Test Bar -- A Straight Edge Alternative

[dusty]

I have not put my Test Bar to the test against other supposedly straight objects -- other than the jointer bed. Tonight I will look for some objects that should be straight (perhaps my 4 foot level or the extruded aluminum edge guide I bought some time ago) and I will test them against my home made Test Bar and report back on my findings.

I totally agree that what is sufficient precision/accuracy will differ for different folks and different projects/applications. .003 is overkill for framing a house. Probably not good enough for cutting an octagonal frame. In between these two applications, there's lots of projects where .003 is fine.

Please make sure you understood what I wrote. " I have chosen .003" as the theoretical maximum error for all of my tool adjustments/alignments".

When I start cuttings framing members, etc - a new set of rules come into play. I measure with a tape measure, mark with a carpenters pencil and cut with a chop saw. Probably the BEST I can hope for is 1/8" repeatability.

When I progress to finish molding and trim work another set of tools come into play and a new set of target standards (aided by sand paper and wood filler).

I helped build a house once. We started with a concrete slab that had been poured by a previous property owner who was unable to complete his project (for reasons unknown to me). The fellow I was helping (actually two brothers) had the idea that they were going to do a bang up job on the framing and the way they were going to achieve this was to use all precut materials. They established an arrangement to have studs cut to one length, still plates, headers, etc cut to their prescribed length (all determined from a set of prints. This was suppose to result in dead on walls, perfectly level ceilings, perfectly square door/window frames, etc, etc.

I learned that this does not work. Good theory - no good in practice. Square and plumb happens in real time and has only a little to do with the length of the boards all being the same. Trim and shim became commonly used verbs.

Thankfully, we passed nearly all of our inspections and when the building was finished we were only six weeks behind schedule.

[loueg]

As a toolmaker - I am familiar with the concept. This concept is used to scrape a flat surface - using three flat surfaces , with bluing- and rubbing each against the other two -you keep removing the "high"spots until the entire plate is touching. I appreciate the info you have given --- now I know an inexpensive way to compare a straight edge. Thanks.

While reading an article in Fine Wood Working on jointer maintenance http://www.finewoodworking.com/ToolGuid ... px?id=2643, I came across a concept I hadn't previously heard of and that I think might be of use to others.

The author wanted to ensure the tables on the jointer were not sagging at the ends, which would have required shimming.

Rather than buy an expensive straightedge with a guaranteed degree of straightness (guaranteed, that is, until you drop it the first time ), he built what he called a "Test Bar" out of three long pieces of MDF (need only be roughly straight) and 9 dry wall screws.

Since everyone isn't going to be able to read the FWW article here's the build:

Rip the MDF into three strips roughly 5 inches wide and cut to whatever length you need to check. My jointer tables beds are about 48 long total, so that's how long mine were.

Drill three holes in one long edge of each board. Put one hole near each end and one off of center. No particular measurement for placement of the holes is important]if necessary[/I] and if you have to make an adjustment, repeat the process. I had to adjust A's center screw a total of two times.

According to the author, now the heads of all three screws in the Test Bar (A) lie along the same exact plane.

Put the test bar on a surface. If all three screw heads touch the surface, the points where the screw heads touch are all in the same plane.

The Test Bar was great for confirming my jointer tables were co-planar and didn't droop (couldn't get a feeler gauge under the screws at either end or near the middle (one reason you don't put the third screw in the middle if you intend to use this to verify co-planar jointer tables is that a perfectly centered screw would probably be over the jointer head!).

For the Shopsmith I could see using the Test Bar to check and adjust the infeed table of the jointer (if you made the Test Bar to the proper length), or to check and adjust the the heights of the main table and any auxillilary tables and also to ensure your main and auxilliary tables were in the same plane (not tilted). Of course, if you have a 510 or 520, I supposed the table connecting tubes doe this for you. However, I suspect I will find other uses for having a long Test Bar which can always be recalibrated if it gets out of whack.

The whole procedure from ripping (I used 3/4 baltic birch), cutting to length, drilling, and adjusting the screws took less than an hour. The cost was nil because I had the drywall screws and a wide enough, long enough cut off of baltic birch to get all three pieces.

Hope this hasn't been too confusing and will be helpful to others. Seems like a much better alternative than buying an expensive precision 48 inch straight edge for this particular use.

Al

[algale]

As a toolmaker - I am familiar with the concept. This concept is used to scrape a flat surface - using three flat surfaces , with bluing- and rubbing each against the other two -you keep removing the "high"spots until the entire plate is touching. I appreciate the info you have given --- now I know an inexpensive way to compare a straight edge. Thanks.

loueg,

Funny you should mentioned the toolmaker connection. The article in FWW actually credits the idea as being adapted from the techniques toolmakers use.

Al