Request For An HSS Lathe Cutter Primer for Boring Bars

[WarrenP]

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.

 

[P. Waller]

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.

 

[WCraig]

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

 

[WCraig]

So, is this a boring tool?



Craig

 

[mikey]

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.

 

[mikey]

@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.

 

[WCraig]

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

 

[WCraig]

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

 

[WCraig]

...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

 

[mikey]

@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

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.