A while ago I designed a Festool MFT style table using the 96mm x 96mm grid concept. Since we’re kind of weird in Canada and use both metric and imperial (metric for most things but imperial still dominates tools and construction) I decided to design my table with both sets of holes. This is actually what got me interested in CNC work as it was hell trying to get it accurate by hand with hand made templates - so I dove in and joined a market space and learned the basics so I could cut it myself.
I had a hell of a time getting the sub mm precision needed for snug but usable holes, even on the big ShopBot, at least with 2D and 3D pocketing operations, which is what I was advised to use. Running the hole cutting pass two or three times mostly got me there, but was an expensive way to do it (due to time and frustration) and some of the holes are “loose” while others are a tiny bit too tight. No bueno. So what are the optimal settings to get perfect dog holes on your CNC?
Now that I’m making a new waste board for the Longmill I decided to use the same concepts and same dogs on a grid to help with alignment and work holding (there are various vertical and lateral clamp options that go through the holes). Kreg make some nice 3/4" (19mm)dogs that come in 3/4" and 1/2" heights (I have both). Lee Valley make some gorgeous dogs in 20mm diamter with a 25mm collar on them (and an 8mm threaded center for accessories you might dream up). They come in long (“hot dogs”) and short versions. There are practical uses for all of them depending on work height and work holding plans - that’s an entirely different topic…
After a much needed tune-up of my V-wheels (I THINK I finally have them tight and correct) as well as adjusting my anti-backlash nuts ,I believe my machine is nicely calibrated now. So it was time to figure out the best way to cut the elusive dog holes so that the holes would be the perfect size coming right off the machine, first pass.
To compare toolpaths, I fired up Fusion 360 and laid out six pairs of holes, each pair having a 19.1mm and a 20mm hole. The Kreg dogs are exactly 19mm and the Lee Valley/Festool style ones are 19.9mm so I figured the -.1mm hint from Lee Valley would be a good plan for the Kreg ones. I was close, as you’ll see.
-I pre-drilled all the hole center locations with a standard 3/8" 135 degree twist drill bit from a $15 Hitachi bit set I got on sale at the local tool store. For the drilling operation I ran the router at 10K RPM with a plunge rate of 1016 mm/min and a retract rate of 2032 mm/min. It only took a minute or two to run. These settings may not be optimal but worked fine for me. I have the 3/8" collet adapter for the Makita which allowed me to use the larger diameter bit. There is only ~5mm of height between the tip of the bit and the 19mm top of the MDF. NOTE: MAKE SURE YOU SET YOUR HEIGHTS TAB TO ALL EQUAL 5MM OR UNDER OR YOU WILL CRASH YOUR MACHINE DURING THE DRILLING OPERATION.
-The material was bog standard 19mm / 3/4" MDF from Home Depot Canada.
-For all my milling tests I used the Sienci 1/4" down cut bit in my Makita RT701C on my Sienci Labs 30x30 Longmill. For now, mine is mounted inelegantly to a piece of 19mm MDF that is Irwin clamped in four spots to a discontinued Ikea kitchen island that I use as a worktable. I do intend to get something stiffer but had no trouble achieving .01x mm accuracy once the machine was dialed in.
-tl;dr SUCCESSS: I was able to achieve an optimal outcome (both size dogs fit perfectly out of the gate) using CIRCULAR tool path with the following settings (takes about 90s-100s per hole to mill):
-Operation type: CIRCULAR (was steered away from this by “experts” previously - go figure)
-Feed rate: 2283 mm/min (~90 ipm) per Winston Moy’s Shapeoko MDF YouTube Video
-Speed rate: ~17,000 RPM, ~3 to 3.5 on the setting dial
-Depth of Cut (Step down) 1.524mm, using even step downs
-Milling direction - Conventional Milling
-Stock to leave: For the KREG 19 mm holes use -.125 mm stock to leave (cut the hole .125mm larger than the 19mm you drew - ignore me drawing them at 19.1, in future I will draw at the correct 19 and use stock to leave as below). For the Lee Valley 20mm dogs no stock to leave is necessary, the hole comes in at 19.95mm-20mm out of the gate and the .1mm undersized dogs (19.9mm) fit snugly right off the machine. If you’re doing both sizes (which I recommend, options are good) then make two different tool paths to avoid over cutting the 20mm holes. NOTE: The Kreg dog seems loose at first until pushed in to full depth where it is perfectly snug. It may be tapered a bit.
Kreg calls theirs Versa-stops, you can see them here: https://www.kregtool.com/store/c75/accessories-and-parts/p475/adaptive-cutting-system-versa-stops/
Lee Valley has these ones which are more impressive to handle but the collar makes them less practical in some situations so I find myself using the Kreg ones more often. Lee Valley calls theirs Veritas Parf Dogs: https://www.leevalley.com/en-ca/shop/tools/workshop/workbenches/benchtop-accessories/71185-stainless-steel-parf-dogs-by-veritas
I also tried these operations: Boring, 2D pocket and 3D pocket but all three of them resulted in holes for both sizes that were unusable and under sized by .1mm to .25mm or more depending on which operation and which hole. It was quite inconsistent which is really frustrating if you’re making a large worktop grid! The Boring operation did give a bang on 19.1mm hole but was a bit too tight (I may try it again with a 19.125 target size just to see) but the 20mm hole was unexpectedly significantly under size at ~19.63. Not sure what is going on there. Likewise the 3D operation was undersized by .15mm or more and neither of the dogs fit. Initial tests used the same toolpath for both sizes so settings were identical.
-Linking Tab: Lead to Center
I’m going to go cut two more exactly as described above to confirm these directions one last time, but that was the magic formula. I hope this pays it forward and helps the next person trying to find this information.