Closed End Mandrel

[jimbob91577]

So I have an idea, and it may have already been built, but my uncle does metal fabrication and I've asked him if he would build something for me. Before I have him do it though, I thought I would ask the folks on the forum for their input.

Basically, I'm looking for a closed end pen mandrel. I'd like something that is flexible enough to be used for multiple kit designs - thus I'm thinking of something like what is pictured below. It consists of a mandrel system and a sleeve. The sleeve, depicted below the mandrel system, will fit over the mandrel, following the appropriate bushing, and be secured by "O"-Rings and set screws. The pen barrel will fit over the sleeve and be secured by "O"-Rings. The Mandrel depth can be controlled by the two nuts on each end of the taper.

My question(s) are as follows:
- Will the taper work itself loose not having any pressure keeping it in the head of the lathe?
- Any problems with using "O"-Rings to secure a pen barrel to the sleeve?
- Any problems with the overall design?

Thanks!

 

[mickr]

check the library here at IAP..lots of info there on making closed end mandrels..see if yours has all the necessary parts to do the job

 

[juteck]

To prevent your MT from coming loose in the headstock, extend the length of the threaded rod all the way through the headstock spindle, and make a knob with a nut to tighten against your handwheel.

 

[glycerine]

What would the set screws be used for?

 

[jimbob91577]

What would the set screws be used for?

Holding the sleeve to the mandrel. I wasn't sure if O-Rings would be enough to hold it without it slipping around the mandrel. Having the sleeve would allow the mandrel to be used for different pen kits. Just slip on a new sleeve.

check the library here at IAP..lots of info there on making closed end mandrels..see if yours has all the necessary parts to do the job

Any articles in particular?

To prevent your MT from coming loose in the headstock, extend the length of the threaded rod all the way through the headstock spindle, and make a knob with a nut to tighten against your handwheel.

If the shaft of the rod is 7mm - would that open up the possibility for flexing in the rod? My headstock depth is at least 10", which makes the rod at least 13-14" long. Will the taper prevent (or restrict) the flexing of the 3-4" of rod exposed for use?

 

[gwilki]

I'm confused by your pic. You show a sleeve over the mandrel with an outside diameter the same as the bushing. That being the case, how do you slide the blank over it? The od of the sleeve needs to be the same as inside diameter of the tube in the blank, if I am understanding your pic correctly.

 

[RAdams]

I really like the adjustable mandrel and single bushing method lately. I use my adjustable mandrel and the bushing on the other end. The only special anything you need is tape to do the bottom. I have a bunch of different sized bolts that have been turned into various cl. end mandrels, but i don't even use them anymore.

 

[jimbob91577]

I'm confused by your pic. You show a sleeve over the mandrel with an outside diameter the same as the bushing. That being the case, how do you slide the blank over it? The od of the sleeve needs to be the same as inside diameter of the tube in the blank, if I am understanding your pic correctly.

Sorry perhaps I didn't describe it correctly, see the attached image - the outside diameter of the sleeve will match the outside diameter (OD) of the step of the bushing. Take for example a Flat Top bushing for Berea's Fountain and Rollerball pens - it has a large OD and a small OD - the sleeve OD would match the small OD allowing the blank to slide over the sleeve and the small OD part of the bushing. The o-Rings would hold the blank to the sleeve preventing slippage, and the o-Rings/Set Screws would hold the sleeve to the mandrel to prevent slippage. (albeit at this point, the set screws might not be necessary...)

All you would need to change to a different kit type is a new sleeve and the appropriate bushing.

 

[glycerine]

I see. But I would think that unless you have set screws on 2 (or 3) sides, the sleeve would not be perfectly centered, correct?

 

[skiprat]

I like your thinking but I suspect you will run out of diameter...
what I mean is that you already have say a 6mm (1/4 ) mandrel shaft, you will then need a little clearance for the sleeve, say 0.25mm
You will need some thickness in the sleeve to accomodate the o-rings.

I'm not near a pen tube and can't remember what a Jr tube inside dia is?

Finally, on the 6mm mandrel you will have grooves to take the o-rings. Even if you find 1mm thick o-rings this will reduce the mandrel to only 4mm inside the grooves. I think this would bend too easily.

Have a look at my O-ring pin chuck in the library. You can make one in about ten minutes for any size tube and you can make it with a wood lathe and a file:wink:http://content.penturners.org/articles/2008/pinchuck.pdf

 

[jimbob91577]

I like your thinking but I suspect you will run out of diameter...
what I mean is that you already have say a 6mm (1/4 ) mandrel shaft, you will then need a little clearance for the sleeve, say 0.25mm
You will need some thickness in the sleeve to accomodate the o-rings.

I'm not near a pen tube and can't remember what a Jr tube inside dia is?

Finally, on the 6mm mandrel you will have grooves to take the o-rings. Even if you find 1mm thick o-rings this will reduce the mandrel to only 4mm inside the grooves. I think this would bend too easily.

Have a look at my O-ring pin chuck in the library. You can make one in about ten minutes for any size tube and you can make it with a wood lathe and a file:wink:http://content.penturners.org/articles/2008/pinchuck.pdf

Actually, I did look at your write-up, and if I had a set of pin jaws for my chuck, I'd be going that route. But alas, the Grizzly chuck I have doesn't have a set of pin jaws for it, and I don't have the funds at the moment to get a new chuck with the appropriate jaws necessary. Further, I don't have a jacobs chuck either - so I'm kinda stuck between the proverbial rock and hard place.

I've also thought about an alternative where I have the taper built to accept a threaded rod (say a 7mm typical rod) and then have the appropriate size bolts machined down (similar to what you describe in your article) and threaded to fit into the taper. Same principle except I'd need a separate mandrel for each kit style (which isn't a big deal considering I'd need a sleeve for my design going the other way).

Still, another alternative would be to do away with the set screws and o-rings on the mandrel and just have the sleeves threaded on the inside. This would lock the bushing in place keeping it from spinning and eliminate the "Flex" problem in having a 4mm o-ring groove on the main mandrel.

I chose the sleeve method because of my circumstances in not having the appropriate chuck setup. I figure I can find and modify sleeves easier than trying to modify a bolt, but I could be mistaken.