The On-Line Magazine of Rideable Model Railroading

  NUMBER 191


© September 09, 2012   

 ©Discover Live Steam. This material may not be published, rewritten, or redistributed without written permission.

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Expanding Mandrel



Written by Dave August

An expanding mandrel comes in handy for many things. Of course you can buy precision ground taper mandrels or 2 piece sliding mandrels, but those cost good money and if you don't have one when you need it, you have to wait for delivery. This simple idea can be made from junk in your scrap box, you can make them to fit any odd ball size and be back at work in an hour or so.

The basic idea here is to use a pipe taper tap to thread the inside of the mandrel end and pipe plug to force it open.

I developed this idea when I needed to turn down the OD and trim to length some bushings for main rods. It's not as accurate as a taper ground mandrel, but is plenty accurate for Live Steam work, especially if it's held in a collet.

There isn't much to making one. I don't have a drawing for this but the pictures pretty much tell the story. Dig around in your scrap box and find or turn to the correct diameter a piece of round stock. Drill and tap it with a suitable size pipe tap. Slit the tapped end open so it will expand when a pipe plug is screwed in.

I prefer to use a commercial pipe nipple for the plug since they have a shallower taper than what you get when you use a pipe die for threading. I tap the opposite end of the nipple and used "Loctite" on an Allen bolt to make it easier to screw in. Since I use the plug for several different size mandrels I didn't mind the extra work required to do this. Squaring the end for a wrench would probably work just fine, and if you are a real hack, I guess you could just grab it with your 10R Vise Grips.

I split the mandrel end into 4 sections, with 2 cuts on my power hacksaw. It would probably be slightly more accurate if you split it 3 ways with a small saw on you mill, but that's more work.

These pictures tell the story. About the only thing strange here is that when I tapped the plug for the Allen bolt I did it backwards from the way I normally do. When I power tap in my lathe like this, I usually hold the tap in the drill chuck and the stock in the collet or a chuck. That's they way I did it when I tapped the mandrel. I didn't have a collet the correct size to hold the pipe nipple, and didn't want to change out the collet closer for a chuck just for this. Since there was no need for any real accuracy here, I put the tap in a collet and the work in the drill chuck. If the Allen bolt head looks turned, it is, and was just more junk from my scrap box left over from some other fixture I made.


Written by Dave August


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