Big drilling

I am having a problem drilling an 11/16 hole. Because of the length of the drill bit (I have had to get a blacksmiths shanked drill bit) after a few turns it becomes apparent that the hole is not straight. I do not have a chuck that will hold an 11/16 bit. Any comments will be very much appreciated. Thankyou.

Please explain what you mean by "the hole is not straight"? Are you attempting to drill the hole without drilling a pilot hole first?

Lathe/mill/drill press? Speeds/feeds/material and set up?

I use el cheapo blacksmiths drills every now and again on the lathe and milling machine and have had no problems. If anything they're rather shorter than the equivalent Morse taper shank drills.

Andrew

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Blacksmiths drills ( I find ) are not the best of quality items and tend to grab and snatch their way through.

How deep is the hole and what material.? - The way I would do it if possible is the hold the part in the lathe, drill out close to size and then bore out with a boring bar.

This would also put far less strain upon our hobby sized machines. A drill of that size depending upon what lathe you are using runs the risk of giving the tailstock a real good hiding if the drill bit is snatching.

Nick

Edited By Nick_G on 08/06/2016 10:43:51

There is already a 1/2" hole drilled. As a result of this problem I was looking at buying a boring bar from RDG. this is described as a 12mm shank boring bar.......but......there are three options CCMT 06 DCMT 07 TCMT 11. What do these options indicate ? RDG state they are all perfect for hole turning.

Nothing wrong with blacksmith drills usually, are yours HSS or carbon steel, if the latter halve the speed. You can turn down the shank of an ordinary jobbers drill, as its not hardened, you just have to be able to get the bulk of the drill in the spindle of your lathe. Ian S C

A boring bar will give a better truer finish than drilling, really depends what you want the hole for.

The three options describe the shape of the carbide insert, personally I would go with CCMT as that is quite common and can be used in a number of different type holders so you can standardise on one type of insert.

A 12mm boring bar probably has a minimum cutting size of 16mm (CCMT type) so check that before you buy or open the hole out with a 5/8" or 16mm drill first

Which is where we came in!

Andrew

I meant to get the 12mm boring bar in without the edge of the insert rubbing but would agree that on light machines 1/2" to 11/16" may be a bit of a big step between drill sizes depending on material.

If this is for the middle of that cam/gear then bore the hole as a drilled one won't be very good.

Edited By JasonB on 08/06/2016 12:39:15

Roger, consider an ER32 chuck that fits in place of your current chuck. It can hold up to Ø20mm tools. This would allow the use of slot drills or larger parallel shank twist drills to be used.

I sometimes use a No. 2 morse taper ER32 collet chuck in my tailstock.

Martin

When I did my industrial training years ago I had occasion to drill large holes, many, many large holes up to oooh, about 1.5inches, and I was instructed to never use a pilot hole because the drill would run truer. I forgot this and on my lathe which I bought about 6 years ago I would drill a pilot and was disappointed when I found that the large drill would wobble as it drilled (I use Dormer drills). Now I use a centre drill to just remove the centre, non-cutting part of the big drill, but I don't go deep, just below the surface, and then the big drill now ploughs through with no wobble and straight as a die.

If you don't have the power to go through with the big drill, then drill a pilot, but only of a diameter equal to the centre, non-cutting part of the big drill, so it will be quite small.

Charles