Holding convex / concave parts during engraving

I'm getting ready to start engraving my patch box. But, because the part is convex shaped, I'm afraid that my vice jaws will bend the part or not hold it securely enough during the engraving process.

I read in one of the books that using some auto body bondo as a glob on a chunk of wood will securely hold it so you can grab the wood in your vice during engraving, and a torch to the metal will release it when you're done. Has anyone tried this technique before? Results?

Why not do it right on the wood ?? Lot easier to clamp the whole gun than fiddle with a vise. I have a =SWivel 6" Vice with a 1" X 6" board that has cleat on bottom, rotate vise outward so you can walk around the work area and have at it. ..Tom

A couple of reasons for doing it with the part detached;
My work area isn't large enough to swivel a 5 1/2' gun that many degrees to get the right angle to bang away on it

the rear portion of the patchbox is unsupported, and I'm afraid I may damage the hinge.

Yes, have used the body filler (technique).

I get the "resin" body filler (not long or short strand stuff which has the fiberglass fibers in it - the resin is just the grey stuff).

Mix it up and as soon as it starts to set up, press the part into it and let it cure (usually on a chunk of 2x4 which is then clamped into the vice or whatever you are using to hold it with.

That leaves everything supported from underneath.

You don't quite need a torch to get it to release - a heat gun works fine.

I do agree with Tom that doing it on the stock when you can is preferable (at least personally), but I have done a couple of domed lid metal boxes that just can't be done that way and in cases such as that, the "bondo" works fine.

I'm no engraver... but have a lot of experience jury-rigging!
Unless someone else can come along and vouch that a reasonable amount of heat will release the brass from the bondo (remember- bondo is a product meant to adhere to metal!), I'd lay a piece of saran wrap over the bondo- to keep from adhering the brass to it- then use your screw holes in the batch box to screw it down to the bondo block... that way you can have the bondo form-fit to your piece- to act as a support- yet be ensured it's not going to be a permanent arrangement!
Wes

The process is detailed in Chapter 33 of "The Gunsmith of Grenville County", which is where I learned the process and have found that a heat gun gave sufficient heat to let the brass release from the body filler although the text suggests a propane torch will allow release "in under 1 minute".

A now-passed bud of mine was an old time engraver. His trick for small things and odd shapes was to float a block of canner's paraffin in hot water just the right temp (I forget what temp) so it softened but kept it's form. Remove the block from the water, press the part into it, and let it cool. He had some stops rigged on his bench top, and used those to snug the paraffin into place while he worked. Slick. A return to boiling water freed the parts.

Using the bondo method works great. Even a hair dryer would work. You only need enough heat for it to release and pop it off. You don't want to use a torch and get it so hot that it melts the bondo. On flat parts, I just use a cheap hot glue gun.

Hi,
If you are doing the whole PB assembled, screw it into a shaped piece of wood that can then be held in your vise. If disassembled, you can use a pitch bowl to hold the parts. You can also glue parts with Elmers white glue to a shaped piece of wood. The glue will let go with a little heat and even a little prying. Bondo works (I've used it) but I cannot stand the smell so no longer do so. I do a fair amount of engraving and thoroughly endorse engraving parts off the gun if practical. Sometimes that is simply not feasible but it is much easier to engrave if you can smoothly spin your work to cut curves. Essentially, you want to turn the work into your stationary hand holding the graver rather than moving your cutting hand around the curve. Having to walk around the work to cut scrolls and curves is a poor substitute and it often shows.

dave

Bondo doesn't melt. And heat will not release hardened bondo very well at all. Spray the part with WD40 before you bondo it on.

That's a lot of bonding surface- why not wax about 90% of the inside metal surface but leave just a small area unwaxed to hold the patchbox in place.

I am not the best engraver but, I have used the bond method many times successfully. Never had a part stick to the bondo or let go while engraving. I do agree it smells bad and it will melt and even burn if you get it hot enough. You don't need that much heat to make it pop off.

I use super glue to attach the parts to curved wood blocks. I do this all the time. Heat gently with a propane torch or heat gun and the part will release cleanly. I also use dental "cold cure" acrylic...don't need to curve the block of wood with this. Also, go to GRS web site and they sell a plastic in stick form (Thermo-lok) that's made for this. Heat the plastic in hot water or in the microwave and stick the part in it. Heat again to release. It can be used over and over again.
Try here:
http://www.grstools.com/workholding/thermo-loc/thermo-loc-sticks.html