cutting a circular groove

[pgk pgkParticipant @pgkpgk17461]

This is to seat a thrust bearing in a 12mm hot rolled plate. bearing OD is 47mm ID 30mm and i want to seat it 7mm deep. There will be a central 10mm through hole.

Now the obvious easy way would be a rotary table on the mill but i donlt have one. the two options i see with available gear is to grind a form tool out of 1/4 HSS and plunge cut it gently in the face on lathe then cut sideways. I take it such a tool would be square ended with a nice 10-12deg relief on end and both sides?. The biggest obstacle i see there is the tool corners dulling and a regrind before finishing the base?

the other thought I had was to drill the centre hole then attach to the mill table with a t-nut, washers each side, double nut with enough slack to be able to rotate the plate and mill the groove. I'd have to find some way of clamping scrap each side of the t-nut under the t-nut groove to stop that moving.

Opinions re the best option (or a third) and which is likely to avoid chatter most, please.

[pgk pgkParticipant @pgkpgk17461]

[EmgeeParticipant @emgee]

Is the plate too big to fit on a faceplate or 4 jaw so the 10mm hole could be drilled and then use a boring bar for the bearing seat ?

Emgee

[HopperParticipant @hopper]

Like Emgee says, I'd drill the 10mm centre hole first and then take facing cuts outwards with a normal turning tool with a bit of extra clearance ground ont he bottom of it to clear the circular groove.

But if you particularly wanted to turn a groove and not a relief going all the way to the center hole, I would still use the lathe, just use a turning tool as above, plunge it in and take facing cuts to the OD of the bearing minus a few thou. Then take a finishing cut plunging the tool in at the OD and then across the face to near the OD. Then fit a righthand turning tool and do likewise to finish the ID.

Hard to explain without pics. But I would avoid plunge cutting with a flat nosed tool like you mention. It is begging for chatter on a small lathe. Not what you want for a bearing seat.

[JasonBModerator @jasonb]

As Hopper says if you have a through hole then dril that and just use a boring bar to open up the the bearing OD and depth required.

If you just want a groove then search the site for trananing as that is basically what you are doing but not cutting all the way through. Also have a look at this thread

If you do plunge then a narrower tool that can be moved back and forth in teh slot will make it easier than cutting full width and more clearance on teh side facing you so it does not catch on teh side of teh groove

Edited By JasonB on 29/03/2015 07:41:57

[pgk pgkParticipant @pgkpgk17461]

It's a groove outside the central hole diameter so opening up to diameter with a boring bar can't be done.

Jason has it right that it is a partial trepanning and his link to three concentric grooves is the right sort of concept with the tool grind he shows being the sort of approach I planned except that I've been avoiding back rake on the tool top after watching vids suggesting it's not that essential on gentle cuts and allows much more resharpening of the tool without having to chop the end off to totally reshape as one wears back on the tip.

I recognised little need for a side rake on the inner groove edge of the tool since the circular face clears but assumed that it could be beneficial when moving inwards to that edge of the groove. a 1/4 tool gives me just under a mm each side for tidying, and I've got one. the reason for thinking 1/4 in was for the increased rigidity of tool and tip for the sideways trimming. perhaps i'll try it and if it causes problems then grind it narrower?

At least i note that everyone dismisses the idea of using the plate as it's own rotary table.

I did trephine an ally plate..3mm thck only and I did that simply by using a 60 threading tool i had and plunging that in and then squaring the edge on the circle I was keeping .. but that;s way softer stuff. I do plan on using the 4-jaw.since it's a rectangular plate. I need a 19mm diameter recess on the other side of the plate aroudn the central hole and plan on boring that width..

[JasonBModerator @jasonb]

The other trick is to use the same tool that does the outside edge to do the inner edge by moving it behind the lathe ctr line and running in reverse, so a small coring bar can be used.

J

[pgk pgkParticipant @pgkpgk17461]

Posted by JasonB on 29/03/2015 09:09:18:

The other trick is to use the same tool that does the outside edge to do the inner edge by moving it behind the lathe ctr line and running in reverse, so a small coring bar can be used.

J

Nice idea!

[EmgeeParticipant @emgee]

If you have one a lathe toolpost mounted live spindle with a slot drill would do the job.

Emgee

[pgk pgkParticipant @pgkpgk17461]

It;s done. Took ages.

the smallest HSS tool I had was 3/16 but one problem was the overhang. The QHTP holders.. well when i make some more then I'll make some wider ones than standard cos I'd had to reverse two jaws on the 4-jaw and leave 2 as they were to get my groove centralised. that also meant a lot of time getting it dead centre and parallel to the chuck face and it meant that needed more overhang than i wanted to clear one jaw.

To reduce chatter it was run at 100rpm and painfully slow feeds and plenty oil. Well over an hour (perhaps 2 – I lost track) just cutting after getting it set up and I did get one dig that broke the tool and then caught it again later in the first dig and broke it again – how it goes.

Final nuisance was my last test fit when the bearing slipped from my hold and fell into the groove…I can't get it out. Even with tiny dental picks… too close a fit. And can't take it out of chuck and lose my concentricity. It's only a couple of squid so if necessary I'll destroy it to remove but I finally tapped it right in and now have to figure some protective covering for it while the (many) further operations get done on this plate – jam some rope in?

Edit: Thinking further – once the concentric operation is done this side I'll have a go at putting a couple of holes through from the back which could be useful for bearing replacement and see if i can push it out – assuming i don't wreck it with the drill. I had given the groove a clean out with some scotchbright but not really polished it.

Edited By pgk pgk on 29/03/2015 23:51:20

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