How much should I scale my model when creating a fixture plate to hold it for OP 2 milling. Trying to find just the right amount of clearance fit.

[longshotsystems]

Hey all,

I'm working on a new product, a steering wheel, and am designing the workholding strategy for OP 2. I think what I'd like to do is fixture the center and a few spots around the ring to a plate from the back side, which will allow me to finish the front without having to swap around work holding.

Since the wheel has a dish to it, I'd like the fixture plate to come up and meet the ring to support it, rather than flexing the ring/spokes by having it constantly pulling it down to the fixture.

I'm designing it by essentially building the fixture block of material, and then subracting my model from the block (see screenshot). I'll then carve out more material around the spokes which shouldn't need that much support so as not to over-complicate the fixture. 

My question is, how much should I scale a copy of the model before subtracting to get a good fit? Or is there a better way to create the small amount of clearance I want so the wheel ring fits nicely into the fixture. Just keep in mind in the screenshots, I haven't removed excess detail/material from the center, and haven't finished with through-holes for threading in the ring from the back. I've just subtracted the model to give you an idea of what I'm thinking. 

Cheers all!

[marcus.toepke]

I think you can use the simple moulded model.
Because the workpiece that you finish will never be 100% like the mould you subtract.

Mother nature has more decimal places than we do! *fingers up

If your workpiece is slightly smaller, it will also lie in the radius, if your workpiece is slightly larger, it will lie on the edges.

However, this will not work as your subtraction has undercuts. you will not be able to fit your workpiece into it.

[programming2C78B · Accepted solution]

program your fixture with -.002" radial and axial on the surfacing toolpath. You'll be fine. 

This assumes your ballnose makes your op 1 model properly to size. 

[longshotsystems]

Thank you. Not sure why I didn't think of this as it's a simple solution, but I appreciate it!

[longshotsystems]

Ty for the reply!