Moldable plastic soft jaws

[kvom]

It's often a problem to hold odd-shaped work in a vise for machining.  Dave Otto had an interesting solution in his build thread:

http://www.modelenginemaker.com/index.php/topic,1326.msg112219/topicseen.html#new

But he has access to a sophisticated 3D printer and modeling software.  Bsed on a thread I saw some time ago on youtube, I decided to do a small experiment.  First, I took a pair of aluminum soft jaws and drilled two 3/16" holes in the one for the fixed vise jaw, and matching 7/32" holes in the moving jaw.  The bronze piece in the photo is what I want to hold in the vise.

[kvom]

The secret to holding the piece is a moldable plastic that melts in hot water and becomes solid again at room temperature.

When melted it can be formed by hand.

[kvom]

Next, I mold it around two 3/16" pins fitted to the holes in the jaw, then press the part halfway into the plastic.  Close the vise jaws and wait about 5 minutes until the plastic turns white again.  The pins keep the plastic in place as it sticks to neither the part nor the vise jaws.

[kvom]

Melt a second piece and mold it around the work and the pins.  The new piece won't stick to the old one. After it's hard the mold can be separated. Or you can just start machining.

In this case facing the short piece went fine, but failed on the second, since the amount being held was small, and it was not entirely below the level of the aluminum jaws.  With only the plastic itself resisting the milling force the work broke free.  Lesson learned.

After completion, the mold can be melted in hot water and re-used.

[10KPete]

kvom!!! That's exactly the issue going through my head since I saw what Dave was doing!! I've had a bag of those fittings for
a couple of years ( I tend to buy things way ahead of actual need ) and it's been driving me nuts trying to figure out how to make the fixturing for machining them. Ells, Tees, Valves, all those bitty parts that don't have handles on them. Mill? Lathe?
Drill press? Vise? Collets? How to hold them; repeatedly and accurately. 

Where did you find that stuff? I've been thinking JB Weld, Lab Metal, Cerrobend, What??? How?

Yer on ter sometin here my friend!

Pete

[kvom]

Got mine on Amazon.

[chris evans]

Looks a bit like the stuff a dentist would take an impression with. It is certainly a good idea.

[sshire]

Dental impression compound is too flexible and not reusable.

[maury]

A trick I have used in the past with castings that don't have a surface to hold to is this. I used a bit of modeling clay to hold the part in a container while I dialed it in for the first cut. When the part was set to the first cut I poured some plaster of paris around it up to the surface I wanted to machine. It holds nice, then you just break off the plaster and hold the part normally using the machined surface as a reference.

maury

[Dave Otto]

Thanks for posting your experiment Kirk; that is an interesting material that could come in handy in the shop.

I also wonder about the Cero alloys; They would trickier to work with but if you had say a small box that you could position your part in then pour in the melted alloy.
Maury I like your idea too; how thick does the plaster layer need to be to resist the machining forces?

Also with 3d printers evolving along with modeling software it is entirely feasible that printed fixtures fixtures could be common in the home shop. I guess there is always Shapeways.

Dave

[maury]

Dave, the part I did this way was the mixer for the Ball Hopper Monitor. It looks bit like a street elbow with some appendages. I almost completely submerged the part in plaster, so, about 3/4" thick. I took fairly light cuts, but in the mill I was able to face the flange and do the bolt holes, as well as making the geometry into the valve seat and the valve stem area. I suppose I could have done most of this in the lathe too, except the flange holes.

The thing is you have to be careful when clamping plaster.

maury

[Dave Otto]

How much trouble was it to remove the plaster and clean up the casting?

Dave