Statics Help Needed - Fixture Design

[KevinSSC]

Good afternoon all,

I have been fighting with a statics problem and am hoping someone can help me figure it out. The problem is shown in the attached picture.

I am trying to determine the length of the base plate required to prevent the entire assembly from tipping over due to the cantilever of the milling head assembly. This dimension is listed as X in the attached picture. I have shown the center of mass for the trunnion and milling head assembly along with their weights. This entire assembly will sit on a table and will not be bolted down.

http://files.engineering.com/getfil...f-a61d-d352d861259a&file=Fixture_assembly.jpg

Please help me find the X dimension required and how you arrived at your solution. I have tried doing a force and moment analysis about the centroid of the trunnion without success.

Thanks for any help you can provide.

 

[MikeHalloran]

Just build it like you drew it.

If you made the baseplate just barely long enough to prevent tipping, that would not prevent collisions between the cutters and the next trunnion in line.



Mike Halloran
Pembroke Pines, FL, USA

 

[KevinSSC]

There isn't another trunnion in line. This is just a fixture to check runout on the cutting head inserts.

I just calculated the length of the baseplate needed and I came up with 6.6" to reach equilibrium. The total length from the back edge of the trunnion to the front of the milling head is ~14.5". My calculation of 6.6" seems a bit short. Any thoughts?

 

[MikeHalloran]

Your calculation goes to hell when a technician rests an arm on the cutter shank and the unit tips over.

To what does one attach the indicator base?

How does one clamp the fixture to the table if desired?

If the fixture slides on the (oily) table, what hits the adjacent fixture, or wall, first?

Your calculation sounds credible, but the point I'm making is that the fixture base should not be sized on the basis of 'just barely not tipping over'.




Mike Halloran
Pembroke Pines, FL, USA

 

[KevinSSC]

Mike,

Valid point. I wanted to find the equilibrium point and from there add a safety factor due to the elements you've described. One reason I wanted to do the calculation was simply to practice something I learned in college and haven't used in awhile. Referring to textbooks was essentially worthless for the purpose of finding a comparable problem.

We have dial indicators on their own base. These will be placed on a granite block.

The fixture will have mounting holes in the base plate if the shop would like to attach the assembly to a table.

Thanks for your input!

 

[hydtools]

As long as the right hand edge is beyond the cg of the cutter it will not tip over if no other load is applied beyond the cutter cg. As long as the right hand edge is at or beyond the face of the cutter, it will not tip over.

Let Lcg be the distance between cg's. Let a be the distance of the trunnion cg from the balance point between the cg's.
16.66*a = 30.87*(Lcg-a), and neglecting the weight of the plate.
Solve for a in terms of Lcg.
Make your plate at least long enough to be beyond the balance point between the cg's.

Ted

 

[KevinSSC]

Thanks a lot, Ted. Once I finished my calculation and input the information correctly, my result was to place the end of the plate at the same location as the CG of the milling head assembly. I think I did something wrong since your solution makes sense. Hmm. I wonder where I screwed up.


I ended up increasing the length so the end of the place is planar with the end of the cutter head. Thanks for the feedback!

 

[hydtools]

You are welcome.

Ted

 

[ornerynorsk]

Practical experience, if you mount the indicator holder on the fixture base, no granite plate is need, and you don't have to worry about the fixture moving when rotational force is applied to the spindle to check runout.

Ditto what Mike said about a shop technician tipping the thing over if it gets leaned on. It WILL happen, spend the extra $5 on material and carry the base out past the cutter :>)

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.