Mounting motor under table?

[JAZZCNC]

The stepper & ratio issue is something I've yet to address. I need to work out the benefits of the various ratios first. I'm pencilling in 1:1 until I've had a good think over a hot cuppa.

If you haven't bought the screws yet then I'd go with 1/1 and get the correct pitch screws best suited to your needs. Mainly for simplicity.
2/1 will double the torque but half the speed and so on. 1/2 will double the speed but half the torque and so on.
One of my machines use's 1/2 with a 20mm x 5mm pitch screw which effectivly gives me a 10mm pitch screw but with reduced torque, the plus side being it's a simple belt & pulley change and I have full torque and high resolution but at half the speed, It works good.
2/1 is usefull if you have small Nm steppers which tend me able to spin faster than larger steppers but with lower torque couple these with a high lead screw and smaller steppers can be made quite strong.

[Rogue]

If you haven't bought the screws yet then I'd go with 1/1 and get the correct pitch screws best suited to your needs.

"The correct pitch screws" should really read "what I manage to find when I have the cash in my hand".

Realistically, C7 1605 screw is what I'm looking at: £20 each for X/Y axis, not sure how much it would cost to get the ends turned down though. No idea about ball nut yet, whether to wait for longer and save for proper ballnuts, or look for a cheap alternative and later replace.

My build is influenced by the fact that I've just completed a course and I'm trying to get back into the workforce, hence not putting down lots of hard cash in one go. I'm hoping to be working again by the time the design phase is finished and the purchasing begins :whistling:

[JAZZCNC]

Personally I'd go with 1610 it's a better all round pitch. Not sure what you mean by only buying the screws and sorting the nut later because it's normal to buy the ballscrew and ballnut together.
RE: Turning. Even thou I have a good lathe and capable of turning my own screws I dont bother and buy them pre-machined from china, It's just not worth me doing it my self for what they charge.

I'd save and buy the ballscrews before buy cheap and replace later as it's often more than just quick swap over job due to the differant way the connect or work, Plus cheap acme lead screw is rubbish and good quality acme lead is more expensive than ballscrews from china.!!

[Rogue]

Personally I'd go with 1610 it's a better all round pitch. Not sure what you mean by only buying the screws and sorting the nut later because it's normal to buy the ballscrew and ballnut together.
RE: Turning. Even thou I have a good lathe and capable of turning my own screws I dont bother and buy them pre-machined from china, It's just not worth me doing it my self for what they charge.

I'd save and buy the ballscrews before buy cheap and replace later as it's often more than just quick swap over job due to the differant way the connect or work, Plus cheap acme lead screw is rubbish and good quality acme lead is more expensive than ballscrews from china.!!

Ah, I don't think my post was as clear as it should have been, my apologies.

I'm planning to go for ballscrew+ballnut, but I noted that, depending on where you look, there seems to be a big jump in price from the ballnuts without preload/zero backlash to the ones that do have it (which I would think are preferable). Going cheaper would get me running sooner, and having a working (albeit lower precision) machine to play with while building up more funds is preferable to me. There also seemed to be suggestions for ways to increase the effectiveness of the lower end ballnuts, though now that I think about it, they may have been talking about the acme setups. Hmm.

As for turning the ballscrews, I suppose that's a matter of perspective. If you have the skills and equipment but your time is money then yes, turning the ends down to fit the bearings probably is better left to the supplier. Would I do it that way? Ask me in 5 years

1610 rather than 1605, you say? I shall run that though the collection of calculations I'm building up.

The following picture is a rough mockup of what it's running - working area was originally planned at 200mmx200mm for the table, will have to be bigger simply to fit the stepper in the location parallel to the screw. The gantry will be close to the table but large so the X can run up quite high and still be supported, for reasons discussed elsewhere. "Scale" is a dirty word as far as this picture is concerned, don't read too much into it. The XY will not be completely seperate from the gantry, it was just easier to draw that way!

[JAZZCNC]

As for turning the ballscrews, I suppose that's a matter of perspective. If you have the skills and equipment but your time is money then yes, turning the ends down to fit the bearings probably is better left to the supplier. Would I do it that way? Ask me in 5 years

Not really I have the time, equipment and the skills but ballscrews are hardened and the indexable tips required cost me nearly as much as what I get charged.
So to me it's not worth the hassle and all thou unprobable still possible risk of error.

RE: 1610 or 1605. I should clarify this depends on the intended use of the machine. If your going to use it to cutt intricate jewelry or want very fine detail then the small pitch is better but for a general allround machine then 10mm pitch works best in my experience.

[Rogue]

All round, I'd guess. If I understood the earlier posts correctly, if I had a system for changing pulleys I could get the higher precision if needed, at the expense of speed? If so, this is one of the things that interest me about pulley systems.

[Jonathan]

RE: Turning. Even thou I have a good lathe and capable of turning my own screws I dont bother and buy them pre-machined from china, It's just not worth me doing it my self for what they charge.

That's precisely what I thought. I would machine a ballscrew if someone asked, but it's just not worth is with linearmotionbearings2008 prices on eBay.

All round, I'd guess. If I understood the earlier posts correctly, if I had a system for changing pulleys I could get the higher precision if needed, at the expense of speed? If so, this is one of the things that interest me about pulley systems.

That is correct. Lets say, for example, you have a standard stepper motor so 200 steps per revolution. The travel per step on a 10mm pitch screw will be 0.05mm with direct drive or 0.1mm or 0.025mm depending on if 2:1 or 1:2. Using microstepping will get finer resolution up to a point.

Though I would normally advise 10mm pitch screws (I probably did earlier!) since this is such a small machine it may not make a lot of difference...having said that the main thing is getting high acceleration, otherwise you'll run out of travel before it hits top speed and has to decelerate. To do that a 2:1 (bigger on stepper) with a 10mm pitch screw would be ideal since the motor has to input only a quarter of the kinetic energy into the screw when accelerating compared to a 5mm pitch screw.

Your 'system' for changing pulleys is just an allen key plus a bit of space.