The Super FMT: Part 2 cont’d – more on the clamp plate

The weather is just about back to normal – we expect 4°F tonight, with (again!) winds around 15 mph. However, today was a good day to be in the shop; the furnace kept the shop at a toasty 65°F. I had a dovetail saw in hand, cutting dovetails for some drawers I must finish. And then it struck me: I had to resolve an issue I ran into with the Super FMT the last time I used it.

I like to check the joinery as I work, because it saves me headaches at dry-fit, and at glue-up time. So, checking each and every single joint is a hard-to-kick habit. After playing around with the Super FMT, moving clamps to and fro as I made mortises and then tenons, I found a misaligned joint:

See the light under the straight edge of the square? This means the joint is misaligned; there should be NO light under the straight edge. Take a deep breath, and start looking for the cause of the problem. My immediate thought: the table and the clamp plate were not at a perfect 90° angle to each other.

Now I must get ahead of the plan I mentioned near the bottom of an earlier thread. As I mentioned in that earlier post, I planned to cover troubleshooting in Part 5 of this series. But I now chose to ignore that part of my plan, and to give you the benefit of my findings right away. So, let us reason this out together – I will wing it:

1. I moved the clamp plate several times, to remove and insert clamps as I switched from routing mortises, to routing tenons.
2. The table moves only on one plane, in the X-axis (left-and-right) and in the Y-axis (front-to-back, and back-to-front). The table is, therefore, a good zero angle reference point.
3. Both mortise and tenon pieces are machined with the same face facing me (I mark these surfaces with chalk).
4. If the clamp plate is misaligned, and both tenon and mortise are machined with the work piece clamped flat to the plate, then the error introduced by a misaligned clamp plate (that is, with the clamp plate at an angle other than 90° ) is essentially doubled. This means that a 1° misalignment of the clamp plate relative to the table will result in a 2° misalignment of the joint. And even such a small error will be blatantly obvious.

Question: Why is the error doubled, as I mention in #4?

Answer: It is doubled because the top faces on the work pieces in the photo above are facing me while I machine them – let us assume each piece will have an error of 1°; the joint later goes together with the marks on the work pieces facing me. Thus, a 2° misalignment results; this is visible without the need for any measurement. But it is impressive when back-lit, as I have shown in the photo above — and it is a good way to for me to show you the results. Once again, the eye can tell and it can see the results of a slight joint misalignment; trust your eyes, and check joint alignment, always.

Solution: Align the clamp plate such that is at 90° to the table.

How to do this: My Wixey Digital Angle Gauge to the rescue. Simply set the clamp plate surface 90° to the table surface every time that the clamp plate is moved. First, the table is made the 0° (zero degree) reference surface:

Then set the clamp plate at a perfect 90° to the table:

After routing the tenon and the mortise, the result is a perfectly-aligned joint when the clamp plate is set to 90° to the table:

I double-checked, and my eyes were correct; I believe you can also tell that this joint is perfectly aligned, by simply looking at the photo above. Trust your eyes, but confirm it with a straight edge — always!

As I kept trying different things, I ran into the following – and when it happened, I felt serious vibration on the router itself, like a strong shudder:

A little more troubleshooting revealed that I forgot to properly tighten the table; I had moved it to align a new work piece.

Edit to add: In addition to remembering to tighten the table, I suggest you also move the table as far left (or as far right) as possible, while still allowing sufficient space to route the mortise. This allows you to move the work piece closer to the clamps; the net result is that the mortise work piece will be better supported by the clamps, and less prone to vibration. And, before I forget, always use two clamps to support the work piece.

After resetting the table, and properly tightening it, I got the following result:

I will continue work on the drawers, aiming to finish them this week. Then on to the rest of that project. And, in between, a little more work with the Super FMT. Hang in there with me!

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—— Al Navas

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