I want to DIY.. some questions please!

[Tenson]

I see, so the motors drop in torque is not proportional to the increase in RPM? If you get more than double the torque at half the RPM then I can understand the argument for 10mm pitch vs. 5mm.

Jazz, have you tried not using the 1:2 gearing and just spinning the motors faster? If you found the slower motor speed is better then I should just follow your advice and go for 10mm pitch ;)

I'd like to go for rotating ballnuts if Jonathan will be kind enough to make the parts for me, as it would be nice to keep the drive motors within the machine frame, rather than stuck on the end. Also I could use ballscrew without end-machining since it would just be held in a clamp.

I can get 2510 ballscrew from Zapp, so can you make a rotating ballnut assembly for that Jonathan?

[Jonathan]

I see, so the motors drop in torque is not proportional to the increase in RPM? If you get more than double the torque at half the RPM then I can understand the argument for 10mm pitch vs. 5mm.

A good approximation is that the torque is fairly constant upto a particular rpm (the 'corner speed') after which the torque does drop proportionately. We've come to the point where hand waving explanations are insufficient as there's too many variables. It will depend on the chosen motor and thresholds.

The graph in the 3nm motor datasheet is helpful:

http://www.zappautomation.co.uk/redi...H88-3008BF.pdf

(They've just gone up in price by 20%!!)

Also Irving's spreadsheet is very helpful to understand which factors affect the feedrate:

http://www.mycncuk.com/forums/showth...otor-do-I-need

Even when critical speed isn't a problem (as is the case with the rotating ballnut) you'll find the 10mm pitch screw wins over 5mm as the torque required to accelerate it is less, due to the kinetic energy of the rotating screw being 4 times less if the screw is only rotating at half the speed.

I'd like to go for rotating ballnuts if Jonathan will be kind enough to make the parts for me, as it would be nice to keep the drive motors within the machine frame, rather than stuck on the end. Also I could use ballscrew without end-machining since it would just be held in a clamp.

I'll work out a price and let you know asap. Though it's theoretically possible to get away without end-machining with rotating nuts I wouldn't advice is as the surface area of the screw in contact at the clamp will be very small. It adds very little to the price with linearmotionbearings2008, so I got my ballscrews cut with the standard end machining, except the same on both ends with the threaded portion. That way you can put a nut on either end to clamp it.

I can get 2510 ballscrew from Zapp, so can you make a rotating ballnut assembly for that Jonathan?

Yep I can do any size, that's what I'm using:



If you want 2510, although I think you'll be fine with 1610, you'd be much better off getting it from linearmotionbearings2008 on eBay. He sells 1605,1610,2005,2505 and 2510.

This is the version I did which is suitable for 16mm or 20mm screws:

[Tenson]

Though it's theoretically possible to get away without end-machining with rotating nuts I wouldn't advice is as the surface area of the screw in contact at the clamp will be very small.

I thought about that too, but it seems easy to overcome. Double-nut the end of it or pot the thread then clamp. I hate waiting for long deliveries so I'd probably get it from Zapp!

Yep I can do any size, that's what I'm using:



If you want 2510, although I think you'll be fine with 1610, you'd be much better off getting it from linearmotionbearings2008 on eBay. He sells 1605,1610,2005,2505 and 2510.

This is the version I did which is suitable for 16mm or 20mm screws:

The bracket on the right of the picture (and in the final picture) looks good for me as I'd want to mount it under the gantry. If you think 1610 will be enough then lets go for that, though the saving seems small (about £15 each screw).

I've started ordering some parts. My plan is to make the Z axis first, then the X and then finally make the machine frame and Y axis. I ordered a 2.2Kw spindle, 250mm ballscrew and some rail and bearings. I'll then make some 10mm plates to hold it all together.

[JAZZCNC]

I thought about that too, but it seems easy to overcome. Double-nut the end of it or pot the thread then clamp. I hate waiting for long deliveries so I'd probably get it from Zapp!

Your joking my last order came special machined from china in seven days. I'll bet you £10 between Zapp and Jonathan you can't get these screws delivered and machined in less time.!!

Sounds a bit swings and roundabouts to me :naughty:

Agreed.!!

[JAZZCNC]

I'll then make some 10mm plates to hold it all together.

Where you going to use 10mm plate.?? Don't use 10mm plate on the Z axis or the gantry sides else again you'll come to regret it. 15mm minimum 19mm better.!!

[Robin Hewitt]

Even when critical speed isn't a problem (as is the case with the rotating ballnut) you'll find the 10mm pitch screw wins over 5mm as the torque required to accelerate it is less, due to the kinetic energy of the rotating screw being 4 times less if the screw is only rotating at half the speed.

Okay you have less rotational inertia from the screw, but twice the pitch means half the driving force on the nut so your max accelerations go down.
Sounds a bit swings and roundabouts to me :naughty: