Flat faces on a cylinder that tapers?


Hi guys, I’m new here and a complete beginner so thanks in advance for your patience and any help you might be able to impart.

So I’m attempting to model a solid part which will eventually be machined on a lathe and mill. It’s fairly small in size (35mm at its largest diameter, and around 100mm long roughly) so firstly I’m not overly sure if I should be modelling a scaled up version of it, if that will help with the face geometry at all?
It’s a pretty simple shape - it has a cylindrical middle section which then conically tapers down at each end to a smaller diameter - around ⅔ the diameter of the original middle section. That part seems pretty straightforward - draw a 2D skyline of sorts and then use the follow me tool around one of the end faces to create the 3D shape.

The problem I’m having is that I then want to make four equal, flat, longitudinal, indexed faces along the length of the shape, which will bisect and subsequently “remove” some the cylindrical AND tapered aspects. Like if I dropped a square-ended rectangular cuboid over the part and “shaved off” what was hanging outside of the cuboid’s faces.

Kind of like if this was a real part the process would be to turn the part on a lathe to make the cylinder with tapered ends, then transfer it over to a mill and machine the flat faces onto it. So the cylindrical middle section would now become a rectangular cuboid with four rounded corners (that are leftover from the original cylinder), but those flat faces would also extend out into the tapered parts also, making them partially flat too.

I haven’t the foggiest idea of how to go about this process. I suspect it might involve perhaps drawing out the rectangular cuboid separately and then moving it into the part so that it intersects with it somehow but at that point I get lost and confused because none of the geometry lines/“faces” of the cylindrical part ever meet up with any of the faces of the rectangular cuboid. So some sort of subtractive process doesn’t seem to want to work. And there doesn’t seem to be any video tutorials I’ve been able to find so far that deal with this in any way.

Apologies for not having any photos but at this point I’m not very far along myself - I’m sitting looking at a cylinder with tapered extremities and have yet to get any further. Accuracy of sizing is currently not critical as the shape itself may change dimensions eventually, it’s more the concept of adding the flat faces I somehow need to get to grips with.

Any help would be greatly appreciated.

Thanks again.



Hi Sam, In general, you can make the turned shape and then intersect the “cuboid” shape you’re thinking of with the turning. Erase the waste afterward. I get from what you are describing that you would basically be turning such that the major diameter is slightly larger than the cross section of the material. Is that right?

Something like this?

I would work at a larger scale. I’d start at the size you want and use the Dave Method.


Something like this?



Hi guys, thank you SO much for your speedy replies!

Yes, simoncbevans that’s exactly what I mean!

I’ve already tried making the model at a much larger scale and intersecting with a cuboid of the relevant size to subtract the waste but for some reason the two sets of geometry just don’t seem to be intersecting so when I try to select the portion of the shape I want to delete, the lines still extend inside of the shape and deleting them creates a gaping hole in the shape.


Did you also use the Dave method for this?


Sorry I didn’t mean to brush over your comment in favour of simon’s, thank you for your reply, your model is almost the same as the one I am attempting except the centre portion of mine needs to begin as a long parallel-sided cylinder, and then taper down from a set point as simon’s example does. But that is neither here nor there, I would imagine the process of the addition is the same regardless. When I place the cuboid over my original shape, is there something I should be doing to intersect the two “officially”? Like is there a menu option to intersect faces?


Yes, if you right click you will see the strangely named context menu item,‘Intersect faces’.


I did it longhand but it didn’t take long. If I were doing it again, I would adopt a slightly different method (as below).

Start by making the cylindrical item and group it. Then you need to make the “cutter”. To do that, I would start with a square the right size to cut the faces you want on the cylinder. Then use the offset tool to make another square larger than the first. Erase the centre face leaving you with the two squares and a surface between. Extrude that upwards as far as you need and make the whole thing another group. Then place the cutter over the cylinder so that its inner face cuts off what you want. Nothing will actually be cut at this stage, you will just have two groups that intersect. But now you can use Solid Tools to cut the cylinder with the square block.

I think Dave’s Method relates to small objects. SU isn’t good at handling small items. What he does is to model at a much larger size and then scale it down to the size you actually want.


Not exactly. The “Dave Method” involves scaling up without need to scale back down (thereby avoiding a source of possible error). His clever method involves:

  1. Put the relevant small geometry into a component
  2. Copy that component to make instance number 2
  3. Scale UP that second instance by 100X or 1000X (I use 100X)
  4. Edit the large instance
  5. Delete the large instance

Because both the original small and the temporary large copy (aka instance) are associated with the same component definition, an edit made to one is automatically reflected in the other (regardless of whatever unique external transformation may have been applied to any of the instances, such as scaling up).


I followed your explanation almost all the way to the very end and it sounds like an absolutely brilliant, out-of-the-box (pun intended) idea of doing what I need!
Unfortunately you lost me when you mentioned “using Solid Tools” to finish the process, as I have no idea what that means! My apologies, I’m an absolute novice so my experience with SU extends as far as the introduction the software gives you when you first install it, and what I’ve gleaned from YouTubing what I think might help me!
I literally had never modelled before until I came up with the idea for this part I’m trying to develop and looked online for something free that might enable me to make my own 3D model in order to discuss the idea with potential manucturers and hopefully keep the development costs down.


Hi Box, thanks for answering my question! I’m a complete beginner so didn’t realise I had to do that. Well at least I’ve got something to be getting on with now!


Also might I ask out of curiosity, what your original “longhand” method was of drawing the part first time around?

Thanks for all your help!


Solid Tools can be found here:

What I meant by longhand is that I didn’t use the Solid Tools method I described. I created the cylindrical element without grouping. Then I created a simple six-sided box over the cylinder, used Intersect with Model, and then deleted unwanted items one by one. That works too but it is more likely that something unwanted will be erased or that something unwanted is left behind.

Solid Tools are so useful that it is worth getting familiar with them. But I do have a word of caution. It is very easy to create a “solid” that is not “true”, especially as the solid gets more complex. Luckily, there is a handy plugin called Solid Inspector that will tell you if a solid is true and will often fix it for you if it isn’t. That saves me an enormous amount of time.


I suspect as @SamTheFarrier 's profile says he is using Sketchup Free (web) that he doesn’t have access to solid tools.


Right. I’m not up to speed on all the various versions of SU. But maybe this could tempt him to invest in Pro?


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