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Constraining a countersunk screw into a countersunk hole?

Forum|Forum|13 years ago
April 29, 2012
10 replies
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I have not been formally taught how to use PTC software so have learnt everything by intuition, playing, a few guides and the help of you on here.

However having said all of that, I am somewhat confused on the method of correctly assembling a counter sunk screw into a countersunk hole. What makes sense to me is to:

1) Make the cylindrical part of the screw centered on the cylindrical part of the hole, or use axis alignment if there is no cylindrical part (material too thin)

2) Make the conical surface of the hole coincident with the conical surface of the screw.

Now to me there is nowhere else that screw can be than fuly mated in the hole. However Creo is saying that the screw is only partially constrained, how can that be?

Help!

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General

Best answer by ColinWriggleswo

I've done it!!!!

Constraint1 = COINCIDENT conical surface of screw head to conical surface of hole.

Constraint2 = TANGENT conical surface of screw head to conical surface of hole.

Screw is not fully constrained (it can be dragged out of the hole or rotated) but by default appears in the right place and will move up and down depending on the geometry of the hole.

No need to create extra datums.

M

MikeFitch

1-Visitor

Can it still spin in the hole? In Wildfire (I haven't yet upgraded), if you check 'allow assumptions' it should show as fully constrained. Alternatively, you could also orient a datum through the axis to one of your assembly datums...

C

ColinWriggleswo

Author

1-Visitor

The is no allow assumptions in Creo like there is in WF.

K

KshetrabasiMaha

1-Visitor

Hi Colin

There is still allow assumption in CREO

Check the image below

K

Kevin

1-Visitor

I can't say for sure in Creo since I'm not using it but Wilfire 5 what I've noticed is when you select the conical surfaces it creates an insert constraint. However, it appears that what it does is aligns the axes and because of this you can flip the direction of the constraint and the part will flip its direction which is the reason it is only partially constrained. You have essentially created the same constraint. You will need to select the mate constraint from the list first and then select the conical surfaces. I believe it's possible to constrain a countersunk screw with a single mate constraint with the allow assumptions checked.

K

KshetrabasiMaha

1-Visitor

Hi Colin

if you are working in CREO then you should check for remaining degees of freedom while assembling.(check the image below)

1.Whether you select cylindrical part to cylindrical part or conical part to conical part ;CREO will consider it to be a co-axial constrain. Even after these 2 constrains you will see one degree of freedom(because it will not consider conical surfaces as mate or aline constrain)

2. you must give another constrain to limit the remaining degree of freedom(a distance constrain(mate and aline constrain in wf) between the top flat surface of the conical part and the flat surface of the counter part will do)

Regards

K.Mahanta

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ColinWriggleswo

Author

1-Visitor

I think I understand what you are saying but my problem still is that I can't dimensionally constrain the top surface of the screw to the top surface of the sheet metal part containing the countersunk hole because that is the dimension I am trying to establish.

When the head of the screw sits in the countersunk hole I need the geometry to establish if it is proud, flush or sub-flush to the surface of the sheet metal part when the conical part of the screw head is sitting tight against the connical part of the hole. If the size of the counter sunk hole in the sheet metal part is changed then the screw head needs to automatically move up and down relative the the top surface.

I think I need a bit more play time.

K

KrisR

1-Visitor

Colin,

A distance constraint that Kshetrebasi is speaking of can be a coincedent constraint between the top surface of the part and the top surface of the countersunk screw.

What Jonathon says below is correct: you'll want to do either an axis constraint or an insert constraint first.

K

KrisR

1-Visitor

In order to fully constrain it, you can create a geometry point on both the diameter edge of the screw and diameter edge of the hole, and constrain those two points.

J

JonathanHodgson

13-Aquamarine

I've used Point On Surface to constrain awkward, conical geometry before. It's usually more reliable to specify the type of constraint before selecting references; then find a vertex at the minor diameter of the hole countersink, and use the conical surface of the fastener head.

You'd want an axis or cylindrical Insert as the first constraint.

A

AriesChen

1-Visitor

countersunk screw:

Create Datum Axis (A_2): Through the two vertices on the edge of the countersunk

assembly component

a. Align: countersunk screw: A_1 + countersunk hole: hole axis

b. Pnt On Line: countersunk screw: A_2 + countersunk hole: Vertex in the chamfer edge

Patriot_1776

22-Sapphire II

If I remember correctly, you can simply use that ertice or line and constrain "point on surface" or "line on surface" without having to create an axis.

A

AriesChen

1-Visitor

"point on surface" Did not calculate the surface extension.

Pay attention to the regeneration.

G

GregFrankland

1-Visitor

Colin,

Does the "centered" constraint give you what you are looking for. I tested it quickly and it seems to regenerate as you are wanting, and it only took the one constraint to place it with the references being the conical surface of the screw, and the conical surface of the countersink.

-Greg

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ColinWriggleswo

Author

1-Visitor

Greg, Centered on it's own doesn't mate the screw correctly for me, but I have now found the solution which works for me.

S

SylvainA.

1-Visitor

If it could help other users, as Kevin said, a "Mate" constraint works.

I make it this way since Wildfire 3 (I'm now using WF5).

You just have to select "Mate" constraint type first, then select the two conical surfaces (screw+hole).

Obviously, "Allow assumptions" have to be checked.

P

ptc-1201103

1-Visitor

I'm working in CREO 2.0 where mate is no longer an option, and coincident doesn't fully constrain the assembly. In WF 5, one could constrain a countersunk screw to a countersunk hole by aligning the cylinder with hole then mate the two countersink surfaces. CREO 2.0 doesn't work the same...no mate.

This doesn't make complete sense to me, but it works and fully constrains the assembly.

coincident cylinders
tangent countersink surfaces
tangent countersink surfaces...again
parallel drive surface...for clocking

P

Pieface

1-Visitor

After numerous attempts I have figured out a way that works for flat head (countersunk) screws in Creo 3.

1. Select the surface on the screw conical, select the vertex of the receiving hole:

Flat Head Screw Constraint 1.JPG

2. Select the 2 axis and make them coincident:

Flat Head Screw Constraint 2.JPG

3. A third and final constraint is required. You can use a Datum of the Screw and a Datum of the Assembly for Parallelism or you can use the constraint “Angular Offset”:

Flat Head Screw Constraint 3.JPG Flat Head Screw Fully Constrained.JPG