Work holding????

[anoel]

Hey, I've always wondered how in the hell a person would go about holding odd shaped things that get completely cut out on the mill.

for instance. Say I wanted to cut out some letters... lets use the letter "S" for instance out of 1/2" thick aluminum 3" in Y and 2" in X. It's decorative so I can't have any superfulous holes in it.

obviously I cant just clamp up a piece in the vise and start cutting and if I were using clamps and a sacraficial base once I get almost at end of the final pass the work piece is going to start rattling and go flying around in the cavity.

The best thing that I can come up with is to "almost" cut it free and then flip it over and mill off the bottom. But that seems like it would be difficult to repeat over and over...

I just finished a Harbor Freight Micro-Mill to CNC conversion and now that I'm able to CNC cut Metal This is something that I might want to do and have no idea how to go about it.

I under stand how to make fixturing for parts that have holes that I can cut/drill first and then Run bolt in to the fixture and then remove the outer clamp downs and then do the cut out contour. But the "non hole having parts baffle me.

On the wood router I would just double stick tape it to the Table and rout away. (Does this work well with metal mills??

[Cold Fusion]

What about running a hold down bar across the face of the part and bolt the ends into the t slot when you are taking off that last little piece of aluminum. This will keep the piece from rattling around and doesn't need holes.

[DareBee]

Adhesive does work well for milling, but I would consider it for special applications only as you will probably take 5 times longer cutting the part.
Parts like your S can be milled in zones by strap clamping it down and then repositioning your clamps.
If you are looking to do a bunch of these I would drill & tap blind holes in from the backside and bolt to a fixture plate; This is the best option anyway but is often too time consuming for a 1 off.

[Rekd]

If you have a vise you can use alum jaws in you could machine the part out of 5/8 or 3/4 mat'l, mill the entire letter all the way to just past the 1/2 depth. Then take it out and machine the same outside shape in jaws so you can hold it. I use this method all the time, works great for round/odd shaped parts.

[anoel]

hmmm... I'm having a tough time visualizing that completely...

Maybe if I had a vise that I could use Aluminum Jaws I'd know what yer talking about..

I'l process that a little while...

[HuFlungDung]

Cold Fusion has a good idea there for doing one-ofs.

If you are actually working in aluminum, you can also mill the part almost through, leaving maybe .003" thickness holding it on. I would usually rough mill around the part once with maybe a .015" offset, leaving .020 on the bottom, too. Then take a light cleanup pass with zero offset and leave maybe .003 in a couple of tabs to hold the piece in. This can be cut through with a knife and then the bottom edge can be sanded a bit, which it most likely would be anyway.

[JRoque]

Hi,

I cut aluminum at .050" depth. That means the last cut will have .050" of material holding the piece in place. After the last cut has started, I pause the program half way thru and clamp down on the already cut section.
Most of the times, however, I simply hold the part down with my hand. At first I thought the part would be violently flung out after being cut off but this has not been the case so far... and I still my both my hands to prove it 8-)

Julio

[Cold Fusion]

Most of the times I hold it with my hands. This works okay except for the little gouge that sometimes happens when the bit cross the last little tab. This will be different depending on your lead in/out as well as the bit. Again, for a one off it's fine but when you start to make many of them you'll need some sort of better hold down system.

[HuFlungDung]

Bad Boys: you don't hold stuff with your hands near a rotating cutter. When it does get you, it'll be so damn fast that you won't even know what happened. And don't think wearing gloves will protect you! It'll likely even get more of you than you thought. Use a clamp! Please!

[Cold Fusion]

The pieces were 7x14 inches. My rapid speed is set at 50ipm so I'm plenty far away in distance and time. (maybe I should have mentioned this the first time)

[HuFlungDung]

Its not just the rapid speed of the table you need concern yourself with. If the cutter gets "a bite" on the piece, it'll be coming out of there like a bullet. Its dangerous! Maybe you've never seen it happen. I have. A simple strap clamp, even clumsily applied, will hold a hell of a lot more than you. I'm sorry to sound harsh, but if you worked for me, you'd get reamed out for doing it. Woodworkers get in trouble most often because they do not use proper clamping methods because "its only wood"!

[Cold Fusion]

I sorry Hu

It was only 1/8 plate, but I guess it really doesn't matter. I'll try to break myself of the habit before I start working in your shop

In a week I will have a new aluminum base which will vastly improve my clamping problem. You guys with T slots have it easy! I went ahead and bought a flat plate for a tapped hole grid because the price of getting aluminum thick enough to mill t slots in was crazy.