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  #11  
Old 07-26-2018, 11:22 PM
rcrawford rcrawford is offline
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Originally Posted by drummerjg View Post
Very clever, Russell. Thanks for posting!

I do have a question. For those of us without vacuum hold downs, would it be ok to use four locating holes drilled into the spoilbord and then use dowels as the holding agent? That way you could still fly cut because you would only be cutting a dowel. Im kind of excited to try this technique out.
The only reason I'd use vacuum hold down is to make sure the UHMW sheet stays flat to the spoil board.
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  #12  
Old 07-27-2018, 10:54 AM
drummerjg drummerjg is offline
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Originally Posted by rcrawford View Post
The only reason I'd use vacuum hold down is to make sure the UHMW sheet stays flat to the spoil board.
Yeah...I thought of that later on. Another half-baked idea. But I still see some possibilities for your solution as long as there isn't much up-cutting going on.
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  #13  
Old 08-17-2018, 03:19 PM
kjoiner kjoiner is offline
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Russell,

If I were to use your method of simply cutting the parts to .688 and then flipping the part over to fly cut and release them, would there be a risk of the parts flying out as the last pass is completed? I don't have vacuum on my machine. The thought of being able to do this without having to install a few dozen screws is interesting.

Kyle
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  #14  
Old 08-17-2018, 11:21 PM
The real JP The real JP is online now
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Just curious. If you fly cut the backside to release the parts.

What keeps them from being released at high speeds? The vacuum loss from the cuts on the underside would negate any hold down I think.

And how would you flip an 80lb sheet of mdf that is almost cut through in many places without breaking it?

Obviously I have never tried something like this.
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  #15  
Old 08-18-2018, 09:13 AM
BradyWatson BradyWatson is offline
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I've had great success cutting both HDPE and laminates where the finished part retains it's 3M waxy paper backer (because finished part has its own adhesive that is part of the product). It can be a real mother trying to hold small waxy parts down...I've had a major manufacturer (for 15yrs now) that sends me this type of material to cut because they & nobody else they've tried, can hold the parts down and get a good clean cut.

I came up with a method that I call 'Vacuum Film'. You take your material and depending on how waxy/slippery it is, you cover the entire back in mounting tape. This is a thin, very aggressive adhesive tape made for mounting vinyl lettering etc. Any sign supply sells it. Then laminate it to a sacrificial layer of 4mm Coroplast. You really need to laminate it good with a J-roller. Then place the entire layup on your vacuum table and suck it down. Cut all the way through the parts into the coroplast slightly. When done, remove vacuum and peel off the parts. Done.

Keep in mind you should use the SMALLEST diameter bit you can on small parts to minimize how much leverage the tool has against your part. This results in less potential for part movement. For instance, imagine if we had a 12" diameter router bit...this would have the potential to launch parts at the slightest vibration or deviation from the cutting path. All that circumferential surface area would be VERY grabby against the edge of the part - however if you used say an 1/8" dia tool, this would be minimized. PLUS - for straight vac hold down you've reduced the kerf width and leakage.

For parts that are tiny but not so waxy, the mounting tape is too hard to get off the back side of the part. No problem. Cover the entire back side of the material in blue painters tape (don't forget to J-roll it) and then cover that with the mounting tape. Fiberglass WOVEN carpet tape can also be used for smaller projects. Then laminate it to a sacrificial material like Coroplast ($10/4x8 sheet) - suck it down with vac, and when you peel off your parts, the light adhesion of the blue tape peels off cleanly.

Keep in mind...ANY adhesive is sensitive to temperature. Too cold and it won't stick. Too hot and it fails. In the winter months, warm up your tape by the heater before you stick it.

Give it a whirl...

-B
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  #16  
Old 08-18-2018, 11:22 AM
The real JP The real JP is online now
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OOhhhhh

You just gave me a ton of ideas.

I had put wide painters tape on my spoilboard and taped the backside of a work piece and superglued the tape to the tape. But a whole sacrificial sheet had never even occurred to me.

The other day I was considering spraying contact adhesive on the taped parts then.
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  #17  
Old 08-20-2018, 09:31 AM
BradyWatson BradyWatson is offline
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Quote:
Originally Posted by The real JP View Post
The other day I was considering spraying contact adhesive on the taped parts then.
Now you're talkin'....I did 50,000 letters 1.5" tall for Federated Dept Stores (Macys etc), using commercial grade laminating adhesive, .090" styrene as a sacrificial layer and SintraTop PVC for the letters. When done, a little solvent wipe on the backside of the letters took off any adhesive still left on. I used vacuum to hold the whole git up down and a straight bit to leave chips IN the kerf to assist in side-to-side wiggle prevention because it was a very tight nest. I lost a few letter 'i' parts, but just would up running an extra 100 of them to make up for it. The glue ups were run through a laminator to make sure they were 'stuck real good' - so keep that in mind if you try it.

Hold down IS the entire job with this work...all else is gravy.

-B
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  #18  
Old 09-05-2018, 10:57 AM
kjoiner kjoiner is offline
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Hello,

So I'm finally moving toward making the fixture. I've drilled a couple of holes in the machine table and set a custom home position. Now I need to make the fixture. Any suggestions on material for fixture? I've got a piece of 1/2" cabinet grade plywood and may try it first. The down side is that threaded inserts are almost that long (.483) and I won't be able to skim it if necessary. Should I just go with 3/4" MDF? The next question is what type of inserts to use. I'm thinking the knife inserts might work (McMaster Carr p/n 92105A675). One piece of good news is that I won't have to skim the UHMW down to 13/16 any more. I have enough clearance to keep the bars at 3/4" thick.

Kyle
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  #19  
Old 09-05-2018, 11:28 AM
The real JP The real JP is online now
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Probably mdf, you will need to resurface it to keep it flat. Life gets to it after a bit.

You know you could build you own vacuum table pretty easy.

If your material is all the same size just use pegboard for the top and whatever for the sides. you would only need to use 1/4 material to build it. Just brace the insides. use a shop vac for the vacuum.

Bradys method of using a sacrificial sheet under the parts is going to be the least labor intensive way to get clean parts.

If you want you could also onion skin the parts and cut them free with a knife then flush trim router or sand them.
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  #20  
Old 09-21-2018, 03:44 PM
kjoiner kjoiner is offline
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I made my first run of parts on the fixture. It worked pretty well. One issue I need to resolve is during drilling, chips are either packing or lodging in the brass inserts. When I run the screws down through the UHMW to hold it in place, if chips are in there, the screw will try to back the insert out the bottom. I will probably need to glue the inserts in place. I may need to revise the toolpath for drilling the holes. Currently, I run a inside profile with a spiral to cut the .260 holes. I may have to go back and peck the holes to provide some clearance and brief cooling time.

Kyle
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