First CNC project

[andrew whicker]

Hi,

I'm getting close to sending off my first flatpack design to the CNC operator... it's Sunday on a holiday weekend so I'll ask here before getting in touch w/ the vendor.

I am making a speaker cabinet w/ lots of dados to help assembly. If the Baltic Birch is 15 mm, I've just been making the dado 15 mm (line to line) on my 3D drawings. What is the standard practice? Does the designer draw line to line and the CNC operator knows to make the dado slot narrower or wider as necessary to match the plywood or is line to line fine?

Thanks and cheers,

[Jim Becker]

Kinda depends on what level of services your operator is providing. If an exact fit is required, IMHO, it's best if the toolpaths are adjusted to reflect the actual material thickness dimension for this kind of thing with just a hair of allowance so you can get the edge into the slot without beating it to death. (CNC cuts extremely precisely, so you can't have a 15mm slot for material that actually measures 15mm, for example. The slot has to be 15mm plus a few hundredths wide to actually allow assembly with glue. If you are providing the material and know it's 15mm on the dot (unusual), if you are just providing vector drawings, you are best served by putting the allowance into your drawing so the toolpaths being created by your service provider will be accurate. I typically use .02-.05mm for this on my own work, but often run test cuts to verify that the "material in hand" actually goes together. Otherwise, you're going to be hand-working those slots wider back in your shop. Discuss with the vendor to understand what THEIR process is and act accordingly.

[John TenEyck]

Further to what Jim said, I would send a piece of the actual plywood to your vendor, or tell them exactly how thick it is so they can make a sample, so they can use it to verify it will slip into the dados properly. We adjust things almost w/o thinking for a good fit when working by hand. With the CNC that is no longer possible so fit has to be defined with the toolpath allowance.

I would send your drawing with 15 mm line to line, if that's the nominal product thickness, but specify they need to adjust the allowance for your actual stock, as discussed above.

John

[Jim Fenton]

I would get a sample too. I always wanted to own a cheap cnc. Good luck.

[Jerome Stanek]

There is also the fact that there could be run out or a resharpened bit that will change the size of the cut. I had some .25 bits that came only .245

[Wes Grass]

It's common with endmills to cut a bit smaller than the shank size. Most I've measured to comp' the size are <.002 under. It's the only practical way to get a sharp edge with clearance. I do have some with a narrow cylindrical land that measures shank diameter. Specific for aluminum IIRC. Might be there to help with chatter.

My couple years at my last job using a router on plywood and 'other' materials made me nervous about trying to make fits too tight. The material varies in thickness, sheet to sheet, and edge to edge, to require some clearance before you even get to dealing with flatness. The thou' or 2 clearance I'd use with metal just 'wouldn't go'.

[David Buchhauser]

In machine shop terms we refer to those as "regrinds". First regrind size will often be 0.010" under the nominal size of the end mill when new. Depending on the application, these can get you into trouble if not paying attention.
David

[Kevin Jenness]

A common practice in cnc cabinetmaking is blind dados. The groove is cut undersize and a tongue cut on the mating piece. The tongue thickness is consistent as it is gauged off the table and independent of the material thickness (which commonly varies considerably in veneer core plywood).

It's important to communicate your expectations clearly to the vendor. Subbing out cnc work is a viable plan but the relationship is crucial. Test samples are never a bad idea. If you're just having one speaker cut out initially the stakes are low.