# Request For An HSS Lathe Cutter Primer for Boring Bars



## WCraig (Jan 13, 2019)

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


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## T Bredehoft (Jan 13, 2019)

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.


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## francist (Jan 13, 2019)

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


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## mikey (Jan 13, 2019)

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.


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## WCraig (Jan 14, 2019)

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


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## T Bredehoft (Jan 14, 2019)

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.


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## WarrenP (Jan 14, 2019)

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


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## mikey (Jan 14, 2019)

WCraig said:


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





WCraig said:


> *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?


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## WCraig (Jan 14, 2019)

mikey said:


> 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


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## mikey (Jan 14, 2019)

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.


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## WarrenP (Jan 14, 2019)

oh i misunderstood, when you said thread on "thread on boring" i thought you meant threading the bored hole..doh...  You probably realize this , but done forget the size of hole your boring, obviously cant get a half inch boring bar if your hole is going to be 3/8 big. I have a boring bar v rey similar to your first picture. I have tried it on a hole much bigger than a half inch but up to that i can say it works pretty well for me.


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## P. Waller (Jan 14, 2019)

Simple, for small holes, .100" to 1/2" diameters use solid carbide bars with the cutting edge ground on the end.
For 1/2" bores less then 5 tool diameters deep steel shank insert bars work well.
For bores deeper then 5 diameters and up to 10 diameters carbide shank insert bars work well.
For bores longer then 10 diameters more sophisticated tooling will likely be required, your best bet is to not bore anything longer then 5 tool diameters, this will save much time and frustration.

As far as grinding HSS boring tools I have no advice, I have not ground a steel turning tool in decades. I do on occasion grind brazed carbide tools for particular forms and inserts that do not quite work for a particular job.


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## WCraig (Jan 14, 2019)

mikey said:


> 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.


The auction lot also includes 1/4" and 3/8" bars along with appropriately sized holders.


> 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.


Noted.


> 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.


I have a selection of reamers.  For the most part, I hope to drill, then ream for most holes.  As I see it, boring is kind of the last resort--only when a non-standard or very large size hole is needed.  Also for making a recess like a lid.

BTW, I plan to mostly work in aluminum and brass (models) with only turning the occasional bit of steel.


> 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.


Noted.  BTW, do you use locking pliers to hold really small bits for grinding?  I think the 1/4" bar requires 1/8" bits.  


> 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.


This whole hobby is emptying my pocketbook at an alarming rate.  Somehow, machinists made do for decades before carbide became affordable for casual use.  I'm going to try to follow their footsteps...

Who knows...the auction prices may fly past full retail and I won't buy any of these bars.  That certainly happened on a couple of auctions I followed recently.  But if I can grab these lots for a reasonable price, I'm going to give it a go.





> 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.


That's the money shot!  Large relief angles; relatively large back and side relief.

Craig


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## WCraig (Jan 14, 2019)

So, is this a boring tool?




Craig


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## mikey (Jan 14, 2019)

I cannot tell what that tool is. It might be a boring tool or a fly cutter. Can't tell without seeing the cutter.

If reamers are good enough for the projects you have in mind then that works. For me, reamers are okay to get me close but if I need a really precise hole then I need to bore it. I hope reamers meet your needs. 

For grinding small tool bits, I slap them in a tool holder and hold onto the holder. Pliers are not sufficient.

Imagine a turning tool for LH turning. Your tool will look like that, except that it must have enough relief to clear the inside of the bore wall. 15 - 18 degrees is usually sufficient for this. Side and back rake varies with the material but I suggest starting at about 15 degrees for both and go up. You need a very small nose radius, the smallest you can shape.


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## mikey (Jan 14, 2019)

@WCraig, my last post was done in a hurry - had to do something for my wife. I meant to tell you that when you grind the relief angles the tool can rub where they meet the bottom of the blank. It helps to round that bottom corner to give you more clearance angle. The other thing is that you want the leading edge of the tool to be the most forward point of contact and a general purpose shape may not be the best option. My tools looked more like a knife tool but with a few degrees of side cutting edge angle. This was enough to cut with the tip of the tool and clean up the bottom of a closed bottom bore. 

I also meant to remind you that reamers can cut a fairly accurate hole but they don't always cut straight holes. This is especially true for deeper holes. The reamer will follow the drilled hole and drilled holes are rarely ever straight. The best way to use a reamer is to drill the hole, bore it straight and then ream it. In addition, many times you will need a reamer that you do not have or that is too expensive to purchase for a single use. In these cases, it is better to just bore the hole so you can get on with it. 

Good luck with this. Keep us posted on how you're doing with these bars. Lots of guys still use them when they fit the job.


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## WCraig (Jan 15, 2019)

Thanks for all the good info.  If I do get some of these auction lots, I'll report back.


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## WCraig (Jan 15, 2019)

BTW, mods...

This group of postings was inappropriately cut out of @mikey's "A Boring Primer" thread.  The thread title "Request for an HSS lathe cutter primer" was NOT my choice and doesn't reflect the discussion at all.  It never reflected the original question I asked.

Can this be fixed?

Craig


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## WCraig (Jan 15, 2019)

mikey said:


> ...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.
> ...


@mikey just wanted to come back to carbide v. HSS.  I'm not using any carbide on the lathe at the moment.  The references I've got say that 300-400 sfm is appropriate for carbide tooling with most steel.  Compared to 100-150 sfm with HSS.  Given that I plan to mostly work with small work pieces, I'd need to be running my lathe at or near it's maximum 3,000 RPM (5/8" cutting diameter or less).  

Then the issue of depth of cut.  I've read repeatedly that carbide 'needs' a greater depth of cut than HSS.  I believe this is to achieve a decent surface finish but, as I said, I have no personal experience with carbide tooling.  If carbide really needs that greater DOC, then that means more strain on the (Zamak) gear-train on my little lathe.  Do carbide insert bits for boring perform differently from carbide insert bits for turning and facing?  

I'm just trying to reconcile what I thought I "knew" about carbide v. HSS cutters on a little machine.

Craig


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## mikey (Jan 15, 2019)

WCraig said:


> @mikey just wanted to come back to carbide v. HSS.  I'm not using any carbide on the lathe at the moment.  The references I've got say that 300-400 sfm is appropriate for carbide tooling with most steel.  Compared to 100-150 sfm with HSS.  *Given that I plan to mostly work with small work pieces, I'd need to be running my lathe at or near it's maximum 3,000 RPM (5/8" cutting diameter or less). *
> 
> Then the issue of depth of cut.  I've read repeatedly that *carbide 'needs' a greater depth of cut than HSS*. * I believe this is to achieve a decent surface finish* but, as I said, I have no personal experience with carbide tooling.  If carbide really needs that greater DOC, then that means more strain on the (Zamak) gear-train on my little lathe.  *Do carbide insert bits for boring perform differently from carbide insert bits for turning and facing? *
> 
> ...



Yeah, carbide needs to run at the proper speeds to cut as intended when turning. You can run slower and it will work but it won't be optimal. As you noted, you will be running at high speeds on small work pieces when using carbide - can't be helped. Give carbide a try, though, because there are times when it is the right tool for the job.

Carbide does not require honking deep cuts to work well and your little lathe can handle it. I don't know where this idea of having to take deep cuts comes from; it certainly isn't based in reality. When we talk about DOC and inserts, we need to keep the nose radius firmly in mind. The larger the nose radius, the deeper we need to go to get the insert to cut accurately. As noted in the boring primer, there is a minimum DOC that an insert can take with accuracy. So, the answer to your question is that carbide needs to only go deep enough to engage and stabilize the nose radius in order to cut accurately. As long as your DOC exceeds the nose radius by about 0.005 - 0.010" or more the insert will cut fine. How much deeper you can go depends on the power and rigidity of the lathe.

The key hassle with carbide is that it requires a minimum depth of cut based on the nose radius to take an accurate finishing cut. This minimum is usually around 1/2NR. Go shallower and the insert will skate; go deeper but less than the NR and the insert loses accuracy or may chatter. You have to determine the minimum depth of cut your nose radius allows and plan your cuts so you can dial it in to finish. It can be a royal PITA to find yourself 0.001" away from final size and you can't cut it with an insert.

The *performance* difference between carbide inserts for turning/facing and boring is not actually about the inserts, which may in fact be identical. The real difference is in the process, the tools and in how the forces are encountered and distributed.

When turning, axial forces hit the bar laterally. Tangential forces push down and radial forces push out but here, radial forces cause the part to deflect away from the cutter. In contrast, when boring, radial forces move the bar away from the wall of the bore. A part being turned requires tailstock support if it is 1.5-2 times the work diameter to counter this deflection, whereas this is not an issue with boring because the axial forces are down the center of the part and radial forces move the bar and not the part. This is why we can usually bore a deep hole with a long piece sticking out of the chuck and not have it go flying out of the chuck. Okay, so this is the general differences between turning and boring - the forces are the same but the direction of those forces differs, and the impact of these forces on the tool greatly differs. This is one of the reasons why turning on the outside is NOT the same as boring on the inside.

Now, considering the direction of the forces with these two processes and the way the insert encounters the work, you can see that the major difference between turning and boring is that the cutting is handled by different edges on the insert. With turning, most of the cutting is done by the side cutting edge and nose radius, while with boring most of the cutting is done by the end cutting edge and nose radius. Since most inserts have identical surface topography on both side and end (this does not apply to an insert with a flat top and positive axial rake), there is little difference in how the insert actually cuts. However, the geometry of the tool holder does make a difference in performance and you really cannot compare the geometry of a boring and turning tool. Bottom line here is that while the insert may be the same, the performance of a turning or boring tool may differ depending on the tool's geometry.

Aside from geometry, there is also a huge difference in how the tool itself is able to handle cutting loads. Because a turning tool is much more rigid, and given that we can use a live center to offset tool deflection, we can take much deeper cuts with a turning tool/insert. With a boring tool, we cannot handle really deep cuts because the bar is unsupported throughout its exposed length; all we really have is extension, bar diameter and bar composition to handle these forces. So, again, in terms of differences in performance, it isn't so much the insert that differs. It is more about the geometry of the tools as well as the rigidity of the tools and how the forces impact on them. A turning tool can take much heavier cuts as compared to a boring tool.

This is also a good place to tell you that whether we are boring or turning, if we take repeated cuts with the same cutting conditions using a carbide insert then we can expect the tool to produce the same results. This applies to both roughing and finishing cuts. This repeatability is important so that we can plan our cuts. *ANY change in ANY of your cutting conditions (DOC, speed or feed) will produce a change in the repeatability of the cut.*

So, Craig, you can certainly use inserts to turn on your Atlas lathe. You do not have the speed, power or rigidity to take big cuts with carbide but these tools will work okay if you keep your depths of cut in a range that produces cutting forces your lathe can handle. It is likely that these tools will not finish as fine as they would on a faster lathe, and you won't be able to hog off huge cuts like the big boys but that doesn't mean you can't use these tools. On the other hand, HSS has some distinct advantages on a lighter lathe that may make it a better choice for turning.

For boring, I recommend using carbide insert tooling for its repeatability.

I hope this helps.

EDIT: I know we're talking in broad terms here and not about a specific insert but I wanted to add that in order to get the chip breaker to work properly you may need to take a deeper depth of cut. On my CCMT inserts, this cut is roughly 0.020" DOC and this will vary with the insert you end up using but heck, even my Sherline lathe can handle that depth of cut. Some chip breakers need more, some less and the insert maker will give you recommended cutting conditions. You, as the operator, have to mess with those cutting conditions to optimize the results you see at the lathe. I mention all of this because there is a whole lot more to using inserts than we're covering here. I also wanted point out that a small lathe has limitations that may limit how useful carbide will be with some materials.

I have used carbide for many years on my Sherline and Emco lathes and they work well for me but they are not my primary tooling. I much prefer HSS on both lathes for most work because I can make a HSS tool do pretty much whatever I need it to do for the kind of work that I do in my shop. Your results will vary.


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## kd4gij (Jan 15, 2019)

I guess I never bought it to the idea that you can't use carbide on small lathe's. That is what I use 99% of the time. Inserts have come along ways over the years.


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## middle.road (Jan 16, 2019)

kd4gij said:


> I guess I never bought it to the idea that you can't use carbide on small lathe's. That is what I use 99% of the time. Inserts have come along ways over the years.


Komet Inserts, the style with the ground chip breaker, have always worked well for me. Very low tool pressure.
I discovered them in the '80s when I was designing a multi-stage boring bar for aluminum road wheels. Created excellent chips instead of strings.


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## kd4gij (Jan 16, 2019)

I run the insert at top.


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## mmcmdl (Jan 16, 2019)

WCraig said:


> So, is this a boring tool?



Yes , it's a boring bar . It is used in a boring head and will work on the lathe as well .


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## NortonDommi (Jan 16, 2019)

The thing in the photo look like a flycutter, the giveaway being the shank.
You may find this useful:


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## Duke (Jan 17, 2019)

Here a quick 3d model I did of one of the bits that I use a lot on my mini lathe:

*Perspective*





*Front Ortho View*




*Top Perspective.*




The rake and relief angles are a bit exaggerated but you get the idea.


For me it works for plastics, brass, aluminium and steel. On plastics you can remove a lot of material in one pass since the leading edge does a lot of the cutting work.



For steel it might be an idea to reduce the angles a bit in order to make the tip more robust.

Rounding the tip slightly with a sharpening stone ensures a good finish as well.

When I got my mini lathe I used carbide, but I prefer the versatility and sharpness of self ground HSS.


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## Duke (Jan 17, 2019)

Found a photo:

The tip looks blunt, but that is just some aluminium that I cut.




I have accidentally brushed my hand against it a couple of times - the thing cuts like a knife.

On the right you can see my 2mm parting bit.

I had absolutely no luck with the carbide parting bit that I got when I bought the lathe.


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## mikey (Jan 17, 2019)

Duke, your model does not include the angles you use. Could you provide them? Also, it appears you don't use any back rake. What are your thoughts on back rake and what are the benefits of not using it?


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## Duke (Jan 17, 2019)

Hi Mikey,

When I ground the bit I did not really pay attention to the angles, but this looks about right: *10 degrees all round.*







I don't have a back rake on the bit, but since I use it as a "Side Cutter" I guess the 10 degrees at the top actually serves as a back rake.

With this bit I normally advance the cross slide to the desired position and then cut in from right to left.

I have a bit that I ground a while back that has back rake, but I rarely use it. Sharpening a bit with back rake lowers the cutting edge which means fiddling around with shims and stuff.

On this one the main cutting edge can be 'restored' by just grinding off less than half a mm from the front.


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## mikey (Jan 17, 2019)

Thanks, Duke! Helps to know the angles so the guys can reproduce it if they choose. 

Lots of guys don't use back rake. Seems they feel it makes it easier to regrind the bit at need without having to refresh all the angles. I have a different opinion on back rake but that isn't relevant here. I was just curious as to why you left it out. The angle on top of your tool is side rake only, by the way.


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## Duke (Jan 17, 2019)

Hi Mikey,

Here's where I got the idea from originally:






Specifically from about 9:30

Adam focuses on chip control, but what I liked is the material removal capabilities of this bit.


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## mikey (Jan 17, 2019)

Understood, Duke. It is always impressive to see them make monster cuts on those huge lathes. I don't happen to have one of them so my tools differ. I happen to think that back rake on a turning tool has great value, especially on a lighter lathe, but that is my opinion. Thanks for taking the time to share.


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## Duke (Jan 17, 2019)

Hi Mikey,

What rake angles do you use? I presume you use different angles for different materials.


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## mikey (Jan 17, 2019)

Yeah, much depends on the material being turned. Instead of giving you the angles and rationale (and to save my fingers), let me refer you to a huge discussion we had on tool angles here: https://www.hobby-machinist.com/threads/models-for-grinding-hss-lathe-tools.62111/

In post #55, there are some pdf files that might interest you.


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## Duke (Jan 17, 2019)

Thank you very much for the link and those pdf files! I missed the reference earlier in this post.

The supplier where I got my grinding vise has a nice selection of hss toolbits at pretty good prices. As soon as I have a chance I am going to get some bits and start grinding.

In my case I ground a couple of bits early on that works ok and after that I never really bothered experimenting further...


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## mikey (Jan 17, 2019)

Maybe do some reading and grinding and we can carry on a discussion if you have questions. I don't think there is a wrong way to grind a tool, provided you know what you're grinding and why you're grinding it that way. 

I meant to tell you that as impressive as it is to watch those big lathes take huge cuts, what impresses me more is when a tool can take a very light and accurate cut on the order of a few tenths to a thou. For me anyway, I'm not trying to remove huge amounts of metal or do a thousand parts. I am trying to make an accurate part that is the exact size I want. To do that, I need the tool to do what I want it to do so I have to know how to grind it so that it can.


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## NortonDommi (Jan 17, 2019)

mikey said:


> Maybe do some reading and grinding and we can carry on a discussion if you have questions. I don't think there is a wrong way to grind a tool, provided you know what you're grinding and why you're grinding it that way.
> 
> what impresses me more is when a tool can take a very light and accurate cut on the order of a few tenths to a thou.



  Shear tool?


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## mikey (Jan 17, 2019)

NortonDommi said:


> Shear tool?



Still haven't had time to grind and optimize a shear tool yet, but I haven't forgotten. Thing with a shear tool is that it is supposed to finish well but is limited to very light cuts. I want a tool that will do roughing and finishing cuts, then finish as well as a shear tool while bringing me in exactly on size. Tall order.


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## ttabbal (Jan 17, 2019)

mikey said:


> Still haven't had time to grind and optimize a shear tool yet, but I haven't forgotten. Thing with a shear tool is that it is supposed to finish well but is limited to very light cuts. I want a tool that will do roughing and finishing cuts, then finish as well as a shear tool while bringing me in exactly on size. Tall order.



I think you need to start with a unicorn horn blank.  

If anyone can make it happen, it's you man. Then I'll grind myself up a few copies.


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## mikey (Jan 17, 2019)

If I can just find the time to dedicate to experimenting with that Unicorn Horn, I can make it happen. So much going on in my life right now. I have NOT forgotten, though.


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## Duke (Jan 18, 2019)

Mikey,  what material do you use for your HSS bits?

Locally there seems to be two options available : HSS and HSS Cobalt.


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## mikey (Jan 18, 2019)

I use mostly M2 HSS and M42 Cobalt.


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## Dabbler (Jan 18, 2019)

I wanted to clean up one thing on the carbide issue.  Many carbide tools holders are negative rake, but you can still find some positive rake inserts for negative rake holders...  An insert/holder combination that ends up with a negative rake cutting surface does need a significant depth of cut to cut well and or evenly.

Holders with inserts that end up wit a positive rake cutting surface and a well honed an sharp cutting edge can take shallow cuts well and cut accurately.  On most small hobby machines these positive rake combinations are well suited for many materials.


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## MarkM (Jan 18, 2019)

Wether it be hss or carbide I think it comes down to tool geometry and the holder is part of that geometry.   Working with hss has given me knowledge to understand tool geometry to help me understand my carbide purchase and it is still my main source of tooling for costs and the fact I can modify if need be.  It can be daunting choosing a carbide tool.  There is a line where the power  and mass is need to turn that cut into a hot knife going through butter.  It pulverises metal and changes it s characteristics and allows it to make the cut.  Smaller machines don t have that ability to make the change in the metal with the negative rake style tools.  It may frustrate you but you ll be better on the other side to work with hss for sometime.  Just my opinion!
I love rake with my hss except with flycutters to remain on centre on regrinds.


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## WCraig (Jan 26, 2019)

As I mentioned, I was following an auction with a tonne of machining tools.  I ended up with a few lots including the boring tools I mentioned earlier in this thread.  All the cutters are HSS; no carbide.  In this post, I've got a few pictures of the tools I plan to keep...along with some questions.  


The two on the left mount in a 3/8" holder but are very slim and can start in very small holes.  I've already used the larger of these in a little project.  It cut aluminum very nicely.  However, I did notice that many of the chips were ejected forward from the bit and ended up clogged in the chuck.  Is that common?

The centre two are basic steel boring bars.  The smaller takes 1/8" bits and the larger 3/16".  The have square and angled recesses at either end of the bars.  The holder on the smaller bar is the same as the one in the quick change tool holder (0XA).  

The two on the right are intriguing.  The shank is 3/8" diameter and will therefore fit the holders to the left.  The bits are round (6mm diameter on the smaller and 8mm on the larger) with a flat milled on the top.  I believe they came sharpened from the factory.  The points are about 55 degrees and extend past the end of the holder.  I believe you could start with a VERY small pilot hole and widen it as much as you wanted.  You could go about as deep as the length of the body of the holder.  Looks like it would be good for making a recess with a square shoulder.  These two don't seem to have ever been used.  I think the bits need honing first.  

Craig


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## WCraig (Jan 26, 2019)

This post is about the boring tools I got in the auction but don't plan to keep.  All of the holders are too big for my 0XA quick change tool holding system.  Before the auction, I knew that the bigger tools weren't going to be compatible.  I didn't realize that the smaller ones (especially the two at the left) were just a fraction too large.  The two on the left are both 14mm square and the next two holders are 15.75 mm square.  The fifth from the left is 19mm square holding a 14mm bar.




I'm unfamiliar with the toolholder at right.  It seems to be an extension bar (?) that might either fit in a larger QCTP system OR in an old-school lantern tool post?  The picture doesn't show it, but it will securely hold either 1/8" or 1/4" tool bits.  It is pictured with a random 1/4" bit that came in another lot.  There are no marks on it to indicate that it was ever actually used.  However, the bar is 11.6 by 18 mm so it is too big to fit anything on my machine.  Can someone explain what this kind of holder is typically used for?



There was also a user-made holder for a lantern-style tool post.  It is too big for anything on my lathe but holds a little 3/16" round tool bit.




Craig


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## Dabbler (Jan 26, 2019)

Yes, it is very common for boring bars to eject the swarf into the hole....  Newer carbide inserts are configured to direct much of the swarf out of the hole - but a lot stays in anyway - it's a part of regular boring -cleaning out the hole...

The tool on the right is an adapter for HSS bits in a lantern or nXA tool holder, depending on the dimensions.


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## mikey (Jan 27, 2019)

WCraig said:


> I've already used the larger of these in a little project.  It cut aluminum very nicely.  However, I did notice that many of the chips were ejected forward from the bit and ended up clogged in the chuck.  Is that common?



Not sure who made that bar in the quick change holder but it is a positive rake tool. All boring bars tend to cut stringy chips, especially if you feed too slow. As with all positive rake tools for aluminum, deeper depths of cut and faster feed rates will minimize this tendency. Coolant, like WD-40, also helps.


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## plunger (Jan 27, 2019)

Duke how long will it take for a guy who has just learnt to watsap on his smart phone and borrows his wifes laptop to go on hobby machinest to learn how to draw those nice green tools you posted.?. Do you draw for part of your job or are you self taught and what programme do you use.


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