Digital tachometer for vertical bandsaw - Page 1
Forum section: electronics / projects

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: January 14, 2025, 07:04:19 pm
content: I just recently purchased a used vertical metal bandsaw but the analogue tachometer is broken so I will need to replace it with a digital one.Originally, there is a brake cable that goes from the main electric motor to the analogue tachometer that reads from 0-300 m/min of blade speed.Someone told me a rotary encoder and a display will do the job but I'm guessing there should be some kind of Arduino in there to process and calibrate the unit.That being said, I have zero experience with either and it seems there are lots of different models of rotary encoders out there and I'm not sure which one to pick.If my understanding is correct, a rotary encoder must be coupled to the brake line that rotates at the same speed as the electric motor which then sends out X amounts of pulses every second that must be interpreted by some kind of computer and then it has to be programmed and calibrated to read the actual speed of the blade in m/min.Can anyone help me with this seemingly simple project?

author: joeqsmith
author_rank: Super Contributor
timestamp: January 14, 2025, 07:20:22 pm
content: Bicycle computer? Small, cheap, adjustable and fairly accurate. Mount the sensor next to one of the drive wheels with a magnet.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: January 14, 2025, 07:27:57 pm
content: Quote from: joeqsmith on January 14, 2025, 07:20:22 pmWon't that just give me the speed in km/h or mph?I want to feed the unit power from the machine and not have to deal with a battery.Also, I want it to look like an OEM part with seamless integration so a large digital display in a 3D printed bracket is the way to go I think.

author: jpanhalt
author_rank: Super Contributor
timestamp: January 14, 2025, 07:41:25 pm
content: Isn't the ultimate metric for cutting, load per tooth? You haven't attached any pictures of how the current tach is attached. If it a rotary encoder on one of the pulleys, then that's the way I would go.If it is something else, , maybe consider a variable reluctance (?) sensor and sense each tooth. That will give you tooth per time. From that, you can calculate load per tooth given your feed rate or linear speed given the tooth pitch. In any event, some computation will be needed, and since you seem to be familiar with Arduino, I am sure that can handle it.

author: floobydust
author_rank: Super Contributor
timestamp: January 14, 2025, 07:51:15 pm
content: Are there any sheaves, something with spokes? Need pics of the bandsaw.I'm thinking of where to put a magnetic Hall sensor like Honeywell 101GT detecting the drive wheel spokes. I've done that before, works great.

author: Martinn
author_rank: Frequent Contributor
timestamp: January 14, 2025, 07:53:04 pm
content: Quote from: jpanhalt on January 14, 2025, 07:41:25 pmhttps://bladeserpent.com/media/asset/s/p/speed-chart-embed.pdfAs far as I know, you determine the speed of the blade (in ft/min or m/min) depending on material and blade material. For other cutting tools this is optimized so that your tool lives a certain time (like 1 h) before being dull. If you are not paid by the hour you might want to keep it slow. I have my bandsaw set to the slowest setting - take it easy, blade lasts forever. Chip load per tooth is more relevant for milling or turning tools, afaik.Quote from: drummerdimitri on January 14, 2025, 07:27:57 pmBike computer: You can set the wheel diameter so you get a correct readout, question is if the adjustment range is large enough to get you in the correct range.Not having a battery is a good point.Maybe you should make a photo so it's clear what optics you are after.Some generic RPM meter might work, although I don't know the adjustment range.https://www.aliexpress.com/item/1005005929715664.htmlSearch Ali for "tachometer"- Martin

author: joeqsmith
author_rank: Super Contributor
timestamp: January 14, 2025, 08:45:56 pm
content: https://www.instructables.com/Tachometer-made-from-a-bicycle-speedometer-cycloc/But maybe the math to create a conversion chart hung on the side is a bit much. Battery wise, mine would last a few years. Took a coin cell.You could also consider a simple digital tachometer with sensor like the one shown.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: January 14, 2025, 08:58:10 pm
content: I like the idea of a tachometer but the ones you guys have mentioned only have direct reading of rpm. I need a way to converter that to m/min as the cutting speed chart on the machine is all in m/min.Such a tachometer would work if it was somehow adjustable or programable but in its current form it does not serve my purpose.I've attached a picture of the gear that is belt driven by the main motor which in turn has a brake line that goes to the broken tachometer.I can totally eliminate this system if I can find a tachometer with a sensor that can be calibrated to an arbitrary known value.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: January 14, 2025, 09:09:16 pm
content: I also found this model that read m/min but I have no idea what type of sensor goes to it. Seems to be adjustable too!https://www.aliexpress.com/item/1005001704761003.html?spm=a2g0o.productlist.main.29.70b85bf1Pzk5YV&algo_pvid=07ad676f-7143-4760-a58b-24dfa0f6a3b1&algo_exp_id=07ad676f-7143-4760-a58b-24dfa0f6a3b1-14&pdp_npi=4%40dis%21USD%218.48%218.48%21%21%218.48%218.48%21%4021015b2417368886544423526e81a3%2112000017225132786%21sea%21LB%212645614429%21ABX&curPageLogUid=LcFCz83YoR0N&utparam-url=scene%3Asearch%7Cquery_from%3A

author: jpanhalt
author_rank: Super Contributor
timestamp: January 14, 2025, 09:09:37 pm
content: Quote from: drummerdimitri on January 14, 2025, 08:58:10 pmPlease explain how counting teeth per second equals rpm. How about the current encoder? How is it connected?

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: January 14, 2025, 09:12:13 pm
content: Quote from: jpanhalt on January 14, 2025, 09:09:37 pmThere is no encoder currently. Only a mechanical tachometer that doesn't work but when it did, the line from the above attached picture went to it and provided rotary movement which then moved the needle on the tachometer.

author: themadhippy
author_rank: Super Contributor
timestamp: January 14, 2025, 09:15:47 pm
content: QuoteDont blades have a tpi or teeth per inch,few sums and voila

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: January 14, 2025, 09:32:07 pm
content: I also found this Shimpo Instruments DT-501XD Panel Mount Tachometer/Rate Meter.Could this be the solution to my problem? Seems like it can take a variety of input signals and a hall sensor should also work with it.I've attached the datasheet for reference.

author: jpanhalt
author_rank: Super Contributor
timestamp: January 14, 2025, 09:34:06 pm
content: @themadhippyOf course they do (you knew that). That why feeds and speeds are usually in load per tooth/cutting edge.

author: SteveThackery
author_rank: Super Contributor
timestamp: January 14, 2025, 10:10:34 pm
content: You don't have a way of measuring the blade speed directly, so you'll have to measure it indirectly and do the necessary maths to get a metres/minute output.For the measurement part you could detect and count the blade teeth passing over a sensor, or you could find something that rotates - a pulley wheel, or whatever - and count the rotations. Forget trying to count the teeth; for one thing, different blades have different teeth per metre so you'd have to remember to adjust the calibration factor whenever you changed the blade for one of a different pitch. Also, detecting the passage of teeth isn't trivial. So, choose a rotating element - a wheel, a pulley, a shaft - and use that.There are two easy ways to sense a rotating shaft: optical or magnetic. Both are easy and work well. However, magnetic sensing is more robust and reliable. You need to find somewhere to attach a magnet, and then mount a Hall effect sensor near to it.You then need to count the pulses from the sensor, convert that into metres/minute, and drive a digital display with the result. A microcontroller will do all of that, and I strongly recommend an Arduino because it's the perfect choice for a beginner.You said you want it to be powered from the machine, and quite rightly. But not yet. One thing at a time. Develop and debug it running off a battery or plug-top power supply first. Then you can look for a power supply that fits inside the machine and is safe and compliant.So you have a number of steps, which you should tackle one at a time.1/ Find a good place to fit the magnet and sensor. Choose somewhere that rotates fairly quickly so the blade speed will be updated quickly.2/ Select a suitable display module; preferably one with driver electronics so you can drive it directly from the Arduino.3/ Get your Arduino to time the interval between pulses in milliseconds and output the milliseconds to the digital display. (This is extremely easy to do with the Arduino.) Confirm that the number looks sensible.4/ Work out how to convert that milliseconds figure into metres/second, and implement in the Arduino.5/ Work out and implement a suitable power supply arrangement, along with all the physical stuff: mountings, etc.This is averyhigh level breakdown; in reality each step involves several sub-steps. We can help flesh out the steps and sub-steps as you proceed. To get help you need to "own" this project and ask for specific help when you get stuck. Nobody here will do the work for you (and I don't believe you would want that anyway).I think I could probably do this project in half a day, possibly a full day. If you are a complete beginner then we need to be realistic - you are looking at several full days of work. The good thing is, when it's done and working you'll get an amazing sense of achievement.Finally, the boring bit.If you want this device to be powered off the mains, then you will have to get the input of a qualified electrician to approve and test it for electrical safety, especially as it is in a work environment.

author: Ian.M
author_rank: Super Contributor
timestamp: January 14, 2025, 11:19:09 pm
content: The output voltage of a PM DC motor driven as a generator is proportional to its shaft RPM over a considerable range. Therefore a tachometer could be as simple as a small low voltage DC motor coupled to whichever rotating part of the bandsaw is easiest (maybe friction drive), a 1 mA analog panel meter and a resistor to set the scaling so the meter reads the blade speed in m/min directly. Maybe add an electrolytic capacitor to smooth the voltage so the meter needle doesn't pulse at low speeds.

author: Poroit
author_rank: Frequent Contributor
timestamp: January 15, 2025, 09:50:47 am
content: G'day drummerdimitri,Can you post a pic of the inside of the band saw that shows the blade drive pulleys.You can use an Inductive Proximity switch with that Shimpo meter( or others) if you can generate enough pulses.Calibration as per SteveThachery's post.

author: joeqsmith
author_rank: Super Contributor
timestamp: January 15, 2025, 02:33:13 pm
content: Do you have the original tachometer that you can post a photo? Could it be repaired? Was it a standard 50s-70s motorcycle tachometer with custom face plate?

author: Ferroto
author_rank: Frequent Contributor
timestamp: January 15, 2025, 09:35:23 pm
content: Stick a magnet on the Wheel, put a reed switch behind the wheel and clock the result into an Arduino. Then output to a 7-segment display.

author: joeqsmith
author_rank: Super Contributor
timestamp: January 15, 2025, 09:40:54 pm
content: https://www.instructables.com/Arduino-Bandsaw-Speedometer/https://www.instructables.com/Add-a-Linear-Speed-Display-to-Your-Bandsaw/http://www.mkctools.com/tachulator.htmhttps://www.bahco.com/int_en/tachometers-for-bandsaw-pb_3870-tacho_.html

author: SteveThackery
author_rank: Super Contributor
timestamp: January 15, 2025, 10:10:17 pm
content: Quote from: Ferroto on January 15, 2025, 09:35:23 pmBeen there, done that. See #14.

author: Poroit
author_rank: Frequent Contributor
timestamp: January 20, 2025, 02:17:52 am
content: G'day joeqsmith,I thought your post on Jan 15 was very goodThat was clever how they targeted the pulley.