# Fixtures for short production runs



## difalkner (Jan 3, 2012)

I've mentioned this before but one of the most enjoyable tasks with the CNC is coming up with efficient fixtures for work holding for short production runs. 

Today I needed to make a fixture to hold 30 small Walnut bases to cut a slot into for an acrylic plaque. The base is 2 3/8" x 4 3/4" x 1 1/4" high with the front cut on a 25° angle. The acrylic plaque is leaning back at a 15° angle. Since my CNC is a 3 axis with no capacity to tilt the spindle that means I have to slant the work piece if I cut on an angle.

The next part is the fixture can't take me all day to make, has to be designed and built in a short period of time. I have many, many times fallen into the Rube Goldberg time-consuming spiral and designed very complicated fixtures. They work but they take forever to design and implement. Simple is better most of the time. It also helps that all 30 pieces are identical so no surprises when it came to holding the base in the fixture.

So this is what I came up with on today's challenge. The face of the Maple hold down board has a strip of 220 grit paper to ensure nothing moves. 








































Including a finishing pass the slot took 33 seconds to cut. It took about 45 seconds to raise the spindle, move it out of the way, vacuum the chips, remove one piece, place the next piece, and then hit 'Go to zero' in Fusion 360 to then start the process over again. And in this case all I had to do was loosen the knobs a little and raise the Maple hold down to remove the Walnut base, then slide another one in its place. Many times I have modified the G-code to raise the spindle and move it out of the way automatically but it's just as easy to do it manually. Now if I had a hundred of these to do then I'd add that code in to do it within the file. 

I tried to sand pieces while others were cutting but the cut was over too quickly to do much sanding. So to do 30 of these took about 40 minutes or so. Tomorrow they'll get sanded and shot with one coat of sealer and one coat of gloss lacquer, delivered on Monday which will be a week ahead of schedule.

David


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## hawkeye10 (Jul 28, 2015)

David, did you cut the Walnut bases on the table saw? Looks like a quick and easy project for a CNC.


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## difalkner (Jan 3, 2012)

Yes sir, cut everything on the table saw except the slots. Far, far faster and less waste than on the CNC. Well, I also used the bandsaw, jointer, and planer... :wink:

David


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## 4DThinker (Feb 16, 2014)

Nicely done. Simple solutions are almost always the best solutions. 

In V-carve/Aspire there is a home/start position offset you can use to have the router start at and move back to any position you want when the cutting is done. This is distinct from the origin point. No need to add your own g-code lines to a file to get the "move out of the way" action you described.

I rarely do the same cut more than once, but when I do this Home/Start Position feature is very handy. 

4D


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## difalkner (Jan 3, 2012)

Fusion 360 may have something like that but I haven't looked. I figure editing the G-code helps me to learn more about the actual movement, how each line affects the toolpath, and generally keeps me on my toes to be on the lookout for something out of the ordinary. Being a hot rodder from years gone by I like being 'under the hood and tweaking the engine', so to speak. :wink:

David


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## chessnut2 (Sep 15, 2011)

The Rube Goldberg Spiral. Now I know the name of what I'm afflicted with. Great idea for a quick, effective fixture!


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## beltramidave (Jan 6, 2016)

difalkner said:


> Fusion 360 may have something like that but I haven't looked. I figure editing the G-code helps me to learn more about the actual movement, how each line affects the toolpath, and generally keeps me on my toes to be on the lookout for something out of the ordinary. Being a hot rodder from years gone by I like being 'under the hood and tweaking the engine', so to speak. :wink:
> 
> David


David,

Are you familiar with G28 & G30 commands. Very simple to set a position anywhere. 
Manually move machine to whatever position you want, then go to MDI and type in either G28.1 or G30.1. This will set position for either G28 or G30.
In toolpath, just enter G28 or G30 (whichever one you set) in appropriate spot. Very easy. Could put in post processor file as well.

Dave


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## difalkner (Jan 3, 2012)

I've seen them, Dave, but haven't checked into using them or fully understanding what they do. I need to add that to my list of 'things to learn on the CNC'.

What I usually do is add a line at the end that says - G01 F400 Y25 - or whatever Y or X value gets the spindle out of the way sufficient to remove a piece. Then I just hit Go to Zero which moves the spindle back to X0 Y0 at 600 ipm and hit cycle start again. Pretty simple, really. I've thought about modifying the Post Processor but have yet to do that, too. 

David


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## papasombre (Sep 22, 2011)

Hi, David.
Your projects are always interesting for me. I don't have a CNC and I am far from get one but in this particular job one the measurements of your walnut bases came timely and handy for some mobile phones bases I am planning to do. 
Thank you.


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## Gaffboat (Mar 11, 2012)

Great jig and excellent idea, David. I bookmarked it for future reference.


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## difalkner (Jan 3, 2012)

Thanks, guys! Since I posted about the fixture it's only proper that I post a couple photos of the finished pieces. I actually finished these a few days ago but just now taking photos.

















Those of you with eagle eyes may have noticed the chips in the edges of the acrylic. They had a tube go out in the laser and just sort of barreled their way through getting this cut so I'd have a test piece. The new tube is installed now and the cuts will be clean on the actual pieces going to their customer.

David


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