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Design for CNC - tips for 18mm Baltic Birch

Jeff Member

I'm designing some furniture that will be cut on an CNC machine out of 18mm Baltic birch plywood. After I get a pre-production prototype I'm happy with I plan to kickstart it with a minimum initial batch of 50.
I've already had one prototype made on a CNC machine, and I learned a lot. But I have questions for the next prototype.
I plan to design it to be assembled by gluing pieces face to face with 5/16" diameter wood dowel pins, 1” long, giving me a 36mm thick slab with features on both sides. This worked well on the first prototype. That said, I only used the pins for location, and screwed the parts together.

Question 1: Will I need to chamfer or otherwise relieve the edges of the holes in case the glue swells the grain around the hole?

Question 2: How accurate can I expect things to be on the Z axis? I thought +/- .010” was reasonable… but this shop was way off. It worked for the prototype, but it wouldn’t work on production runs. Am I asking too much of CNC? I don’t think they had a vacuum hold down system.

Question 3: Due to the number of holes (3 different sizes), I know I will need to send this to a shop with a tool changer. Should I strive to make sure the majority of the non-hole features can be cut by a 3/8” bit instead of a 1/4” bit?

Question 4: I’m plan to do my own nesting, primarily to make sure I can keep my materials cost down. If I am cutting two parts next to each other, I assume I should keep the parts farther than one tool diameter apart so you don’t cut both edges (one climb, one conventional cut) at the same time? So that both parts can be cut climb or conventional as fits the bit / material?

Question 5: Any face to face joining techniques I should look at other than dowel pins? I am worried about glue squeeze out.

Question 6: Other than making parts self-align and making it easy to assemble by keeping part count and confusion down, what else can I do to make this cheaper and easier to manufacture? I’m planning on a clear polyurethane finish.

Thanks for any help you can give!


5/10/20       #2: Design for CNC - tips for 18mm Balt ...
Tom Gardiner

My thoughts are:
1. No need to chamber the dowel holes more on that later.
2. I strongly recommend finding a shop with vacuum holding. You cannot depend on Baltic birch to lie flat on the table otherwise and even sometimes with vacuum it will have enough warp to lift off the table. Another slow method would be to use the cnc to drill pilot holes in the the sheet to hold down.
Once you have reliable holding the cnc will have a Z axis accuracy of < .005" easily.
3. A tool changer will be much faster as the drill routine is quicker per hole, however, a 1/4" bit can route holes of any size larger by using a spiral plunge cut. Bet on 2-3 seconds per hole.
Using a down shear will reduce the fur around the holes but can cause excessive heat build and possibly fires with the chips being forced down into the holes.
Tool size is a variable that will determine the feed speed of the router. Each router can handle feeds and depth of cut differently depending on the rigidity and power of the machine. Talk with the machine operator to find out what his/her machine can handle. My router is a light industrial 4 x 8 and is happy routing 18 mm BB in a single pass with a 5/16" compression bit. A 1/4" bit would take two passes.
4. By all means do your own nesting. My software doesn't offer the option of using a common line to cut parts. I offset my parts by ~.010" when nesting. You may want to include a last pass tolerance into the toolpath to achieve the best finish so you will have to allow for it in nesting. Once again work with the cnc operator to get the best results for that machine and the operator's preferences. I know I am more likely to give a favourable quote to a job when I am confident in the way it will run ( ie it's the way I have cut before)
5. Glue squeeze out can be controlled by how much and how glue is applied. A random bead of glue squeezed liberally around is a recipe for messy glue ups and patches of unglued material. I have a hopper glue roller to apply a consistent film to the entire surface but a paint roller will be sufficient for little investment. If you are screwing together your parts I suggest you drill clearance holes for the screws with the cnc. I like to see well placed screws if they must be used.
6. Sanding and finishing will likely be the largest labour cost in any furniture job done with plywood. Choose the best ply you can. All BB is not created equally - some have large voids that show in the edges. Top sheet sanding can be brutally bad and requires heavy sanding to be acceptable for a finish. Take the time to test the cnc footpaths for best finish possible. Don't be going after the fastest run times only to face hours of edge sanding.
Birch plywood furniture looks great on paper /screen but the proof of its lasting appeal will be in it's finishing details. How does the edge feel? Is the top lustrous or show sanding marks? If you must see screws are they part of the appeal or do the distract or seem to be randomly placed and driven poorly.
Make use of the cnc's ability to do repeatable tasks quickly. Don't think that you can do an operation later faster /better.
Surface sand your material before cnc at least up to the second last grit. Even better find a shop with a wide belt sander.
Spray the finish! It will be tougher, look better and take a tenth of the time.
That is it for now.

5/10/20       #4: Design for CNC - tips for 18mm Balt ...
Jeff Member

Thanks Tom!

All the parts will be cut "good side down" and then get glued together to show the good sides on the outside.


5/12/20       #5: Design for CNC - tips for 18mm Balt ...
Dropout Member

90% of what we cut is baltic birch.

Any design that needs a tolerance of 0.010 Z using wood needs a rethink - too many factors involved.

The quickest way to the best finish is a 1/2 compression spiral chipbreaker as a 0.020 oversize first pass and a 1/2 downspiral finish pass. We run the first at 750 or 800 IPM and the finish (if needed) at 500 to 600.

I leave 13mm between parts on a nest.

You don't mention the size of your parts, but we do many small parts. We cut these 16.5mm deep and then separate on a trim router. We use an upspiral flush trim bit for best edge finish.

Nesting yourself sounds like a good idea, but if the shop you deal with has decent nesting software don't waste your time. They can do it better and quicker than you can.

Hourly rate is a bad way to have people quote. I charge $140.00 per hour and am usually lowest cost per part when compared to people in the $100.00 per hour range.

Where I am 5X5 BB is less cost per square foot than 4X8 so look for 5X10 table. Load/unload and tool change time divided by 50 square feet instead of 32. Over time it's a big deal.

5/13/20       #6: Design for CNC - tips for 18mm Balt ...
John Meachen Member

If you are using full sheets it might be more efficient to use vacuum bagging to bond them together unless you have a press.All you need is a plastic sheet and a flat surface.A piece of breather cloth is handy too and the stack can be as many sheets as you like.Even clamping pressure and much greater force to minimise the glue line thickness and glue usage.You can use masking tape to align the edges or 2 small pins per sheet.I have found online nesting available for rectangular shapes and you set the parameters to match your needs.

5/20/20       #8: Design for CNC - tips for 18mm Balt ...

Only thing I can contribute to what others have said is to consider gluing your sheet goods together before the cnc. We do this on a regular basis, just apply glue to both pieces, align them and place them in the press. CNC time will be slightly decreased over-all due to only handling half the number of parts, even though the actual spindle run time may be exactly the same. I also find that two pieces glued together seem to stay a little flatter on the vacuum table than a single sheet.

6/1/20       #9: Design for CNC - tips for 18mm Balt ...
Jeff Member

Thanks everyone!

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