oval thread

[riverracer]

i need to machine a true oval / eclipse thread insert to be used in a mold to allow for the part shrinking when it cools.
example size 80 x 80.5mm with 2.35mm pitch
it needs to have blind start / finish ends.
ie; position tool to center at depth-feed out to dia-cut thread-feed back to center-retract
how do i go about this in mc x3? (mill level 3, solids)
i can do a 80mm dia single point thread no problem, but i cannot figure out how to do an oval. or wont mc do it?
and no i cannot just do a circle thread and move my centers a bit each way to achieve a butchered oval.
we presently thread mill these on a lathe with a tracing attachment following a saw tooth like profile, lot of stuffing around with setting up.

any ideas? contour with ramp maybe!

thanks

[Superman]

Me thinks you would have to draw the ellipse as the OD of the shape you want

Try getting something going with a circle, then you may be able to scale up the circle in the X axis to give an ellipse

using threadmill, circlemill would create round features, so I'd stay away from these and use a 2D/3D_Contour and ramping ( but the above suggestion may work-put the endpoint not on the axis you want to stretch - rotate the circle 90° )

if these above don't work
2 options using a perp. lead in/out lines only
----2D_contour with ramping would give you conventional ( up-cutting ) movement ( ramps in the Z- direction )
----3D_Contour ( draw the actual elliptical helix from lowest point up 1 pitch then Xfrom translate 3D up ( you should get Z+ ramping , and climb milling )

I'd say you'd be using a non-standard pitch not covered by a threadmill
This then IMO would need to be done with a single point 60° vee tool ( eg internal LH threading bar and threading tips in an ER collet - check what hand bar you would need )
A single point can do a RH or LH thread ( you just need to make it climb mill M3/M4, spiralling down or spiral up )


PS---Is there a possibility that you can scale individual axes on the machine for just to do the threading
ie program it for a Ø80 thread and then apply a scale factor X1.0625 Y1. Z1. ( 80 x 1.0625 =80.5 )

[Kiwi]

I know this isn't MasterCam, just trying to help.
I have a program that will generate points around an ellipse. From these the Z points can be added. Here is the code for 20 points on a ellipse of 80.5 X 80.

G01 X40.25 Y0
G02 X37.823 Y-13.681 I-39.7596 J-0.0024
G02 X30.833 Y-25.712 I-37.4933 J13.7375
G02 X20.125 Y-34.641 I-30.7028 J25.9348
G02 X6.989 Y-39.392 I-20.0973 J35.0314
G02 X-6.989 Y-39.392 I-6.989 J39.8655
G02 X-20.125 Y-34.641 I6.9613 J39.7824
G02 X-30.833 Y-25.712 I19.9948 J34.8638
G02 X-37.823 Y-13.681 I30.5033 J25.7685
G02 X-40.25 Y0 I37.3326 J13.6786
G02 X-37.823 Y13.681 I39.7596 J0.0024
G02 X-30.833 Y25.712 I37.4933 J-13.7375
G02 X-20.125 Y34.641 I30.7028 J-25.9348
G02 X-6.989 Y39.392 I20.0973 J-35.0314
G02 X6.989 Y39.392 I6.989 J-39.8655
G02 X20.125 Y34.641 I-6.9613 J-39.7824
G02 X30.833 Y25.712 I-19.9948 J-34.8638
G02 X37.823 Y13.681 I-30.5033 J-25.7685
G02 X40.25 Y0 I-37.3326 J-13.6786

The program can be altered if this is of any use.

[cadcam]

Like Superman was saying draw the ellipse and use a thread hob to create the threads.

[riverracer]

cannot use a thread hob, as it is a special form buttress shape.
we use a single point fly cutter, with specially ground tips
also the first pitch has a 2deg taper as a lead in.
will see if i can get a pic to show. (added)
will try superman's suggestion of scaling in one direction and see how that goes.

i dont make things easy do i !

thanks all

pic 1 shows 1 half of a pair, showing stop/start in insert on same side
pic 2 shows style of fly cutter we use ie: homemade

[niggle]

Have a look at this, http://www.uwgb.edu/DutchS/MATHALGO/Ellipses.HTM in particular the 5-centres method of constructing an ellipse.

This method splits the ellipse up into seperate arcs, with being in seperate arcs you can write this in g-code in simple G2/3 codes to generate the path of the ellipse.

Now you have a path of the ellipse the tricky part is to ramp up while milling the arcs and still acheiving the correct thread pitch. The way to do this is to work out the angles of the intersecting arcs that make up the ellipse and calculate the z height of each point using the degrees of the point as a percentage of the pitch of the thread. Ie, say one of your intersecting arcs is at 120degrees; 120/360 = 0.33333% of the total ramp up = 0.783, so your line would look something like G3X_Y_Z0.738R_.

Good luck!

[niggle]

Or if your machine has an M code to enlarge the scale of an axis (I'm pretty sure our Okumas do), you could just stretch the X-Axis out by the appropriate amount - similar to what superman said.

[Kiwi]

Originally Posted by niggle This method splits the ellipse up into seperate arcs, with being in seperate arcs you can write this in g-code in simple G2/3 codes to generate the path of the ellipse.

That's how the program referred to in post #3 works.