Using biscuit joiner to cut miter slots

What would cause loose joints from a biscuit cutter and how do you make them stronger?

I have a biscuit jointer that cuts slots for 0, 10, and #20 biscuits. I upgraded to the newer unit because I had a smaller unit that cut slots where the biscuit would literally flop around in the slot, making a good joint impossible (unless I did something wrong). Anyway, with this new jointer, the biscuits still flop around, only not as much. But a tight fit there isn't. When I watch videos the biscuits fit tightly, with no flop. So my question is this: Are there consistency standards in the biscuit manufacturing industry, or are the machines that cut the slots using different size cutters? I am annoyed when I purchase a joiner like this and can't get a good outcome. Or am I doing something wrong? Please help. - Tom M.

Rob Johnstone: While there is certainly such a thing as too loose in the slot, biscuits should slide easily into the opening. The biscuits themselves are compressed and expand when they come in contact with white or yellow glue, thus fitting more tightly after glue-up than before. I don't think you are doing anything wrong as long as you are not trying to "free-hand" the cuts without supporting the biscuit joiner on a surface of some sort. It is true that the high quality biscuit jointers (for example, a Lamello) cut more accurately and have more exacting tolerances. It is possible your cutter is loose or bent, thus making a slot that is too wide for a typical biscuit, but I sure couldn't know that without looking at it. I hope this helps.

Tim Inman: Have you tried a different batch of biscuits? This sounds too simple, but it may not be the tool, but rather the insert biscuits that are undersized If you get a sloppy fit from more than one source of biscuits, then the tool - or the tool user - need to be questioned. My biscuit joiner leaves me with a nice, snug fit - with multiple sources of biscuits. If I need a looser fit, I can manipulate the tool and make the groove sloppy. So from that, I can say the operator does have control over the fit. Maybe you're making multiple cuts on the same groove? Be sure the guide surfaces on the cutter are flat and seated before you plunge the cut.

Chris Marshall: The upside to biscuit jointers is that you can cut slots lickety-split. But speed can lead to sloppy cuts. As with the other answers here, I can't be sure if it's your machine or a bad batch of biscuits that could be to blame. However, make sure your technique is solid, and you can eliminate one variable from the equation. The tool's contact surfaces need to be planted firmly against the edge of your workpiece, and they can't move at all when plunging the cutter in. Jiggling the tool even the slightest bit will widen the slot more than what you want. Work slowly and carefully and see if that helps you achieve better results.