Need Jig help please...

[John Stephens]

Friends,

I need to make a jig that has a square hole cut out of the middle.
Square pieces of material will fit very snuggly in that cut out which
will be somewhere in the middle of the table.

I am trying to figure out the best material to use for the jig. I will want
the design to basically bleed very slightly off of the peice on all 4 sides.

I know that I can cut it out of an acrylic sheet but when the design
runs off the side and over the jig(very slightly) obviously it will be burning the acrylic. Consequenlty, I will be constantly replacing the jig I would think as it is gradually burned away. The pieces I will be burning are about 1/4" thickand about 8" square.

Ideally, I would find something that the laser will not burn. Of course if that is the case I will not be able to cut the jigs out with it. I thought
about cutting the jig out of acrylic and coating it with something that the laser won't burn.

Any ideas would be greatly appreciated.

Thank you,

John

[Kenneth Hertzog]

how about aluminium
ken

[Frank Corker]

I have to be honest and tell you I don't understand this properly. A jig is for making certain that you put the working piece in exactly the same place every time that you want to engrave or cut. This means a cut out shape that will hold a defined work piece that fits into it with ease. If that is the case why are you going to get bleed? The only thing I can think of is that the working piece is of an irregular shape or can roll around.

Okay, if you make an acrylic square hole which remains in a constant location everytime that you place it in the laser. Make another square that fits inside of the first jig, but has another hole which is smaller, but allows the workpiece to be placed inside. This fixes the location of both parts of the jig. Put little tabs on this second part of the jig so that it can be lifted out of the first jig and over the top and of the way of the working piece. Would that do it?

[Doug Griffith]

I'm not sure this applies but...

I've used perforated aluminum sheet (McMaster Carr) cut to fit over my honeycomb table. I register it to the corner and then register a piece of acrylic on top of it. I offset the cut path .030 each direction and cut all the way through the acrylic. I then weed and glue it to the aluminum in position. After that, I create my standard jig and glue it on top of the first layer of acrylic. The cut path runs inside the weeded grooves.

This method keeps jig components in place and does not get cut by the laser. It also raises the part to be cut up from the base so bounce back is minimal. Using perforated aluminum still allows the gases to be sucked out through the bottom.

It's a little work but for large runs or proprietary products is worth it.

Cheers,
Doug

[Stephen Beckham]

John,

I'm a bit lost on the purpose as well, but a combination of the suggestions above might work. If I had to make this jig, I'd use the Acrylic, Wood, Matboard or Romark plastic (scrap of course) that easily cut with the laser.

Once the hole was there, I'd use (more scrap) sheet aluminum left over from Trophy plate cutouts. Double Stick them right up to the edge of your cutout. You may have to trim two sides so that they fit, but four small pieces of the scrap would allow you to keep the jig from getting burned.

I would like to add a caution though. When you allow bleed - you are going to get flash burns on the sides of your target. Remember your beam in hourglass shaped and will touch the edge of your target as some point. Even if the beam does not touch the edge - your flash back from the metal (or non-burning surface used to protect the jig) will also have to go somewhere - if your target is snugly against the edge, it too might cause some flash burns with a nice clean edge just below the jig-line... Careful with the bleeds...

Good luck!

[Mike Null]

I would use a thin material for the jig that would be out of focus. That may not be posible in your application but may be worth consideration.

[Paul Woodrich]

Cut your fixture opening from plywood, cardboard or acrylic. All are relatively accurate and not too costly. If burning your fixture is a problem / concern then tape a piece of aluminum foil over the fixture. Regardless of the fixture medium chosen, you are good to go.

[Frank Corker]

I'd be careful with the aluminium foil, I had a great 'bounce back' with that. Still there permanently etched onto the opening door of my laser

[Richard Rumancik]

I think John is doing something rather artistic here where he wants the graphic to go off the edge of the workpiece. I have seen this done with text on photo frames for an interesting effect.

John: your workpiece is .250" thick. If you make the rectangular cutout in the fixture from say .188 stock there will be about .062 protruding. You could cut strips of .062 or .094 baltic birch (or arborite or other material) and park them up against the workpiece. They could be attached with spray adhesive, small screws, tape, or whatever you prefer. This sacrificial strips would be replaced when they got too burned up.

However, I would be concerned that even a small gap between the workpiece and the strips would let the beam penetrate the gap. This would result on an unsightly vertical burn on the side of your workpiece. There will be tolerances on your workpiece and on your fixture.

Solution? Perhaps 2 fixed and adjustable strips (screwed down) to close the gap to a negligible amount. Maybe also applying some self-adhesive foil tape (aluminum, copper etc) to the sides of the workpiece to shield it from damage in the areas where the bleed occurs.

Example of bleed in image: