Expanding mandrels

[NeoTech]

So i need to make a small tool to holding a small aluminium part in my lathe.
And well never done this before..  Could i get some input on this idea.

[Pete.]

It needn't be that elaborate especially for a single setup. If you set your compound and turn a tapered slug, then just chuck up some ally in the 3-jaw, bore it through, turn the taper for the slug, turn the OD for a close fit to your part with a back-shoulder, cut the slots, de-burr then mount the part up and tap in the slug. It won't move.

To release, tap the slug out from behind with a long knock-out bar through the spindle.

[NeoTech]

Oh its not for a single setup im gonna do 24 identical parts and the "lip" that i can clamp to its only 20mm wide, 2mm deep on the inside and isnt going through the part.

Would your solution still work? *sounds alot easier* =)

[vtsteam]

Another very simple option I've used, though maybe not as applicable to production, but possibly of interest to others doing a few parts is to turn up a close sliding fit on a stub mandrel, leaving a shoulder and then mill off a flat slightly deeper than a dowel pin you insert in the gap. Twist the part on the arbor to lock it in the turning direction, then twist it the opposite way to unlock.

Oh- it's not going to work on a blind hole -- I'm just adding it here in case someone else checks this thread and needs a quick solution.

Pictures are better than words:



Above are the arbor quickly turned up and the dowel pin and a couple of steel blanks -- one has been turned on the arbor



This is what the arbor (and work) looks like when set up on the lathe. The pin locks the work when you give workpiece a twist one way, and unlocks it when you twist it the other way.

[awemawson]

Turn up a stub with suitable shoulder to seat the item. Axially drill and tap only using a 'second' tap not a bottoming one. Saw the end of the stub radially with two cuts so the end on view looks like a cross. Screw in a grub screw. As it reaches the incomplete female threads it will tighten and expand the stub gripping your part. Obviously the grub screw needs to be of suitable dimensions that the wall of the stub can flex.

[ozzie46]

Another very simple option I've used, though maybe not as applicable to production, but possibly of interest to others doing a few parts is to turn up a close sliding fit on a stub mandrel, leaving a shoulder and then mill off a flat slightly deeper than a dowel pin you insert in the gap. Twist the part on the arbor to lock it in the turning direction, then twist it the opposite way to unlock.

Oh- it's not going to work on a blind hole -- I'm just adding it here in case someone else checks this thread and needs a quick solution.

Pictures are better than words:



Above are the arbor quickly turned up and the dowel pin and a couple of steel blanks -- one has been turned on the arbor



This is what the arbor (and work) looks like when set up on the lathe. The pin locks the work when you give workpiece a twist one way, and unlocks it when you twist it the other way.

  Now That is an amazing idea!!!
So simple yet I can't believe I never thought of it. Oh yes I can. I'm not as smart as I wish I were.


 Ron

[vtsteam]

Well I certainly didn't think of it either. Just passing it on! 

[vtsteam]

Another quick way to make an expanding arbor is to tap the end for a pipe plug, and then slit it.

Put the workpiece on the arbor and then thread in the pipe plug. It has tapered threads and it will expand the arbor fingers.

That one is from David Gingery.

 

[PekkaNF]

I have used concrete anchor bolt to expand slit mandrel. Saves me from makin tapers. This type:
http://www.confast.com/images2/products/main/drop-in-anchor.jpg

Haven't tried this one:
http://www.confast.com/images2/products/main/sleeve-anchor-hex.jpg

I should mention that I had about 50% success rate...for some reason does not work allways very accurately. Looks like good idea to tighten and loosen just only eneough...no extra 1/4 turn "just in case".

Pekka

[Rob.Wilson]

Few mandrel photos here http://madmodder.net/index.php/topic,2584.0.html 

Rob

[fixerup]

Neotech
I think your diagram looks like the right idea. The tricky part is, 2mm is not much to grab onto depending on what machining you intend to do to it.
I found this tubing expanding mandrel. I wish we could see a breakdown of parts to give us a better idea how to make one.
http://www.ryanairconspares.com/refrigeration-tools-copper-tube-pipe-expanders-c-9_22.html
Vt steam, thanks for sharing that one.

Cheers !
Phil
 

[NeoTech]

fixerup, well that what the little "lip" in the end was for i thought. But i may need to give it some clearance and make it a bit sharper so it can bite into the material better from the inside.

the parts need to be outside faced and a radius is gonna be on it.. its a 15mmx30mm piece with a 20mm hole that is 2mm deep. the radius is wanted to be 12mm. so its basicly a knob more or less.
the recess will have a 4kg neodymium magnet pressed into it.. and the set is three "knobs" and a block with three magnets fitted. they will make up a corner. So that plastic can be wedged in between them for spraying cabinets. (i make these for a friend that had this idea of a portable spraying cabinette)

[philf]

fixerup, well that what the little "lip" in the end was for i thought. But i may need to give it some clearance and make it a bit sharper so it can bite into the material better from the inside.

the parts need to be outside faced and a radius is gonna be on it.. its a 15mmx30mm piece with a 20mm hole that is 2mm deep. the radius is wanted to be 12mm. so its basicly a knob more or less.
the recess will have a 4kg neodymium magnet pressed into it.. and the set is three "knobs" and a block with three magnets fitted. they will make up a corner. So that plastic can be wedged in between them for spraying cabinets. (i make these for a friend that had this idea of a portable spraying cabinette)

NeoTech,

I think I can picture the item you are trying to make.

Why not tap a blind hole in the part and screw it to a spigot (the end of which is a good fit for the recess) which can be clamped in a 3-jaw or a collet. The hole would be hidden by the magnet.

I think there could be a potential problem with your design in that, if the wedge is more than 2mm down the tapered hole, there would be little clamping force on your part. A ring (thick washer) around the stepped diameter at the inner end of the mandrel would help.

Also I think the tapered hole in the fixed part is unnecessary. The wedge as shown is too big to go in. The screw you show would need to be a very good fit in its hole to retain concentricity.

 

Phil

[PeterE]

... and yet another idea that I have seen in use somewhere ...

I have no pictures so I have to try to describer it 

The idea is much the same as an ordinary collet that one puts material into and it contracts around the material when pulled into an arbor (= lathe headstock) but working opposite...

Instead of having a tube with the cone on the inside fitted to the headstock as mandrel, the cone should be on the outside. Then a "collet" with an outside look of a step drill (but larger) having an "inside" cone matching the one on the mandrel. So when pulled toghether the "step drill part" being pulled over the outside cone will expand (if it have been properly slitted of course) and open up instead of contracting. That will become the grip end for a recess and can be used to hold an item in the lathe for turning for example.

I hope you understand how I mean, if not I will have to try to draw it quickly.

Just as a thought ....

BR

/Peter

[fixerup]

I missed that lip details, Yes that would definitely work.
Cheers!
Phil

fixerup, well that what the little "lip" in the end was for i thought. But i may need to give it some clearance and make it a bit sharper so it can bite into the material better from the inside.

the parts need to be outside faced and a radius is gonna be on it.. its a 15mmx30mm piece with a 20mm hole that is 2mm deep. the radius is wanted to be 12mm. so its basicly a knob more or less.
the recess will have a 4kg neodymium magnet pressed into it.. and the set is three "knobs" and a block with three magnets fitted. they will make up a corner. So that plastic can be wedged in between them for spraying cabinets. (i make these for a friend that had this idea of a portable spraying cabinette)

[NeoTech]

PeterE, i do think a picture is needed. 

[BillTodd]

This is essentially the same as awemawson's idea :

1, saw a long slot in a piece of HRS 
2, Drill & tap half way through
3, fix in chuck
4, turn & face to size (by doing this at this stage it will be absolutely concentric and square to the axis)
5, insert screw & tighten to fix piece

[PeterE]

PeterE, i do think a picture is needed. 

OK, here is a quickly scribbled down sketch which I hope will reveal my thoughts.

The stepped collet is of course slitted lengthwise like any other collet and then pulled onto the conical end of the mandrel which will make the collet open up.

I guess it is a more elaborae version of PekkaNF´s idea with the expanding masonry bolt.

BR

/Peter

[NeoTech]

PeterE, so if i get this right, the cones are matching, the stepped cone part is turned to ID, and then "pushed" into the work. When pulled over the mandrell cone it will seat and expand to its exact dimension and there for put X amount of pressure on it. Doesnt this mean it needs to be slightly oversizes so it would need to be compressed slightly to fit into the work. I guess this would make a really proper concentric fit though.

GraphiteCad - is better than nothing. =)

[PeterE]

Well, I managed to make the size difference between the two parts a little too small.

The mandrel cone shall have a larger diameter than the "collet", then the collet will automatically expand when pulled into the mandrel (towards the right in the picture).

The idea is to get an internal collet that provides an equal pressured on the inside of a recess like holding a pipe on its inner diameter.

The "stem" of the collet above of course goes through the lathe spindle and is pulled like a normal MT-collet.

Hope this completed the picture.

BR

/Peter

(Shall see if I can make a better sketch using "GraphiteCAD" or even an electronic tool    )

[NeoTech]

I managed to figure out that a size difference is needed and came up with this. =)

[philf]

I managed to figure out that a size difference is needed and came up with this. =)

Neotech,

I very much doubt this design would work at all. If you are gripping inside a 2mm deep recess there would be very little clamping force available that far away from the taper (which, in my opinion, is too shallow even if you were trying to clamp the part near to the taper).

Phil.

[NeoTech]

I have my doubts as well.. But its interesting so i will try to make several of em this weekend and test them. =)

[PeterE]

NeoTech,

You have understood the idea, but you need a cone angle that resembles an ER or 5C collet, or somewhere between 10 and 20 degrees to get enough expansion force. I would also suspect that the fit between the stepped end of the "collet" and the workpiece to be quite close from start. That way you do not need to open the collet very much.

I must also add that I have only seen this thingy described but never in use IRL so how well it works is anybodys guess I think.

What is the depth and diameter of the recess you want to grip?

BR

/Peter

[fixerup]

NeoTech,
I believe your first design would work, I would make the cone almost flush with the edge or even have it protruding a little(.005"), so that when you draw the cone in it would put the clamping force right were you want it.
Cheers!
Phil

[philf]

Neotech,

Sorry.
Following what fixerup just posted I had another look at your first design and I originally misunderstood the construction.  (In my defence, a section in a 2d drawing would be shown hatched and the slotted part wouldn't have been.)

I thought the expanding part was seperate to the body. Now I see that the line is just the bottom of the slot.

If you can stop the cone from turning when you tighten up the screw then there's no reason why it wouldn't work.

Cheers.

Phil.

[NeoTech]

Yeah, tomorrow will be experimentation day i think.. I have some steel, and i will photograph each setup. =)
This whole thread expanded into a much bigger discussion than i imagined. ;)

[fixerup]

This whole thread expanded into a much bigger discussion than i imagined. ;)

To me, this is a very interesting discussion, because learning different ways of holding a workpiece on the lathe is a great asset to know.
Good luck making your part and thanks  everyone for sharing

 


Phil

[PeterE]

NeoTech,

Even though I have described a relatively complicated stepped inside mandrel/collet I think you were on the right track from start.

After having thought a bit more, my design would be something like below:

A short housing with a wider "collar" at its lengthwise center to be stop for the chuck jaw or collet nose. The back end would be drilled out so I could fit a long nut. The business end would be turned to a close fit in the part it shall hold and then drilled/bored out so it is just about 1.5 mm thick. Then cross cut to be expansionable.

A cone with suitable diameter to fit inside and a bit of threading to pull the cone in using the long nut. The cone shall preferably not protrude outside the thin end so the collet can bottom in what it shall hold.

Now pull the cone into the collet and there you have it.

I would rather use a nut to pull the cone in insterad of a screw as there would be a risk that the screw will push out the job if too long.

When it comes to measures I would go for the same length of the business end as its diameter (if not very large), the support collar about 3 to 5 mm in all sizes and the back end around 20 mm to be good enough for chauck jaws and collets.

Just another idea...

BR

/Peter

[NeoTech]

So i made a "version" of my mandrel design anyway. instead of making a cone with a thread i made a bolt with a conical cap and put a nut on the backside. And it actually works. =)

And this is the result.. it takes frickin forever though removing mount it do the outside radius and remove it file it and the next one.. 6 out of 24 done.. ;)