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View Full Version : I think my mill power feed may be a failure...



The Artful Bodger
02-04-2012, 02:05 AM
.... or at least a semi-failure.:confused:

I got a nice case made to cover the motor, the worm and wheel, the dog clutch and to mount a couple of controls. Actually, I was rather pleased at how the dog clutch panned out as that is my most complex milling project so far.

But I seem to have messed up somewhat in my choice of speeds as the slowest I can move the table is one inch per minute (a tad less, 24.3mm in fact). This seems fine for fly cutting aluminium but I have a feeling that will be too fast for work on harder metals such as steel?

macona
02-04-2012, 02:26 AM
One inch per minute is way slow enough. I never cut that slow.

Evan
02-04-2012, 02:41 AM
There will be some situations where that isn't slow enough. In particular, any sort of cutting with very small cutters, 3mm and below, especially solid carbide cutters.

dp
02-04-2012, 02:56 AM
Your anti-gloat is meaningless without pictures! :)

The Artful Bodger
02-04-2012, 03:08 AM
Macona, Evan, thanks, it was the small diameter cutters that I was afraid of breaking through feeding too fast and unfortunately my mill does not have a high spindle speed with a maximum of 2800 plus changing the belt speed is a real pain, in fact I have never used the highest speed so I guess I will need to make myself a little stool to reach those dang pullies!:)

The Artful Bodger
02-04-2012, 03:15 AM
Your anti-gloat is meaningless without pictures! :)


Ouch!..............;)

The Artful Bodger
02-04-2012, 03:50 AM
Your anti-gloat is meaningless without pictures! :)


Ouch!..............

http://farm8.staticflickr.com/7143/6815857019_5f0672386c.jpg (http://www.flickr.com/photos/58195047@N04/6815857019/)
IMGP9632 (http://www.flickr.com/photos/58195047@N04/6815857019/) by MrJohnHill (http://www.flickr.com/people/58195047@N04/), on Flickr
General view...



http://farm8.staticflickr.com/7012/6815856171_7584377c95.jpg (http://www.flickr.com/photos/58195047@N04/6815856171/)
IMGP9631 (http://www.flickr.com/photos/58195047@N04/6815856171/) by MrJohnHill (http://www.flickr.com/people/58195047@N04/), on Flickr

Cover off showing the dog clutch.





http://farm8.staticflickr.com/7023/6815855733_9e30b97f04.jpg (http://www.flickr.com/photos/58195047@N04/6815855733/)
IMGP9629 (http://www.flickr.com/photos/58195047@N04/6815855733/) by MrJohnHill (http://www.flickr.com/people/58195047@N04/), on Flickr
I took a panel off the mill base and fitted the power supply in here, there is room for further developements.

The Artful Bodger
02-04-2012, 03:54 AM
http://farm8.staticflickr.com/7028/6815854875_da73f7e264.jpg (http://www.flickr.com/photos/58195047@N04/6815854875/)
IMGP9628 (http://www.flickr.com/photos/58195047@N04/6815854875/) by MrJohnHill (http://www.flickr.com/people/58195047@N04/), on Flickr


Shows worm and wheel.


http://farm8.staticflickr.com/7016/6815854129_bab17e522e.jpg (http://www.flickr.com/photos/58195047@N04/6815854129/)
IMGP9627 (http://www.flickr.com/photos/58195047@N04/6815854129/) by MrJohnHill (http://www.flickr.com/people/58195047@N04/), on Flickr

The cover is back on..

John Stevenson
02-04-2012, 06:05 AM
Looks to me like a DC motor ? If so what about fitting some electronic speed control ?

macona
02-04-2012, 06:58 AM
Only 2800 on that big of a mill?. That is kind of surprising. If it has a three phase motor you could overspeed it to at least 4200 like most R8 knee mills, they all use the same bearings.

ME Consultant gives me about 4ipm for an 1/8" 3 flute carbide end mill at 2800 RPM. I have found the main thing with little end mills is getting the chips out. That's what causes the most broken end mills for my and why I added a second mist head to my mill.

Evan
02-04-2012, 10:21 AM
ME Consultant gives me about 4ipm for an 1/8" 3 flute carbide end mill at 2800 RPM.

That depends entirely on what you are cutting. In steel that size bit could easily be run at 8000 rpm. Running at the calculated maximum chip load with those bits is a good way to break them. I do a lot of work with small cutters and most of them end up broken before they wear out. Since I am using CNC I know the cutting rates are accurate.

J Tiers
02-04-2012, 10:43 AM
Looks to me like a DC motor ? If so what about fitting some electronic speed control ?


Exactly........ In fact, did we not have a discussion about that only a little while ago?


http://bbs.homeshopmachinist.net/showthread.php?t=52082

jkilroy
02-04-2012, 10:57 AM
I don't think I have ever cut as slow as 1 ipm. 3 ipm is usually where I start and work up from there. Even with slabbing mills I cut faster than that. I could see the need with tiny cutters and low rpm in tough materials you might need it, I just don't do that.

The Artful Bodger
02-04-2012, 02:43 PM
Looks to me like a DC motor ? If so what about fitting some electronic speed control ?

Thats in the programme John but it will have to wait until I get to the 'big city', 50 mile drive. Meanwhile I am using PWM generated from the serial port on a PC but I dont want to always do that, or I have to get organised and incorporate a an old PC in a convenient fashion.

The Artful Bodger
02-04-2012, 02:59 PM
Exactly........ In fact, did we not have a discussion about that only a little while ago?


http://bbs.homeshopmachinist.net/showthread.php?t=52082


Yes we did and thanks to all those who gave advice, I have been using a PC to generate the PWM but that is rather incovenient, the plan is to build a knob controllable PWM when I can get the bits.

The Artful Bodger
02-04-2012, 03:03 PM
Regarding the mill speed, I have not yet grasped the nettle of 3 phase and VFDs (a VFD here seems to be much more expensive that in North America) and of course I am cursing myself for taking a load of 'useless' 3 phase motors to the scrap yard a few years ago.:rolleyes:

The belt change on this mill is extremely awkward, so much so that I am considering changing it to have a layshaft and a pair of cone pulleys outside the mill's belt housing, when I get a round tuit.:cool:

darryl
02-04-2012, 04:50 PM
I have a rudimentary drive system on my mill. Just out of interest, I checked the IPM- on the maximum fixed speed I get 8 inches/min. X and Y axis drives are the same. I have arranged for a plug-in controller which allows me to vary the speed down from that. One thing I don't like is that without increasing the power supply voltage, I can't traverse the table any faster than that. I end up unlocking the clutch, then hand cranking to position, secure the clutch again for the next pass. Not a big deal, but it seems that if I was going the effort of building the drive package in the first place, I would have built in a higher voltage to allow faster table positioning.

I can always upgrade the power supply, now that the mechanisms have been in place and tested over the last few years. These are DC PM motors operating through gearboxes, and having a simple one-bolt clutch engagement method. Should I ever get around to it, these bolt heads will have some wings added so I can just use my fingers instead of the wrench. There is no feedback, no positioning sensors- just apply voltage and watch the tables move. Simple, rugged, and proven.

I can see that I would like to have some form of electronic speed control of the motors for when the speeds are slow. The lower the voltage you feed the motors, the greater is the tendency to slow even further when a load is on. In one sense this isn't a bad thing, but it does mean that you aren't able to lock into a specific speed. If there's increased drag on the table, the movement will be slower, etc.

This is a long way from CNC, but it's been very useful as you don't have the intermittent variation of table positioning that you can get when you hand crank. Plus of course, you don't wear out your arm cranking the table around for multiple passes.

Just for information- my system is derived from actuators used in satelite system dish positioning. The ones I used have needle thrust bearings on the leadscrew, which I've transferred to the leadscrews on the mill. The gearboxes have been grafted onto the side of the tables, leaving no part of it above the surface of the table to get in the way. The handcrank is grafted onto the leadscrew shaft in much the same was as it was originally. If I have the drive engaged and turn the handcrank, it will still turn as the gearbox does not have any worm and wheel or other locking type of geartrain components. The only part of this that does protrude above the table surface is the hand crank itself- nothing has changed in this regard. If I have a long workpiece that must overhang, I just remove the handwheel and use power only.

The Artful Bodger
02-04-2012, 06:58 PM
Darryl, I am uncertain what the maximum ratio between fastest and slowest speeds might be for a PM DC motor operating under PWM. Although these are fairly high class motors from old main frame mag tape transports they do not seem to have much low speed ability (meaning that high to low speed range is not very great) leading me to abandon any ideas of powered high speed positioning.

darryl
02-04-2012, 09:07 PM
Good question. I have more experience with supplying variable voltage DC than with PWM. I would think though that if your pwm circuit had the capability to use a feedback voltage, you could tell it to respond to that. It would then adjust the pwm ratio to maintain that level of voltage. It would probably require a capacitor/resistor possibly diode combination to average out the voltage without having to put a capacitor directly across the motor leads- something which some pwm speed controls would not tolerate.

I don't have the real answers here, just speculating on possible problems and solutions. Feedback is done in different ways, of course- another way would be to use an external sensor and circuit to monitor rpm and feed back to the pwm circuit.

It may also be true that it's no big deal- you might find that you can run open loop and still have good control of speed, even at very low speeds. If the torque your motor and gearbox can provide is much larger than would be required to move the table against cutting loads, then this would probably be true. In it's favor, if the very low speed is chosen to make it safe while using small diameter end mills, your torque requirement would also be lower to begin with. There may be no need to go overboard in trying to regulate the feed rate so exactly.

Evan
02-04-2012, 10:57 PM
PWM will permit operation of a DC motor at low rpm while still providing a reasonable amount of torque. It will not make it possible for the motor to provide full torque though. Feed back is the only way to do that and it is extremely effective. I posted results of some experimentation with adding feedback to a DC brushless motor that I was using to drive the spindle on my mill. That was a couple of years ago or so. The feed back system is very simple to implement.

The Artful Bodger
02-05-2012, 12:19 AM
PWM will permit operation of a DC motor at low rpm while still providing a reasonable amount of torque. It will not make it possible for the motor to provide full torque though. Feed back is the only way to do that and it is extremely effective. I posted results of some experimentation with adding feedback to a DC brushless motor that I was using to drive the spindle on my mill. That was a couple of years ago or so. The feed back system is very simple to implement.


I have some motors with tacho generators mounted on them but unfortunately they are far too small for this job but maybe what is really needed is an optical chopper disk?



[I feel I am standing at the start of a very steep learning curve...!:) ]

lakeside53
02-05-2012, 12:26 AM
Servo (brand) and BP 6/8F power feeds get along just fire without any feedback. Dead simple DC motor control.

The Artful Bodger
02-05-2012, 12:29 AM
Servo (brand) and BP 6/8F power feeds get along just fire without any feedback. Dead simple DC motor control.


Do they feed and position without changing the mechanical gearing?

MaxHeadRoom
02-05-2012, 12:32 AM
Are you looking for closed loop or open loop (feedback)?
If open loop, check with your local Baldor or KB motor distributor for one of the simple DC SCR controllers, if you buy bare bones without enclosure it is cheaper.
Also many on ebay.
Max.

The Artful Bodger
02-05-2012, 12:36 AM
Thanks Max but I am not even sure what I should be looking for right now!:D

macona
02-05-2012, 12:44 AM
That depends entirely on what you are cutting. In steel that size bit could easily be run at 8000 rpm. Running at the calculated maximum chip load with those bits is a good way to break them. I do a lot of work with small cutters and most of them end up broken before they wear out. Since I am using CNC I know the cutting rates are accurate.

OOps, forgot to say that was steel.

macona
02-05-2012, 12:48 AM
I was over at a friends place and he was working on a little bosch wood lathe. It actually had a closed loop motor drive. The motor has a small brushless tach feeding back to the small motor drive board. I was really surprised to see that in there.

The Artful Bodger
02-05-2012, 01:02 AM
I suppose a little stepper motor would make a good feedback generator?

lakeside53
02-05-2012, 02:00 AM
Do they feed and position without changing the mechanical gearing?


Speed is via pot. You control the postion with the fwd-off-rev lever. No problem with torque or speed.

The Artful Bodger
02-05-2012, 02:32 AM
Speed is via pot. You control the postion with the fwd-off-rev lever. No problem with torque or speed.


Thanks, obviously they can get a good range of speed from the one motor.