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[4]

Expanding mandrel for musical instrument key cups

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fluxcapacitor

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#1
I'm looking to make an expanding lathe mandrel to turn the outside of a pad cup after the inside has been machined from nickel silver. Here's an example of a pad cup: https://www.windplus.net/supplies/pad3.gif

I've seen expanding lathe mandrels that use a socket head cap screw to tighten, but that works better for ring or tube shaped work. Since I wouldn't have access to the face end of the mandrel, I thought it might work to counterbore a hole in the shank (straight shank would be held in a collet) for a cap screw that pulls a cone shaped wedge. I'd have to remove the mandrel to remove the part, but I'm okay with that. I'd also cut a shoulder into the front outside of the mandrel to provide a square face to register the part up to.

Any suggestions or thoughts of what material to use for the mandrel? I'm thinking a cylindrical shaft of about 3/8". The parts would be 3/8" - 7/8" diameter and less than 1/4" overall thickness. I've thought of using 12L14 leaded steel or C360 brass. It's not something I would be using often or making a lot of parts with, but if I'd like it to hold up to at least a dozen uses. Brass would probably be easier on my slitting saw and won't rust, but I don't know if it would hold up.

-- Nate
 

markba633csi

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#2
So if I understand correctly the pad has a concave recess which you would use to hold it for machining the outside edge? If so then I think
you have the right idea: a mandrel or "gripper" which you would then grab with a collet- and you would remove the mandrel to remove the part.
You could however make a mandrel with a tightening screw or collar accessible from the outside edge so you wouldn't have to remove it each time.
Mark
 

kd4gij

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#3
What material are you making the pad cups out of? And I have used the bolt out of these anchors to make expanding mandrels.
Oh and what collet system are you using?

1527725799592.png
 

Jimsehr

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#4
I have been using expanding collets for years . If you have a 5c collet set up you can buy off the shelf expanding collets on eBay and they are machinied to fit the part and can be used thousands of times. If you have a levered collet system on your lathe you can load and unload in seconds. The parts run true and you can hold dead lengths with them. Hardinge makes them but they are pricey. They are made by other makers like Rovi collets in the Los Angeles area. I know that Rovi make 5c type that go from small less then 1/2 inch up to 6 inch. They also have a video showing how to machine and run. Google Rovi collet.
 

David S

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#5
Joe has a pretty good video on expanding mandrels and after about the 8 minute mark he shows how to hold parts like yours.


David
 

fluxcapacitor

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#6
Thanks guys!

I think the split mandrel in the video is spot on what I'm looking for - pretty simple and well implemented. The o-ring version would be great for when I get around to making clarinet barrels out of ABS.

The material I'll be working with is C792 nickel silver.

I'll have to browse through the Rovi site, looks like they've got some interesting stuff.

As far as collet system, I'll be using 3C on a Sherline (just ordered their 3C headstock). At this time I just don't have the space for something larger. I've used a cheap MT1 ER20 collet chuck with a drawbolt - works okay, but sticks out a ways from the headstock. Being able to use a collet work stop and lever closer would be a big plus as sometimes I need to make identical small parts such as standoffs or spacers for computer and electronics projects.

-- Nate
 

Boswell

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#7
Another alternative might be to use super glue. check out the videos from Clickspring (here). In several of his videos he uses superglue to hold a small part (or large in one case) for machining. He releases the part with a little heat. I have not tried this technique but would definitely give it a try if I had the need. Looks like a simple and effective part holding technique for small parts where you can't easily clamp the part. Sorry I can't point you to a specific video and time stamp. but on the other hand his videos on building the Antikythera mechanism are amazing.
 

philip-of_Oregon

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#8
If You DO use CA, the easiest method to remove it from your machining is Temperature Differential.

The difference of material expanding Differently causes a clean separation. Then use a Razor Blade to scrape the SuperGlue off, and save using the Acetone normally Used for when you glue your fingers together, OR at least if you are like ME, I'll do that goofball trick every 3rd or 4th time!

philip from the Great Pacific NorthWET.
 

Dredb

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#9
Inside chucks (collets) are (were?) commonly used on watchmakers and instrument lathes.
 

fluxcapacitor

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#10
Yeah, the clickspring videos are phenomenal. I stopped bookmarking the individual videos because I like most all of them. I remember seeing him use glue in multiple videos, and have definitely considered it as an option.

I noticed a number of step collets for watchmaker lathes. I'll have to keep my eye out for inside work holding for those.
 
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