Cutting on two sides of a sheet- help

[andrewm]

I need to make some parts that are going to need to be routed on both sides. I am cutting a number of grooves on one side, then flipping the sheet and cutting out my parts on the other. Despite carefully registering my sheet when I flip it, I am consistently off by about 1/8". I keep going back to my CAD files, but they seem to be dead on. I have also moved my bot around checking to make sure it is lined up with the corners of my sheet, and it too looks perfect.

Has anyone ever done this, and if so, do you have any tips for getting both sides to line up?

Andrew

[beacon14]

Andrew,

I have done this successfully once or twice. My suggestion is to use your CAD file and the ShopBot to create some kind of registration system, ideally using some feature that you need to rout into the first side of the part anyway, and make a fixture with a matching male/female fitting (a dowel protruding from the fixture into a hole in your part, or a ridge fitting into one of your grooves, or even pocketing out the entire shape of the part into a fixture). If you plan it right, it doesn't even need to be at the same point on your table as the where first side is cut, so you can leave the fixture in place while cutting the first side. The point is that by using the original CAD file to create the fixture for the second side with the fixture already secured to the table, the fixture will automatically be positioned properly for the subsequent machining to be done.
By the way, if you are always off the same amount, you might be doing it 99% right, but missing something basic. The kind of thing that when you find it, you'll slap yourself on the forehead and say "how could I be so dense as to miss that? (I speak from experience here.)) I'd be more concerned if it misses by some random amount each time.

Hope this helps
David

[gbatten]

Andrew,

Check the location of your graphic image in your software. If the lower left corner of your graphic is not EXACTLY at the 0,0 point on your screen then you will be "off" when you flip your part over. I just learned this the hard way with an expensive sign.

...Gene

[Vince F (Unregistered Guest)]

Gene,

Is it possible it may have something to do with using a 1/4 inch cutter and not allowing for it when you flip the part? Just a quick thought.

Regards, Vince

[billp]

Gene/Andrew,
One way to get the 0,0 placed properly without straining your eyeballs is to make your sign "blank" the exact size of your file in your software. Then if you are lucky enough to have a "center vector" feature in the software it should place both sides of the piece in the exact position.....

[artisan]

I have two suggestions here. First....VERY similar to what Bill has just said, make your page size or work area (software) the same size as your sheet goods. You should then be able to "register" your piece exactly in the software with the x and y spatial coordinates on each side. Second, add a tab or tabs to your piece along the centerline which can then be routed off in the final toolpath. You should be able to drill these tabs for accurate alignment and use a simple jig to hold your piece and flip it....D

[andrewm]

David, your suggestion got me right on track.

The solution:
1. Have the Shotbot drill one hole at each end of the sheet about 2 inches from the end in right in the middle of the sheet. (the axis point of your flip)This hole goes about 1/2 inch into the top MDF sheet.
2. When the sheet is flipped over, put a dowel rod into these holes and line it up so it fits in the holes made in the table. Bingo - its lined up and you can put your clamps on the peice and cut the other side.

(Note: I was cutting a full size 4'x8' sheet into lots of 2 sided parts.)

Cut 4 sheets so far with everyone registered perfectly!

I found that blocking didn't work well. The sheets were each just a little different in size and the little slop worked out to a slot of sloppy parts. The key is getting the point of rotation of your flip accurate, and the pegs worked like a charm. These holes should be drilled by making them in your CAD file in the very center of our drawing. This is key.

By the way, Shopbot may not know it, but this forum is worth its weight in gold. You guys were a great help. As a pilot and aircraft owner, I often found that the best airplanes were ones that there were forums with vocal pilots who liked to talk about their aircraft. If it wasn't a good aircraft, the owners didn't want to talk about it. This forum was a big decision in my purchase. I hope I can help contribute.

Andrew

[robtown]

I needed dados on both sides of some shelf partitions for a case I was building. I just took the part, flipped it, and used blocks the size of my router bit to wedge it back into place in the large sheet that it was cut out of (still screwed in place), and ran my dados again. Worked perfectly.

[beacon14]

Rob,

Sometimes it's the simple solution that gets the job done right, and right quick to boot!

Andrew,

Glad I could help.
Working alone it's too easy to get stuck, often it takes a "nudge" of an idea from someone not in the middle of the problem to make the leap from frustration to success. Such is the beauty of the forum.

David