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internal grooves for o rings

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larry phelan 101/06/2020 18:12:41
719 forum posts
14 photos

Good evening all,

I am looking for advice regarding the cutting of internal grooves for o rings. The groove in question needs to be 3mm wide, so I made a cutter or tip 3mm wide and rounded the end. Fixed this to a shaft, something like a boring bar and proceeded to cut the groove, having first bored out the workpiece.

The problem is that while the cutter makes a start on the job, cutting fairly well, it soon starts to rub, very similar to the way a parting tool behaves at times..

I tried several different speeds, which made little or no difference. Checked that the tip was set at centre ht and that the projection fron the tool post was not over long, in order to avoid flexing or springing. All to no avail.

I have no doubt I have missed out on something and someone out there knows what !

Any ideas ?

old mart01/06/2020 18:18:04
1756 forum posts
138 photos

Grooves for o rings rarely have a radius, they are normally square. You should post the full dimensions of your bore. O ring manufacturers have suitable groove sizes on their websites.

bernard towers01/06/2020 18:20:05
15 forum posts
43 photos

Not sure what use you are putting your o rings to but I personally have not ever seen an o ring groove with a rounded profile, all the ones I have seen are square bottomed to allow it to deform when fitted and usually they are wider than the ring dia. Not sure if this will help but food for thought

larry phelan 101/06/2020 18:27:14
719 forum posts
14 photos

For my first attempt, I did use a square shaped tip but it did not cut very well, that,s why I thought it should be rounded. What I really want to know is why the tool rubs rather than cuts. As I said, very similar to the way parting tools do at times. Is my speed wrong. The bore is 40mm, and I might add that this exercise is just to learn how to do the job, not for any real job.

pgk pgk01/06/2020 18:37:28
1791 forum posts
288 photos

I've not tried that job but the problem with most form tools is that the load goes up as the tool shapes the space. first thought is the rigidity of the boring bar and how far in the bore you need to get to to minimise stick-out of your tool and keeping the tool tip as short as possible. You can also try cutting the groove with a narrower tool as two bites and then traverse the bottom.


Martin Connelly01/06/2020 19:02:17
1369 forum posts
159 photos

Have you got relief for the sides as well as below the cutting edge. The cutting edge needs to be the widest part of the tool going into the groove. HSS tool with steel at 40mm diameter wants a speed of about 190-200rpm to avoid overheating the cutting edge and blunting it. Are you using any cutting fluid?

Martin C

Ady101/06/2020 19:23:40
3688 forum posts
514 photos

It's a bit of a pain but maybe a pointy 3mm first, then a round 3mm after would reduce stresses


larry phelan 101/06/2020 19:41:30
719 forum posts
14 photos

It,s not so much the size of the groove, more the difficulty of cutting it. When the tool begins to rub instead of cutting, you know there,s something wrong.

No 0ne mentioned speeds, but since I tried many different speeds with no effect, I presume speed is not a factor

I know that form tools are hard on machines, as PGK points out [found this before ] and I did think about using a narrower tip, again very much like the problems when using a wide parting or grooving tool on outside work.

Not sure what bearing the bore size would have as long as the tool had plenty of room to operate.

This was more a case of "How to" before it became a case of "Need to ", It,s nice to know how to do these jobs in advance.

There is a lot more to learn here, but in the meantime, my thanks to all who offered help.

Martin Connelly01/06/2020 19:56:43
1369 forum posts
159 photos

I mentioned speeds. Bore is relevant as the bigger the bore the bigger the boring bar you can use. Bigger is always better. A 25mm diameter bar would good a 10mm diameter bar not so good. PGK was wondering what the boring bar stick out was as shorter is always better than longer, 15mm stick out good 150mm stick out bad.

Martin C

not done it yet01/06/2020 20:02:15
4645 forum posts
16 photos

Speed will not alter rubbing - it will only rub faster or slower.

Try raising the cutter height a tad (at a time). That may help.

Bill Dawes01/06/2020 20:09:33
322 forum posts

Hi Larry, do you have the right front clearance on the tool, remember on that internal turning the curve of the material is going into the tool rather than away from it as on external turning. do you have any side clearance?

Bill D

Neil Lickfold01/06/2020 20:15:36
613 forum posts
102 photos

With internal O ring grooves, I use a tool with 0.4mm corner radius and is 1.5mm thin. On the last pass, move it along to make a finish pass. A 3mm groove I take in 3 passes and leave 0.1mm for the final clean up on along the diameter. Having a vfd and finding the best rpm is the easiest way. Having the tool slightly above centre I found is needed for internal grooving, around 0.1mm usually does the trick. At centreline, as the tool drops, it is actually taking a deeper cut. Above centre line the tool as it drops, takes a lesser cut, unless it goes too far and then gets deeper clearly.

larry phelan 101/06/2020 20:26:57
719 forum posts
14 photos


My thanks for your input, it arrived just after I had shut down.. Yes I was using cutting fluid, but I take your point regarding side clearance.. The overhang from the toolpost was kept as short as I could, I know the danger there, but perhaps I could go for a bigger dia bar, plenty of room for one.

All food for thought and all welcome. One thing did occur to me, does the same situation not arise when cutting grooves for circlips?

Plenty more for me to experiment with !

Again, my thanks to everyone.

old mart01/06/2020 22:02:59
1756 forum posts
138 photos

I have recently bought a couple of internal grooving tools with 16mm shanks, MGIVR 2016-2 and MGIVL 2016-2 (l & r handed) which take the common double ended MGMN 200 inserts. They are 2mm wide, 1.5mm and 3mm are also available, but you have to buy the matching holders for each width.

I would use the narrower 2mm to produce the 3mm groove by alternate multiple overlapping cuts. Tool height needs to be about 0.002", 0.050mm above centre, and minimum possible tool overhang. Keep blowing the swarf away as you cut.

Edited By old mart on 01/06/2020 22:04:31

Mike Poole01/06/2020 22:30:18
2575 forum posts
60 photos

If the tool is rubbing the the presentation of the tool needs looking at or the clearance needs to be increased. The clearance angle is a compromise between enough clearance to cut freely and not so much it weakens the cutting edge. You could blue up the tool and see where it is rubbing and grind the high spot away, rinse and repeat until the tool cuts cleanly. Boring tools need a surprising amount of front clearance as the bores get smaller, its curious that you start clean cutting and then rub, this may mean the side clearance is the problem, check that both sides have clearance and the tool is at the correct angle. The O ring groove is usually square and the pressure you are trying to contain is what makes the O ring work as it gets behind the O ring and squeezes it to seal against the shaft and the wall of the groove.


larry phelan 102/06/2020 09:10:41
719 forum posts
14 photos

Good morning everyone,

Such a lot of new information and all helpful. Yes, it seems that clearance is the big problem and setting on centre height may not be the best idea either [so much to learn ! ]

To Old Mart, where did you buy those grooving tools ? I never knew such things were available.

Neil, I think you may have put your finger on it alright ! Perhaps you have "Been there ,done that, and ruined a teeshirt ?"

There is plenty of food for thought here and I will attack the task again, a lot wiser this time around.

Thank you one and all for your time and patience answering my silly questions.

Martin Connelly02/06/2020 09:52:45
1369 forum posts
159 photos

Internal grooving insert. This is an example but there are other suppliers and types available.


This is my 1mm circlip grooving tool.



Martin C

Edited By Martin Connelly on 02/06/2020 10:06:08

larry phelan 102/06/2020 10:10:50
719 forum posts
14 photos

Hi Old Mart,

Just been checking out those holders on Ali Express. They sell them as boring bars, but of course suitable for grooving as well.

Surprise, surprise, the cost of postage is more than the cost of the item itself !

Came across this a few times before. At least I now know what I,m talking about and looking at.

Martin Connelly02/06/2020 10:14:59
1369 forum posts
159 photos

This is a close up of the tip of my circlip tool showing the side relief behind the cutting edge.


Martin C

larry phelan 102/06/2020 13:09:28
719 forum posts
14 photos

Thank you for that Martin, I have just been checking out those grooving tools, never knew there were so many types available, well worth investigating. I can see now that my attempts were crude to say the least.

Plenty of that stuff around on Ali express but getting anything from China right now is a hit and miss affaire except for a virus, of course.. I have three items "ON THE WAY" one of which is "In the country", but which country ?.

Just a case of wait and see. Still it,s good to know what is available.

Take care.

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