Sunday, August 2, 2009

Metric measurement in a world of US measurement system

I have never thought about the type of screws in my power tools, let alone the measurement system used the measure them.

Things changed this weekend. This weekend I was planning to mount DeWalt DW621 router on Kreg precision router table. I had to get longer screws than the ones originally mounted on router to mount the router to the insert plate of the router table. The insert plate for router is 3/8” thick. Kreg provides a video to install a router on the insert plate.

I have an inch-system micrometer which measures thickness in fraction of inches. I used this micrometer to get an approximate measurement of the screw. I thought the screw is 5/32” thick at its threads. I could not find any odd fractional machine screw at my local Lowes. I picked up a few 8-32 ¾” screws. When I came home and tried the new screws on router, they did not fit.

The next step was to do some online research on this particular screw. eReplacement has a nice picture of all the router parts for DW621

Fig # 7 in the above link is the screw type I was looking for but twice its original length. The Part Number is: 945121-00. The length is 10 millimeter.

It is a M4X0.7 screw. What is a M4X0.7 screw after all? In the metric measurement world, M is followed by the value of the nominal diameter D and the pitch P, both expressed in millimeters and separated by the multiplication sign “×”.

I need a M4x0.7 screw with 20 millimeter length. I could not find 20 mm M4X0.7 screw online but I found a 25mm long M4x0.7 screw at

Once I get this screw, I will use a hack saw to cut it down to the required length.

I also ordered a metric drill bit to drill a hole for this 4mm screw.

Next week I will be experimenting with the metric drill bit and metric screw from

Couple of interesting links on metric measurement system, I came across during my research:

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