Request For An HSS Lathe Cutter Primer for Boring Bars

[WCraig]

Um, I don't see much on using HSS cutters? I've got a little Atlas 618 and HSS seems to be the best route for it. I don't even have a boring bar at the moment but there is an online auction I have my eye on!

Would someone be able to discuss the geometry for HSS bits? Differences for steel, AL and brass?

Thanks,

Craig

 

[T Bredehoft]

Mikey, here on this board did a magnificent job of teaching all of us how to grind HSS lathe tools. I'm sure if you PM him, he will see to it that you get the information you need. The thread he started on the subject was around for a LONG time.

 

[francist]

https://www.hobby-machinist.com/threads/models-for-grinding-hss-lathe-tools.62111/unread

There you go -- a bit more than a primer but you asked.....
Your Atlas Manual of Lathe Operations has good info too, that's what I used when I started out.

-frank

 

[mikey]

Thanks for the kudos, Guys!

Craig, we've had a lot of discussion about the very thing you're asking about. Go to post #55 in our model tools thread and scroll to the bottom of that post. There are some pdf files there that will get you started. The first file, Grinding Lathe Tools on a Belt Sander, will give you most of what you need. Then you can read that humongous thread on model tools. Post questions there and we'll give you a hand.

 

[WCraig]

I guess I didn't make myself clear. I asked my question in the context of the thread on boring:

A Boring Primer

I'm interested in the geometry of the HSS cutters fitted to a boring bar. @mikey makes the case that the loads on a boring tool are rather different from those on a regular turning or facing tool. Unless I'm missing something, the pdf's in the 'models for grinding hss lathe tools' don't address bits for boring bars.

Perhaps a mod could rename this thread to "HSS bits for boring" or somesuch.

Craig

PS I haven't really studied the 'models for grinding HSS...' thread in great detail. I found a page on steves-workshop.co.uk that has a concise and well-illustrated tutorial on HSS tools before I found the 'models' thread. http://www.steves-workshop.co.uk/tips/toolgrinding/toolgrinding.htm

 

[T Bredehoft]

The cutting geometry for borinig bars is the same as OD cutting, only the relief is different. You have to be certain that below the cutting edge, the tool will not interact with the work. This, of course, is dependent on the radius/diameter of the bore.

 

[WarrenP]

or sounds like you want to name it something like "primer on Bore threading "?

 

[mikey]

I don't even have a boring bar at the moment ...

I'm interested in the geometry of the HSS cutters fitted to a boring bar. @mikey makes the case that the loads on a boring tool are rather different from those on a regular turning or facing tool. Unless I'm missing something, the pdf's in the 'models for grinding hss lathe tools' don't address bits for boring bars.

Yeah, the context of your question was not clear, Craig. Let's see if we can make it even more clear before we try tackle the answer, just so we know what we're addressing.

You don't own a boring bar at the moment but given that you have a small lathe, you think that you need to use HSS boring tools with that lathe, correct? Rather than use conventional HSS boring bars, you wish to use bars that use a HSS blank that is fitted into the end of a boring bar and locked there with a set screw. You are specifically interested in the geometry of the HSS tool that is mounted like this, correct?

While we wait for your response, let me tell you why I don't use these bars anymore. There is nothing wrong with them and they have been in use for decades, and quite successfully. Many are made in home shops but some commercial makers had them, Everede first amongst them. The reasons I don't use them is 1) because they have the same constraints that all bars have, which is the extension restrictions of the steel bar these things are made of, 2) the geometry depends on how the cutter is mounted (perpendicular to the bar or at an angle that can range from 30 degrees or more) so you have to individually grind each cutter depending on how it is mounted and 3) chip clearance is an issue with these bars and becomes a nuisance, at least for me. Finally, and not as a bias against them but more a personal observation, solid bars work better and inserted bars work even better, at least for me.

Like many hobby guys, I read all the old books, especially those penned by the British guys who made these bars so popular back in the day, and I actually made several of them and ground quite a few HSS cutters for them. Of course, I didn't know much about boring back then but I eventually formed some biases against them as I listed. I'm willing to discuss what I remember about grinding these bits but please answer the questions above first, okay?

 

[WCraig]

Yeah, the context of your question was not clear, Craig. Let's see if we can make it even more clear before we try tackle the answer, just so we know what we're addressing.

You don't own a boring bar at the moment but given that you have a small lathe, you think that you need to use HSS boring tools with that lathe, correct?

Right

Rather than use conventional HSS boring bars, you wish to use bars that use a HSS blank that is fitted into the end of a boring bar and locked there with a set screw. You are specifically interested in the geometry of the HSS tool that is mounted like this, correct?

I thought I was describing a "conventional" HSS boring bars. No? This is similar to what I've seen:


https://www.busybeetools.com/products/boring-bar-d-e-1-2-sq.html

There is an auction closing this week with a couple of sets of such boring bars on offer. Each of the different diameter bars has provision for a square HSS bit--held at right angles at one end and an acute angle at the other. My tiny Atlas 618 lathe doesn't have the speed/power to use carbide cutters.

Given the radial and tangential forces that you talked about w.r.t boring, I thought maybe the HSS cutters would work better if ground to a specific geometry. I would guess that more side rake (ie back towards the tool post) might be good? Given the need for relief on inside of the hole, I guess that you can't have a great deal of back rake. Otherwise there won't be much support for the cutting edge. But maybe I'm overthinking things again!

While we wait for your response, let me tell you why I don't use these bars anymore. There is nothing wrong with them and they have been in use for decades, and quite successfully. Many are made in home shops but some commercial makers had them, Everede first amongst them. The reasons I don't use them is 1) because they have the same constraints that all bars have, which is the extension restrictions of the steel bar these things are made of, 2) the geometry depends on how the cutter is mounted (perpendicular to the bar or at an angle that can range from 30 degrees or more) so you have to individually grind each cutter depending on how it is mounted and 3) chip clearance is an issue with these bars and becomes a nuisance, at least for me. Finally, and not as a bias against them but more a personal observation, solid bars work better and inserted bars work even better, at least for me.

Like many hobby guys, I read all the old books, especially those penned by the British guys who made these bars so popular back in the day, and I actually made several of them and ground quite a few HSS cutters for them. Of course, I didn't know much about boring back then but I eventually formed some biases against them as I listed. I'm willing to discuss what I remember about grinding these bits but please answer the questions above first, okay?

Is a "conventional" HSS boring bar one that is ground from one solid blank of HSS?

BTW, there was a couple of cutters in one of the auction lots that I'm not familiar with:



I take it this is a form of boring tool? Wish I had a better picture of it.

Craig

 

[mikey]

Okay, I think I catch your drift now. First of all, you have to realize that in order to use these bars you have to be boring a pretty big hole, right? With a 1/2" bar plus the stick out of the cutter, you're looking at boring 3/4" holes and larger, to a depth of about 2" deep. This is a pretty narrow range of requirements so I hope this fits your needs.

It might be helpful to clear up the lathe thing first. A small lathe, even a 6" Atlas, can use inserted carbide boring bars very effectively. Heck, I use them on a Sherline lathe and they work great. At the depths of cut we are dealing with when boring, power is not a real issue. Nor is rigidity; the bar deflects, not so much the lathe. Speed is the main problem with carbide but when boring we can make up for some of the loss in speed by altering feed so I don't find this to be all that big a deal for this particular lathe operation.

The configuration of the bars you're interested in requires some stick out of the cutter from the bar. In most cases, this stick out will be at least 1/8" and in most cases it will be closer to 3/16" or more. That really chews into the space the bar can fit into so you're really looking at boring relatively large holes. Moreover, these bars are made from steel so they have an extension capability of 4:1 before deflection is a major issue. So, you must bore large and you cannot go really deep with these bars.

You also have to grind cutters for the angle you're using. That is, different tools are needed for both 45 and 90 degree tool orientations. This can be done; I've done it. However, it is a hassle and you really have to know what you're doing because all of the grinding will be done on about 3/16" of the tip of the tool blank. On the plus side, it takes all of a few seconds to hone the tool to keep it sharp. On the minus side, they will dull faster because all the wear occurs in a very small area. This might seem to be a minor deal but consider that this wear occurs while you're boring and that leads to inconsistency. When the cutter dulls, you have to re-hone it and that changes the tool geometry a tiny bit such that the cutter may not cut the same with the next pass. This is one of the key reasons I stopped using HSS for precision boring because above all, I need consistency.

These bars you're considering are an old design that are easily outperformed by more modern designs - yeah, I mean inserts. If you must use HSS then I highly recommend you look at the tools from AR Warner. They can use HSS (or carbide) inserts and their bars are very, very good. A 3/8" bar will bore a 1/2" hole or larger and can theoretically go 1.5" deep. However, I have pushed this bar to 4" deep using a HSS insert and held very, very tight tolerances in aluminum.

Anyway, if you are dead set on grinding HSS cutters then we can discuss that. You are basically grinding LH tools with large relief angles and a relatively large amount of side and back rake. Not hard to do.