Drill press motor swap

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I have been threatening to get a tach for a while now. I'll probably get an optical tach, but one that gives me the option also getting a linear speed. It looks like they can be had for around $50-$60 for a budget model.

Why not use a Hall effect tachometer, leave it permanently attached, I'm using 3 on different machines . You only need a 12V DC power supply .
Google search , I have the ones from Aliexpress .


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I've already got an rpm display on the servo, but it only displays in 100's of rpm. The tach will let me determine whether I'm varying the speed set point in 100rpm increments, or just displaying to the closest 100rpm.

If I set the speed and it's displaying 1700rpm, but I'm reading 1725rpm on the tach - then I'll be able to determine if it's the setpoint, or the display. If I can vary the pot setting a bit, and both the servo display and the tach remain constant, then I'll know that they're changing the speed setpoint in 100rpm increments. Otherwise, I'll know that they are just displaying to closest 100rpm.

I've got a used Ametek 1726 tach coming, I've used them before and they're a good tach. I found a listing for a 1726 that was dated 2010, so they've been around for a while. The 1726 is a dual use tach, both as a non-contact optical tach and as a contact type tach. It's got a ridiculously high non-contact rpm range, 0-99,999rpm. When you slip on the contact type converter it'll let you measure up to 20,000rpm. It also has a little wheel that lets you measure linear speed, up to 20,000 feet per minute. The 1726 will also do the math for you and convert the reading into just about any Imperial or metric reading that you could want. That'll let me measure just about anything I can dream up right now.

I don't think that there's anything I can do about the motor speed/speed settings in the servo, this is just for my own curiosity right now.

When I 3D printed the pot/switch enclosure I didn't get the 0-100% speed arrow correct. It increases in the right direction, but it's upside down. I'll fix that on the next version, and add some text to the print for the switch and the pot. Plus, I STILL had to file the cable strain relief opening - I'll open him up a little more too.

I re-printed the enclosure, I even got a white paint-pen and painted the top of the text so that it stands out a little more. Some numbskull forgot to scale the print up to 103%, so I had to "adjust" the fit of the cover. Other than that, everything worked out OK.

I used the drill press a little bit over the weekend. I know that using a 1Hp drill press to drill itty-bitty holes in plastic parts is overkill, but at 100rpm you get a nice clean hole with no melting from friction.

I tried changing the speed setting and I could hear the pitch of the motor change, but the "Real-time" rpm indicator didn't change. I'm pretty sure that this thing is varying the speed in a linear manner, but only displaying the speed to the nearest 100rpm. The tach is supposed to show up by Thursday, so I'll know for sure then. Either way, I can live with this arrangement.

A word to the wise... If you do a similar motor change, and use the same hobby machinist circuit that I did, be sure you always leave the Run/Stop switch in the Stop position when you turn off the servo drive. If you just turn off the servo drive, when the servo boots up the next time you'll get a "Pedal Error" because the speed setting is not at the minimum position.
Two months down the road now. So how is it doing? Lessons learned? We are counting on you, LOL. I have been using mine almost everyday in a test fixture, but have not made any mods to it.
Thx, Lloyd

OK, two months in and I don't have much to report. I've mostly been drilling small holes in plastic and that variable speed is soooo nice - no more melted plastic from running the bit too fast. I haven't had a reason to try drilling any big holes in anything yet but I'm sure that will help there also.

Lessons learned, a few come to mind immediately, in no particular order:

1) The Rpm indicator on my servo drive only displays to the nearest 100 Rpm. It's got a 4 digit display so you've got to wonder WHY? I know that this is displaying the actual motor speed and not the speed setpoint because I can slightly change the speed pot setting and hear the motor pitch change, but the display doesn't change. This isn't really a big deal though, as it's a WAYYYYY more better indicator than I had before - which was Nada. Any of these drives by a different manufacturer will probably be different, so you may - or may not - get an Rpm display.

2) The speed pot circuit in the Home Machinist link, referenced earlier in this thread, replaces the Hall Effect sensor speed controller and is a good starting point. You may want to do some experimenting with the resistors to the 1K pot. These resistors control the minimum and maximum voltage that the pot's wiper will sense. Mine currently goes to minimum speed at about 15-20% of the pot's travel instead of closer to 0% where I would like it. The same is true for the upper end, I get full speed at about 80-85% instead of 100% where it should be. I probably should decrease both of those resistors, but I decided that I'd already rebuilt that circuit enough times and that I could live with it - as is.

3) When I started trying to replace the Hall Effect sensor speed control with a speed pot I had assumed that this was a simple voltage divider circuit. I didn't bother to measure the current on any of the connections. I tried building a voltage divider circuit with the parts I had on hand and it didn't work. I was getting the correct voltages, but apparently my speed pot circuit was not able to supply enough current to drive the circuit within the servo. Moral of the story? Use a 1K pot - it works.

4) The Home Machinist circuit includes a Run/Stop switch which bypasses the speed pot setting and stops the motor. Be sure to switch the motor to Stop before turning off the servo drive, you'll get the "Pedal Error" fault if you don't. DAMHIK

5) I'm not sure about other servo drives, but mine came with a "Brake to Stop" feature and it sounds like most other drive are the same. The range of this setting was 0-5 and it defaulted to 1, I set mine to 0. This causes the drill to coast down to the minimum speed setting, THEN brake to a stop. It MIGHT be amusing to watch the drill chuck spin open or closed when the motor brakes to a sudden stop... But a lathe chuck, filled with a big honkin' hunk of steel, unthreading and spinning across the shop would be less than amusing.

6) I've got my minimum speed set to the drive's absolute minimum of 100 Rpm. It's a lot easier on the equipment to brake to a stop from 100 Rpm than it is to brake to a stop from 3000 Rpm. And we're talking about coming to a full stop within 50-100 milliseconds, it takes about 100-400 milliseconds to blink your eye. I think you might also be able to increase the braking time so that the servo would brake the motor to a stop a little more slowly. I might have to check into that and try it. My servo drive doesn't like running at 100 Rpm, the motor speed oscillates. It doesn't smooth out until about 300 Rpm, but I don't really RUN the drill at 100 Rpm unless I'm getting ready to shut it off.

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Thanks for the write up. It sounds like the changes you made were pretty helpful, and almost necessary. It will be interesting to see what it does when you load it up, but no matter what, a lot better than a 4 step pulley.
I thought of one other modification that I made to the servo drive on my drill press. My servo drive came with a receptacle labeled Lamp. I thought this was nice of them so I plugged my drill press light into it. But in its' stock form the "Lamp" receptacle was on whenever the servo drive was plugged in, not exactly the way I wanted it to happen. I re-wired the receptacle so that it would be controlled by the switch. Now when I turn On/Off the servo drive I'm also controlling the drill-press light.