CNC drill bit conundrum...

[greenz123]

I have a requirement to to drill 4.2mm holes to a specific depth in acrylic using my CNC spindle (I don't have a drill press & the depth of the holes have to be pretty accurate).

My problem is, 4.2mm drill bit shaft is too thick for a 4mm collet, yet too small for a 5mm collet (& I don't have a 4.2mm collet!) ...besides I'd rather not have to faff about changing collets where possible (that's a real time sump!). In the main I use a 3.2mm collet.

I'm having great success with these type of drills (which have a 3.2mm shaft)...



But they only do 3mm, 4mm, 5mm & 6mm :-(

I'm therefore considering, buying some cheap (soft) 4.2mm drill bits turning down the shaft using my lathe (once it works!) so the shafts fit in a 3.2mm collet ....is this plan flawed? (runout aside!).

Is there a better way? (ie somewhere I can buy 4.2mm bits with a 3.2mm shaft!)

Hi Hank,

I have come across this exact same conundrum, the ebay link in the thread doesnt work anymore. I dont suppose you found anywhere that does drill bits with 3.175 shanks which have a larger diameter ?

[HankMcSpank]

Hi Hank,

I have come across this exact same conundrum, the ebay link in the thread doesnt work anymore. I dont suppose you found anywhere that does drill bits with 3.175 shanks which have a larger diameter ?

Wow, that was an old post.....in the end I found a good Uk source for drill sizes larger than 3.175mm (but with a 3.175mm shaft), here you go...

http://www.acsind.co.uk/ ...I was dealing with - Dave Cornell (Technical Sales Engineer)...I don't want to post his email on an open forum (else bots will start spamming him), but if you want to email him direct, shoot me a PM)

...they do both new & re-pointed, being a tightwad, I bought re-pointed (but frankly I got great results with them & couldn't tell the difference vs. brand new), they have all manner of sizes available & the prices were excellent as were their postage charges (no affiliation!). I mainly bought sizes below 3.175mm as the cost does go up somewhat for sizes greater than 3.175mm...but I did order a spattering of larger sizes (e.g. 4.0mm 4.2mm 5.00mm 6.0mm & 6.35mm) all with 3.175mm shafts....a great time saver when you're only drilling acrylic.

Edit: I've just found the email with the prices I paid back in 2011 (just to give you an idea)...

10 off Repointed 1.60mm drills @ £4.49 in total (i.e. 44.9p each)
10 off Repointed 3.00mm drills @ £4.49 in total
10 off Repointed 3.175mm drills @ £4.49 in total

2 off Repointed 3.40mm drills @ £4.36 in total
2 off Repointed 3.60mm drills @ £5.14 in total
2 off Repointed 3.80mm drills @ £5.14 in total
2 off Repointed 4.00mm drills @ £5.14 in total
2 off Repointed 4.50mm drills @ £5.90 in total
2 off Repointed 4.80mm drills @ £6.68 in total
2 off Repointed 5.00mm drills @ £6.68 in total
2 off Repointed 5.50mm drills @ £7.56 in total
2 off Repointed 6.00mm drills @ £8.36 in total
2 off Repointed 6.35mm drills @ £9.00 in total

[greenz123]

That's brilliant thanks, it will save a lot of time ! Much appreciated.

[cropwell]

Can't you just cut 4.2mm pockets, or do you need to go to great depth ?