Oval, ellipse or something else?

[Perry]

I have been asked if I could make up a brass trigger guard to replace a plastic one on an air rifle. Easy peazy. Nope. lol

The opening for the trigger area is causing me to think. (more then I'm used to, lol)

Google has failed to give me an easy answer of a formula for this shape. It appears that each quarter of the shape is made of three different radius. This site

https://www.twinkl.com.mx/teaching-wiki/ellipse-shape got me to realize that this not a true ellipse. Is there a name for this shape that might refine my searches?

A rectangle with rounded corners is close, but the longer side of the rectangle should have a slight arc to it.



Cheers, P.

 

[YYCHM]

Are you going to CNC the replacement?

 

[Perry]

Are you going to CNC the replacement?

Yes, that is the plan. He is happy with the rectangle with the rounded corner approach. I'm just more curious about the shape. lol

This youtube video is interesting and shows how to draw ovals. I like this kind of stuff. This shape just caught my attention. There must be an easy way, because you do see it used a fair bit.

 

[YYCHM]

It's an oval but it's not an ellipse If you took the ellipse as pictured in the video and drew a line across the top of the two opposing circles you would have an oval that's close to the shape you're trying to achieve.

 

[PeterT]

There's probably a few different ways to make a shape like using primary geometry (circles, arcs, lines). Here is an example showing minimum number of dimensions which fully define the shape. The top & bottom arcs are tangent to the left/right circles. Tweaking the relative size alters the aspect ratio.

If you want to nerd out in CAD, Splines offer a lot of interesting shapes opportunities that can be defined by points, but the points can have properties like tangency, loose or complete intersection, relative vector weight of the spline elements. Goes on & on. Example spline pic where I honor majority of the circle/arc driven geometric points but deviate with the lower left one & adjust the weight. This version could be for the 'fat finger' model LOL

 

[Perry]

It's an oval but it's not an ellipse If you took the ellipse as pictured in the video and drew a line across the top of the two opposing circles you would have an oval that's close to the shape you're trying to achieve.

You have a rectangle with a radius in each corner. The length of the radius is 1/2 the width of the rectangle.

 

[YYCHM]

You have a rectangle with a radius in each corner. The length of the radius is 1/2 the width of the rectangle.

No, you would have a truncated flat topped ellipse.

 

[Perry]

There's probably a few different ways to make a shape like using primary geometry (circles, arcs, lines). Here is an example showing minimum number of dimensions which fully define the shape. The top & bottom arcs are tangent to the left/right circles. Tweaking the relative size alters the aspect ratio.

If you want to nerd out in CAD, Splines offer a lot of interesting shapes opportunities that can be defined by points, but the points can have properties like tangency, loose or complete intersection, relative vector weight of the spline elements. Goes on & on. Example spline pic where I honor majority of the circle/arc driven geometric points but deviate with the lower left one & adjust the weight. This version could be for the 'fat finger' model LOL

Peter, Adjusting the dimensions will still give you a oval I believe. An oval has and arc on each side. Basically 4 arcs.


For lack of a better name I'm calling my shape a race track. The race track has a long curved straight away, corner and short curved straight away. (This can be mirrored horizontally and vertically to complete the full shape.

I'm sure I could do this with the splines, but my curious mind has me thinking. This is a shape you see in many places. It can't be that hard. One place that comes to mind is the fuel access panels on aircraft.


Fat finger model. lol

 

[Former Member]

If you can draw it manually freehand, scan it as a pdf and import it into CAD dimension it and save it as a dxf and let the CAM software do its stuff.

 

[DPittman]

You better make 2 or 3 while your at it, someone else will want one when the see it.

 

[Chip Maker]

Can you not digitize it on your mill?

 

[Bluebird1337]

Hi, in F360: Insert -> Canvas
Select the picture

Right click on Canvas -> Calibrate to size

New sketch using the canvas as the pattern. Play with Arc, fillet and the tangent constraint.

 

[StevSmar]

New sketch using the canvas as the pattern. Play with Arc, fillet and the tangent constraint.

I’ve also had good success with offsetting, which might be helpful for the top inner part.

@Perry ,once you trim the inner part you’ll break the offset and you’ll then be able to draw the bottom inner part and connect it with tangent constraints. It also looks like your part has symmetry? If that’s true then I’d only draw half of it and then mirror it for the completion of the sketch.

 

[RobinHood]

Could it be called an “ellipsoid” or an “oblate spheroid”?

 

[historicalarms]

Without CNC machinery, If I were tackling that trigger guard & the customer insisted on close to the original design, I would still start with two identical radi at each end with a flat section in the middle then press bend the bows in both top & bottom to match original. leave one side wide when cutting the piece from stock so it can be still cut flat where it mounts in the gun stock after making the bend.

 

[PeterT]

Peter, Adjusting the dimensions will still give you a oval I believe. An oval has and arc on each side. Basically 4 arcs.

That could be its name, I'm not really sure. I just took a guess at how to construct something similar by eyeballing the shape. Yes 4 arcs, 2 symmetrical in either axis, 4 tangency points (orange ticks). Just erase the middle construction stuff & you're left with the perimeter shape. Yes, adjusting the dimensions yields a different size and aspect ratio, that's what I was getting at. For example if you adjust the dimensions of a square (defined by one single dimension), its still an equal sided form (same shape). But for this form you have control over side arcs & vertical arc radii which changes the aspect ratio, kind of like an ellipse, but not exactly the same. For this shape there are limits to the relative dimensions before tangency cannot be met.

 

[Perry]

You guys are right. I use Solidworks and there are many ways to approach this. I can use the spline tool or insert a photo in the background and trace it out.
Many ways to the end result. It's cold outside and my mind was stuck on how this shape is created. (Pre CAD, in the old school days.)

No clear answer yet on the "how" it was done.

I ended up on using the rectangle with the rounded corners for two reasons.

The flat side of the trigger guard makes it easier to line up the piece in the vise to do the final machining on the base.

The gent that wants it likes this aftermarket version of it.


Just sitting down to finish the CAD on it. This is where I stopped last night.




Thank you for the replies. I'm sure I will get distracted later and fall into that black hole of the internet looking for an answer.

 

[thestelster]

Too funny, I had to make a trigger guard for a customer a couple days ago. I have no CAD/CAM. I just traced the old one on a piece of aluminium and went at it.

 

[Perry]

Yours looks good. Mine is still virtual. I'm not getting paid for this, so it will get worked on in my free moments.

(I should probably spend less time searching google on the different types of ovals. )

 

[thestelster]

I wish I had the time to learn that stuff. I think its so amazing. There are a lot of shooters' rifles out there with broken trigger guards that aren't available anymore who are willing to pay hefty prices for replacements. Keep at it, it looks great.