creative work holding

[Bob W.]

I need to machine a thin part that will be difficult to hold on to. It is the cover that surrounds the LCD on a laptop computer prototype. I have read about folks using the 'picture frame' approach where one side is machined leaving an uncut border, the cavity is filled with Bondo, faced when bondo is cured, and flipped over for side two machining. This would work great but how does one remove the bondo after side two is done? This will be a delicate part so significant force cannot be used.

 

Thanks,

Bob

[Colin Gilchrist]

Hi Bob,

 

Bondo and delicate do not go together. I have done what you are describing with machinable wax before. For the wax you can imerse the part in a hot water bath and melt away the wax. The other thing you should consider is making a vacuum fixture to hold the part. Basically you would machine the back side of the frame, Make a mating vacuum fixture, mount the machine stock to the fixture and machine away the rest of the material. How thin are you going? If you are trying to hold any kind of tight tolerance and the finished part thickness is less than about .04, forget the wax and go for the fixture. Let me know if you need help on this one. I've done lots of thin wall plastic and aluminum parts like this.

 

Thanks,

[MultiAxGod]

Only need to make one? If not, then a vacuum fixture would be the best for a delicate part if made right. If your good with them, it only takes from a few minutes to a couple of hours to make them.

Something else to try might be double back (carpet) tape, depending on how much you have to take off and what the part configuration is. Bondo or any type of bonding agent sounds like it will damage the part after use trying to get it off.

Good Luck!

[Bob W.]

I had planned on using vacuum fixturing for the first ops for sure. I have the chuck but I was hoping to avoid fabricating an additional fixture. Ths thickness will be ~.040" and the material is magnesium.

 

Thanks,

Bob

[Colin Gilchrist]

Is .04 the finish thickness of the wall dimension or the thickness of the stock you are starting with? You could hold the plate down with strap clamps for the first operation, mill the inside of the display bezel surfaces, and profile the inside and outside leaving some material tabs. The make a vacuum fixture for the second operation that accomodates the stock from your first operation. You could then finish the bezel from the front side and be finished.

[crazy^millman]

Sorry Colin, but I have to disagree. Bondo and delicate are a perfect match. I got taught this by one of the best model makers I ever worked with. Get some rice paper tape. Make the lip around the part do no go below the parting line. Leaving the picture frame inside and outside the part for support. I would use 2 dowel pin holes across the X or Y axis your choice on the inner part of the part. Now you have a valley where the part is. I would do the outside 1st and do the inside to cavity part 2nd. Now you need to get some rice paper tape, or a very thin masking tape. Here is link: Rice Paper Linky

 

Now you fill in the void with Bondo and small chucks of plastic or model board to not have to use so much bondo. Then face like you thought. Now flip over glue the part down to your fixture block using super glue(or the vacuum plate should work). Now run the 2nd side leaving the break through cut to last. You can even run coolant using this method. Now without coolant cut the part through. Now get some acrylic cleaner and use it to dissolve the glue holding the tape to the part. Almost all plastic have no problem with acrylic cleaner and should be good to pop it right out perfect right out of the window frame of plastic still sitting on the table with a negative of the part you just made. Now depending on how big it is you can do places around a part. Do the bondo in stages is putting on a lot to keep the heat down.

 

HTH

[Colin Gilchrist]

Thanks for the tip Ron. I stand corrected. I love this forum.

[crazy^millman]

Colin was a cool trick when I was showed it. Javier was the guy who showed me this one.

[Bob W.]

I need to make sure I understand correctly.

 

1. I would machine one side first, leaving the picture frame border

 

2. Mask the machined surfaces with rice paper tape

 

3. Fill with bondo (adheres to tape), face and then flip the part for the second side machining

 

4. Complete second side machining

 

5. Use acrylic cleaner to break the bond between the tape and the finished part

 

Is this right?

 

Thanks,

Bob

[Chris Rizzo]

wow, that was hard to follow, but sounds like a good idea.

 

probably already thought about it, but are their any threaded bosses on the inside of the part? I've made some very simple fixtures utilizing such bosses (the mounting holes for lcd screens and keypads) .

 

The parts were for a local company too.

[Bob W.]

There are a few bosses but not enough to hold the part securely. This is a mag part and the finished thickness will be ~.04" so it will need to be pretty secure to keep it from chattering.

 

BTW, I did finally get my ball screws replaced with standard pitch. It took slightly over a year for Selway to come through.

[cherokeechief79]

i sure would like to see a pic of this being done.any of you have any visual examples?

[MACHINEMASTERG]

ive had vacummed formed thin walled plastic parts i had to do finish machining on and i filled them with water and froze them then machined them very quickly the ice was my support !!!

[crazy^millman]

Yes Bob that is the correct way. Only thing is not break through on the 2nd side until almost done.

 

HTH

[MotorCityMinion]

I've milled .04 thick glass fabric using a sacrificial piece of stock and toe clamps. I was able to mill .02 off the top, and bury the e-mill on the profile, maintaining a .002 tolerance. Leave a tang on one end, mill as much of the profile as you can. Use can use the e-mill to punch holes as well. M00, put a clamp on the finished side, remove the other clamp from the tang, then mill off the tang. Clamps for lathe inserts work good for this if you have some lying around. Use a penny (stone or sand it so it doesn't leave marks, or a chip breaker to spread out the clamping forces. Use a stone on the part in between moving clamps to give it a brushed looking finished. You could also get creative with a small e-mill to finish the surface with swirls, curves, diagonal lines,and so on.

 

[ 02-26-2008, 12:58 PM: Message edited by: MotorCityMinion ]

[nofalloff]

FWIW Lacquer thinner dissolves bondo. I've done one offs with epoxy and double sided tape. The tape lets go with denatured alcohol and a heat gun takes care of the epoxy. I've done this with aluminum to which the heat involved is no problem. I've never machined mg.