The spiral mill tool path with a 1/4 or 3/8 end mill would be my choice. You could spiral ramp in with a roughing cut then do a finish profile at the bottom with a tangent lead in - lead out. That should give a nice smooth hole that's on size
I'm working on a project in which I need to drill lots and lots of 1/2" diameter holes into several large panels routed from 3/4" baltic birch plywood (see attached drawing, with inches as unit) to make an oversized, wacky "Lite Brite". I'm currently trying to figure out how to cleanly cut all of these holes without requiring too many hours of machine time. I also am hoping to find a way to cut these holes so that they don't require any cleanup afterwards (like deburring or sanding or anything) - there will be about 4000 holes total, so post-processing that has to be done really adds up.
I've learned about an operation called 'peck drilling' that seems like might work, though I do need to source an end mill that is ever so slightly larger than 1/2" to give some clearance fit for my Lite Brite pegs (made of acrylic rod). Does peck drilling with a compression cut end mill still produce good top and bottom finish, or is that only when doing horizontal movement?
Another technique that I've been thinking about is using a 1/4" compression cut end mill and an aggressive ramping profile to progressively spiral down into the material (see attached screenshot - blue for step downs, red for ramp), taking advantage of it's horizontal cutting preference to get that smooth top and bottom finish. It'll take longer than the peck drilling, but maybe it'd be worth it. Does this seem like a viable strategy? What can I do or pay attention to in order to get it right?
I'm open to other suggestions too - I'm pretty new to CNC machining in general, so my knowledge of operations and end mills is far from complete!
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The spiral mill tool path with a 1/4 or 3/8 end mill would be my choice. You could spiral ramp in with a roughing cut then do a finish profile at the bottom with a tangent lead in - lead out. That should give a nice smooth hole that's on size
Sweet. Do you recommend a compression cut end mill for both the roughing and finish passes?
It might be OK if the top surface is just a little bit less clean than the bottom, if that opens up more possibilities. The top surface will be the "inner" surface of the finished sculpture, so I can run a stiff brush over it to knock off burrs without much of a problem. But the less work the better :P
If the top surface is the inside (hidden) surface, then an upcut bit might be better. It should leave no burr on the visible side.as long as the work is tight to the spoilboard. The finish cut should eliminate any burr left by the rough cut.
I would do the rough and finish pass in one operation. Setup the tool path so it steps over at the bottom to do the finish pass. The trick is the set the cut width larger than the tool. If the tool is 0.250 then set the cut width at 0.270 and that will give you 0.010 per side to cleanup on the finish pass.
What kind of machine are you working with, and how are you holding the sheets down?
Gerry
UCCNC 2017 Screenset
[URL]http://www.thecncwoodworker.com/2017.html[/URL]
Mach3 2010 Screenset
[URL]http://www.thecncwoodworker.com/2010.html[/URL]
JointCAM - CNC Dovetails & Box Joints
[URL]http://www.g-forcecnc.com/jointcam.html[/URL]
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)
Couldn't you just use a waste board underneath and a 1/2" forstner bit?
What kind of machine are you working with, and how are you holding the sheets down?
If possible, I'd like to use a very hacky 4x4' machine that I have access to at a local makerspace. I created a lot of documentation for it, so if you have specific questions let me know. There are some photos and info here: https://www.instructables.com/id/Using-the-4x4-CNC-Router-at-Twin-Cities-Maker-Hack/
That machine has T-tracks for clamps, though it isn't all that easy to secure full 4x4' sheets.
I also have tracked down two other machines in the area that have reasonable hourly rates, but I'd rather not spend the money if I don't have to. One is an industry-grade 5x8' with vacuum hold down, and another is a repurposed plasma cutter table with something like 8x10' bed and DIY clamps.
Couldn't you just use a waste board underneath and a 1/2" forstner bit?
I don't have any experience with Forstner bits, so I couldn't say. Can they be used to produce holes slightly larger than 1/2" to give my pegs some clearance?
Additional waste board would be tricky, as the 4x4' machine I have access to has very, very little Z height. Its a bit of a struggle to just use 3/4" plywood once an end mill is installed!
Last edited by jasonwebb; 09-01-2017 at 10:13 PM.
I don't think you can use Forstner bit in a router, they are a slow speed tool. Routers turn much too fast. Another option might be a brad point bit. They make a clean cut and are available in 1/64 increments. Also metric sizes. But again spinning at router speeds might be a problem, to much heat generated. I've never run one at more than 2500 rpm or so and even at that they get a bit warm.
You are going to have to very carefully control the Z height without a spoil board. Maybe you have room for at least an 1/8 inch or so. I don't know how to get a smooth edge without going through. The machine with a vacuum table would be a better choice.
Any drilling operation is going to cause issues on the backside, unless you use vacuum and surface the spoilboard between each sheet.
I'd probably use a 1/4" compression bit. Start by spiraling down to rough out a 3/8" hole, then a full depth finish pass. The bottom edge will most likely be cleaner than the top edge.
Gerry
UCCNC 2017 Screenset
[URL]http://www.thecncwoodworker.com/2017.html[/URL]
Mach3 2010 Screenset
[URL]http://www.thecncwoodworker.com/2010.html[/URL]
JointCAM - CNC Dovetails & Box Joints
[URL]http://www.g-forcecnc.com/jointcam.html[/URL]
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)