Clamp from the bottom, contour the outside

How To Workholding
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I’m looking to make some simple circular coasters, and I’m wanting to hold them from the bottom so I can machine the top and around the full outer contour. I have no problem machining a small feature on the bottom (e.g. a circular pocket) to enable the work holding.

As an added bonus (but not essential), I’d also like some registration feature so I can align the coaster repeatably each time. Maybe two metal pins in a known orientation.

Has anyone had any success with a work holding setup like this?

I know a custom vacuum fixture would work, or a chuck pulling from the inside out. There’s also specialised clamps for this kinda thing (bore clamps/expansion clamps) but they tend to be expensive.

Anyone done something similar or have any lessons to share? Even better, pictures!

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The typical way would be to clamp outside the circle. Cut any detail on the top of the coaster, then cut out the profile with tabs. Break or cut the tabs, then sand the outside edge.

You could add a registration mark. Usually only done if flipping the part to cut the other side.

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Then cut a jig using the same vector cuttint a pocket for the coaster to fit into. Sand the registration mark off when done.

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Yeah this is kinda what I’ve been doing, exactly like you say:

  1. Clamp the rectangular stock, engrave the top, and then contour round the circle, leaving tabs.
  2. Cut it out using a multi tool
  3. Flip the part, clamp it at the circumference of the circle and cut around the contour to get rid of the tabs (but not to the full depth of the contour as that would hit the clamps).

Maybe this is the best option and I like your idea of the locating bump, I’d not thought of that.

My main issue with the above is that there is always a mark left in the wood where the tabs are so I’d be keen to leave a little radial stock and then do a finishing pass around the whole depth of the circle. Maybe I just need to make a vacuum fixture! (sidequest begins)

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Maybe quicker to just clean up the tabs with a trim router and a tiny bit with a bearing on the bottom.

This is the one I use:

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That’s a reasonable approach although I’m ideologically against manual labour when I have a CNC. It does the work, I watch it - this is the way :smiley:

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When you’re holding a hammer, everything looks like a nail! :wink:

CNC is just another tool in the shop. Use it accordingly :smiley:

To be fair, I’ve done things on the machine that would be much quicker by hand. But more as an academic endeavor.

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I understand that desire, but I can do it by hand faster than I can change the bit on the CNC.

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I have using masking tape and CA glue more and more for holding down stock. I have been pleasantly surprised just how strong this hold down technique is plus it is fast to set up. I sometimes clamp down a piece of scrap wood down onto the work surface first. Then I slap on wide masking tape on both surfaces, spray CA accelerator on one surface and CA glue on the other surface and set them together. I can start cutting with a few minutes. The nuisance is often just pulling them apart afterwards because the joint is so strong. I have learned to use less CA glue.

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I’m a fan of 2 sided woodworking tape to hold down my smaller projects.

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I use a orbital sander with 240 grit on tabs. It takes a very, very light touch so as not to go too deep but it works great.

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