What causes slitting blade mistracking?

[jmkasunich]

I was feeding by hand and would get stalling, overspeeding etc at anything but the sloooooowest hand feed rates. It could be the controller for the mill is defective. The controller board is potted, and there are no adjustments to set. However, I will set it up again and try once more.

Honestly, it has to be something I am doing that seems (perfectly) reasonable to me, but isn't something that you do. Or the capabilities of the machines are totally different. It's me (and what I am doing) or the machine, are there other possibilities?

For one, should I be conventional cutting or climb cutting?

I have significantly less problem with cutting aluminum, as I can run the saw faster. At the higher RPMS there are a lot less motor control issues, the speed tends to be relatively constant even with low to moderate load. The saw behaves ok, sort of like you are describing. This is not the case when slitting steel.

It's the machine. That spindle drivetrain was NOT designed for torque at low speed. Use the smallest diameter cutter that you can get away with to bring the speed up. Then feed by hand fast enough that it can actually cut.

 

[jmkasunich]

Do you recall what your speed was? In mild steel, you would be running somewhere near 300-350 rpm.

For a 2-3/4" diameter HSS cutter? That would be 215 to 250 SFPM....

 

[WobblyHand]

Let's walk through both speed and feed. Earlier you said 150 RPM, just now you said 125RPM. The 2.75" cutter has a circumference of 8.63" or 0.72 feet. So 150 RPM is 108 SFPM, and 125 RPM is 90 SFPM. To be honest, I'd go a little slower if possible. But if you have a variable speed motor and no gear/belt reduction then spindle torque will be a problem...

Now the feed. At 150 RPM and 0.05 inches/minute, that is 0.00033" per revolution. Even if you only had three teeth, that is 0.0001" per tooth. With the 80 or so teeth that the cutter actually has you are talking a few millionths of an inch per tooth. I'm willing to bet It was rubbing, not cutting. That creates heat, and the rest follows naturally.

Did you see chips coming out of the cut? Or just shiny dust? They aren't going to be big chips, but they should be chips.

Due to runout a slitting saw isn't going to cut on every tooth. But it does need to CUT, not rub. I'm thinking you want to be feeding at least a couple thou per revolution, maybe as much as 10-20 thou per rev.

Getting back to the spindle speed and torque - I just looked up the manual for that machine. It has two mechanical speeds, with a 2:1 ratio. I assume you were on the low range. But even so, the torque is going to be pretty sad at such a low speed. That machine just isn't made to swing 2.75" diameter cutters. Both speeds are belt drive, so limited torque before the belt slips, even if the motor has good low speed torque.

How deep do you need to slot? Can you use a smaller diameter saw, perhaps something that is 1" or 1-1/2" diameter with a 1/2" arbor hole?
Like this maybe?
View attachment 403374

Or maybe something like this keyseating cutter? With a 1/2" cutting diameter you can spin it at 500 RPM for 65 SFPM surface speed. You'll have at least 5 times more force at the tip of the teeth because of the smaller diameter. Of course, it might not reach where you need to cut....
View attachment 403373

Thank you for your thoughtful reply.
1. I was trying to cut a little too much. 5/8" deep cut in steel. That pretty much rules out the smallest saws. Larger saws mean one needs even lower RPM.
2. I had the low speed pulley on. Yes, the torque utterly is awful at this low RPM. The speed control is poor. The resolution of the pot is poor. I am lucky if I can get within 40-60 RPM of target. So maybe it was 120 rpm for a little while, or 145 for a different time. The motor/spindle stalls out under the slightest of loads. It simply has no torque nor adequate speed control in the 80-200 RPM range. This is the crux of the matter. Once there is a stall the motor overspeeds and 5-8 seconds later finally slows down.

 

[jmkasunich]

Thank you for your thoughtful reply.
1. I was trying to cut a little too much. 5/8" deep cut in steel. That pretty much rules out the smallest saws. Larger saws mean one needs even lower RPM.
2. I had the low speed pulley on. Yes, the torque utterly is awful at this low RPM. The speed control is poor. The resolution of the pot is poor. I am lucky if I can get within 40-60 RPM of target. So maybe it was 120 rpm for a little while, or 145 for a different time. The motor/spindle stalls out under the slightest of loads. It simply has no torque nor adequate speed control in the 80-200 RPM range. This is the crux of the matter. Once there is a stall the motor overspeeds and 5-8 seconds later finally slows down.

I don't know enough about that class of machine to know whether it just is what it is, or whether something is defective. PM is supposed to have good customer support - try calling them and describing the behavior. It's never going to have the torque of a geared spindle, but they advertise it as going down to 50 RPM, it ought to have useful torque at that speed. Maybe something is broken and can be fixed.

 

[WobblyHand]

It's the machine. That spindle drivetrain was NOT designed for torque at low speed. Use the smallest diameter cutter that you can get away with to bring the speed up. Then feed by hand fast enough that it can actually cut.

It's pretty clear it is the machine. I was using a 2.75" saw so I could make a cut of 5/8" deep. That drops the RPM to the low speed range which is torqueless and gutless. Once in that "regime" any feed rate is too fast. Only thing that I can think of that might help is to make a new pulley set, so the motor can run faster, but the spindle is slower than the stock ratio. That or replace the motor and controller.

 

[WobblyHand]

I don't know enough about that class of machine to know whether it just is what it is, or whether something is defective. PM is supposed to have good customer support - try calling them and describing the behavior. It's never going to have the torque of a geared spindle, but they advertise it as going down to 50 RPM, it ought to have useful torque at that speed. Maybe something is broken and can be fixed.

I wouldn't be surprised if it is a little of both. (The way it is AND the possibility of a defect.)

Have to laugh at the thought of 50 RPM torque on this mill, bet it is very low. How could I test it? Didn't Clough42 make a torque stall tester?

Might as well put this in: RPM = SFM * 12/ (pi * D), so if SFM=90 FPM for steel, and D=2.75", RPM = 125

 

[jmkasunich]

I wouldn't be surprised if it is a little of both. (The way it is AND the possibility of a defect.)

Have to laugh at the thought of 50 RPM torque on this mill, bet it is very low. How could I test it? Didn't Clough42 make a torque stall tester?

The "wimpy torque, then stall, then overspeed for several seconds" behavior is suspicious. The overspeed indicates that the drive realizes that it has stalled and starts to pour on the coal, but it happens too late. Makes me think something might be tuned wrong in the drive. I would start by by calling PM and see what they say in response to that description.

 

[WobblyHand]

The "wimpy torque, then stall, then overspeed for several seconds" behavior is suspicious. The overspeed indicates that the drive realizes that it has stalled and starts to pour on the coal, but it happens too late. Makes me think something might be tuned wrong in the drive. I would start by by calling PM and see what they say in response to that description.

Yeah, don't have much to lose at this point. It's like the servo control system is improperly tuned. Seems like a bad controller. And there are no knobs to turn on this. The controller is potted and there are no adjustments.

From watching BlondieHack's experience with this, I'm not all that anxious to go through what she did. She had bad rheostats (check), bad controllers (maybe), displays that were matched with controllers, and a down mill for quite a while. It was quite a mess.

 

[WobblyHand]

If you feed by hand so that you feel a slight resistance to the feed then there is no way you're going to stall the motor. Forget power feed, forget the DRO. Feel the machine cut.

It will stall the motor if you didn't use the slow speed position, ie. change the pulleys.

I must of had a dyslexic moment, could have sworn I was in slow speed mode, but I wasn't. So changed the belt position used a 2" diameter cutter, (1/2" arbor) 0.063" thick blade, and was able to cut the stock. The mill is still no powerhouse, but I was able to make a 6" long cut through 5/8" thick 1018.

With the correct belt position, I could indeed feel the machine cutting. In the wrong belt position, this wasn't possible, since the motor would stall at even a very slow feed rate.

 

[WobblyHand]

Ok. Post mortem. Had two main issues, with a possible third.

1) Defective arbor. Arbor screw drilled parallel to but not concentric with the spindle axis. 0.036" TIR on screw. Screw head when tightened caused interference with arbor cap which tilted the arbor cap and blade. Blade then followed path of its own until the blade broke.
2) Operator error. Thought the mill was in low speed mode, but it was in high speed mode. Mill had insufficient torque to cut with low feed rates. When operated in low speed mode, using 2" cutter, was able to cut through 5/8" 1018 over roughly a 6" long cut.

3) Lesser issue. Poor motor speed control especially in low speed mode and low speeds. Unable to set motor speed to proper speed in low speed setting. Could not set speed to 171 RPM for steel. Only could set to 220 RPM or 140 RPM (rough numbers). Seems there's a hole in the potentiometer range. Ran at 220 RPM, which under load was roughly 180 RPM. Poor speed regulation under changing load.