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Turning (approximating) a Domed Surface

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Howard Lewis28/09/2021 11:33:54
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50 mm radius on a 25 mm bar sounds like a job for a radius turning tool

Either a special just for this job or ma more sophisticated one for other, later, jobs.

At a pinch, maybe a boring head mounted on the toolpost, so that it can rotate about its axis, might suffice.

(Making a tool, to make a tool, to do a job! )

More ways of skinning a cat, (Not that I am suggesting making a tool for skinning cats. mFirst objection would mbe from ours! )

Howard

JasonB28/09/2021 12:12:18
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It's not on the end of a 25mm bar, look at the images posted.

Being quite a thin item if held in a chuck most ball turning tools will foul the chuck

Howard Lewis28/09/2021 13:25:32
5298 forum posts
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Misread the bit about 25 mm diameter.

Approaching the problem from another point, make up a concave form tool, run slow and feed very gently along the lathe axis?

If you really fancy your chances grind from a 1/2" HSS toolbit!

Put some gauge plate, off set in the 4 jaw and carefully turn a concave to make the form tool. Harden / temper as you see fit. Having no clearance the tool would need to be mounted at angle to provide a clearance angle..

For brass, with care, and only one or two to make, you might even get away as is.

Howard

Dr_GMJN28/09/2021 14:20:42
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I didn't buy castings for these end caps, I got some cast iron bar, and will make them from that.

I'm going to go for an arc, not an ellipse, because I think the apex looks more defined like that (rather than a bit flat if I use an ellipse).

I'm also going to make the apex of the dome project about 1 mm from the bolt face rather than having it flush (as per instructions), again to make the form a bit more defined. If it looks wrong I'll just try again with different settings.

JasonB28/09/2021 14:28:50
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If it's of use this is the profile I drew for the Victoria, radius and protruding

vic cyl cover.jpg

Dr_GMJN28/09/2021 22:11:25
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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.

JasonB29/09/2021 07:07:43
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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_GMJN29/09/2021 13:26:16
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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?

JasonB29/09/2021 15:03:52
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Looks like you may just have enough clearance

cover dcmt.jpg

Dr_GMJN29/09/2021 17:01:21
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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.

JasonB29/09/2021 17:28:47
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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_GMJN04/10/2021 22:17:49
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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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