Using digital RPM tachometer for DOALL bandsaw blade speed.

I'm planning on mounting a digital tach on my DoAll 16" bandsaw that I am renovating. The old Doall gearbox driven speedometer never worked and it too costly to repair.

I was thinking of using this unit Amazon.com: Digital Speed Meter, AC 110V 220V 4Bit Digital LED Tachometer RPM Speed Meter High Precision with Proximity Switch Sensor: Computers & Accessories

If I mount the hall sensor/magnet on the bandsaw wheel, I would get final drive RPM. I could then use a spreadsheet chart to convert RPM to surface speed with Surface Feet Per Minute. SFM = .262 x D x RPM.

It seems to be a inexpensive way to verify saw blade speed. Any better ways to do this?

tonystoolroom said:

I'm planning on mounting a digital tach on my DoAll 16" bandsaw that I am renovating. The old Doall gearbox driven speedometer never worked and it too costly to repair.

I was thinking of using this unit Amazon.com: Digital Speed Meter, AC 110V 220V 4Bit Digital LED Tachometer RPM Speed Meter High Precision with Proximity Switch Sensor: Computers & Accessories

If I mount the hall sensor/magnet on the bandsaw wheel, I would get final drive RPM. I could then use a spreadsheet chart to convert RPM to surface speed with Surface Feet Per Minute. SFM = .262 x D x RPM.

It seems to be a inexpensive way to verify saw blade speed. Any better ways to do this?

Click to expand...

This sounds good. You might want to mount the sensor INSIDE OF THE BANDSAW,away from chips and dust. This might not be possible. I don't remember if the final shaft that the lower wheel is mounted on is exposed. Edwin Dirnbeck

Thanks Edwin. I was thinking of mounting the sensor at the upper bandsaw wheel as it stays pretty clean up there. The lower wheel is where all the chips & swarf accumulate.

Put it in the back cabinet on the drive pulley for the lower wheel. On edit- a 16" wheel is pretty close to 4' (4.18' to be more precise) per revolution, so if you put 4 flags on the wheel, your readout would be almost directly in FPM. Great idea though!

Dan, great idea- adding 4 magnets to the wheel would provide a direct FPM readout with negligible error at the speed i'm usually running the saw at. Very cool. No more ruined blades from running the saw too fast !!

tonystoolroom said:

I'm planning on mounting a digital tach on my DoAll 16" bandsaw that I am renovating. The old Doall gearbox driven speedometer never worked and it too costly to repair.

I was thinking of using this unit Amazon.com: Digital Speed Meter, AC 110V 220V 4Bit Digital LED Tachometer RPM Speed Meter High Precision with Proximity Switch Sensor: Computers & Accessories

If I mount the hall sensor/magnet on the bandsaw wheel, I would get final drive RPM. I could then use a spreadsheet chart to convert RPM to surface speed with Surface Feet Per Minute. SFM = .262 x D x RPM.

It seems to be a inexpensive way to verify saw blade speed. Any better ways to do this?

Click to expand...

I have a used tach out of a 16" DoAll ML saw that still works if your interested?

Kevin

Here's a thought.

SFM = 0.262 X D X RPM

Any RPM reader normally shows the RPM, using ONE pulse per revolution.

Refining the above equation, using a 16" wheel diameter in your band saw I get:

SFM = 0.262 X 16 X RPM
SFM = 4.188 X RPM

OR, with an error of less than 5%

SFM = 4 X RPM

So, if you put FOUR magnets (or whatever is needed by the sensor) on the 16" wheel, instead of just one, then your RPM meter will directly read SFM with an error of less than 5%. In my book, that is close enough for most purposes.

If you really need a more accurate number, which I doubt, your actual SFMs would be 4.7% higher than that reading and you could come very close by multiplying by 1.05. This could be done in your head by taking 10% of the reading, dividing that by 2, and adding it to the number displayed. Example: Meter reads 490. 10% of that is 49 and half of that is 24.5. Rounded that is 25. 490 + 25 = 515.

But the displayed number should be accurate enough for 99.9% of all shop situations.

You may need to evenly space the magnets, depending on the internal workings of the RPM meter.

How's that for a better way?

PS: The magic number here for the wheel size would be 15.27". 15.27 x 0.262 = 4.00074. Other wheel sizes which would work would be 11.45" with three magnets, 7.63" with two magnets, and 19.1" with five magnets.

tonystoolroom said:

I'm planning on mounting a digital tach on my DoAll 16" bandsaw that I am renovating. The old Doall gearbox driven speedometer never worked and it too costly to repair.

I was thinking of using this unit Amazon.com: Digital Speed Meter, AC 110V 220V 4Bit Digital LED Tachometer RPM Speed Meter High Precision with Proximity Switch Sensor: Computers & Accessories

If I mount the hall sensor/magnet on the bandsaw wheel, I would get final drive RPM. I could then use a spreadsheet chart to convert RPM to surface speed with Surface Feet Per Minute. SFM = .262 x D x RPM.

It seems to be a inexpensive way to verify saw blade speed. Any better ways to do this?

Click to expand...

I like post #4 myself, but that's just me.

tonystoolroom said:

Thanks Edwin. I was thinking of mounting the sensor at the upper bandsaw wheel as it stays pretty clean up there. The lower wheel is where all the chips & swarf accumulate.

Click to expand...

Yes, that would be better.But, look out if you do a lot of hi speed sawing the blade can break and get jumbled up in your wires. If I go thru the trouble to re-engineering something, I want to never fool with it again. But surprising things allways happen anyway. Edwin Dirnbeck

EPA, all the math explained in a nice manner for him is great, but that is an awfully long post with an awfully glaring error. The formula of [(C. S. x 4) ÷ Dia.] is not correct for an accurate number - that's only for ballpark calculations in your head. The actual formula for correct numbers is [(C. S. × 12) ÷ (π × Dia.)] or shortened [(C. S. × 3.82) ÷ Diameter]. If he's going to bother to set the magnets at a diameter other than the O.D., might as well get them in the correct place. It's worth noting that the O.P. had it right in his first post also: (1 ÷ 0.262 = 3.82).

Edit: And I just discovered that I misread your post and that you did set it up right at the end. That's what I get for skimming. Oops, sorry.

Sounds like a lot of BS to me. There are available parts. Keep it original. It will maintain its value much better. All that screwing around to solve a non-existent issue is a waste of money and time.

If he's got a vfd running the saw it might make sense. If it's on a 4 pulley setup or something like that then maybe you're right. But it's his saw, he can do what he likes with it. Personally I would look into the mechanical tach offered above first.

Edit: just realized this is a vertical bandsaw, doh! So it has a constantly variable speed. An electronic tach might be nice, but a lot less hassle to grab the one offered above.

There are tach units that can be programmed for the math and will read directly in SFM, or any other units you like. The only modern unit I have used is a Red Lion on my South Bend lathe, using a magnetic pick off on the large back gear. I assume you don't want anything with vacuum tubes and Nixie display. I am in the process of equipping a cylindrical grinder with digital readouts on both the wheel head and work head. These are voltmeters reading signals from VFDs which most have built in now, a simple approach. My Sheldon lathe has a DC generator read by a digital voltmeter. There are photo pickups that read a mark painted on any shaft. The point is that there are a lot of ways to skin the proverbial cat.

I would look at Red Lion first. Pricey, but industrial grade equipment.

Bill

Kevin, that is really generous of you. I would take you up on that offer but my speedometer has 3 range rings for low/medium/high gearbox positions so not sure if your speedo would indicate proper speed. What do you think?

Thanks for the suggestions Bill !

No VFD onboard.. its a 220 single phase 5hp WEG 1800rpm motor that I just installed and swapped out an old 3phase.

Steve, the speed indicator doesn't work at all. It's 70 years old at this point. I got quoted $400 to repair it from a speedo shop. Doesn't seem like that's a good value to me. I need to know the speed of the blade. If I can spend $20 and get a cool digital readout its not waste of money or time. What is a waste is knocking teeth off an expensive blade from overspeeding on tough metal.

This is a big saw with hydraulically operated table/feeds. It can go from real slow to high speed spark shooting friction sawing on HSS. It can run file bands too, which I have. I need a speed indicator.

eKretz,

This is a digital tachometer and it is counting 1 revolution per pulse. One rev is one rev regardless of the magnets position on a radial line on the blade drivewheel. So the magnets position radially is not relevant as long as they are equally spaced circumferentially.(In our case we would use 4 magnets so the tach will count 4 pulses for every one rev.) The bandsaw blade is running on the 16" diameter and that's the surface speed we are calculating. So each rev of the wheel will indicate 4 pulses or 4 feet. The tach will calculate based on how much time elapsed between pulses.

Yes there are probably digital tachs where the pulse count is adjustable, and then I could use one magnet. Like a VDO car speedometer. But they are pricey , and magnets are cheap. And i'm not losing sleep over a marginal FPM error - like the original speed indicator was ever that accurate.

I sent the original tach for my Doall ML off to a speedometer shop. They went completely through it, cleaned the face, gave me a new needle and replaced the glass. I may have gotten a new cable at the same time as well. With shipping, the whole affair was right around $100. The cable is a pretty standard mechanical speedometer cable.

Personally, the original blade-speed tach is the only thing that will ever look right on an older Doall saw. Its a vertical bandsaw... I can’t imagine needing better precision than what the original tach offers. It’s not a production machine, you don’t need to “optimize” anything, you need to not tear the teeth off of your new blade immediately. Easiest done by starting slow and working into it.

The world is full of opinions, above is mine and I’ll finish by recommending to not bother with the air pump, during your renovation. It provides little in the way of pressure or volume. Depending on what you have left from the original plumbing, literally ANY other air source will work infinitely better. I’ve got the original Doall mount and nozzle (as best I know) and would be glad to share pictures if you’d like. I’ve also shared the original Doall belt placard from my ML elsewhere on this site.

Doall has always made good saws, especially the vertical ones and in my opinion they found a great blend of form and function on their older machines. Doall’s original system worked as well as any you should need and even if it needs replaced in entirety, I feel doing so would be the best use of your energy, overall.




Be safe and stay healthy




Jeremy

If I could get my original speedo fixed for $100 it would be a no brainer but as I posted ^^^ the speedo shop wanted $400 and that's costly for a 70 year old speedo that could break again next year.

Yeah the air pump chugs away under the motor cabinet,driven by a small vbelt off the driveshaft. Seems to blow enough for its purpose but in 15 years using this machine i never used the air to blow chips away, just one of those nice gadgets.