Oh man, don't let that sucker spin in your tube. You'll end up with a short tube really quick.
@Mortalis . . . What I am about to say might be perfectly obvious to some. . I think it is still worthwhile saying.
Mr. Mortalis, I think we are finally getting to the crux of the issues with TBC now, sir !! . . . The light has finally come on !!!
Please forgive my wordiness. . Also, "Someone" whose name appears above in this thread is going to say "I told you so .... many times before."
A DETAILED DISCUSSION OF POTENTIAL PROBLEMS ASSOCIATED WITH TBC ( TURNING BETWEEN CENTERS )
The friction between the diamond cone and the brass tube, which the diamond cone directly contacts, is the key.
This friction due to the diamond particles goes a long way to prevent the blank from spinning ( slipping ) on the cone shape.
This friction is the reason that I have had as much success as I have had with TBC because I have always used the diamond cone.
With REASONABLE pressure from the tail stock I have not experienced troublesome spinning of the blank on my diamond cone dead center.
The problem described and STUNNINGLY ILLUSTRATED in this thread has brought some fundamental matters to light.
The fundamental RISK associated with TBC is now elucidated with good clarity ..... finally. . The RISK is BLANK BREAKAGE.
The fundamental RISK is the destruction of the blank being turned on the lathe, as seen above, whether it is a wood lathe or a metal lathe.
The problem in understanding the RISK involved with going to TBC is that there are two (or more) things happening at the same time.
I believe that the essential points are as follows : . ( FORGIVE MY WORDINESS .... I NEED TO GO THROUGH THIS STEP-BY-STEP FOR MYSELF )
Using conventional bushings :
1. The use of bushings inserted into the brass tube ends has a problem ... namely the bushings do not EVER fit 100% properly ... there's always slack.
2. A technically more precise way of saying this is that bushings can not be made to fit into the brass tube perfectly.
3. The bushings have to be made a LITTLE SMALLER in diameter than the I.D. of the brass tube ... some might call this a tolerance consideration.
4. Also, if a mandrel is used, there is a tolerance consideration for the fit of bushings on the mandrel ... I.D. of bushing hole to mandrel diameter.
5. Hence, the inherent inaccuracies with using bushings ...
..... When a blank is mounted with bushings, the blank axis may not exactly be on the rotational axis of the lathe => off center, or eccentricity.
Using the TBC ( Turning Between Centers) method :
6. Someone then recommends the elimination of mandrel + bushings .... use conical centers at both ends of the blank => dead + live center.
7. But now, the conical centers introduce new issues ... there is now a radially outward force on the inner surface of brass tube due to conical shape.
8. Previously, the bushings applied a purely longitudinal force to the blank ... flat end of bushing "ridge" against flat end of pen blank
.... No force problem, but the problem might be that the blank is not centered on the rotational axis of the lathe => off-center shaping of blank.
9. With TBC, using conical 60-degree centers, we can get the blank "perfectly" centered on the lathe rotational axis ....
.... but .... the contact area between the cone-shaped dead center in the head stock and the brass tube in the blank is now greatly reduced.
10. The reduced contact area results in a reduction in the FRICTIONAL driving force at the head stock end of the blank => blank may slip on cone
11. So, with TBC, to prevent the slippage at the head stock end, the LONGITUDINAL FORCE applied by the tail stock MUST BE INCREASED.
...... This increased force increases the friction to prevent the slippage.
12. But .... there is a downside : because of the shape of the cone, some of that increase in longitudinal force becomes a radially outward force.
13. This radially outward force is applied to the inner surface of the brass tube .... THAT IS POTENTIALLY VERY BAD FOR THE PEN BLANK.
14. THE PEN BLANK CAN BREAK due to this outward force .... as seen above.
15. The outward force is actually applied by the dead center cone to the inside surface of the brass tube at the end of the brass tube.
16. Brass is a SOFT METAL and it can and does expand outward due to the force .... the radial expansion of the brass tube end is called "flaring".
17. The OUTWARD FORCE, which causes the "flaring" of the brass tube end, gets passed on to the BLANK MATERIAL.
18. The BLANK MATERIAL CAN BREAK, as we can see has happened in the above example of the use of TURNING BETWEEN CENTERS.
19. The glue between the blank material and the brass tube will not prevent this breakage of the blank ...
20. The breakage is internal to the blank material. . STABILIZATION of the blank material MIGHT HELP ... but don't count on it.
So there we have it .... POINT-BY-POINT .... or step-by-step. .
USING the TURNING BETWEEN CENTERS ( TBC ) method of blank turning HAS A SERIOUS RISK ASSOCIATED WITH IT ... BLANK BREAKAGE.
The use of SPECIALIZED TBC BUSHINGS may have a VERY GOOD PURPOSE WHEN your pen BLANK is FRAGILE AND SUBJECT TO BREAKAGE.
However, WITH special TBC BUSHINGS, you STILL HAVE a possible PROBLEM with your BLANK TURNING SLIGHTLY OFF CENTER.
I think I am going to create a new thread in the "Penturning sub-forum" with the above text repeated, almost word-for-word.
I acknowledge the frequent warnings about TBC that have come from
@jttheclockman .
I have now come to the realization that his warnings PROBABLY relate to the risk of blank breakage resulting from brass tube "flaring".
However, I needed to go through this step-by-step detailed explanation for my own sake .... I hope this may be a help to others also.
The reason I have never experienced the problem associated with TBC is that I almost always use a very sturdy blank material.
I generally do not use materials that have a low tolerance to breakage, such as the "pinecone" blank material used in this thread.
In the above example, described in this thread, that was not the case. . The blank material is weak and breakage resulted.
Thank you for your patience .... all of you who have read this "thesis" .... and especially you, too, John
@jttheclockman .
John, PLEASE ..... if you insist on saying "I told you so !" ..... go ahead .... but .... I ALREADY HEAR YOU LOUD AND CLEAR !!
All of the above may be perfectly obvious .... it was obvious to both you and me, John. . But I think that some may benefit from reading the words.
The fact that this thread shows the problem(s) so clearly makes this a very valuable thread ..... I hope that my words have value to some also.
One final word of warning :
The potential problem associated with TBC will be exaggerated if your brass tube is not glued into your blank with the ends accurately aligned.
If the blank material extends out beyond the end of the brass tube a "little bit", the conical drive centers used in TBC may contact the blank.
If they do contact the blank material at the ends, your BLANK is HIGHLY LIKELY TO BREAK due to the outward pressure from the conical centers.