# Expanding Mandrels Tips And Tricks?



## tomw (Feb 12, 2016)

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


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## Wreck™Wreck (Feb 12, 2016)

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.


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## JimDawson (Feb 12, 2016)

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.


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## Chipper5783 (Feb 13, 2016)

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


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## Cavediver (Feb 13, 2016)

Wreck™Wreck said:


> 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.


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## planeflyer21 (Feb 13, 2016)

Wreck™Wreck said:


> 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.


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## TomS (Feb 13, 2016)

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.


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## rgray (Feb 13, 2016)

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.


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## tomw (Feb 14, 2016)

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


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## stupoty (Feb 14, 2016)

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

Stuart


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## tomw (Feb 14, 2016)

Staurt, the big problem right now is machining the inside surfaces of the journal bosses to the dimension indicated here:




My main problem is getting a set up that allows me to find the center, have the base square to the table, and be very secure.

I am sure this is not that difficult, but I am having a hard time figuring out what the hell to do.

Cheers,

Tom


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## stupoty (Feb 14, 2016)

tomw said:


> Staurt, the big problem right now is machining the inside surfaces of the journal bosses to the dimension indicated here:
> 
> View attachment 122109
> 
> ...



You could drill and tap a hole in each side approximately centered and use buttons to get it dead on for position. 

I'm talking about the bearing hole by the way , sorry about that I see the piston clearance space is what your talking about.

I recently had to line bore a part to get concentricity and that was a bit of a faff to get it secure enough.

Stuart


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## TomS (Feb 14, 2016)

This is how I would do it.  Using an angle plate clamp your set up shown in Post #9 to the plate.  Adjust the horizontal plane and clamp it tight.  Using an edge finder locate off the edge of the 1-5/16 diameter and move the Y axis to establish centerline.  
again using the edge finder locate off the end of your part to establish a reference in the X axis.  Move the X axis 4" and you will be centered on the crankshaft centerline.  With this setup you will be able to bore for the crankshaft, mill the outer crankshaft boss face, and the cap mounting surface.  Flip your angle plate over, reset your X and Y axis centerline references and do the same to the other crankshaft boss.

Milling the inside crankshaft boss is a bit trickier.  I'd make a fixture plate with a register that fits the 1-5/16" end of your part.  Clamp your part to the fixture then clamp this to the table.  Clamp a precision rod in the previously machined crankshaft bores and  adjust your setup to get the bores square and parallel to the X and Y axis.  Remove the precision rod and machine the inner crankshaft boss faces.

I can't tell from your pictures and drawings if you can drill and tap the crankshaft boss from the bottom.  If you can then your part is already setup.  If not you will need a very long drill and tap guide to drill and tap from the top.  

Hope this gives you some ideas on how to approach machining your part.


Tom S.


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## tomw (Feb 14, 2016)

Tom,

Thank you for the ideas. I was trying to through-bore the crank journals, and couldn't do this because the journal caps are not aligned on the inside, so the drill would deflect upon getting to the second journal. Thus I wanted to clean up those inner surfaces first. 

I guess I could bore them separately and hope for the best. Or, I could file the inner boss surfaces flat-ish but still oversized and do the through drilling.  Hmmm.

Thanks tons for your help.

Tom


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## tomw (Feb 14, 2016)

I decided to try the filing thing. I filed off the opposing journal's inner face as flat as I could make it. I then "line bored" using a center drill, drill, then long center drill, then drill, and then reamer to make the 5/16" crankshaft bores. Below are three pictures.

The first shows my general set up. I have the frame mounted to a fixture that has a .030 recesses that positions the piece with the base edges aligned with the mill table. Also present are some cute custom strap clamps. This is then clamped to an aligned 90° angle plate. 




The second is showing using the long center drill to spot the far side boss, the last shows a 5/16" drill rod as a temporary crankshaft. It is a tight fit, but it turns. 








Thank you all again for your help. I am going to start a new thread called "How would you center this under the spindle." If anyone still has comments on expanding or non-expanding mandrels, please continue here.

Cheers,

Tom,


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