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"Homemade" digital bevel gauge

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Forum topic by JNP posted 961 days ago 2747 views 2 times favorited 15 replies Add to Favorites Watch
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JNP

105 posts in 1181 days


961 days ago

Topic tags/keywords: bevel gauge angle finder miter gauge

Hello All,
Thought I’d post this idea that I had for a bevel gauge. I was online going to purchase the Wixey when I thought I check to see if “there is an app for that”. Well, there is…the one I downloaded is free and called “Smart Protractor” (no affiliation).

It has really worked great and I see no downside to using this over a store-bought. Any thoughts or cautions, please let me know.
You can zero the setting at any level. I just attached to a jointed board with double sided tape and zeroed it on the saw top.

Squared up that blade and held to block flush to the blade to get a true 90…I need to change my stops a bit.

Roll it to whatever angle you are looking for and you’re ready to cut.

Also, please let me know if this post should be somewhere else.

-- Jeff


15 replies so far

View BobAtl's profile

BobAtl

49 posts in 1297 days


#1 posted 961 days ago

One question: The teeth on my TS blade protrude just a bit from the side of the blade so that they are not in the same plane as the face of the blade. When you position the edge of your squared block against the blade and teeth, as for your 70 degree angle above, won’t that result in an error since the edge of the teeth and the face of the blade are not in the same plane? I use an IGage and am very careful to keep the magnets attached on the face of the blade and not touching the teeth for that reason. That’s after zeroing it on the saw table, of course.

I may be wrong so this is as much a question for my information as a comment. Thanks for posting as this opens up many possibilities for using technology in the shop!

-- Bob, Atlanta

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JNP

105 posts in 1181 days


#2 posted 961 days ago

Bob, the board that I jointed is thin enough to fit between the teeth of the saw so all of the contact is against the flat part of the blade.

-- Jeff

View Alan S's profile

Alan S

172 posts in 1921 days


#3 posted 961 days ago

Looks like a great idea! It looks to me like he’s got his board narrow enough to fit between the saw teeth. Awesome!
Alan

View BobAtl's profile

BobAtl

49 posts in 1297 days


#4 posted 961 days ago

Ahh… looks like a winner, then!

-- Bob, Atlanta

View lorna's profile

lorna

15 posts in 997 days


#5 posted 961 days ago

Class, now all i have to do is buy a smart phone to go with my piece of plywood! Great idea though

View Bill White's profile

Bill White

3352 posts in 2564 days


#6 posted 961 days ago

And I’ll bet that the batteries won’t go crapper like those on the other gauges.
Good tip.
Bill

-- bill@magraphics.us

View JNP's profile

JNP

105 posts in 1181 days


#7 posted 961 days ago

:-) Yes Lorna, it might be time to move up to a smartphone…the gauges are $40 + batteries (compliments of Bill) and the phones are free…you just have to sign away your life to get one! As a bonus, I’ll send you the plywood

-- Jeff

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JNP

105 posts in 1181 days


#8 posted 960 days ago

CR1, I’ll research a bit but I’m pretty confident that there are sensors in the phone that operate the level. I know there are accelerometers and magnetometers. There is no way a gps could read 10ths of degrees of angle. It seems to be a more accurate way then holding a combination square to a blade.

-- Jeff

View Alan S's profile

Alan S

172 posts in 1921 days


#9 posted 960 days ago

This certainly is not using GPS. Smartphones all have accelerometers built into them now. That’s what these apps use. The accelerometer measures the direction of the pull of gravity. So the accuracy will depend less on the application you use and more on the quality of the accelerometer within your own phone. Since you are setting a zero angle baseline, you are doing a relative measurement, and relative measurements can be fairly accurate.

The biggest source of error I see is the jig rocking toward or away from you since the plywood is fairly thin. Maybe adding another piece of wood to make a T would eliminate that possibility. You’ve just got to still allow the jig to fit between the saw teeth.

Alan

View CharlieM1958's profile

CharlieM1958

15670 posts in 2822 days


#10 posted 960 days ago

One caution: Don’t poke yourself in the eye with the mounting board when answering your phone. :-)

-- Charlie M. "Woodworking - patience = firewood"

View DS's profile

DS

2131 posts in 1024 days


#11 posted 960 days ago

Another word of caution: ALL accellerometers are inherently NON-LINEAR!
Even though, by design, they attempt to be as linear as possible, there is ALWAYS an error.

An application can attempt to correct this in two different ways. It can offset and scale the transducer readings for a best-fit curve to minimize the error along its entire range, or, it can characterize the transducer with a digital lookup table. The latter requires a precision reference table and gauges, which is not likely to be the case with your smart phone.

(Setting the phone at zero and setting it there, then setting it at another known angle creates offset and scale.)

The question is, how linear IS the smart phone transducer and will it be good enough for what you are trying to do? If it is linear to within a degree at 45 degrees, I would be very surprised.

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS251

View JNP's profile

JNP

105 posts in 1181 days


#12 posted 960 days ago

Emailed the developer who said the accuracy is affected by the environment and the quality of the accelerometer. My phone is the HTC Incredible which has a good sensor (according to him). For the environment, the magnetic field is the primary concern. If the values are within 40-60uT, it is perfect. Given my phone and a magnetic field of 40-60uT accuracy will be within 1-2 degrees. You would expect the magnetic fields in a shop to be much higher than 60uT.

Now I truly have no idea how this stuff works so I did the simple man’s accuracy check…I zeroed the device, tilted the blade to 45 degrees, checked that w/a combination square to where I was comfortable that it was a good 45 and checked w/the phone and got a reading of 45.1. That’s pretty good in my book.

DS251, my question to you is how would the wixey gauge differ? Would they use different transducers and accelerometers or be based on a completely different concept?

-- Jeff

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JNP

105 posts in 1181 days


#13 posted 960 days ago

CR1, you can calibrate the accelerometer to an acceptable maximum and if you (your phone) exceeds that it can make an emergency call for you. The instructions were something like, place the phone on a flat surface and slide back and forth as hard as possible. That sets the max…then you program in the numbers you want the phone to call…not my phone (that I know of) but it’s out there :-)

-- Jeff

View DS's profile

DS

2131 posts in 1024 days


#14 posted 960 days ago

Typical settings for offset and scale are at Zero and 45 degrees. But if the transducer’s linearity is 1 degree per 45, the reading at 22.5 degrees could be 21.5 to 23.5 degrees, even if 45 degrees is spot on.

While I am not familiar with the specs for the Wixey Guage, a device like that is designed for a single purpose and the transducer is typically 0.1 to 0.5 degrees per 45, depending on the manufacturer. Another factor in acuracy is called ‘repeatability’. Depending on the technology of the transducer, this can be 0.1 to 0.2 degrees over the entire range.

The transducer in a phone was probably envisioned to shuffle your playlist, flip your viewscreen, or play a video game and not so much for the purpose of precision measurements, so the linearity and repeatability requirements are probably much more lax than a more expensive, more precision, dedicated device.

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS251

View LotsofChips's profile

LotsofChips

6 posts in 960 days


#15 posted 960 days ago

I agree with DS251 and wouldn’t trust the accelerometer in a smart phone to be accurate enough. They are cheap chips meant to flip a screen when rotated not accurately set a saw blade.
- Mike

-- - Mike

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