Mounting motor under table?

[JAZZCNC]

Assuming there are no clearance issues with the table/nut, are there any disadvantages to mounting the motor under the table, parallel to the leadscrew?

Another of my soon-to-be-infamous-for-low-quality example images:

(not sure if the picture worked, ISP is being a pain today)

I'm hoping that a pulley system of some form would be suitable to belt-drive the leadscrew from that location.

Hi Rogue,

I often use timing belts on machines and in the configuration you plan on using.
I've built several machines which use's them on all axis inc Z Axis and they work great without any issue's. They have all the bennafits Jonathan states plus they have one other little known plus. They help reduce resonance, which if bad can affect motor performance.
I've found 20 tooth 15mm wide HTD to be the best for 1/1 connection. They wear the best and easy available at any bearing company. They also have the best size boss on the pulley for using a decent sized grub screws or small bolts for gripping the shaft.

If you want to up the ratios's say 2/1 or 3/1 then 15 tooth is about the smallest I would use on the driven side using steppers, thou be aware the boss size is too small for decent grub screws so you have to then drill between the teeth and make sure the head of the screws is below the surface.

Also be aware that a lot of company's supply the pulleys with a small pilot bore which you then bore to the size required. Drilling with a decent drill press is possible but very easy to get wrong and create runout and if done this way then I'd recommend drilling smaller and reaming to exact size. . . Job best done on a lathe.

[Jonathan]

Yes they do a good job at reducing resonance.

Also be aware that a lot of company's supply the pulleys with a small pilot bore which you then bore to the size required. Drilling with a decent drill press is possible but very easy to get wrong and create runout and if done this way then I'd recommend drilling smaller and reaming to exact size. . . Job best done on a lathe.

Get them delivered to me and I'll bore (or ream) them to the right size on my lathe for you. Just pay me the postage...it's only a 5 minute job, if that.

[Rogue]

I've found 20 tooth 15mm wide HTD to be the best for 1/1 connection. They wear the best and easy available at any bearing company. They also have the best size boss on the pulley for using a decent sized grub screws or small bolts for gripping the shaft.

If you want to up the ratios's say 2/1 or 3/1 then 15 tooth is about the smallest I would use on the driven side using steppers, thou be aware the boss size is too small for decent grub screws so you have to then drill between the teeth and make sure the head of the screws is below the surface.

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.

Get them delivered to me and I'll bore (or ream) them to the right size on my lathe for you. Just pay me the postage...it's only a 5 minute job, if that.

I may well avail myself of your services, though it might be some time away. Thanks!

[Jonathan]

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.

Smaller pulley on stepper = better resolution, which is generally not that big a benefit.
Larger pulley on stepper = higher speed, up to a point which I've not yet managed to reach.

The main thing is as you increase the speed of a stepper motor the torque to start with is fairly level, then drops. You want to size the pulley such that the motor is operating mainly in the region before the torque drops off.

Also I expect a smaller pulley on the stepper motor will get better acceleration.

Just for example with a 13T pulley on my Y-axis stepper, and 12T pulley (bit too small really) on the 10mm pitch screw I get 36,000mm/min (1417 ipm). Changing the 13T to 30T gets about 55m/min (2165 ipm), and changing to 42T gets 60m/min (ipm = lots). Clearly the 42T pulley isn't worth the extra <10% speed given the significant reduction in resolution. Microstepping the stepper motors to artificially gain resolution is really only cheating yourself after about 1/4, maybe 1/8 of a step. The accuracy of micro-stepping depends on the motor and what it's connected to. In general apparently it is not that accurate, but still use it for general smoothness.

You can do some pretty impressive things with 10mm effective pitch as I'm sure JazzCNC will soon demonstrate...

[JAZZCNC]

You can do some pretty impressive things with 10mm effective pitch as I'm sure JazzCNC will soon demonstrate...

How would you like me to do that Jonathan.? With my little Aztec friend. . Lol

[Jonathan]

How would you like me to do that Jonathan.? With my little Aztec friend. . Lol

Yep, that's just what I was thinking. I'd like to try one of those on my machine.

[JAZZCNC]

Yep, that's just what I was thinking. I'd like to try one of those on my machine.

Ok just for jonathan Here's what 10mm pitch gets you.!!

Johnathan if you want the code your welcome to it, or any body else for that matter. It's a 17mb file thou and over a million lines of code, If I remember correct it took about hour n half.
I did another after that but with the Welsh dragon on the back side but didn't take any pics. . . . Looked good thou.!!

Enjoy and the pics don't show all the detail or do it justice. . . 150mm diameter 10mm Ali plate cut with a 6mm 90deg carbide chamfer/spot drill.

[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.!!