Torque required for a small mill power feed

I am looking to add a very basic powerfeed to my X2 mill X axis. The table is a bit small to be adding a wiper motor. There are lots of smaller geared motors out there but I have no idea of the expected torque requirement. Anyone have any ideas?

Wrap some string etc around the rim of the handle if you can and pull it with a spring balance. Or tape.

It's also possible to get a rougher idea by pulling in the handle from just below top dead centre and noting the reading when it starts to move.

It'll depend to some extent how tight the gibs are but I suspect on a mill of that size they are best set a little stiff.

John

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Torque required will really depend on how much metal you are trying to remove.

Its a bit of a trade off between feed speeds from the gearbox and torque. To get a decent return speed you need a high ratio but that starts to get stalled when you lower the motor speed.

The one I'm testing at the moment could do with a lower ratio to give it a bit more power for larger cuts.

Edited By JasonB on 18/08/2016 15:58:12

I don't have a spring balance (I do remember them from school) so how do I use tape? Thanks. Im also assuming that the torque required needs to be measured at the crank as it will be a direct attachment rather than from the handwheel? Im confused.

Bags of sugar or buckets of water are an alternative to the spring balance.
I was rather pleased with myself the other day wrt force measurement. A few weeks ago I evaluated several hundred items for the force needed to move them. I then measured a selection of them with a balance to give a numerical value to the ones I considered weak or hard. Then a few weeks later I had some more samples. I found I was generally accurate to about 5 Newtons in my estimates of these and consistent to about 3 Newtons spread showing I could be 'calibrated' as a weighing machine.

Hope this diagram helps. You can measure torque at any distance from the crank because, for a given turning resistance, weight * length is a constant. If length is short, you need a large weight. But as length increases you need proportionally less weight to turn the crank so you get the same answer.

In the absence of a spring balance, position the handwheel at 3 o'clock or 9 o'clock and slowly increase the weight until the handwheel turns.

Dave

For a medium sized mill i would rule out 12v altogether, it isn't powerful enough to drive it so i'd start with 24v. I learned the hard way unfortunately.

Michael W

The handle rim has a radius so the torque needed to cause movement can be calculated. Same if the handle is used.

Tape is just an alternative to string which might be easier to wrap round.

Or various weights as Muzzer mentioned.

Jason has a point too - really need to do it into a cut.

Maybe you have something that you use to weigh luggage when going on holiday? I prefer the spring types as the ones used by fishermen usually a have a sliding part that remains where ever they are pushed to on the scale.

I've also done it by pouring water into a bucket or jar etc and then weighed that.

John

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Looking at some of the gearhead motors similar to whats on mine one with a 7.5:1 reduction gearbox has a torque of 10kg.cm max so at 1cm from the feed screw axis it will apply 10kg of force. so say your handwheel is 10cm dia or 5cm radius this would be equal to 2kg applied at the rim.

However this is at the motors full speed where it makes the most torque. Its well known from the far eastern variable speed lathes that as you reduce the motors speed they loose torque. So if you are wanting a feed rate of say 100mm per min you have to run the motor at about 1/6th of its optmum speed so that torque that seemed nice and high will actually do bu**er all at the lower speed and may not even be enough to overcome the friction in the ways let alone take a decent cut. It will give you a nice 600mm per min fast return speed though.

So what to do. You can get many different ratio gearboxes so lets say instead of 7.5:1 we try a 15:1. this will have a torque of 20kg.cm max, double the higher ratio. Therefore at our 100mm per min desired feed rate will the motor will now be able to run at 1/3rd its optimum speed and therefore be able to make more of its possible max torque. Only down side is that now at full power the fast return will only be 300mm per min.

I could live with the slower return speed as the ability to take a slower or heavier cut would be more desirable.

Edited By JasonB on 18/08/2016 20:06:30

So as a starting point a torque of 20kg/cm should work with a direct gearbox to feedscrew axis join? I dont take big cuts !

It would be a good starting point, but depend on the speed the motor is running at when it developes that torque.

I'm inclined to think that a Stepper Motor has better torque characteristics for a job like this.

MichaelG.

I think you have things the wrong way round Jason. 10 kg at 1cm radius is the same a 1kg at 10cm radius. The extra length increases the leverage.

If some one knows what feed per min they need and the torque it should be possible to work that back to watts. Then say assume 50% efficiency for the motor. I don't think any of the small geared motors with 6mm shafts are powerful enough and where ratings are given it may mean assuming that the motor will be red hot.

I'll leave the sums to some one else.

John

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Do you not know any one who goes fishing? They all have one . Cheap as chips & it helps the " one that got away " stories for back home

Thanks John, yes longer lever should need less force, I've altered the post.

how about this on a well known auction site

12v 24v High Torque Worm Motor Geared Motor DC motor Robot GW31ZY

Taking the 25kg.cm one for example which would be around the sort or torque we are talking about that is going to be running at 22rpm so if you have a 10tpi leadscrew you are only going to get 2.2" per min, not bad for afine cut but it won't return the table any quicker.

Minimum sort of return rate I would want is 12" per min so 120rpm but they are only producing about 5kg.cm at that sort of speed and a lot less once you put a speed controller onto them to get teh feed rates required

As the X2 mill is frequently converted to CNC with at least acceptable results surely simply slecting a motor of at least similar, preferably greater, torque than the preferred size of stepper motor would be effective.

Going all round Robin Hoods Barn to get measurements of a torque hopefully somewhat equivalent to that needed to drive the mill table under respectable cut conditions then searching for a motor of useable size whose specifications are acceptably close to the measurements seems to involve far too much faff and wasted time.

Personally I'd just copy a known to work CNC conversion stepper motor set up, hook the stepper driver up to a simple variable pulse rate generator for speed control and call it "Job Done". Initial prices might look more expensive than ferreting around but you get it all in one hit and know exactly what you will spend. Leaving aside the time invested in finding and making which could be better spent on what you really want to do ferreting around always seems to involve extra costs for odd bits'n bobs that sorta got overlooked in the initial planning stage. Its easy to spend almost as much, often more, on an OK(ish) DIY contraption than buying properly sorted commercial kit.

This old fogey in particular has a hard job keeping up with how relatively inexpensive quite sophisticated components and gadgetry are these days when compared to the 1970's when he first started taking notice!

Clive.

I would look at car power window motors or the motor / gearbox unit from an old battery drill both have more than enough torque for the job .

I have the seig power feed unit which cost five tomes what it was worth but it works - sort of !

On thing to watch out for is if your motor is underslung it can hit the base when the table is wound fully to the right on the x axis , my mill has the DRO scale is on the same side as the power feed motor so i had to make an extension bracket so the motor did not crash into the scale unit .

Ian

I measured the required torque on my Naerok at 0.55 Nm, which is 0.41 lb.ft for the imperialists. I found I needed to add a feedback loop to keep constant speed with varying cut, others have managed without. I wrote an article about it all, but suspect it had too much of the dreaded electronics in it. On my latest miller I used a stepper motor, much the best solution, and it allowed me to make more electronics!!

If you send me a PM I'll send a copy of the write up on the Naerok if you're interested.