small drill bits ?

cessna · Sep 14, 2010

I thought I had drill bits covered, I have from 1" down to #60, but the carb I am working on calls for #72 and #76. Where is the best place to buy these small bits? What are the tricks in using bits of this size, I have a Sherline lathe and mill, the part is set up in the lathe right now, will this work or just break the bits?
Terry ???

stevehuckss396 · Sep 14, 2010

cessna said:
I thought I had drill bits covered, I have from 1" down to #60, but the carb I am working on calls for #72 and #76. Where is the best place to buy these small bits? What are the tricks in using bits of this size, I have a Sherline lathe and mill, the part is set up in the lathe right now, will this work or just break the bits?
Terry ???

All I can tell you is get a set of quality drills from 61 - 80 from a tool supplier. Do not buy a cheap chinese set, they are horrible and will not work well. I tried them way back when and they break very easy and were not very sharp.

ruzzie · Sep 14, 2010

Hi
I bought mine from ebay they where solid carbide in a pack of 50 maybe worth a look

Size Diameter Qty Size Diameter Qty
#83 (.0120) 5 #76 (.0200) 5
#80 (.0135) 5 #75 (.0210) 5
#79 (.0145) 5 #74 (.0225) 5
#78 (.0160) 5 #73 (.0240) 5
#77 (.0180) 5 #72 (.0250) 5

Cheers
Paul

GWRdriver · Sep 14, 2010

cessna said:
the carb I am working on calls for #72 and #76. . . . What are the tricks in using bits of this size, I have a Sherline lathe and mill, the part is set up in the lathe right now, will this work or just break the bits? - Terry

Terry,
The most deceptive thing about small drill bits is the speed at which they should be run . . . 1000's of rpm, so the problem for most of us will be getting sufficient rpm out of our machine tools which are designed for general machining rather than high speed operations. The #60+ wire drills are particularly troublesome.

I'll compute an example for you. HSS in mild steel (C1020) should be run at between 60-100 SFM (surface ft/min) according to one chart I have. So taking a #76 drill at .025" diameter = around .078" circumference or .078" travel per revolution. Let's say we wanted the bit to cut at 80 SFM, 80 x 12" = 960". Divide 960" by .078" and you get a target speed of around 12,300 RPM! That's the Gotch'a for most of us and why small drill bits often give us fits.

All the above is theoretical and the reality is that one-off small hole drilling can be done at sub-sonic speeds so I wouldn't let the above discourage you, just try to get all the speed you can from your setup. Also it wouldn't hurt to used a good tapping fluid (I use Tap-Magic) and withdraw the drill and clear the chips very often. I withdraw about every 2X diameter as a guideline and adjust my feed based on what I feel the drill doing.

cessna · Sep 14, 2010

Thanks for the help guys, I just ordered a set of 61-80 bits from KBC tool.
Terry

don-tucker · Sep 15, 2010

I had to drill a very small hole in a gas jet once,the only way I could do it was this,sorry I have no photos as I gave the jig away,I mounted a Dremel at one end and the jet in a sliding member at the other,so I could feel the job onto the drill running at a few thousand revs,I think the drill was .012" dia.Sorry to be so vague but I thought it may help.The jig was made horizontal so easy to see and no gravity to worry about.
Don