Randy_
Member
I spent quite some time on the drawing in this post although it probably doesn't look like it. (I only have a crappy drawing program.) It was done in response to another thread; but is buried where a lot of folks might miss it. I didn't want to waste the effort so I decided to start a new thread that might get wider exposure.
Lots of new pencrafters get in trouble for lack of a proper live center and, sadly, this is a subject that is not always adequate address by the manufacturers in their kit instructions or catalogs.
If you are going to craft pens and use a mandrel of some sort, it is best if you have a 60° live center for your lathe. (We will ignore the subject of mandrel-less turning; but having a proper live center is important there as well.) I will say that some folks use the stock LC that comes with their lathe which most commonly comes with a 30° tip; but the majority of pencrafters(based on posted comments) much prefer to use a 60° live center.
Using a 30° LC is mechanically unsound and has caused a lot of people some grief. The tip that comes with the JET live center (and with most other wood lathes, as well) has a 30° point and is intended to be inserted into wood blanks. (unless JET has changed their tips recently) It "IS NOT" designed to be used with the center dimple in the end of a pen mandrel. For the center dimple in in your mandrel, you should use a 60° live center for best results!!
Below is a rough sketch that shows the fit of 30° and 60° tips in a center dimple and why a 60° is the preferred choice.
The upper drawing is the 30° tip and you can see that the bearing surface between the tip and the mandrel is very tiny. Because the bearing surface is so small the wear is concentrated and causes the tips to wear out prematurely. It is also possible that extra pressure will be needed to create enough friction to make the LC spin and this could lead to a bowed or bent mandrel.
In the lower drawing, you will notice that the angle of the 60° tip exactly matches the angle of the sides of the center dimple thus creating a much larger contact area. Less wear and more friction....that is the way the system was designed to function.
Sometimes, the center dimples are drilled with clearance holes that are too shallow. I have drawn a purple line to simulate where the bottom of the clearance hole might might be if it is to short. With a little imagination it is easy to envision that the extreme tip of the LC point would rest on the bottom of the clearance hole and there would be no contact between the dimple wall and the tip. This situation would totally defeat the purpose of the centering dimple as the tip would dance around on the bottom of the clearance hole and the wall of the dimple would provide no centering effect at all. Additionally, this would cause severe wear on the tip of the LC. As you might imagine, this possibility is more likely to occur with a 30° tip.
If you do get a mandrel with a shallow the clearance hole, the easiest solution for most folks is to file a bit off the end of the tip.
I hope this clears up some of the confusion surrounding live center tip angles and mandrel fit.
Lots of new pencrafters get in trouble for lack of a proper live center and, sadly, this is a subject that is not always adequate address by the manufacturers in their kit instructions or catalogs.
If you are going to craft pens and use a mandrel of some sort, it is best if you have a 60° live center for your lathe. (We will ignore the subject of mandrel-less turning; but having a proper live center is important there as well.) I will say that some folks use the stock LC that comes with their lathe which most commonly comes with a 30° tip; but the majority of pencrafters(based on posted comments) much prefer to use a 60° live center.
Using a 30° LC is mechanically unsound and has caused a lot of people some grief. The tip that comes with the JET live center (and with most other wood lathes, as well) has a 30° point and is intended to be inserted into wood blanks. (unless JET has changed their tips recently) It "IS NOT" designed to be used with the center dimple in the end of a pen mandrel. For the center dimple in in your mandrel, you should use a 60° live center for best results!!
Below is a rough sketch that shows the fit of 30° and 60° tips in a center dimple and why a 60° is the preferred choice.
The upper drawing is the 30° tip and you can see that the bearing surface between the tip and the mandrel is very tiny. Because the bearing surface is so small the wear is concentrated and causes the tips to wear out prematurely. It is also possible that extra pressure will be needed to create enough friction to make the LC spin and this could lead to a bowed or bent mandrel.
In the lower drawing, you will notice that the angle of the 60° tip exactly matches the angle of the sides of the center dimple thus creating a much larger contact area. Less wear and more friction....that is the way the system was designed to function.
Sometimes, the center dimples are drilled with clearance holes that are too shallow. I have drawn a purple line to simulate where the bottom of the clearance hole might might be if it is to short. With a little imagination it is easy to envision that the extreme tip of the LC point would rest on the bottom of the clearance hole and there would be no contact between the dimple wall and the tip. This situation would totally defeat the purpose of the centering dimple as the tip would dance around on the bottom of the clearance hole and the wall of the dimple would provide no centering effect at all. Additionally, this would cause severe wear on the tip of the LC. As you might imagine, this possibility is more likely to occur with a 30° tip.
If you do get a mandrel with a shallow the clearance hole, the easiest solution for most folks is to file a bit off the end of the tip.
I hope this clears up some of the confusion surrounding live center tip angles and mandrel fit.
