Work holding ideas for a granite bed router

[kb58]

I built a granite bed router, partly as a challenge, and partly to not build another due to the first not being sufficient. With it done now (as done as any home-built machine ever is), and with basic checkouts nearly complete, there's the issue of work holding. The router will be used to machine wood, plastic, and aluminum. There are many YT videos on hold down solutions, but they all seem to focus on spoil boards in machines used solely for woodworking.

The current thinking is to start out by boring four holes in the corners of the granite bed, just outside the work space so the cutter can't contact the bolts. Epoxy in threaded inserts (or studs), then bolt down either an MDF or aluminum tool plate. Either can have threaded holes place wherever, but this is where I'm confused: the granite (a used surface plate) is extremely flat, yet bolting down very non-flat MDF or aluminum sheet seems counter productive. The purpose of the granite base was to have the mass and stiffness for a better surface finish and to lessen chatter, but don't know how well that's succeeded yet, but I find it ironic how an ultra-flat surface isn't necessarily benefit. Yes, future additions may include mounting holes for one or two machinist vices, so for that it would be a benefit, but otherwise, surfacing a spoil board seems like it offsets the same benefit as having an uber-flat surface below.

Sorry for the muddled thoughts; this is mostly a stream of consciousness post, but I suspect some of you have similar machines and have figured out a good solution. Please share so that I don't waste time going down badly-considered rabbit holes.

[suntravel]

You will need more than 4 holes in the granite table to mount a waste board or an aluminum T-Slot table....

Here is an example:



Uwe

[kb58]

Hah, so hundreds of holes...

[suntravel]

to mount a plate 100x100mm will be ok...

[EastMemphis]

I'm curious what you're planning on machining on that granite bed? If one of your end mills comes in contact with that material, it's toast. So you'll need a spoil board if for no other reason, to protect your expensive end mills. Most jobs require a cut through to achieve good surface quality on the backside of the work, so you'll need to have a spoil board for that too.

I've had a CNC for more than ten years and use a two sheet birch plywood base. It's plenty sturdy and is fit for the machine's tolerances. So unless you have some sort of miracle machine that's rigid as a diamond, I doubt a granite bed will do anything but make life difficult.

[suntravel]

Possible reason for a granite router:



Uwe

[kb58]

Thank you, Uwe, that's exactly why. The surface plate was bought used, so wasn't much more expensive than making the base via other means.

Sorry if my confusing post was.... confusing. I never intended to not use spoil boards and fixture plates of some sort. What I'm facing right now is figuring out if I must drill all the holes in the granite myself, or have the router do it. The more I think about it, the more the latter seems like a bad idea, mostly for tool feed reasons.

[suntravel]

Drilling granite will not need much torque, I do this with 2000rpm and diamond bits. The key is flood cooling with plain water.

Uwe

[kb58]

Since I have yet to run a CNC router at all, it's rather intimidated (read: fear of crashing and/or snapping tools).

My concern about boring the holes with the router (9.5 mm diamond core drills) is feed speed, knowing "how hard to push?" Is it as simple as listening to the sound of the spindle?

On a related matter, after installing the threaded granite inserts... then what? Is an MDF intermediate plate most appropriate, then an aluminum fixture plate on top of that? I'm speaking "in general" and realize that special jobs may require specific setups.

Thanks in advance.

[suntravel]

No Risk No Fun

2000rpm 10mm/min 1.5mm peck drill should be ok to start. Depends on the quality of the drill bit and on good flushing

No MDF, directly mount a fixture plate 20mm flat milled AW5083 or cast iron

If you use a vise, directly mount on granite

Uwe