Bullnose a 4" HDPE circle - Router Forums
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post #1 of 5 (permalink) Old 01-14-2020, 01:20 PM Thread Starter
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Default Bullnose a 4" HDPE circle

Hello all, wondering if you guys can help me make a job more efficient.

A few times a month, we run about 1000 3/4"x4" HDPE circles with a .154" center hole. I cut the blanks 288 at a time on a CNC router and then they go over to a CNC mill where 2 at time, a full 3/4" radius is put on the edge of the circle. This is time consuming and I am being tasked with finding a faster solution to this. I have already suggested a dedicated vacuum fixture that does the same thing the mill does, just 24 at a time instead of 2. Boss likes the idea of just running it over a table router one at a time, a few seconds each. I like this too, but I know that I can't use a bit with a bearing because it needs to machine the entire edge. A template would be too time consuming. Can't use a 3/8" radius tool because there can't be a flat on the edge. They come out perfectly on the mill. So I have to match that while maintaining a .030 tolerance.

I am thinking of using the center hole as an axis to rotate the circle's edge into the bit. It's too dangerous to start with the piece already contacting the bit, so I'd have to figure out some kind of T-slot sliding fixture that puts the radius of the circle in the same place every time. Any other ideas that are obvious and I'm just an idiot?

Company is willing to buy new equipment for this job specifically, so if it requires new equipment no big deal.

Thanks in advance!
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post #2 of 5 (permalink) Old 01-14-2020, 02:15 PM
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You have a sound idea if a router table is to be used...but...I'm a bit concerned about manipulating a 4" slippery HDPE disc against a bullnose bit by hand. I like your idea to engage the disc with some kind of mechanical hold down but I doubt it will be just a few seconds. The disc will need to be mounted, secured, turn on the router, push into the bit on some sliding device, turn the disc, pull it away, turn off the router, dismount the disc...

You are wise to use some form of slide to push the disc into the spinning bit...but, I would add some machinery to rotate the disc rather than rotating it by hand. A few times a month times 1000 makes for some statistically good chance of somebody putting their fingers into the bit.

You could add a small rubber pad on the mount (say 3" mount) to grab the disc on the bottom side and maybe a clamp to the center/top to hold down the disc. Friction would allow the mount platform to spin the disc. You would then need a way to spin the mount platform as it approaches the bit. A stop would provide repeatable cuts. I'm thinking something like the mechanism on a portable CD player...or...maybe something like a capstan against the edge of the disc to spin it into the bit...? Sort of like a tape player...remember those...? LOL

Might sound like overkill but it's all to keep worker fingers away from the bit...

A real interesting project...good luck with it...


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post #3 of 5 (permalink) Old 01-14-2020, 02:44 PM
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Hmmm, How about a fixture that uses the router table. The idea is a bolt (#12? M3?) with .154 sleeve that secures the disk to a rotary shaft that you can crank. The rotary assembly would be held in place by a bracket that allows it to turn. The bracket is encased in a box that is open on the bottom. You place the box over the exposed bullnose bit, push the disk against the spinning bit and then rotate. There would be a stop (use a fence?) that correctly positions the disk against the bit. Set time is securing the disk to the rotor arm and removing it after. Routing would be reasonably quick. Care would be needed to make sure the rotary shaft won't move other than rotating.
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Last edited by PhilBa; 01-14-2020 at 02:46 PM.
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post #4 of 5 (permalink) Old 01-14-2020, 04:46 PM
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There`s a simple jig that allows cutting perfect circles on a bandsaw that you can use as a basis. It uses a pin like you have in mind and the pin is attached to a piece of wood that slides in the miter slot something like shown in this you tube video.
Add a stop to limit the forward travel of the jig and yourè set.

I also would be concerned with trying to turn something as slippery as HDPE by hand. It`s a large enough project that I wonder if it wouldn`t be worth tooling up to jig that has a motor to drive the disks and it along with the pin and something sticky enough on a foot that presses down on the disk and turns it for you at the right speed. A DC drive motor like the ones on drum sanders for controlling the belt feed speed would do it.
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post #5 of 5 (permalink) Old 01-14-2020, 06:12 PM
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cut the bullnose on a 2nd CNC operation...
use a vacuum clamp to hold the disk...
place an indexing pin that fits the center hole of the disk into the center of the clamp..
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