Finish Furniture and anyone who likes tight fits

I am building a bunch of fancy furniture lol, its out of plywood but it still takes the time. I am working on tolerances and trying to get prefinished ply (birch to fit snuggly.) I am onto the 1/8" Compression bit and ran some work tonight but still not happy with my rule of thumb. I heard you just add .01 inch to the female inserts, but it seems like vertical panels into horizontal need more. Just want to start a conversation and see if anyone has advice on how to get as close as possible joints with ply and how you offset your line work to ensure tight fits?


James NM

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One thing that could help is a test board of slots with different tolerances. That’s probably the most straightforward way.


I like that idea!



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On my old machine I had a diy vacuum hold down system. I used it more to hold sheet material flat when cutting speaker cabinets. I would set my files up not just to mill the slots but also the end that would be inserted into the slot to a specific thickness. Generally the thickness of your material is going to vary enough that a one size fits all file will not give you super tight tolerance. This was the only solution that I could come up with that gave me consistent joints without having to measure and adjust my file each time. The MDF I was using was around 0.76 at the time. The files were set up to be zeroed from the table surface to 0.74. Slots would be milled about 5-7 thou over. Basically perfect every time with just a little room for glue. This only works if you have some sort of vacuum hold down, which can be done super cheap if you are just trying to hold material flat. Lighthouse Brand Vacuum Motors

On the left side click the link for Shopbot motors and I think there are links to their forum where Brady Watson did a ton of work figuring out DIY vacuum hold down solutions. To just hold sheets flat an old shop vac can work well. You just want a little 1/8" or so hole on the inlet side for leakage to help cool the motor.

Cut a slot with your bit, and measure it with calipers. I bet it isn’t 1/8", and probably a little bit undersized. There are several ways to handle this, but the easiest is just to put the measured diameter (ie: the width of the slot) as your tool diameter. This may help a lot with precision. I find that some fairly new bits are a good .003-.004" off. Some of it has to do with feeds & speeds.

Once you have done that, cut test pieces and modify tolerances until it fits. You can start out with doing this, but if you switch to a new bit you may find out that things don’t work again, so doing the first step will help you a lot.

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