Workholding -- itty bitty polycarbonate cones

[Tim Wescott]

Not a model engine question, but for work:

I need to make some little (about 1/2 inch in diameter) "chicklet" prisms that fit into cone-shaped holes, don't stick out much, and have nice flat, sloped faces on the outside.

I'm planning on machining a sloped flat on the end of a rod, then cutting a conical end with the rod in a fixture that holds it at the proper slope. Then I need to cut the nascent prism off the rod, and finish the big end. At this point, all my workholding ideas end up filling my imagination with splintered, melted, or otherwise destroyed polycarbonate parts, or with polycarbonate parts flying all over my shop from insufficient application of force. I'm left with hand-sanding them one-by-one, and sanding off my fingertips in the process.

Any suggestions? If these were aluminum chicklets I'd consider making cone-shaped holes in a piece of aluminum and gluing them in with superglue or epoxy, doing my machining, and then knocking them out (if superglue) or baking them out (if epoxy).

Should I do my cone-shaped hole thing with a plenum behind it and learn how to vacuum clamp? The area I have to work with is very small, so the clamping force, wouldn't get above two or three pound -- it may work for gang-sanding those faces flat, but I don't think the clamping would survive contact with an ordinary cutting tool.

 

[jack620]

I Googled chicklet and I'm still none the wiser. Can you provide a drawing of what you're making?

 

[BaronJ]

Hi Tim,

Normally small Items would be held in a wax chuck ! These normally use shellac to secure the workpiece, but you could do the same with other types of wax. Once the work is released a solvent is used to clean the wax off the work. Meths works for shellac.

 

[goldstar31]

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Deleted.

 

[Tim Wescott]

I Googled chicklet and I'm still none the wiser. Can you provide a drawing of what you're making?

It was our optics guy who came up with the name. Don't ask me. Drawing to follow.

 

[Tim Wescott]

Hi Tim,

Normally small Items would be held in a wax chuck ! These normally use shellac to secure the workpiece, but you could do the same with other types of wax. Once the work is released a solvent is used to clean the wax off the work. Meths works for shellac.

How strong is the bond? Could I glue them in and then run a milling cutter across the top? I assume "meths" means methanol -- that should be compatible with polycarbonate.

 

[Tim Wescott]

I should have done my web search first -- isopropyl alcohol removes hot glue. I'm still concerned about the strength of the bond, but -- hmm.

 

[BaronJ]

Hi Tim,

It all depends upon size. Shellac will secure a few mm but you do need something like a hotplate to get the shellac molten. Machinable wax could be used but like shellac you will need something to contain any depth. Also be aware that cold shellac will shatter like glass !

 

[aarggh]

Could you do one face first leaving a stem, then grip the stem in the chuck and do the other face, then knock the stem off and lightly touch up the nub?

You could even use some some quick and easy soft jaw inserts to grip for the second operation to make it easier.

 

[Badhippie]

Can you use a vacuum table to hole it down.
or 2 face 3m tape. I have had to use the 2 face tape method before on my mill for some Teflon pieces. You’ll want the 3M body molding tape it holds very well
Thanks
Tom

 

[Circlip]

A drawing is going to save a thousand assumptions.

Regards Ian.

 

[stanstocker]

Greetings,

Polycarbonate is prone to all sorts of solvent issues. I don't have a good answer for holding, although shellac seems a likely adhesive. Before going too far down the rabbit hole on any approach, here's a link to solvent effects on polycarbonate that is quite extensive. Might save you having a bad time after investing a lot of time on machining and polishing at least.

https://www.bayplastics.co.uk/downl...carbonate Chemical Resistance Information.pdf
Take care,
Stan

 

[Tim Wescott]

... Before going too far down the rabbit hole on any approach, here's a link to solvent effects on polycarbonate that is quite extensive ...

Oh, thank you. I was not looking forward to the necessary web searching to find a document like that.

 

[Tim Wescott]

Here's an idea of what I'm making. Half inch diameter, 104 degree included angle on the conical part.

Our optics guy has repeated to me enough times that a 40 degree included angle is insane, and I finally heard him -- so that'll change. But this should give an idea of the workholding challenge.

Zip file contains a step file, if anyone wants to get that deep.

 

[Tim Wescott]

FWIW, the included angle doesn't have to be exact -- 20 - 25 degrees is fine. I'm not a machinist, I have no pride to uphold in that regard. I just need surfaces that are flat and polished, and a conical section that fits a conical hole.

 

[BaronJ]

Here's an idea of what I'm making. Half inch diameter, 104 degree included angle on the conical part.

Our optics guy has repeated to me enough times that a 40 degree included angle is insane, and I finally heard him -- so that'll change. But this should give an idea of the workholding challenge.

Zip file contains a step file, if anyone wants to get that deep.

View attachment 126759

Hi Tim,

On the basis that they are wider than the height shellac will do what you want perfectly. Depending upon how many you need to do, you could put quite large numbers of them on at once. Use a flat steel plate to press them onto the shellac, wait for it to become cold and hard, machine the surface and remove.

The trick here is to place a sheet of paper on the chuck first then get it hot enough to coat with shellac, place your parts and press them in place. machine as required, then heat up the chuck again when finished. If you do it properly you will be able to lift all your parts off the chuck with them still attached to the paper. Lay another piece of paper on the hot chuck apply shellac and put the inverted parts on the paper and press in place. When cold you should be able to remove the paper clean and machine.

The paper is used as a carrier for the parts. But I'm sure you get the idea.

 

[Tim Wescott]

@BaronJ : You're assuming I'll make the big flat first, and then machine the rest. I'll think about whether that's the right approach.

I had been thinking that I'd machine the little flat first, then the cone, then I'd glue the thing into a fixture with conical holes in it and finish off the big flat. Obviously it requires a fixture and makes the use of the paper sheet impossible -- so I'm seeing definite advantages to doing it your way.

I'd have to figure out how to machine the cones if I did it your way, though.

 

[johnmcc69]

Hi Tim, I'm not sure of some of the details, such as how many are "few", what specific tolerances may be required, or the machines you have available but...
Can you do a number of these on a 4th axis on one side of a round bar & slice the bar along it's axis from the machined part? Again, depending on tolerances, you may be able to achieve a good result.

john

 

[James Barker]

If these prisms of sorts do not need to be optically pure, this would seem to me to be a perfect application to use a 3D printer.

BC1
Jim

 

[James Barker]

If nothing else a blank could be made with a 3D printer, final finished to specs manually, and then used as a pattern in a silicon or latex mold using lucite as the material of choice.

BC1
Jim