As far as I know the Bosch rig is the only system in which both the tool rest and the boring bar are anchored at a single point, the banjo. This makes it very convenient to set up and its robust engineering makes the most of the concept, but it does present a limit on overall stiffness which can be alleviated by using a separate tool rest.
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Bosch hollower hack
- Thread starter Kevin Jenness
- Start date
Bill, for Rick's comment about hollowing systems "balanced on one point", you have added your "Without a doubt!" to Mike's "AMEN".
Perhaps you can answer the question I posed to Ric: What hollowing systems are you talking about that are "balanced on one point"? While some systems (such as captives) might hold the tool more firmly, do not all captive and articulated systems have two points of contact: the tool rest and some place that holds the tool?
On the (reasonable) assumption that the banjo is as solid an attachment point as the separate attachment point for some other hollowing system, I don't see how it could matter.
Subject to correction, as requested above, all hollowing systems are attached to (or near) the lathe at one point, and have two contact points with the tool: the place that holds the tool and the tool rest. The only difference with the Bosch Stabilizer is that it doubles-up on the place at which the system is attached to the lathe and the tool rest, using the banjo for both.
We have all grown accustomed to the concept, in things subject to mechanical stress, that two points of attachment should be spread apart, but that concept is not in play here: there is no mechanical advantage in having the attachment point of the system distant from the tool rest.
That is to say, assuming a reasonably capable banjo and lathe bed, the attachment of the articulated arm to the banjo does not degrade the banjo's ability to support a tool rest, and the banjo's support of the tool rest does not degrade its ability to hold the articulated arm. (I suppose it is possible that the forces on the articulated arm could cause the tool rest to move a tiny amount, theoretically impacting the tool that sits on it, but when you think of how much the tool is moving at its tip, I doubt that would be a material factor.)
The end of an articulated arm, where the tool is attached, is just as stiff (or not) whether the arm is attached behind, beside or in front of the end. So, why not be efficient and use the banjo, which is already there for the tool rest, for the attachment point of the system? It looks different, it might look like it is "balanced on one point", but that is a misperception: the leverage / control is provided by the spread between the tool rest and the place where the tool is held, not by a spread between the tool rest and the attachment point of the system. Those two points can be (as with the Bosch Stabilizer) one-on-top-of-other. Despite our habits of perception in other contexts where things are attached and we want the attachment points spread apart, these two points function separately, such that there is no mechanical advantage in having them a distance apart.
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The single point of attachment is the banjo to the bed. With two banjos you have two points of attachment. All forces are ultimately transferred to the banjo(s) therefore spreading the force across two banjos has to be more stable than one.
I would say no Mike, because even with two banjos the system only attaches to the lathe bed at one point, not two.
Both arrangements have an attachment point and a tool on the tool rest. The Bosch Stabilizer uses the same point for the attachment and the rest, but as with any other hollowing system, it has one attachment and one resting point.
And as I explained in my earlier post, in this context using the same point is mechanically the same as using two separate points, because the point is used for different reasons that do not compromise each other. There is no additional stability achieved by having those two things (the attachment to the lathe bed and the tool rest) at different points.
Both arrangements have an attachment point and a tool on the tool rest. The Bosch Stabilizer uses the same point for the attachment and the rest, but as with any other hollowing system, it has one attachment and one resting point.
And as I explained in my earlier post, in this context using the same point is mechanically the same as using two separate points, because the point is used for different reasons that do not compromise each other. There is no additional stability achieved by having those two things (the attachment to the lathe bed and the tool rest) at different points.
I agree with that Mike, I mentioned this above in an earlier post. Attaching the articulated arm at the banjo will transmit the stress to that point.
When you think of the short, beefy components at that location, I would be surprised if this force deflected the banjo or tool rest more than a few thou. In all events, it would be a very small amount relative to the movement at the tip caused by other factors, such as the flex in the bar and the various moving parts.
Though I think that this would be a very small source of movement, if at all, it is something I will be looking for as I experiment with the configuration that Kevin suggested.
When you think of the short, beefy components at that location, I would be surprised if this force deflected the banjo or tool rest more than a few thou. In all events, it would be a very small amount relative to the movement at the tip caused by other factors, such as the flex in the bar and the various moving parts.
Though I think that this would be a very small source of movement, if at all, it is something I will be looking for as I experiment with the configuration that Kevin suggested.
Much depends on how substantial the banjo is. Some of my first lathes the banjo left much to be desired, if not down right dangerous. The current lathe I built myself and probably got a bit carried away due to earlier experiences. As it is a short bed bowl lathe, there is little room for dual anchor points. But I agree with your point Mike
See post #26.
Gord after looking into Kevin's post because I did not understand what he was showing I saw the difference between what I use and how the Bosch is different. The Bosch system goes into the banjo and has its own tool rest so it is a single point system. Every hollowing system I own and have owned are two point hollowing systems where the system is tied to two points on the bed of the lathe. Either by having a receiver at the back of the lathe and the tool rest on in the banjo in front of the piece or in the case of the original Elbo Tool which connected to the tail stock for the second point on the bed. Tim has changed that and has made a receiver to hold the system at the back of the lathe. I'm not saying the Bosch system is bad I'm just saying that the two point system is without a doubt a sturdier setup. I cannot imagine that there would be the same amount of chatter using scrapers in the Bosch vs the two point system, I believe there would be more chatter with the Bosch as only being tied to one point on the bed. That chatter is why I now use the Hunter carbides because they cut and not scrape for my hollowing.
Yes Kevin, I like the idea of this experiment, but judging from your pictures, your two setups in post #26 are different in important ways, so there are limits on the conclusions we can draw from your data.
Recalling that the question--as I think it was correctly identified by Mike--is whether, and if so how much, the "normal" setup of the Stabilizer might allow for movement of the banjo / tool rest (which would explain the movement you measured at the holding point of the tool), we should not have the banjo that is holding the Stabilizer made partly of wood, as you have in your setup. A less significant but still material factor is that you also have a plywood plate on top of the tool rest held by this banjo. For your alternative setup, the front banjo is a normal all-metal banjo with an all-metal tool rest.
So, post #26 compares a metal-and-wood structure to an all-metal structure. When we are measuring in thousands of an inch, this is not an ideal comparison.
With that said, even with two proper banjos there could be a difference in the two setups. Also, you noticed a performance difference in the real-world scenario of turning a difficult piece of wood. So, I would like to track this down. I will try some measurements with two solid banjos.
I still don't get it (nor do I care to). I'll just use the Bosch system as it was designed,
To your points, I repeated the experiment taking the wooden parts out. I guess I should explain their presence in the first place. The plywood/laminate shim atop the Bosch tool rest is to adjust for different boring bar thicknessses without changing the tool rest height, which is a finicky process. For this test I replaced it with a steel drill bit. I have two banjos, one shorter than the other, and the Jamieson tool rest post is too short for the smaller one, thus I mounted the jig in the shorter banjo with a wood spacer. In this case I used another tool rest and put the jig in the tall banjo.
I measured deflection at the same point as before on the link bar as well as at a point on the boring bar 9" from the tool rest (where it really counts). With the standard setup I measured .005" on the link and .093" on the boring bar. With a separate toolrest I got .0025" and .075" at the same points.
Static testing is obviously not the same as actually using the system, but I believe the tests show a couple of things. One, there is less deflection at the cutting tool using a separate tool rest. Two, the stiffness of the system is greater with the jig mounted in a taller banjo, with less projection of the mounting post. The results in use are more convincing to me.