Hello to all on this forum, I hope you can help a nOOb with a headache.

Could you please advise me on which wizard would be best to make a simple dome on the end of bar or bolt heads etc.
I have tried OD Fillet 2 and OD Arc for ages with all the settings I thought made sense to a manual lathe man but no luck.
I think no luck as none looked right in the toolpath window.

My goal is to start cutting at the centreline outwards to around -2mm z at the outer edge. Creating a shallow curve.
Any help in layman's terms would be great as I have only been into CNC a week.

Hi Druidan,

This can be done, I have done it, at least for a hemisphere.  I'll look at the wizards on my machine and see.  What version of Mach 3 do you have?  I know there was an issue with the wizard on an earlier version of "Turn" - I upgraded mine and it worked OK.

Hello, Thankyou for looking into it.

My Version R3.043.062

It is not a hemisphere I am after. Imagine a pizza, If I cut it in half I get a hemisphere around the edge of the crust.
What I am after is the curve you would get from the pizza being cut into segments.
I.E something shallow you could create with a file on the end of the stock.

Yes I know what you are looking for I think - I was just saying that I have used the wizard and it works - at least in the version I have (I'll check which that one is).

OK, suppose you want a dome with a radius of curvature of 50 mm, so it's a "cap" sliced off a 100 mm dia sphere.  This will be on the end of a 25mm diameter bar.  In the OD Sphere wizard you set the sphere diameter to 100 mm and the Z centre position to -50 mm.  X start is 25 and X end is 0 mm.  Now you need to set Z end to the height of the dome.  The distance of the dome centre to the centre of the bar end and to the dome edge will both be equal to its radius, but the edge will be set back from the bar centre by the height.  If the sphere radius is R and the bar radius is r, then the height h is given by:

h = R - sqrt(R^2 - r^2)

where R^2 is "R squared".

For R = 50 and r = 12.5, h works out to 1.588 mm, so you put -1.588 for Zend.

The only problem with this for me is that it cuts edge to centre, with Z moving in a positive direction.  My lathe has an ordinary Z leadscrew with significant backlash, and the cutting forces would be in the wrong direction.  The X would be OK as it has a ballscrew.  Really a better version would cut centre to edge.  Anyway here's the G code the wizard generates.

G18 G64 G80 G90 G40 G49
M3
G00 X26.2
G00 Z0.6
F30
G00 Z-0.2577
G01 X25
G01 X25
G03 X22.8511 Z0 I-12.5 K-49.7423
G01 Z0.6
G00 X26.2
G00 Z-0.5155
G01 X25
G01 X25
G03 X20.4901 Z0 I-12.5 K-49.4845
G01 Z0.6
G00 X26.2
G00 Z-0.7734
G01 X25
G01 X25
G03 X17.833 Z0 I-12.5 K-49.2266
G01 Z0.6
G00 X26.2
G00 Z-1.0314
G01 X25
G01 X25
G03 X14.7204 Z0 I-12.5 K-48.9686
G01 Z0.6
G00 X26.2
G00 Z-1.2895
G01 X25
G01 X25
G03 X10.7639 Z0 I-12.5 K-48.7105
G01 Z0.6
G00 X26.2
G00 Z-1.4844
G01 X25
G01 X25
G03 X6.3277 Z0 I-12.5 K-48.5156
G01 Z0.6
G00 X26.2
F15
G00 Z-1.5877
G01 X25
G01 X24.9999
G03 X0 Z0 I-12.5 K-48.4123
G01 Z0.6
G00 X26.2
M5
M30

Having looked at this I realised that actually for my hemisphere I used the OD Fillet wizard for the above reason, but that only cuts full quarter circle fillets so won't work for you.  Depending on your backlash you may be OK.  Oh, and I think I have the same version as you - with the original one I used the wizard reliably crashed Mach 3.

Hello, Thank you for the reply. I think I need to brush up on my maths!

I had a chance to have a 'play' today and it leaves a flat part at the edge going in about 2.5mm
Like you I have the woes of lead screws Harrison saw fit to have them from new, hmmmmm.
Ball screws are on my wishlist though.

It also cut from edge to centre. Just to rub it in there is a video of my lathe from a previous owner machining a dome a treat and I noticed he managed to get it to do it from centre out. http://www.edproject.co.uk/CNClathe.html

I am going to try and get in touch with him.

I'm afraid that some maths is needed when you're doing CNC, even when using wizards!

OK, the flat is consistent with backlash on the leadscrew.  I suspect that either the previous owner used CAM to generate the g-code or wrote it himself.  Probably the latter as the method of generating the dome looks a bit odd - I would make the tool follow the dome profile several times at reducing Z offsets, so it starts by cutting the outer edge and as the Z offset decreases eventually cuts from the centre.  Making the tool follow the right path is a simple G03 command.  Or you could even edit the above g-code.  Of course a problem with cutting centre to edge is that now backlash on the cross-slide becomes a problem, but I have a balllscrew on that!

Might not be what your looking for but it gave me a challenge on a rainy day.

Here is a rough idea of excel code I created for computing points using Johns numbers
uses tool tip radius and a couple variables so I can cut in either direction on most domes.
pretty much all of my gcode programming is done with variables.
I use this method to give me a start on creating the gcode.
I start with a cad drawing and then verify the dimensions with the calculations.
Once you know the computed variables they just get updated/created when ran for the known set Radius and part diameter variable dimensions.

I find variable programming a lot faster than posting out a program or repetitive paths, In this case id probably use a G10 in the loop and keep incrementally shifting Z0 over until part face dome cleaned up.

Cool!  Any chance you could post the Excel file please?

I use Excel a lot for generating odd curves - most recently the pallet profile for a gravity escapement and an equi-angular spiral cam for lifting the gravity arm of a Synchronome-style clock.