View Full Version : Tool angle for boring with lathe

brian Rupnow
04-16-2008, 04:38 PM
Today I have been playing with my new lathe, and decided that I would try out the boring tool I purchased when I bought it. I chucked a peice of 1.5" pipe in the 3 jaw chuck, and attempted to bore it out to a larger diameter. I set the tool so that the cutting tip was parallel to the floor, and at center height. On close examination, it seemed that the "heel" of the tool was going to contact the inside of the pipe before the cutting edge. This being the case, I rotated the tool in the holder, (tool has a round shank) untill the cutting edge was tipped down on the front side of the lathe about 15 to 20 degrees. This seemed to work fine, and I did a bit of boring as shown. Is it normal procedure to have to tip the cutting tool a bit like I did?http://i307.photobucket.com/albums/nn294/BrianRupnow/TOOLANGLE001Medium.jpg

04-16-2008, 05:05 PM
Actually its normal to use a tool with a smaller "head" when boring smaller bores. Without going too overboard, you might be able to grind some more clearance on the bottom...but you are removing strength and rigidity.

You are machining something that doesn't machine well (cheap steel pipe--aka "black iron pipe") with perhaps a less than ideal boring tool. Those are most commonly used in a boring head for boring holes with a mill etc. In the ideal, with the brazed carbide piece parallel to the ways rather than tipped as you have it, it has zero rake. Pointed down it goes to negative rake at which point it's really just scraping metal off rather than shearing it. HSS tools made just like the one you have can be ground for some positive rake. More often, boring bars that hold a small, standard HSS tool bit might be used. You grind the bit to the architecture you want, cut it off pretty short, and then insert it into the hole in the boring bar and tighten a set screw.

Don't get me wrong, you can use those tools you have to bore in a lathe. I have done it although I typically turn it to provide a few degrees positive rake....but be careful to keep the point just above center:
You want to be just maybe a few thouandths high from center with the cutting edge so that as it digs and flexes downward, the point is pushed into less material rather than more. If it went into the material deeper when it flexed under load then it would bite harder, causing it to dig deeper....etc....making for a broken tool or botched work.

The up-side to what you are doing with negative rake is that the tool is not apt to dig as is often the case with a not-so-carefully-adjusted boring tool.


04-16-2008, 05:48 PM
The cutter angled down like that would make your compound dial graduations not accurate...

Good way to "add relief" to a tool though. I'll have to remember that. Would be handy to rough out a bore then swing it parallel and finish

brian Rupnow
04-16-2008, 05:51 PM
Thank you for that information. I purchased a "set" of those boring tools when I bought the lathe. The "set" was relatively inexpensive. The sales people at "Busy Bee" are helpfull, but they are salespeople, not machinists. I have a couple of books dealing with "The Amateur Machinist", and they both show the type of tool which you are describing. Since I have the tools already, I will probably take your suggestion and grind a bit more "rake" on these tools and use them untill I purchase a proper boring bar set-up.---Brian

brian Rupnow
04-16-2008, 05:58 PM
Do I need a special stone to grind more rake on that brazed carbide?

04-16-2008, 06:20 PM
Special "stone"? Yup. Here in the states, you can buy some inexpensive diamond hones...even at places like WalMart...often in the pocket knife department. I have a few in different grits. They are a flat plastic handle with a piece of diamond impregnated metal glued to the end. That piece is maybe 3/4" wide by maybe 1-1/2" long.

Otherwise, diamond impregnated grinding wheels are available...even the cheap imports are in the maybe $70US and up range. If you have much to remove, its going to be a good bit of work if tackled by hand, however.

I think the delimma you will have is that you have a relatively thin carbide "chip" out on the end. I was really advocating buying maybe one or two HSS bars of the type you bought. They will likely be in a name brand, however, so you may pay what you spent on the ubiquitous carbide set on just one or two bars. I found some used ones on Ebay just recently for just a few bucks.

I would be tempted just to rotate to the point of getting positive rake in the contact angle and call it good. Save your grinding for the back side of the steel portion of the bar to get some needed clearance. You are in a catch-22 since using a smaller headed bar for clearance usually means a thinner bar, reducing your rigidity. To regain some of that rigidity, you can use a smaller one. One advantage of the HSS tool bit holder type I mentioned in the other post is that they are the same diameter along the length, so you can choke up on the bar just leaving enough out to do the job.


04-16-2008, 06:33 PM
An old guy told me when it comes to setting the cutting edge at center line "better inside above line and outside below line" than twisiting the tool to provide rake. It works and if you do not go to extremes you won't be that enough off to mess up your feed dials.


04-16-2008, 06:40 PM
Mark-- I always viewed the height of the cutting edge as one issue (and agree it ought to be above center) and the issue of the tool angle (for best cutting action) to be a separate issue. Center height is easily adjusted after getting the angle correct, by either using shims or buying a QC toolpost whose holders are inherently adjustable for center height.

I think the original poster was rotating the tool not for tool height reasons, but for back-side clearance reasons on the material he was boring.


04-16-2008, 06:55 PM
Hi Brian.

Just to put it in graphic rather than textual form - have a look at this as a guide to get you going.


I'd suggest using high speed steel (HSS) ground to suit.

As said by others, make sure that the part of the tool below the cutting edge does not rub on the job - you should have a look at the geometry of a wood circular saw blade as it will be more visible there. An end mill cutting edge and clearances are another example.

I have the same lathe as you do - it is a fine machine and capable of some very good work - and good value.

brian Rupnow
04-16-2008, 07:22 PM
Thank You gentlemen. I will probably use my boring tools "as they are" and save my money---Instead of buying a special grinding wheel for carbide, I will put the money towards the purchase of a proper "boring bar" when it becomes necessary to have one.---Brian

04-16-2008, 07:45 PM

If the hole your are boring is not much more than say and inch deep, you can make a perfectly adequate "boring bar" from a piece of spare HSS tool steel. I can tell you that it works fine - without the cost or inconvenience of a formal "boring bar". No need for other than your trusty pedestal grinder with its (grey-coloured) wheel/s - as that will get you by. Its all I use most times. I use a simple hand or oil stone to get a fine edge when needed.

In short, you probably have adequate "stuff" right now to do the job.

"Boring" is no big deal or mystery - at all. It is only turning on the inside of a job instead of the outside - no more and no less. Just make sure that the cutting edges are there and that there is no inadvertent "rubbing" of the tool on the job.

Make the set-up as rigid as you can and then just "feel your way".

Most stuff is plain old common sense and a bit of planning. And that includes screw-cutting and parting off - both of which when all boiled down are are only variations or "form tooling".

A few "experiences" - good and bad - tend to keep things in your mind - a sort of "learn by doing" event!!.

I try to not see things as mysteries or mystifying - most can be broken down into a series of smaller easier bits to "chew" or "digest".

I took the opportunity to have a look at your web site. Anyone who can sort out things at that level should have no real problems with "machining" in any of its manifestations.

You are doing fine.

04-16-2008, 08:02 PM
FWIW, another useful boring trick is to angle the tool. You have a pretty big opening there, so there is room. The idea is to rotate your tool turret there so that the boring bar goes in at an angle and the shaft is not parallel to the axis.

Why do it? Because the forces are such that the bar acts stiffer to the cut.

This is a tip from Machine Shop Trade Secrets, but I do try to angle the bars when I can. It may even help with the "head clearance" problem you describe.



Paul Alciatore
04-17-2008, 12:42 AM
It is probably best to set the tool's rotation angle and height to maintain the proper rake angle for the metal you are cutting. This will produce the best finish.

As for getting the proper clearance, that can be a problem with ready made tools and small bores. I also have a couple of sets of the brased carbide boring bars. One was new and the other was a E-bay set that had been used somewhat. I use the used set for any job where I need to make special mods for extra clearance.

I have one of those home supply store type diamond knife sharpening flats and I do use it for touching up the carbide edges, but they are dead slow if you are trying to change the shape of the tool. For that, I have one of the Dremel diamond wheels (about $10) that I use in my Dremel tool. It is about 1 1/4" in diameter and just big enough to do a little carving on the carbide tool. I use the side of the wheel and a fairly high speed. I am careful not to cut the steel that the carbide is brased to with this wheel to avoid the carbon/steel/heat interaction. I use the reinforced abrasive wheels in the Dremel for that and I make this cut first. I usually go for a radiused clearance with a radius that is less than that of the rough size of the hole I am boring. I have several boring bars with extra and special clearances cut into them.

I just don't know how I ever survived before I got that Dremel.

All the usual disclaimers.