Turning (approximating) a Domed Surface

[JasonBModerator @jasonb]

If it's of use this is the profile I drew for the Victoria, radius and protruding

[Dr_GMJNParticipant @dr_gmjn]

Thanks Jason, I drew something out on CAD earlier, I think it will be OK. Just have to work out the co-ordinates. I think 25 increments of 0.020” on the cross slide should be good, using a 0.8 tip radius tool. Might need back-gear for this one:











Couple of questions:



1) The minimum section ends up at about 2.6mm. I say this should be OK to take the cylinder pressure. The original minimum is about 3mm, but I’d expect a large safety factor within that?



2) I want to keep distance x to 7.5mm. Reason is that the front covers are that, and that dimension keeps the bolt PCD in the middle of the faced area. The angle of the tool tip means I can’t touch on the O/D and wind inwards by a set amount before going into the end. And I want that profile generated by the tip going in to form the edge of the dome. Any suggestions? Maybe start at the centre and work outwards – but that means determining the exact centre using the tip I guess.

[JasonBModerator @jasonb]

The 2.6mm will be OK and still likely to be more than the valve chest cover with it's recessed surface

Make sure your DCMT insert won't rub it's edge to start with. 

You will probably have to start from the outside and work inwards as the tool will rub on the smaller diameters if working outwards.

I'd be inclined to rotate the toolpost further so that the edge of the tip nearest you is parallel of just beyond the axis of the lathe, you can then hold something against the edge of the cover and touch the tip against that as you wind the cross slide towards you and as the OD is known you can move 7.5mm in to the first cut

Edited By JasonB on 29/09/2021 07:35:29

[Dr_GMJNParticipant @dr_gmjn]

Thanks Jason. I might turn the tip as you say, and might even draw out a starting geometry that I can rough machine out, to make sure the edges don’t rub. I guess you can see where the cutting edges end by looking carefully at the tip?

[JasonBModerator @jasonb]

Looks like you may just have enough clearance

[Dr_GMJNParticipant @dr_gmjn]

Ah, I misunderstood – tool rubbing on its flank, on the ‘I/d’ of the step?

Do DCMT inserts have a standard flank angle (to stop rubbing) that you’ve modelled?

Thanks.

[JasonBModerator @jasonb]

Yes, the C in dCmt signifies the angle which is 7degrees and by far the most common though others are available. This is why you see the insert boring bars tilting the insert at an angle so the side does not rub in a small bore.

[Dr_GMJNParticipant @dr_gmjn]

Just to wrap the thread up, I used the co-ordinate method. I used the side of the insert touched on the side of the blank cap, and the tip touched on the PCD face to get datums for everything:

Roughed the domes out:

Then using co-ordinates, approximated the profile (nice example of chatter here – solved by using back gear). Also I made a slight error with a CAD measurement which showed up as the wide band. Didn’t make a deal of difference in the end though:

Then using a combination of a dremel, files and abrasive paper:

Smoothed to shape:

Parted off using the home made rear post from earlier this year (worked great):

Then made a spacer ring and reversed in the chuck to turn and mill the inside details:

Took five attempts to get two good ones, mainly due to finger trouble, but eventually I got a decent result. As far as I can measure – with height gauge, callipers and more importantly, eyeballs – they are pretty much identical.

The domes are primed; they’ll be painted dark blue eventually.

Thanks everyone for the advice. Job done with your help – again.

Edited By Dr_GMJN on 04/10/2021 22:20:01

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