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Boring 16mm hole in stainless bar

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Twf03/05/2021 22:03:35
16 forum posts

I need to make up a couple of 50mm spacers out of 32mm 316 stainless round bar, with a 16mm hole bored through each. I had a go today at making one as a test, I used 6mm Cobolt drill bit in tailstock of my Super 7 with the back gear engaged on the second slowest speed, then moved up to a 12mm Cobolt drill bit with no issues. The only 16mm drill bit I had was one of the reduced shank “blacksmith” drill bits, which eventually chewed and scraped its way through, but left a very poor finish.

Presumably I want to be splashing out on a 16mm Cobolt drill bit?


David George 103/05/2021 22:18:16
1572 forum posts
482 photos

Hi have you thought of finishing it with a boring bar from 12mm drill. A carbide tipped boring bar will only take three cuts to finish it with less strain on the lathe.


Andrew Johnston03/05/2021 22:22:16
6055 forum posts
671 photos

If it's only a spacer with a clearance hole does the finish matter? If It does, I agree with David, use a boring bar, although I'd drill bigger (say 15mm) first and not bother with the 12mm drill. There is a third option, drill and ream, but I already have a 16mm reamer.


ega03/05/2021 22:46:57
2140 forum posts
176 photos


I take it you are using a suitable cutting oil?

I would expect to be able to cut 316 with "ordinary" HSS and doubt that cobalt is essential.

Twf03/05/2021 22:57:01
16 forum posts

I have been using CT90 oil. In all honesty the 16mm drillbit is a cheap and nasty thing. I think my boring bar is larger diameter than 12mm, I will have to check.


Hopper03/05/2021 23:00:40
5505 forum posts
137 photos
Posted by Twf on 03/05/2021 22:03:35:

... I used 6mm Cobolt drill bit in tailstock of my Super 7 with the back gear engaged on the second slowest speed, then moved up to a 12mm Cobolt drill bit ,,,

WAAAAAAY too slow. Second back gear is about 50 RPM. Try 500 RPM would be more like it. Or as you are using an oversized drill bit with a reduced shank, plus stainless can be a bit grabby, maybe the next slower speed of 200rpm. So leave back gear out of it.

Look up how to calculate your rpm to get the desired cutting speeds for different diameter jobs and materials. It is essential to all machining. Drilling a 6mm hole at 50rpm must have been painfully slow?

With the correct, or at least higher, RPM your drill bit will cut better and the feed rate relative to each turn of the job will be much finer, both of which will give you a better finish. Which as said is not critical inside a spacer but treat it as a learning experience.



Edited By Hopper on 03/05/2021 23:06:54

ega03/05/2021 23:42:28
2140 forum posts
176 photos


I don't know CT90 but it sounds good.

Plus one for Hopper's speed point - I actually wondered if you meant the reference to back gear.

Another point is that some suggest a pilot drill no bigger than the chisel point on the final size drill and no intermediate drills if the machine will stand it; this is said to be kinder to the flutes of the larger drill particularly if it is carbon as opposed to HSS (or cobalt).

If you are going to drill rather than bore the ideal would be a three flute core drill (a suitable boring bar would be cheaper and more versatile).

John Reese08/05/2021 06:16:46
934 forum posts

If itis a spacer and the bore will be hidden why worry about finish,

SillyOldDuffer08/05/2021 09:19:42
7229 forum posts
1593 photos
Posted by Hopper on 03/05/2021 23:00:40:
Posted by Twf on 03/05/2021 22:03:35:

... I used 6mm Cobolt drill bit in tailstock of my Super 7 with the back gear engaged on the second slowest speed, then moved up to a 12mm Cobolt drill bit ,,,

WAAAAAAY too slow. Second back gear is about 50 RPM...

Just a thought, but drilling dead slow would tend to cause rubbing rather than cutting and many Stainless steels work-harden under those conditions, which would blunt the drill in short order.

As Cobalt drills are rather harder and tougher than ordinary HSS, I wonder if they survived the mistake, whilst the softer Blacksmith's drill went straight to perdition?

Stainless steel is much fussier about cutting speed and feed-rate than mild-steel, and it pays to get both right. Cutting Fluid helps enormously. To avoid blunting, the tool must cut continually, requiring determined steady pressure at the right speed with lubrication and sufficient cooling.

In my guilty past I ruined a lot of drills by pussyfooting until Andrew Johnston put me right. I've learned so much from this forum!


Chris Evans 608/05/2021 09:36:12
1923 forum posts

For non fussy holes, clearance for bolts etc. I tend to go from pilot say 6mm to final 16mm but grind the 16mm drill a little off centre to make it cut oversize. This tends to stop the flutes rubbing but means drills doctored in this way need storing separately.

Simon Williams 308/05/2021 12:37:00
595 forum posts
81 photos

It's all very well exhorting the OP to go faster, but my S7 wouldn't drill from 12 mm to 16 mm in direct speed in 316, simply from lack of torque. If you just feathered the drill so the spindle didn't stall you get into the work-hardened nightmare in a very short order, as it would be doing little more than scraping the surface.

Agreed back gear is too slow but at least you can get a cut on.

I feel the boring bar suggestion is a better bet, though putting something stiff enough to be useful down a 12 mm pilot bore 50 mm deep is a bit of a contradiction in terms.

This is more difficult than it seems, within the limitations of an S7.

noel shelley08/05/2021 13:09:32
570 forum posts
18 photos

IF it's only a spacer why worry to much about the finnish ? In 316 the drill MUST be sharp and MUST cut, it will need cutting fluid ! If it rubs atall it will go glass hard and your in trouble. My Cleveland drillers book tell that for 16mm in stainless it's 40 Feet /Min so 244 RPM. Hope that helps. Noel.

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