Bench Dog Tightness

I've really never used bench dogs much so unfamiliar with fit required. I'm having an MFT style MDF slab CNC'd to use with my track saw system. This is known as a 20mm system. What should be the fit between the bench dogs and top holes?

From my investigation I found many use 20mm drill bits, 20mm router bits, 20mm hinge boring bit, and 20.01mm for CNC. I had a test hole bored and the 20.10mm hole size that is recommended by some seemed too loose using my bench dog as a plug gauge (my bench dogs are 19.89mm to 19.90mm). The CNC guy was nice enough to bore six more holes at 0.001" smaller increments (I know metric/English). So I had holes at 0.791", 0.790", 0.789", 0.788", 0.787", 0.786", and 0.785" (note: these were sizes he punched into his CNC, no measurement made to verify their exactness). At 0.788" the dog did not drop right in under it's own weight but still had noticeable side-side motion. At 0.787" with slight force it slid in the hole, no side-side motion, and pulled out without much effort.

I choose this hole but in hind site is this too tight? The tables haven't been machined yet.


Thanks for your input,

Mike

20mm holes should work just fine with dogs designed for the 20mm system. The dogs should be slightly smaller and fit just fine. They did with the 20mm holes I bored using my Festool OF1400 in a project last year and if I were going to cut something similar with my CNC now that I have it, I'd still cut them 20mm exact.

Quote:

---

Originally Posted by Jim Becker

20mm holes should work just fine with dogs designed for the 20mm system. The dogs should be slightly smaller and fit just fine. They did with the 20mm holes I bored using my Festool OF1400 in a project last year and if I were going to cut something similar with my CNC now that I have it, I'd still cut them 20mm exact.

---

Thanks Jim - have since found the older MFT's had 20.00mm holes and the newer ones 20.15mm and indeed some complain about the slop in the newer tops. Will stick with the 20.00mm.

Thanks

And if it's MDF...it will "relax" with use. :) I'd rather a dog be snug than not-snug...

I'm actually mentally designing my next bench top and plan on incorporating a full MFT compliment of holes plus a reasonable array of .75" holes so I can continue to use my beloved hold-downs. :)

Picked up the tops last night and my Veritas Parf Dogs fit nicely. Glad I didn't fold in to comments I heard elsewhere and go bigger.