER collet chuck quality might be so-so but for this kind of application they are perfect: i.e

https://www.banggood.com/C16-ER16-100L-Straight-Shank-Collet-Chuck-Holder-100mm-CNC-Milling-Tool-Holder-p-1065923.html?rmmds=search

[BCRider]

Add a cheapo ER collet set and straight shank ER collet chuck and you get even more odd but very rigid and versatile boring bar holder.

Cheapo asian ER collet chuck quality might be so-so but for this kind of application they are perfect: i.e
https://www.banggood.com/C16-ER16-10...l?rmmds=search

Back when I originally made these two tool posts the Aloris clones were still way off in the future. It was the same thing with cheaper ER collets. But these days with the low cost of reasonably decent ER collet tooling being affordable adding a 1 inch stub shaft ER chuck to my quiver sounds like a fantastic option. Thanks for the idea.

[Illinoyance]

I am surprised nobody else brought this up. The insert has a negative rake presentation to maintain clearance under the cutting edge.

Personally on a small lathe I prefer HSS boring tools.

[danlb]

The tool holder for this lathe is so sized that an 8mm high tool has its tip exactly on the centre line of the work.

The shaft of this new boring bar is an 8mm round rod with two flats milled opposite each other, presumably to give the tool-holder and its screws something flat to grip. These flats are around 7mm between faces.

The insert has pronounced negative rake with respect to those flats, so its tip is only about 4mm above the bottom flat rather than the 8mm which would put it on centre.

As I read this, you are using the stock "turret" style 4 position tool holder which is designed for the 5/16 inch turning tools. Every round shank boring bar that I've used has the cutting edge at 1/2 the height of the shank. Some square shank boring bars have the cutting edge at the top of the bar, but then the bottom corners of the bar are ground away to provide clearance in the hole. This effectively reduces the stiffness to that of a tool with half the size.

The solution is to shim the bar, just as you do with your normal turning tools. The tool post has a 16mm tall slot, so there is plenty of room for the 8mm shank and the 4mm shim under it.

BTW, I looked at the C3 manual on LMS and it's MUCH better than the one that they provided 15 years ago.

Dan

[old mart]

Its normal for boring bars to be lower than expected for their nominal shaft size, unlike ordinary lathe tools.
The tip should be at or a few thousandths of an inch above the centre line.

[MattiJ]

I am surprised nobody else brought this up. The insert has a negative rake presentation to maintain clearance under the cutting edge.

Personally on a small lathe I prefer HSS boring tools.

That what I was trying to say

[Mike Burch]

Thank you all for your helpful replies.
I'm sorry my explanation was not completely clear. I have tried to post a photograph to clarify things, but for some reason the site won't allow me to.
I'll try all your suggestions and see how I get on.
Thanks again.

[BCRider]

I guess the real question is does the opening in your tool post have enough clearance to let you shim it up as far as it needs to be to get the cutter edge at the right height.

I know that on my own 4 way it took 1/2" tools but only had roughly 1/8" of room over the tool for shimming. So I could not mount a 1/2" or even a 3/8" round boring bar and still get up to the proper height..... which is what led me to make the dedicated tool post and shop made boring bars I showed above.

[Illinoyance]

That what I was trying to say

Matti,
I went back and read your post again. Yes, you did say that. If I had followed the link you posted it would have been obvious to me. Sorry I missed it.