Expanding Mandrels Tips And Tricks?

[tomw]

Dear All,

I am using the expanding mandrel idea in a current project. However, said idea is, when applied, less than solid. I am trying to hold a piece with a 3/4" bore. The piece is a small CI steam engine frame. My tool is a Sherline lathe (3.5 x 17).

So:

1) What are good vs bad metals to use for mandrels, given the size of the problem?
2) What is the most expansion one should target (as a proportion of diameter)?
3) What is the best way of getting expansion?
-How do you actually design you mandrel to expand? If using NPT taps and fittings, how do you judge depth of hole, depth of tap, distances, etc?


Thanks,

Tom

 

[Wreck™Wreck]

I have worked in machine shops full time for over 30 years and have never used an expanding mandrel, what exactly is the application that would require such a device?

I am merely curious.

 

[JimDawson]

I have used both steel and aluminum for expanding mandrels. I have not made one where the mandrel is longer than the hole size (1x diameter). I normally use a pipe plug as the expander bolt, size predicated on the mandrel size, but normally 1/8 or 1/4 NPT. I try to go full depth with the tap. I normally make the mandrel a bit oversize, drill & tap the hole, slit the end, then screw in the expander plug just over finger tight, then finish the diameter to a snug fit. That way when you put the part on it doesn't take much tightening to grab the part tight and the mandrel stays pretty round and on center.

 

[Chipper5783]

If you are concerned about expansion, make a plane mandrel, slip fit and glue the part on with Loctite (heat to release).

 

[Cavediver]

I have worked in machine shops full time for over 30 years and have never used an expanding mandrel, what exactly is the application that would require such a device?

I am merely curious.

Turning or making cuts on a part concentric to a blind hole? I have done it with wood turnings, but have no clue if that is a valid operation with metal parts.

 

[planeflyer21]

I have worked in machine shops full time for over 30 years and have never used an expanding mandrel, what exactly is the application that would require such a device?

I am merely curious.

Check out myfordboy's youtube channel. He uses all sorts of setups with mandrels, lots of it with sheet metal.

They're much more useful than I would have thought.

 

[TomS]

I have used expanding mandrels but I find it more convenient to use a plain mandrel. I chuck a piece of stock in the chuck, turn the length slightly less than the length of the part and to a snug slip fit, drill and tap the end of the mandrel, and use a bolt and washer to secure the part to the mandrel.

Tom S.

 

[rgray]

Plain mandrel is the way to go for accuracy. Type that you turn between centers.
They are inexpensive. I've never had to glue or loctite any thing on.
Always cut toward the direction the part is driven on, so your not trying to push it off the taper.

 

[tomw]

Dear All,

Thank you all for the help.

Jim, I think I will try your approach.

Plane, I will look up that Youtube channel.


What I was trying to do was hold a model steam engine part to machine some features that are referenced off the bore I am putting the mandrel in.

The model is the PM research #5 engine.

I am working on the central frame that holds all the important bits:


I am trying to find a way to machine the areas that are circled on this here plan:



I have successfully done the bore and recess on the top. The other features are all relative to that bore, and the tolerances are tight. I want to create a way to hold the part and be able to center my spindle relative to the bore. I need to be able to center both horizontally and vertically, and with the frame upside right (as pictured) and upside down (to machine the journals for the crankshaft).

Here is the assembly drawing:


Getting to the area under the bore to hold with a non-expanding mandrel won't work. There is just no room.

I have thought of using CA glue or epoxy, but I was worried about rigidity.

I am using Sherline equipment.

Again, thank you all so much for your help.

Cheers,

Tom

 

[stupoty]

Could you attach the base to the cross slide and drill it with a drill / reamer in the lathe chuck ?

Stuart