[levelzero]

The stock is 4x4x0.125" square tubing. Length is 16" and is being held in two vices with 2" deep jaws. The cut I am making is to remove 15.5"X3.75" from one side. Obviously this is chattering like a ........ due to the walls flexing. Anyone have any tips to make this less painful?

[Delw]

Fill it with wax

[Superman]

Yep,
cut 1/2" off the end- throw the big bit away, and weld a 4" x 1/8" x 15.5" long plate onto the box. LOL


Any chance of using a long 3" square up the guts and clamp everything to the table, eliminate the vices ??
or
Oxy-cut the majority excess, and clamp as above and then machine ??

after-thought
what about an internal spacer to allow the vice jaws to actually grip the walls ??
a couple of bolts with nuts on the end ( bottle jacks ) would work as spacers.

[LeeWay]

Cut a block of wood to slip fit in there. It would be semi reusable for several parts, but would become worn after awhile. That would stop the flex and cheap and fast.

[mozmozmoz]

If you're not cutting the whole side away you can fill it with something, like they say above. If you can block the ends then (dry, no-salt) sane would be fine, or TIG filler rods if you have enough of them (I've bent tube that way), or a combination of TIG rod and bigger bits.

But if you're actually just chopping out a chunk of the whole wall, why not make cuts right through? Even milling it with a small cutter would be better than trying to cut away that much face metal. Not sure from your description whether you're trying to cut a window into it, but if you are then just drill a hole near one corner and run round the outside cutting a channel slightly deeper each pass should work and would be a lot stronger right up to the final pass. Small cuts add time but reduce cutting forces (and you suck the big end ) I'd still try for a clamp-down rather than vise fix (like superman says).

[levelzero]

Here is an image of what I am talking about.

I thought about some sort of jack on the inside but the wood is a great solution, especially as I only need to make 4 more right now.

[levelzero]

Opps forgot the image.

[mc_n_g]

Put one or two of the small machinist screw jacks underneath it to reduce vibration and crate a little lift/support.

only an example of the jack I am referring to.
http://www.use-enco.com/CGI/INSRIT?P...&PARTPG=INSRHI

[LeeWay]

Yep. Snug fitting block of wood. Mill in two passes and leave two small tabs on the ends.

[Geof]

Two blocks of wood, wedge shaped, slide them in from opposite ends until they are tight; much easier to get in tight and then out than a single block.

If you are doing many parts use steel wedges and just make sure you are wedging them between sides that do not have the weld.

[LeeWay]

I agree. A pair of wedges would be ideal.