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Internal grooving tool?

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Chris Parsons25/04/2014 10:09:39
118 forum posts
37 photos

Need to cut an internal groove for an O ring in a 5/16 bore?

I am thinking about making a lathe tool by drilling across a round bar and grinding a square ended insert (with the appropriate relief) from an old drill bit held in by a couple of grub screws - any observations/comments anyone?

The groove needs to be 0.094" by 0.062" (2.3mm by 1.5mm)

I have an indexable internal threading tool but it was the square bottom and sides of the groove I was thinking about...


Michael Gilligan25/04/2014 10:34:43
15470 forum posts
668 photos


That sounds fine ... 'though I think a single [axial] screw at the end of the bar would probably do the trick.


Andy Pugh25/04/2014 10:57:48
47 forum posts

Horn make some nice grooving cutters:

And someone is selling cheap holders:

For smaller holes thye do this style:

Ian S C25/04/2014 12:23:02
7468 forum posts
230 photos

I,v got one a little bigger than you need, but it could be scaled. It's just a bit of 3/8" bar(scrap bin), with a chip off a broken industrial power hacksaw blade brazed on the end, I needed it for a one off job, and its been used for a number of jobs since. Ian S C

Neil Wyatt25/04/2014 17:30:28
17714 forum posts
697 photos
77 articles

Easy way is to turn a disc of the required width on the end of a silver steel bar. File out a 90-degree notch to slightly below centreline, harden and temper to straw.

Mount in a boring tool holder and rotate so cutting edge is on the centreline.


jonathan heppel25/04/2014 22:53:59
99 forum posts

Did some about 10 years ago for 1/4" bore out of a 6mm HSS engraving cutter blank on offhand grinder and angle grinder held in a vice. Material was 316. Neutral rake worked just fine. Job's been working (dymamic, not static) 40 hrs a week ever since. Diameter is the critical dimension with O rings, width is more about type of duty but anyway is less important. Surface finish must be adequate but nothing special. It's easier than first appears.

Edited By jonathan heppel on 25/04/2014 22:56:02

Andy Pugh25/04/2014 23:18:22
47 forum posts

The lathe + silver steel is probably the quickest and cheapest approach. Though if you happen to have a Woodruff cutter lying about then that set up right ought to work too.

Oompa Lumpa26/04/2014 06:31:43
888 forum posts
36 photos

I have just acquired a lovely 6mm ISCAR Boring bar kit that would do this.

You don't want to be buying one for a one off. Lovely bit of kit though.

I would go with Neil's suggestion.


Edited By Oompa Lumpa on 26/04/2014 06:32:57

Steve Withnell26/04/2014 10:08:23
796 forum posts
215 photos

Offhand grinder, make any queer shaped tooling you need. Both intentionally and otherwise!

The grooving tool I used for eccentric straps was an old letter stamp, ground to shape.


NJH26/04/2014 11:20:45
2314 forum posts
139 photos

Hi Chris

I did just as you say.

tiny boring tools.jpg

The tool on the right I ground up from an old No.1 centre drill and held it in a holder like the one on the left - which is nothing more complicated than a end of square bar cross drilled for the centre drill and with a hole tapped in the end for a grub screw. Just a few minutes work and it works fine. Mine was for the same job as Steve - eccentric straps. I guess that in a 5/16 bore though you might have some rigidity problems as everything will need to be pretty small so I think Neils method is favourite. ( Well he should know - after all he did get the job!)



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