Cutting polyethylene

[dubliner]

I've got a possible job cutting out a 1" x 4" x 1/4" simple part(its a spacer) with 2 x 1/4" holes in it, & as many as I can get out of 2 x 96/48 sheets of 1/4" poly, is that going to be a $%^&# to hold down? are tabs realistic? plus packing & shipping. Any ideas how to handle this one

[gabepari]

Table saw and drill press, a good plastics blade should give you good edge quality.

Gabe

[ken_rychlik]

Neville, Tabs are realistic but take some time to clean up. If the parts are square, I think I would run them on the table saw. Then make a jig and let the bot do the holes.

[dubliner]

It will have to be CNC though as two corners are radiused.( or would laser be better I dont have one, but may refer it to one of you with one),

[Brady Watson]

I've done a few jobs like this. Here's how I would do it:

1) Rip the sheet in strips, 1/16" wider than you need.
2) Chop saw to length, 1/16 larger than you need.
3) Make a gasketed vac jig with islands to compensate for holes being drilled, and then drill/inside profile the holes, then machine perimeter in a single pass.

You can setup multiple vac 'pods' for parts that will allow you to do maybe 10 at a time (in a row) and then code the last lines in your file to move the machine out of your way to unload & reload. Cycle times will probably be something like 2 to 3 minutes per 10 part run + how long it takes to reload parts. Not too long ago I cut 6,000 parts like this 10 at a time (in a row) and it took me all of about 2 days to bang them out.

-B

[mikeacg]

Brady,

What do you use for gasket materials? I am cutting 12 x 24 sheets on my laser and wanted to move them to the Bot but I can't seem to hold them down properly.

Mike

[mikeacg]

Clarification: I am cutting 27 small tags with rounded corners and a strap hole in each out of each 12 x 24 sheet. The holes cut good and the outside of the tag is good right up to the release point. I hate to use tabs because of the clean-up time.

Mike

[Brady Watson]

Mike,
It depends - usually I don't need gasketing because I surface my jig totally flat before laying the parts on there. When I do use gasketing, I use AllStar spoilboard cover. It is self-stick and works great. Be sure to order the correct thickness to avoid the 'jello effect'.

The second factor is what type of vacuum you use. A Fein will only output about 7"Hg at best. Higher vacuum may be needed. If you are cutting out 27 parts from a 12X24" sheet, you'll have to make sure that there is enough meat left on the scrap to hold those tabbed parts - high density nested parts + problematic/borderline adequate vacuum = frustration.

One thing you can do is create a toolpath that tabs the start point of each part at ideal cut speed. Then, go back and run a machine along vector at a very slow speed to clean up those tabs. You'll have to mess with cutter geometries to come up with a bit that works for this, as an upcut spiral can really fight your workholding efforts. A straight or downcut might be needed (a bit change) when you run the MAV to cleanup the tabs.

Experiment as much as possible, using your own magic joo-joo & observation skills to SEE exactly what is happening during cutting. Pull off dust collection & wear safety glasses with side-shields. When you SEE what is happening, it clears up a lot of the mystery when there is a problem.

-B

[knight_toolworks]

for something like this drill the holes use sheeetrock type screws to screw them to the spoilboard and then cut them out. I think this will take less time then any other method. I do this with thin plastic where I only cut the part once so it is not worth the setup time.

[gundog]

Neville,
Do you have a vacuum table? If you do here is what I would do I would cut the profiles and leave a .030-.050 skin first then drill the holes. Rough cut the parts out of the sheet with a knife or skill saw / jig saw. At this point you should have a little thin edge sticking out all the way around the parts. Use a laminate trimmer bit or any guide bit mounted in a manual router table and trim the profiles flush. If you don't have a router table use your spindle/router just bore a hole in a piece of plywood for a portion of the guide bearing to fit inside use the CNC like a pin router with the bearing guide to trim the profiles flush.

I do a modified version of this with all my plastic parts I am cutting I cut a minimum of .5” sheet and I can just use a laminate trimmer with a flush trim bit, I just flip the sheet upside down with the skin facing up .250” is not thick enough to do this unless you let the parts hang off the bench because the bearing on the flush trim bit will hit the surface the part is laying on and the cutting edge won’t reach to cut off the scrap.

Mike