mandrell diameter

[peter1958]

For penturning i have some different mandrells. For closed end and open ends. A standard bar in a MK2 collet and a nut on the other end is one of them.
I use it with the bushings that i bought for every kit.
Today i was busy with some penblanks and i put some bushings on the mandrell but i noticed there was wiggleroom between the bar and the bushings.
The battery on my calipers died so i don't have the exact play. The standard calipers showed 0.4 mm play. Is that normal?

Sanding a alsmost finished blank shows the ends have a little difference in wall thickness. The tube is out of round, What to do?

 

[magpens]

Do you really mean 0.4 mm play ? . . that's nearly 0.016" !!!

That seems WAY too much.

The diameter of the mandrel that most people commonly start off with is 0.246", if memory serves me correctly.
I believe this mandrel is called the Type-A mandrel

There is another, slightly larger mandrel ( Type-B) which is less common, but I forget its diameter..
Correspondingly, the bore of the appropriate bushings must also be larger.

Is it possible that you have put the larger bushings on the smaller mandrel ?

On the Berea Hardwoods website, I found both Type-A and Type-B mandrels offered. . Here is a link :-

Mandrel Systems, Mandrels and Arbors

Berea Hardwoods Offers a large collection of pen making supplies including Precision shop pen kit with Berea Hardwoods

www.bereahardwoods.com

EDIT:

Also, I found a link right here on IAP that deals with the two different mandrel sizes :-

Different size mandrels?

I first started turning pens about 20 years ago however I've not turned much in the past 5 years or so. Most of those have been 7mm kits. However many years ago I turned some pens that had larger tubes and used a "B" size mandrel. Mine is bent, and I want to replace it but I dont see mandrels...

www.penturners.org

 

[NJturner]

So I am not sure about the diameter of the bushings vs the mandrel, but I can tell you no matter the diameter, the bushing/mandrel clearance would not be the cause of the blanks being out of round. The out of round condition would be caused by either the mandrel being bent slightly or your headstock and tailstock being out of alignment. This could be caused by overtightening the mandrel (if you have a morse taper on one end and a 60 degree live center on the other side of the mandrel), or the actual mandrel is bent.

Since the blank is turning at a reasonable speed, the cutting edge is approaching the blank at the distance set by the mandrel more than the bushing, as the bushing could move laterally on the mandrel as it is pressured by the cutting tool, but the mandrel cannot move.

I would look at your mandrel and check your headstock to tailstock alignment.

 

[egnald]

Greetings. The larger, "B" sized mandrels were only made by Berea. They were designed to provide more rigidity for turning very large pens and were spec'd at 0.291 inches +0 /- 0.0002. The standard "A" mandrel is spec'd at 0.247 inches +0 / -0.0002 inches. The "B" mandrel never really caught on with suppliers other than Berea and equivalent "A" sized bushings can now be found for most kits that specify a "B" mandrel and bushings.

For mandrels my go to is a D Drill rod (0.246-inches diameter). Even though the mandrel is under the specified diameter a few of my bushings feel too tight on it and I would have to push very hard to get them to go on. When this happens I back off to a C Drill rod (0.242-inches). I am guessing that a lot has to do with the tolerances used to make the bushings. A 7/32 inch Drill rod should be a close replacement for a "B" sized mandrel measuring in at 0.2187 inches.

Dave

 

[Curly]

I would suggest you get a 60º dead centre for the headstock and a 60º live centre for the tailstock. Use your mandrel and sloppy bushings and turn to 0.5mm or 0.3mm of the final sdiameter. Then take the blank off the mandrel and put it on the conical centres, without the bushings, to complete the pen. You only have to put just enough pressure on the tailstock to turn the last little bit off. As you have done all the rough work you don't need a lot of force to spin the blank. There will not be any out of roundness as there are no bushing and mandrel slop nor any bent mandrel factors to worry about. Use callipers to get to the final sizes.
An example of the dead centre.

Morse Taper Dead Centres - Arc Euro Trade

Used in the tailstock when turning longer work or in the headstock and tailstock when turning between centres. MT1, MT2 & MT3 - hardened and ground.

www.arceurotrade.co.uk

One other factor not mentioned about mandrels and bushings is if the blank ends aren't perfectly square they will "kick" the blank over a touch and instead of turning along the centreline it turns an oval, giving you an out of round condition.

 

[peter1958]

thank you all for helping me. A new battery in the calipers shows 0.43 mm play at 1 set of bushings.
I have the same set bought earlier and that one has 0.18 mm play.
I measured 50 sets and the mandrell at several points. Mandrel is 6.19 mm (6.16-6.22 mm)
Bushings are 6.25-6.32 mm. Maximum play is 0.13 mm.
I bought some plastic boxes for the bushings so i can find them faster.. I have 3 doubles.... The mandrell seems ok, little bent but corrected that for a new pen.