Milling taper with AlibreCAM

[jcdammeyer]

I can use 3D Horizontal finishing to create a tapered hole. But the outside perimeter ends up being horizontal no matter what I try. I must be missing something but what? The attached photo shows how the tool path goes down in a purely vertical fashion while the pat is tapered.

I've added the part. It's a simple 1/2" plate to be cut out of MDF. The inner hole has the taper. The perimeter doesn't. If I try any of the 3D operations it just mills out the hole even though I've selected the outer segments.

What's the trick?
Thanks
John

 

[Nick952]

If I suppress or delete all the holes I get an "Unknown Internal Error" pop up message, when trying to generate the outer profile using 3D Horizontal Finishing.
Might be worth trying to redraw the model sketches and try again. If there is still a problem, contact MecSoft and they should help, even if you are not on maintenance with them.

 

[jcdammeyer]

I suppose someone from AlibreCAD should then investigate why you get an unknown internal error. I see that once in a while too. I save my work often. I'll try it here. Maybe the file was corrupted while uploading to this forum.
Thanks for trying.
John

 

[Nick952]

Hi John,

I've had a chance after my shift to have a play and even on a simple part I got the same error.
However after sketching a rectangular containment/drive region (larger than the cutter dia) on the base, the 3D Horizontal Finishing generated correctly.
Looks like it may be a bug in AlibreCAM, so might be worth reporting it to them.



Nick.

 

[KMoffett]

Did you Scale the part to create a taper?

 

[Nick952]

Not on my part, I just used the Draft Angle function in the Extrude, as did John in his design. I believe the Scaling he did, is to allow for shrinkage in the final casting.

Nick

 

[jcdammeyer]

I've asked a friend to import the drawing as a step file and generate the G-Code using Fusion-360. I'll report back what the results are.

I'll supress the scaling and see what happens but I agree that this appears to be a MecSoft issue. I've reported it to them. No answer back but then it's also been over 8 months since I reported that a feed rate on fourth axis should remain constant regardless of the part diameter and it's the rotational rate that should change as the diameter of the part changes. They said it's on their list.

 

[Nick952]

Just tried your part, using the same method as on my test part and with a .25" cutter got this:-
Edit:- Just checked and MecSoft have replied in the user forum.

 

[jcdammeyer]

So my guess is that, although I think that the box stock should serve as a adequate boundary for this, it in fact doesn't. And rather than report that it can't it blows up instead. I'll cut up another piece of 1/2" MDF and give it a try.
Thanks.
John

 

[KMoffett]

Just tried to make your base plate. My Z zero in the top. I used the base as one sketch, the large hole as a second sketch, and the screw holes as third sketch. Used chamfer to create the taper. Guessed on the dimensions. In AlibreCAM I Profiled the large hole to remove most of the material. Then Horizontal Finishing for the taper. Sectioned to show the taper.

 

[jcdammeyer]

When constrained, the horizontal finishing profiles the holes without needing to removed the middle. Extruding the module with draft lets one set the the ideal angle for the pattern so it pulls from the green sand correctly.
Ideally I'd set 3 degees of draft and then use a single pass 2D profile to remove the steps from the horizontal finishing. Using a ball end mill (router bit in this case) also makes the steps less visible if I didn't (which I don't) have the tapered too bit. I've been thinking of getting the tapered bit to try.
https://www.the-carbide-end-mill-st...ed-end-mill-3-tps-htc-96716-3fl-uncoated.html

 

[KMoffett]

Yup. I replaced the Chamfer with Draft and it worked fine. Is this for a casting pattern? I wondered because of the scaling.
Ken

 

[jcdammeyer]

Yup. I generally do the finished part as machined first. Then make a copy of that, scale it and in some cases change dimensions. For example scaling made the center hole larger so I had to reduce it in size so that once cast and machined the raised edge of the AC servo motor will register in the hole.

And yes I could have bought a piece of 1/2" aluminium and just milled it but where's the fun in that. And I think the price of the plate was 1.5 x one bottle of scotch.

 

[jcdammeyer]

Primed with 3 degrees draft and ready for paint before casting.