How to tell if strait edge is strait?

tvrgeek

Scott
Corporate Member
Google did not help much. Most answers were social, not technical.

So, I have this strait edge. I do not know it's history, if it was dropped, abused, or anything. Do not know the specs.
I think it is good. Verified none of my yardsticks or levels are very strait. ( all different, so no trend)

But, is there any easy way to know?

I discovered my issue with my jointer is in fact, outfeed table slopes up a bit. Makes sense with the struggles I have had.
 

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mdbuntyn

Matt
Staff member
Corporate Member
Go see Mike anyway, but you can check a straightedge similarly to how you'd check a square.

Put it on a piece of paper, or wood, and mark a line against it with a fine-tipped pen, mechanical pencil, or marking knife. Flip the straightedge to the other side of the marked line, and align the ends of the straightedge with the ends of the line. If the marked line matches the straightedge for its entire length, it's good. If the line is covered by the straightedge, there's a belly. If there's space between the line and the straightedge, there's a hollow.
 

tvrgeek

Scott
Corporate Member
Go see Mike anyway, but you can check a straightedge similarly to how you'd check a square.

Put it on a piece of paper, or wood, and mark a line against it with a fine-tipped pen, mechanical pencil, or marking knife. Flip the straightedge to the other side of the marked line, and align the ends of the straightedge with the ends of the line. If the marked line matches the straightedge for its entire length, it's good. If the line is covered by the straightedge, there's a belly. If there's space between the line and the straightedge, there's a hollow.
Hard to do on a 4 foot, and we are talking thousands for a reference edge. Actually on a Starrett, half a thou. This is a reference for machine setup, not something to mark wood with. A yard-stick does fine for that. Same with my precision machines square. It is a reference. For layout, I use a Japanese try square, though I drool over that silly expensive Bridge City "just because". I liked my Marples, but it was not quite square enough.
 

ssmith

New User
Scott
Given the straightedge you're using, I'd second Matt's recommendation to go see Mike. To pile on though;

"Normal" straightedges used as cutting guides or to mark cut lines can easily be checked using Matt's method.

More accurate straightedges intended for machine alignment can be checked against a high-end reference like Mike's straightedge. That eliminates parallax and marking line width errors, but you will still get transfer errors between the two. This method is probably good to a few thousanths and is a great option if you have access to that type of reference.

Next - buy your own reference straightedge. That eliminates the transfer error but you need deep pockets to take advantage of it (several hundred $$$). Starrett specs Mike's at ± .0002” per foot (assuming I'm looking at the right one).

More accurate ways of confirming accuracy exist, but few of us would be crazy enough or have the means to use them.
 
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mkepke

Mark
Senior User
Easiest way (imo): call up a machinist shop and ask if they have a large reference plate they can compare your straight edge to. Prolly cost you a few $bux for the shop time.

-Mark
 

chris_goris

Chris
Senior User
Not sure exactly how, but Ill bet you can use a laser somehow. I would also say a Dietzgen would be as good quality as a Starret.
 

ssmith

New User
Scott
I discovered my issue with my jointer is in fact, outfeed table slopes up a bit. Makes sense with the struggles I have had.

A bit off-topic, but I use an older model of this same jointer (a JP06000) and have difficulty getting the jointed/planed side perfectly flat, even though the outfeed height is set correctly.

Because of that and given your statement, I checked the outfeed table alignment with infeed using a good straightedge. Same issue, it slopes up slightly. There's no gap at the end of outfeed table but about 0.006" gap right next to the cutterhead.

I'm wondering;

1. Is that amount of out-of-flatness too much for a jointer this size?
2. Is there any way to adjust it? No way I see - the alignment seems fixed by the dovetail ways machined into the body and infeed/outfeed tables.
 

Mike Davis

Mike
Corporate Member
Since the outfield is usually 'set it and leave it' I shim that side to match the infeed on the small less adjustable jointers.
 

bob vaughan

Bob Vaughan
Senior User
Dietzgen made quality drafting instruments back in the day. I suspect that's a draftsman's straightedge and should be plenty straight enough to get a 6" Asian jointer adjusted.
 

tvrgeek

Scott
Corporate Member
A bit off-topic, but I use an older model of this same jointer (a JP06000) and have difficulty getting the jointed/planed side perfectly flat, even though the outfeed height is set correctly.

Because of that and given your statement, I checked the outfeed table alignment with infeed using a good straightedge. Same issue, it slopes up slightly. There's no gap at the end of outfeed table but about 0.006" gap right next to the cutterhead.

I'm wondering;

1. Is that amount of out-of-flatness too much for a jointer this size?
2. Is there any way to adjust it? No way I see - the alignment seems fixed by the dovetail ways machined into the body and infeed/outfeed tables.
You do so by thin shims under the top edges of the dovetail. Tilt by adjustment of the gibs. Do check your gib snugness first. I was able to bring mine in with just the gibs. There are several U-tube videos on this. Be sure you get the ones for the dovetail, not parallelogram jointer. Watch several. The parallelogram you adjust the infeed. One is actually using the exact jointer, but all of them were made by Emerson and later Technitool, so they are pretty much identical. Only difference is a small change to the cutter diameter. ( as described by the great folks at mywoodcutters.com) One strip of a beer can is probably too much, but you can check. A tip is to get a cheap set of feeler gauges for shims.

As I mentioned, I got mine in tune by using a dial indicator on a beam method, again from a U-tube. I know it worked as I can run two pieces, put them face to face and they are good. I run a helical head and the cutter is .0005 higher than the outfeed. I have some short aluminum bars. When laying across and I turn the head, they don't really move, but I can hear the cutter touching. The small height is probably compensating for the wood spring back.

Besides the cutter head, I use a decent Gates belt. Really helped the vibration.

I had two goals. One to align my tool, the other is to verify my gauge bar is in fact strait as I do not know it's history. It may be fine, but it might have been dropped and damaged. Only way is to verify it.
 
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Jclrk

Jclrk
Corporate Member
Go see Mike anyway, but you can check a straightedge similarly to how you'd check a square.

Put it on a piece of paper, or wood, and mark a line against it with a fine-tipped pen, mechanical pencil, or marking knife. Flip the straightedge to the other side of the marked line, and align the ends of the straightedge with the ends of the line. If the marked line matches the straightedge for its entire length, it's good. If the line is covered by the straightedge, there's a belly. If there's space between the line and the straightedge, there's a hollow.
I learned something that's why I'm here
 

kelLOGg

Bob
Senior User
Scott, you sound serous about straightness so consider contacting NIST (National Institute of Standards and Technology. They sell standards for many things and have what you need.
 

SteveHall

Steve
Corporate Member

ssmith

New User
Scott
@tvrgeek – apologies for getting wordy in your thread. Please let me know if it'd be best to reply elsewhere in the future.

Scott, you sound serous about straightness so consider contacting NIST (National Institute of Standards and Technology. They sell standards for many things and have what you need.

@kelLOGg. It’s that obvious eh? Until I retired last year, I was responsible for the technical integrity of the measurements made by a rather large calibration lab. So yes, being interested in measurement processes is an occupational hazard. Now, I’m just trying to become a halfway competent woodworker. :) Still have lots to learn, but one thing I've noticed already is that many of the woodworkers here use surprisingly accurate tools and methods to get measurements done.

To your point, many calibration labs can calibrate a straightedge using a granite surface plate with feeler gages, a height gage, or an interferometer depending on how accurate it needs to be. Some labs also use a CMM. A quick check of a few labs in the triangle area showed it would cost $100-200 to confirm the accuracy of a straightedge - probably more than any forum user would want to spend. That’s why I didn’t suggest it above. Instead, most people seem to prefer buying a quality tool to begin with and then assume it stays "good" forever. It may or may not, but if a measurement tool became suspect, it can always be compared to someone else's instead of sending it to a calibration lab. As far as NIST goes, they can certainly check straightness but they're a rather extreme option. While you literally can’t do any better, our lab only used them for certain ultra-high accuracy items since they're extremely expensive – if memory serves, the cheapest calibration we had them do was several thousand $$$.
 

Mike Davis

Mike
Corporate Member
We have a company that comes into our weaving plant and checks calibration on a couple dozen machines, scales, and measuring instruments every 6 months. I have no idea what they charge.
 

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