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Bosch hollower hack

K

Kevin Jenness

Joined
Nov 15, 2020
Messages
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Location
Huntington, VT
Bosch alt setup.JPG

I was having a heck of a time hollowing this spalted maple vase, 8" d. x 12"h. using a 3/4" Jamieson boring bar in a Bosch Stabilizer. The workpiece is heavily spalted on one face and sound on the other, alternately dense and soft, and unbalanced- I could spin it at only about 450 rpm without shuddering and the tool was bouncing badly even with a slow feed rate. I got about 2/3 of the way in and was seriously wondering if I would be able to complete the piece at all. I decided to try a setup I had used in the past to extend the reach of the boring bar by using a separate tool rest in a second banjo, which did the trick.

I really like the Bosch hollowing jig for its compact size and easy setup, but it turns out that combining the tool rest and the boring bar mount on a single 1" post is not as resistant to leverage and vibration as the alternative shown here. If you have a spare banjo this can extend the limits of the Bosch rig.
 
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Alan Weinberg said:
Your setup looks weird to me—Bosch has the tool rest very close to the piece as per this picture. Not sure why you need the second tool rest—maybe I am missing something.
Click to expand...
With the standard setup, when the boring bar is as deep as it can go in the work the anchor point that resists the downward force on the cutting tool is just behind the tool rest. With a separate tool rest the linkage can extend well behind the fulcrum and better resist deflection. I was surprised at the difference in performance, but it is real.

One can also reach slightly deeper with a separate toolrest because the upper post is not held back as far as by the wide integral toolrest.
 
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Kevin Jenness said:
View attachment 74879

I was having a heck of a time hollowing this spalted maple vase, 8" d. x 12"h. using a 3/4" Jamieson boring bar in a Bosch Stabilizer. The workpiece is heavily spalted on one face and sound on the other, alternately dense and soft, and unbalanced- I could spin it at only about 450 rpm without shuddering and the tool was bouncing badly even with a slow feed rate. I got about 2/3 of the way in and was seriously wondering if I would be able to complete the piece at all. I decided to try a setup I had used in the past to extend the reach of the boring bar by using a separate tool rest in a second banjo, which did the trick.

I really like the Bosch hollowing jig for its compact size and easy setup, but it turns out that combining the tool rest and the boring bar mount on single 1" post is not as resistant to leverage and vibration as the alternative shown here. If you have a spare banjo this can extend the limits of the Bosch rig.
Click to expand...

So I assume the second banjo is out of the photo, to the lower right corner?

I'm curious, the neck of the vessel there, was that a mistake, or is that just how the piece is at the neck there?

Regarding the feed rate...wouldn't the slow feed rate actually exacerbate the bouncing of the tool? A lot of the time you can get around a bouncy tool with a faster RPM. Or at the very least, make the magnitude of each bounce smaller, which I think tends to make it more manageable. I haven't used one of these systems, though, only speaking from experience using tools on a single tool rest. Still, I wonder if, despite the imbalanced nature of the workpiece...if a higher RPM would help minimize the jitter in the tool?
 
Jon Rista said:
So I assume the second banjo is out of the photo, to the lower right corner?

I'm curious, the neck of the vessel there, was that a mistake, or is that just how the piece is at the neck there?

Regarding the feed rate...wouldn't the slow feed rate actually exacerbate the bouncing of the tool? A lot of the time you can get around a bouncy tool with a faster RPM. Or at the very least, make the magnitude of each bounce smaller, which I think tends to make it more manageable. I haven't used one of these systems, though, only speaking from experience using tools on a single tool rest. Still, I wonder if, despite the imbalanced nature of the workpiece...if a higher RPM would help minimize the jitter in the tool?
Click to expand...
Both banjos are visible in the photo.

If the neck is a mistake it is an intentional one.

By feed rate I mean the rate at which I advance the tool into the work. It's true that higher rpms tend to smooth out the cut, but because of the inherent imbalance in the blank I could not safely spin it faster.
 
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Kevin Jenness said:
View attachment 74879

I was having a heck of a time hollowing this spalted maple vase, 8" d. x 12"h. using a 3/4" Jamieson boring bar in a Bosch Stabilizer.

I really like the Bosch hollowing jig for its compact size and easy setup, but it turns out that combining the tool rest and the boring bar mount on a single 1" post is not as resistant to leverage and vibration as the alternative shown here. If you have a spare banjo this can extend the limits of the Bosch rig.
Click to expand...
I really like the Bosh stabilizer as well. Recently used it with an inch and quarter bar to hollow 18" deep. I have had the same trouble with bounce you did. Your solution seems to redistribute the tool support balance and at the same time provides a way to use the swing arm with less restriction. Using this solution I may be able to go to 20" deep. Great, now I need to figure out how to acquire another banjo. :rolleyes:
 
I do a lot of hollowing and have found that most or perhaps many hollowing rigs cater to the max, which is fine. But you need to keep the length of the hollowing tool to the minimum for the vessel being hollowed, all my hollowers the shaft can be lengthened to suit vessel. Add to that the tool rest as close as practical, also consider the cutting action, slicing is always better than scraping. That's why I have several hollowers, its horses for courses so to speak.
I have had a couple of swing arm type captured hollowers and have moved away from them due to the fact that there is often some degree of flex where you don't want any. But then I admit the species you are hollowing can defeat even the best of rigs. The other is speed and going slower for me is often the only way I can get it done, one thing about about hollowing I have found is it will always be slower than spindle turning.
 
hughie said:
going slower for me is often the only way I can get it done, one thing about about hollowing I have found is it will always be slower than spindle turning.
Click to expand...

That is an understatement :). I have been trying to follow a maxim of Army logistics, "slow is smooth, smooth is fast", but "fast" is relative.

What do you use for holding your boring bars? I have a couple of captured contraptions but have been going back to hand-held hollowing occasionally for the experience.
 
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Kevin. I think this is a brilliant idea! I love the Bosch hollowing rig but always wished for a bit more stability when hanging out there. Time to mock up a tool rest contraption from some ironwork :)
 
Kevin Jenness said:
That is an understatement :). I have been trying to follow a maxim of Army logistics, "slow is smooth, smooth is fast", but "fast" is relative.

What do you use for holding your boring bars? I have a couple of captured contraptions but have been going back to hand-held hollowing occasionally for the experience.
Click to expand...
I use hand held for much of what I do, with long handles so to allow long shaft to slide in and out for each vessel. I'm in the process of rebuilding my captured rig, it will mounted on the banjo with a depth capability of around 350mm or 14". I also use a portable type support set up that can be moved any of my hollowers, something of my own design.
Yeah I agree fast is relative, my average slow speed is around 4-500 , but then I like to turn what most folk burn or throw out
 
After two days of looking at your photo I had to go to the Bosch website and see the Stablizer to comprehend what you were talking about, so now I understand. My Monsters have a receiver that you are using the second banjo for so I couldn't get it looking at your photo without knowing how it worked alone. A very good solution if you have a second banjo. I don't know if a receiver like other systems use at the back of the system to hold it would be a cheaper solution.
 
I guess I’m the only person who doesn’t understand this setup. The first banjo, which holds the Bosch stabilizer, goes near the opening of the piece being formed. Where does the second one go? Maybe another picture from the side will help me understand.
 
Alan Weinberg said:
I guess I’m the only person who doesn’t understand this setup. The first banjo, which holds the Bosch stabilizer, goes near the opening of the piece being formed. Where does the second one go? Maybe another picture from the side will help me understand.
Click to expand...

Trent's Stabilizer normally is one piece. It has its own tool rest and the arm to stabilize the tool. The arm holds the tool near the handle. When hollowing near the depth of the tool, the arm is holding right next to the tool rest and offers little support. That's the normal setup.

What Kevin has done is move the banjo holding the stabilizer back away from the hollow form. He's rotated the stabilizer's tool rest so it's no longer used. He then added another banjo with a regular tool rest near the hollow form opening. So the Stabilizer arm is holding the tool farther away from the hollow form where it can get some leverage.
It's really clever.

The front banjo and regular tool rest is marked in red. The back banjo and stabilizer is cyan (blue-green) (can't see the bottom of this banjo, just the top). Hope it helps:
Bosch alt setup.png
 
Dave Landers said:
Trent's Stabilizer normally is one piece. It has its own tool rest and the arm to stabilize the tool. The arm holds the tool near the handle. When hollowing near the depth of the tool, the arm is holding right next to the tool rest and offers little support. That's the normal setup.

What Kevin has done is move the banjo holding the stabilizer back away from the hollow form. He's rotated the stabilizer's tool rest so it's no longer used. He then added another banjo with a regular tool rest near the hollow form opening. So the Stabilizer arm is holding the tool farther away from the hollow form where it can get some leverage.
It's really clever.

The front banjo and regular tool rest is marked in red. The back banjo and stabilizer is cyan (blue-green) (can't see the bottom of this banjo, just the top). Hope it helps:
View attachment 74931
Click to expand...
That helped greatly. When hollowing deep, though, why would this help since where the tool is being held and the front tool rest would then be very close to one another?
 
Alan Weinberg said:
That helped greatly. When hollowing deep, though, why would this help since where the tool is being held and the front tool rest would then be very close to one another?
Click to expand...
I think it just gives the arm a better shot at providing support. Also looks like it'd spread the load between two banjos. So the single banjo isn't taking all the torque - less opportunity for flex.

I'm really interested in trying this - gonna require cobbling together another banjo or something to be the forward tool rest.
 
Kevin, maybe you can help me out here. I have the same equipment you have (Bosch Stabilizer—which I love—and 3/4” boring bar), and I have a second tool rest that I can use to recreate your setup. But, I have been configuring them on my lathe to see how you managed to get an advantage with the second tool rest, and I don’t get it. (Thanks Dave for trying to explain, but it hasn't helped me yet.)

The Bosch Stabilizer, when extended, provides full support for a 19” long bar with its own integrated tool rest, That is, the when the tool rest is at the mouth of the piece, and the linkage is fully extended, the whole length of the 19” bar is used. For each inch you hollow into the piece, you must move the mounting point an inch closer to the mouth of the piece, where the tool rest (the integrated Stabilizer rest or a second tool rest) is located.

So, I don’t see how I would get any more leverage or control with a second tool rest.

When Alan queried you on this point, you said that the separate tool rest allowed you to locate the linkage further behind the fulcrum, but—and perhaps this is the point that Alan was making—I can’t see how this is possible. As I say, the Stabilizer linkage already extends far enough to allow for the mounting point to be located as far away from the integrated tool rest as the (19”) length of the bar permits. A longer bar would allow further distance, but it would be better just to move a longer bar up in the mount on an extended linkage as the hollowing progressed--no point in a second tool rest.

If the second tool rest can provide an advantage I will certainly deploy it, but can you (or Dave) explain how this could be possible?

Gord
 
Separating the stabilizer post from the tool rest means that the stabilizer post can be mounted farther back when the boring bar is fully extended. This allows for better resistance to uplift at the anchor point where the bar is attached to the linkage in opposition to the downward deflection of the cutting tool. Using the integral toolrest means that the anchor point is close to the toolrest when the bar is extended and the bending force is absorbed by one banjo instead of being spread over two points. If that isn't clear I am sure the fault is in my inadequate explanation, and you should probably experiment for yourself. It's like horseshoes, it works whether you believe in it or not.

I don't want to appear critical of Trent Bosch's design, I think it's extremely clever and well engineered. This alternative gives it a little more reach and stability in extreme situations at the cost of a little more setup.
 
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Thanks for being patient with me Kevin, but I am perplexed by your opening sentence: "Separating the stabilizer post from the tool rest means that the stabilizer post can be mounted farther back when the boring bar is fully extended."

If that were true I could see how this works, but I can't get there on the math, or in the real world of my lathe bed, so maybe I am misunderstanding something.

I can't see how any configuration can result in the mounting point being farther back than the length of the bar. A typical 3/4" boring bar is about 17"-19" long, and the Bosch Stabilizer allows the post to be a bit more than 19" from its integrated tool rest (and hence the mouth of the piece). And when you move into the piece, the mounting point has to move up in step, so nothing changes when you are inside the piece hanging over the tool rest, whether it is the integrated tool rest or a free-standing tool rest.

I have laid out my Stabilizer and taken measurements with its integrated tool rest and with a free-standing tool rest, and I cannot get the mounting point any further from the tool rest in either configuration and at any point of penetration into the piece. The determining factor always ends up being the length of the bar.

Put another way, as long as the configuration allows the distance between the mounting point and the tool rest to be at least as far as the bar is long (as the Stabilizer does with a 19" bar), I can't see any difference whether one uses the integrated tool rest or a free-standing tool rest.

So, if the second tool rest added some control for your troublesome wood, I don't think it was from any increase in leverage. I have pondered Dave's point that the second tool rest might add some stability, but both configurations have the tool rest supported directly by the bed ways and the tool supported at the end of the articulated arm, so I doubt that there is much difference. (BTW I find the Stabilizer quite solid on its own.)

Sorry Kevin, if I am being dense, but I am not seeing how a second tool rest could help. As you say, I will have to try it to see.
 
I believe Kevin is using a longer bar (from his Clarke hollowing system), not just the Bosch system. So assume a 24" bar with a 3/4" adapter at the back and a tool projecting from the 1-1/4" diameter rod at the front. There's a previous post on his setup somewhere on this forum (I am going from my memory.....which is .....poor :)
 
Hmm. Kevin said he was using a Jamieson 3/4" boring bar, which I think is 17", but if he has a 24" bar, that would make a difference.

As an aside, with a 3/4" x 24" bar I would keep the distance between the mount and the tool rest at about 16" to minimize bar flex, advancing the tool through the mount as the tip advances into the piece. No second tool rest is required with that approach.
 
I have used a 1 1/4" x 18 "bar in this system, but in this case I was using the shorter 3/4" bar due to the opening size, and the length was pretty well maxed out going 12" deep.

Here is a link to my previous post about setting up the Bosch rig for different size bars. setup The second photo shows a side view of the two-banjo arrangement. As you can see, the mounting point of the stabilizer in the second banjo is much farther back from the tool rest than it ordinarily would be using the integral tool rest, and that is why it resists deflection better. It's like having a longer handle on a shovel. In that case the opening was large enough to use a box rest that extends inside the vessel's opening, improving the mechanical advantage even more.
 
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The second photo is useful, but I actually think it describes my point: the articulated arm is folded up. If it were unfolded, the mount could stay right where it is and the integrated tool rest of the Stabilizer could be where the second tool rest is. The second tool rest is not allowing the mount to be further back.

As I see it, the Stabilizer extends 19", so as long as the bar is not greater than 19", a second tool rest cannot allow the mounting point to be further back: I think it is just math, or geometry, or something like that.

Here is what I did, perhaps you can tell me where I am going wrong: I put a 19" bar in the Stabilizer and extended it 8" over the integrated tool rest. I noted the position of the bar mounting point on my bed ways, and measured its distance from the integrated tool rest (and thus from the mouth of a notional piece of work). Then I put in a free-standing tool rest and moved the integrated tool rest of the Stabilizer out of the way. If I maintained 8" over the (now free-standing) tool rest, the mounting point of the Stabilizer was in exactly the same position as it had been (except for a very small difference because of the different widths of the tool rests).

Thanks for bearing with me on this.
 
Gord, we seem to be speaking different languages. What I call the mounting point of the stabilizer is the post that fits in the banjo directly under the integral tool rest.

When the boring bar is fully extended and the tool is cutting the downward deflection of the bar is resisted by the front edge of the integral toolrest and there is an opposite and equal upward force on the linkage behind the toolrest that is transmitted back to the banjo. The combination of upward and downward forces on the mounting post tends to bend the banjo and toolrest toward the work.

When I use a separate tool rest that mounting point is necessarily farther back than the tool rest. That distance plus the distribution of the vertical loads between two banjos is what gives the modified arrangement better leverage and stability. I believe that putting a dial indicator on the upper post while pushing the tool tip downward in the two scenarios would show a significant difference. I'm no engineer so my understanding of the mechanics may be lacking, and I apologize if my explanation is unclear, but results are what is important.

Some photos or a drawing of your mockup might help me understand your point.
 
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I decided to test my understanding by experiment. I set up the stabilizer with and without a separate tool rest. I hung a 30# weight on the end of my boring bar at full extension and measured the upward deflection of the forward linkage bar just behind the post that holds the boring bar. With the standard setup I measured .012", with the separate toolrest only .008".

Incidentally, I was overestimating the increased reach available. The angled toolrest allowed only 1/2" additional compared to the 1 1/2" wide integral rest. I could reach about 14" with that particular bar with a minimal protrusion on the back end for attaching a handle.

That Jamieson tool rest with threaded post is invaluable for getting the tool located at dead center at the bottom of a vessel.

DSC_0003.JPGDSC_0005.JPG
 
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Yes, I see the confusion in the terminology, so let's call the place where the tool is held at the outer end of the Stabilizer's articulated arm the "holding point". The integrated tool rest is at the fixed end of the articulated arm, mounted in the banjo. I have made a diagram of the normal Stabilizer configuration and the one in the second photo you provided of your setup.

In its normal configuration, the Stabilizer has its integrated tool rest mounted in the banjo in front of the workpiece, and the tool held in the "holding point" back from the integrated tool rest, at the end of the articulated arm (in midair, so to speak).

You can (as you have shown in the second photo of your setup) turn the Stabilizer around, so that the "holding point" is (in midair) beside the integrated tool rest in the banjo, and put a free-standing tool rest in a banjo in front of the workpiece. The integrated tool rest is now far back from the mouth of the piece.

But, as I have tried to show in my diagram, the "holding point" is still in midair at the end of the articulated arm, at exactly the same distance from the (now free-standing) tool rest as it was from the integrated tool rest. Yes, the integrated tool rest is now mounted further back, but I don't understand how this is relevant to the question, given that it has been taken out of the operation. The distance between the place where the tool is held by the Stabilizer (the "holding point") and the (now free-standing) tool rest is the same in the two configurations (the distance "x" in the diagram).

If you are somehow resting your tool handle somewhere on the arms of the Stabilizer, this might help in some way, and if you are doing this that might be why it feels more stable. But the height differences (of the articulated arms) would require some fiddling, and as the force on the handle is upwards and the arms are underneath the handle, I wouldn't expect this to be a significant factor.

Perhaps with this information you can understand my confusion better.
 

Attachments

Gord, the location of the "holding point" is not the controlling factor, rather it is the location of the "mounting point" of the link arms relative to the tool rest. The photos in my last post are as clear as I can make it. That's my story and I'm sticking to it.
 
It is always true that a hollowing system that is anchored at two points fore and aft, is gong to be much more solid than a system that is balanced on one point. Balancing a system on one point allows flexing and vibration.
 
Ric Taylor said:
It is always true that a hollowing system that is anchored at two points fore and aft, is gong to be much more solid than a system that is balanced on one point. Balancing a system on one point allows flexing and vibration.
Click to expand...
Agree but I consider the standard Bosch setup anlready anchored at two points—the tool rest and where the tool attaches, no?
 
I own Trent's Stabilizer and I've never had any tool related problems. It works well, takes much of the stress off my arms and shoulders when hollowing, and, coupled with his Visualizer, provides me the tools to get a great hollowing result almost every time I do a hollow form. When I first started using the Stabilizer, I did have some problems, but found out that these were due to my inexperience with the tool. It's a great accessory/tool for hollowing. There are certainly other hollowing systems available that are likely to be just as effective. Having started my hollowing adventures with one that works well, I wouldn't even consider trying to modify it or spend the money on a different one. For me, practice has always been the key to success with any tool.
 
Ed Weingarden said:
It looks like your steady rest is 2 wheels on the left side of the HF. I think you would get better stabilization with a 3 wheel steady rest with the wheels at about 8 o'clock, 12 o'clock, and 4 o'clock.
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Ed, the arms on a 3 wheel steady rest should be placed at 1, 5 and 9. By having the wheels at these setting you have room for your laser/camera arm to move freely to get a perfect vertical position. This is extremely handy when you are working at either the top of the HF or at the bottom of the HF.
 
Kevin Jenness said:
Gord, the location of the "holding point" is not the controlling factor, rather it is the location of the "mounting point" of the link arms relative to the tool rest. The photos in my last post are as clear as I can make it. That's my story and I'm sticking to it.
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I admire the strength of your conviction Kevin, and I think that your reference to the mounting point of the link arms gets to the nub of this discussion. But I would say that this is because that point is more or less immaterial to stability, not because it is decisive.

Other than your hands on the tool handle, the only two points that the tool is controlled / stabilized are (1) the tool rest and (2) the place at which the tool is held--the holding point. The location at which the articulated arm begins is not material.

Indeed, the genius of the Bosch Stabilizer is Trent's insight that the holding point at the end of an articulated arm doesn't know or care where the articulated arm begins, as long as that point is as solid as any of the options. Thus Trent was able to design the Stabilizer with the articulated arm anchored at the banjo / tool rest rather than with its own support on the bedways. This not only makes the Stabilizer compact, it makes positioning it more flexible for lathes with sliding headstocks, and makes it indispensable for lathes with rotating headstocks.

Thanks for this discussion, it has been fun.
 
Where I think Kevin's insight is useful (for me) is that I could use a larger bar, stepped down to fit the 3/4" opening of the Bosch retainer and use a second rest at the front to support the bar. I LOVE the Bosch system and use it routinely in my hollowing but it's nice to have a little more reach for those forms that are an inch or two too long :)
 
Edward Kerslake said:
Where I think Kevin's insight is useful (for me) is that I could use a larger bar, stepped down to fit the 3/4" opening of the Bosch retainer and use a second rest at the front to support the bar. I LOVE the Bosch system and use it routinely in my hollowing but it's nice to have a little more reach for those forms that are an inch or two too long :)
Click to expand...
Yes, if you wanted to use a >19" bar, a free-standing tool rest could be helpful. I love having options, and I am going to play with Kevin's idea some more on my lathe.

But note that, to get to the bottom of a very deep form, the normal configuration of the Stabilizer should allow you to use a >19" bar too, without a free-standing tool rest, it is just that it could be awkward: you could back the banjo / tool rest away from the mouth of the piece, insert the bar in the piece and then slide the banjo / tool rest back up to the mouth of the piece. This could be inconvenient for clearing chips and you wouldn't have the same range of motion, but it should be doable in a pinch.
 
Kevin Jenness said:
That Jamieson tool rest with threaded post is invaluable for getting the tool located at dead center at the bottom of a vessel.
Click to expand...

That threaded tool rest looks pretty awesome. I'll have to bookmark this for future reference. I have been wanting to turn some larger vases, but have not invested in a hollowing system yet. This threaded rest seems really nice for finely adjusting the resting point of the tool...

I assume you can just loose the lock on the post a bit, then turn that nut, to get really fine tuning of rest height?
 
Ric Taylor said:
It is always true that a hollowing system that is anchored at two points fore and aft, is gong to be much more solid than a system that is balanced on one point. Balancing a system on one point allows flexing and vibration.
Click to expand...
Ric, is there a hollowing system that is "balanced on one point"?

All of the articulated and captive systems I can think of have the tool (1) held at one point and (2) resting on the tool rest. The captive systems will tend to hold and control the tool more robustly, but it seems to me that all captive and articulated systems have two points of contact: the tool rest and some place that holds the tool. Are there some that are otherwise?
 
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