Small Pilot Bits for the CNC

There are several workable options for pre-drilling for #8 screws with CNC equipment. December 10, 2007

Question
Is anyone pre-drilling the holes for #8 screws? If so, can you please explain? I cannot find drill bits that small for the CNC. Currently we are using the 5mm bit to make a dimple on the material to locate the screws.

Forum Responses
(CNC Forum)
From contributor A:
3mm bits are readily available.



From contributor M:
The other option, which I use, is to buy drill bushings (Amana Tool carries them), which are just a 10mm aluminum bushing that is drilled out to the diameter of drill you want, and split along the side so it compresses when the set screw is tightened. Very simple, and I have used these and made them for 15 years.


From contributor J:
Don't use split collets. Purchase a drill bit adapter that has a 10mm shank. They're available for 2mm, 3mm, 4mm, 5mm, 3/32, 7/64, and 1/8 drills.


From contributor O:
3mm and above are available in drill bits with 10mm shank. For anything smaller you will want an adapter, as mentioned in the previous post. I purchased mine from Southeast, I think. The 3mm is solid carbide and a bit more money; the 4mm is a drill bit with carbide tip for around $10 and lasts a long time also.


From contributor L:
You can also buy a drill chuck/tool holder and use any size drill bit you want.


From contributor U:
Don't forget the LH and RH directions.


From the original questioner:
Thanks everyone. I think I'll try the 3mm bit; seems like the easiest solution.


From contributor M:
Depending on the material you are drilling into, you may find the 3mm is too large of a pilot hole for a #8 screw. If it is hardwood you should be okay, but if it is almost anything else it will be too big and the screw will strip the hole. You can get around this problem if you only spot the hole 1/4" to 1/2" deep and let the screws do the rest of the work.


From the original questioner:
Thanks for pointing that out. I will try drilling only halfway through. If you drill halfway through, will the screw drift, or will it pretty much stay its course?


From contributor M:
The screws should stay straight since they have a partial guide to begin with. If they do drift, that would mean that they are bending within the wood or you would see the pilot hole stretch. I've never seen it happen, but you never know.

The last option would be to use 2mm or 2.5mm drills instead. A little testing might be warranted based on material and type of screw you are using (wood screw, sheet metal, euro, drywall, etc.).



From contributor K:
I use 1/8" (3.175mm); works perfect.


From the original questioner:
Who sells 2mm or 2.5mm bits?


From contributor R:
We run a Rover 20 PTP and have been using Lance bits (1/8" and 2.5mm) for years for pre-drilling all our parts. You will need an adapter. We purchase ours from the company that does our saw blade sharpening.