PDA

View Full Version : CNC Router vacuum jigs - need plastic advice



Skip Weiser
02-06-2008, 12:08 AM
The two piece 18" x 32" vacuum jigs I've made, like this one: http://www.sawmillcreek.org/attachment.php?attachmentid=74429&d=1193886174

I'm not happy with the HDPE plastic. It's just not rigid enough. I mean it makes great jigs but there must be stress in it or something because it won't ever lay flat on its own. I know I could mount it to plywood but I didn't want to do that.

I have another similar size jig that is made from two 1" thick pieces of UHMW plastic that I screwed together. It lays and stays flat as can be. Someone gave me this stuff luckily because it's rather expensive.

Can someone suggest a stronger plastic to use that won't bow like the HDPE and doesn't cost an arm and a leg like the UHMW? PVC maybe? I thought I'd ask those of you who may be more familiar with what's available than I am.

Thank you,
Skip

Steve knight
02-06-2008, 1:41 AM
acrylic works great. but in thicker pieces I find scrap by the pound. it costs way too much to buy it by the sf

Keith Outten
02-06-2008, 5:59 AM
I can't be much help at this point as I have not been able to finish assembling my vacuum pump that I started a couple of months ago. I have been considering trying to use Corian for vac jigs, I know 1/2" is a bit thin but I was thinking the vacuum route could be machined on the top of one piece then a second piece glued on top with holes placed for holding the work. Corian stays flat and I have several thousand pounds of it so it is essentially free material that I can make as many custom jigs as necessary.

My thinking may very well be flawed since I haven't built one yet but it seems like a good plan from my inexperienced vantage point :)

.

Brian Backner
02-06-2008, 7:40 AM
Skip,

Assuming that the bed of your CNC machine is flat, a simple solution is to rout a groove around the edge of the HDPE on the back side. Install a gasket into this circumferential groove - when you place this back side down on the bed, and your workpiece(s) on the top side and put the whole thing under vacuum, the jig will be pulled flat to the bed.

Brian

Skip Weiser
02-06-2008, 10:55 AM
I can't be much help at this point as I have not been able to finish assembling my vacuum pump that I started a couple of months ago. I have been considering trying to use Corian for vac jigs, I know 1/2" is a bit thin but I was thinking the vacuum route could be machined on the top of one piece then a second piece glued on top with holes placed for holding the work. Corian stays flat and I have several thousand pounds of it so it is essentially free material that I can make as many custom jigs as necessary.

My thinking may very well be flawed since I haven't built one yet but it seems like a good plan from my inexperienced vantage point :)

.

Hi Keith,

Your description is exactly how I made my jigs. The bottom piece of material gets grooved for the vacuum route, then drilled and tapped on the end for the air fitting. The top piece of material gets pockets routed into it with holes drilled through the pockets into the vacuum route grooves of the bottom piece.

I didn't try gluing the top to the bottom. Instead I put 1/32" adhesive backed spoilboard gasket from Allstar Adhesives between the two pieces and screwed the top to the bottom. I did that so I could change the top piece if it ever got too thin from being resurfaced over the years. That way I could just make a new top if needed instead of having to make the whole jig again. But, if I used a more rigid material, like the Corian you mentioned, once it was surfaced (if needed) it should stay flat and level forever, right? Gluing two pieces of Corian together though would make a nice leak free seal, no doubt.

Steve, thanks for the acrylic suggestion. I'm going to price some.

Brian, I have MDF as my spoilboard and I don't think that would hold enough vacuum to keep the jig flat without my small pump running constantly. Thanks for the thought though.

Thanks guys,
Skip

Michael Kowalczyk
02-06-2008, 1:49 PM
I 2nd that Brian. But if it is still cupping and Skip does not have enough CFM to hold jig and parts, he is back to square 1. I use HDPE every day and it will cup from stress of cutting grids on one side. I have to wedge them flat in our storage rack to keep them semi flat.
Now keep in mind that I am working with full 4x8 sheets of HDPE and laying up full sheets of Melamine or Plywood on them with our smaller 75 +-CFM Busch pump, so I can deal with the cup easier. PVC is a little more $$$ but it lays flat. I use it for my main grid that the HDPE goes on. If you need more than that then maybe look at phenolic. Depending on your environment and need for accuracy, my first choice would be Phenolic. By far the most stable. PVC is second but will still expand and contract with temperature but HDPE will move even more. So if you are going to use the jig several times a year and you need the accuracy, step up to PVC or Phenolic. PVC machines easier than Phenolic. Oh and Phenolic really smells when you route it and your IPM will have to slow down also

Solid Surface is an interesting possibility also as Keith has mentioned. At $7.00+- a foot it may be cost effective but you are limited in width of a solid piece. I know you can seam them but under vacuum they never last like a single sheet does. trust me, I learned this one first hand.

Skip what are you holding your jig with to your table when you use it?

Skip Weiser
02-06-2008, 2:42 PM
Hi Micheal,

Right now I have to use deck screws in each corner of the jig to pull it down flat. I put one in the middle of each side edge too, 6 total. That works, but it never goes to exactly the same position with each use. It's not really a problem if I'm just doing perimeter cuts, but the handles pictured in the jig link above also get a 3/32" R around the perimeter. Being even a couple of thousandths off will make a difference as to whether I get a nice full radius on one end of the jig, and cutting too deep and leaving a ridge on the piece from the radius tool on the other end. Or good ones on one end and shallow on the other. I'm striving for perfection ... ha I guess that's a good thing though.

I'm going to look into the phenolic that you mentioned, and the PVC also.

Thank you,
Skip

Steve knight
02-07-2008, 1:39 AM
the disadvantage of using vacuum to hold down the puck is if you cut through the part bot the puck and the piece can go flying. plus you hae to have the right surface to suck down onto.
I find it is best to just screw the jig in place. I have made most of my pucks from acrylic 3/4" thick or glued up to get that thick as I have been getting it as scrap for 2.00 a pound. it works pretty well. though you may need to check for thickness as it tends not to be super duper flat.

Matt Zettl
02-07-2008, 5:24 AM
Steve,

At a recent Camp ShopBot, Brady Watson showed a vacuum jig made of synthetic decking material, like Trex. I believe he used the brand sold at HD, and said something about it being better than Trex. Anyway, I made one from Trex, and it seemed to work well. Very easy to machine, stable, and the best part is that it is cheap (relative to sheets of acrylic, PVC, etc.). The only problem is it's only available in widths up to about 6". I epoxied two pieces side to side to make a jig 10" wide, and that worked well. After glue up, I surfaced both sides, cut the vacuum grid, and voila!

Hope this helps.

-Matt Zettl

Keith Outten
02-07-2008, 7:46 AM
Hi Keith,

Your description is exactly how I made my jigs. The bottom piece of material gets grooved for the vacuum route, then drilled and tapped on the end for the air fitting. The top piece of material gets pockets routed into it with holes drilled through the pockets into the vacuum route grooves of the bottom piece.

I didn't try gluing the top to the bottom. Instead I put 1/32" adhesive backed spoilboard gasket from Allstar Adhesives between the two pieces and screwed the top to the bottom. I did that so I could change the top piece if it ever got too thin from being resurfaced over the years. That way I could just make a new top if needed instead of having to make the whole jig again. But, if I used a more rigid material, like the Corian you mentioned, once it was surfaced (if needed) it should stay flat and level forever, right? Gluing two pieces of Corian together though would make a nice leak free seal, no doubt.

Skip

Thanks Skip,

It would seem that I am on the right track. Corian is manufactured to some very tight tolerances and is indeed flat from the factory unless it has been improperly stored. I also find that it is almost always 0.495" thick when I check it with my caliper.

I should have my vac pump finished in a few days and will be able to start designing a few special jigs for jobs I do frequently. I purchased the laser pointer that Steve recommended and I have the super zzero fixture ready to go...assuming I don't have a conflict with the ShopBot software that Steve indicated.
.

Steve knight
02-07-2008, 11:01 AM
I have been drilling and tapping my fixtures for 1/2" threads. I have been using as small of tubing as I can. I also have a vacs gage so I can make sure I have a good seal. one of the more critical things is using the right gasket it makes a big difference. http://www.allstaradhesives.com/index.php (http://www.allstaradhesives.com/index.php)

Skip Weiser
02-07-2008, 8:29 PM
Thanks everyone for your input.

Here's something kind of interesting. I talked to a guy who makes vacuum jigs from two layers of 3/4" cabinet grade plywood. He seals them with melted pariffin wax and claims it works great. I'm going to have to think about that.

Steve, I use the 1/32" adhesive backed spoilboard gasket from Allstar Adhesives on all of my jigs. You are right, it's great stuff.

Thanks again,
Skip

Steve knight
02-07-2008, 9:48 PM
i use 1/16" for solid wood. I also use yellow glue to seal any porous wood. a lot of times vacuum leaks on solid woods like beech and others. I put on some yellow glue and scrape off all the excess. this gives me a perfect seal then. it works to seal mdf too when it leaks air. it dries faster then a finish does too.