Gantry advice please

[Crunch]

Hi
I have introduced my self in the new comers section ,
I have a few question to ask.

I had planned to use box steel for gantry,
Cut a letter box in the front so I could put ball screw inside.
The box is 200 x 100 x 8 not the one shown in photo.
I wanted to make this as rigid as possible as I want to be able to cut aluminium.
What is an acceptable weight of a gantry with every thing on it including spindle, it terms of performance, I understand the heavier it gets the more effort it takes to start and stop it when moving due to the mass. Do you just deal with the extra weight with gearing or bigger steppers? Or is there a sweet spot in weight ?


Also which ball screws would be best to use,

2 x 900mm 1605 or 1610 with pulleys y axis. ( front to back ) or

2 x 900mm 2005 or 2010 with pulleys y axis. ( front to back ) would you get whip on this length and would these be better with a heavier gantry?

1 x 1200mm 2005 or 2010 with pulleys on x axis ( left to right ) again for whip due to length .

I have planned dual steppers for y axis .
Any advice would be great .

Regards Paulhttps://uploads.tapatalk-cdn.com/20190502/9b74219c0b3383d8283afd9fea0ba2c4.jpghttps://uploads.tapatalk-cdn.com/20190502/5193071ab3443967225f0b374302a5b1.jpghttps://uploads.tapatalk-cdn.com/20190502/d037a1a77a4e5cbc1cb05965b7e18fa3.jpghttps://uploads.tapatalk-cdn.com/20190502/c43136dbf2e4e3075ffe9c7f1b4e68c2.jpg

Sent from my SM-G900F using Tapatalk

[Voicecoil]

Hey, you've got some tidy welding there! If you look here:

http://www.mycncuk.com/threads/1524-What-size-stepper-motor-do-I-need?highlight=stepper+motor+size

there's a very useful spreadsheet that enables you to play with and optimise weight/ballscrews/motor size. I'm not quite sure what you mean by cutting a letterbox in your gantry box, but do remember that the torsional rigidity of a box section will be significantly reduced if you start cutting longitudinal slots in it, it will become more a like a C-section/channel.

[Crunch]

Thank you
I'll have a look later .


Sent from my SM-G900F using Tapatalk

[pippin88]

Don't cut the tube. Add diagonal braces and end caps (welded, glued, or bolted).

Don't worry about weight. My dual drive 381 Oz in steppers will throw around my 100kg gantry system at 20metres/min.

[Crunch]

Don't cut the tube. Add diagonal braces and end caps (welded, glued, or bolted).

Don't worry about weight. My dual drive 381 Oz in steppers will throw around my 100kg gantry system at 20metres/min.

Thank you for advice,
how big is your machine ?
what size pitch and ball screws you using to move gantry, have you used pulleys, if so what ratio.?

Regards Paul

[routercnc]

If it helps while you wait for the time difference my gantry must be close to 100kg as it is double beam with lots of thick aluminium plates on Y and Z, double screws etc. I have 2 ballscrews driving it at 1605 size and pitch with no issues but would happily go to 1610 if I did another one to get the speeds up for wood. Then swap pulley out to 2:1 for aluminium if required. They are just over 1000 mm and no whip issues.

Each screw is driven by a 3Nm stepper of about 3mH inductance at around 70V. A very strong set up. I’m running legacy (old!) analogue stepper drivers but for a new build go digital.

If you are yet to order the screws and want to go belt drive then make sure you ask for enough machined length on the end to get the pulley on as the standard offerings can be a bit short. Check the drawings they provide against how you will use them to be sure.

Your other question about the 1200 mm ballscrew on Y - this is getting long for screws but I would say still doable. I’ve not built anything with screws that long but my thoughts would be stay with 1610 to avoid having too much inertia and provision for another ballscrew support at the floating end to add some pre- tension. Also I’d think about adding 2 ballnuts on the Y axis unit spaced as far apart as the Y/Z axis allows to hold the ballscrew in 2 places and give slightly less free length when the Y axis is off to one side.
If someone has direct experience of 1200 mm screws then let that be your decider as the above are just my thoughts based build quite a few slightly smaller machines.

As for the letterbox idea it is tempting but as has been mentioned you will loose a lot of stiffness. Mount the screw somewhere on the outside even if you have to space that axis away from the gantry a bit.

Looking great so far by the way �� (and welcome to the site)

[Crunch]

If it helps while you wait for the time difference my gantry must be close to 100kg as it is double beam with lots of thick aluminium plates on Y and Z, double screws etc. I have 2 ballscrews driving it at 1605 size and pitch with no issues but would happily go to 1610 if I did another one to get the speeds up for wood. Then swap pulley out to 2:1 for aluminium if required. They are just over 1000 mm and no whip issues.

Each screw is driven by a 3Nm stepper of about 3mH inductance at around 70V. A very strong set up. I’m running legacy (old!) analogue stepper drivers but for a new build go digital.

If you are yet to order the screws and want to go belt drive then make sure you ask for enough machined length on the end to get the pulley on as the standard offerings can be a bit short. Check the drawings they provide against how you will use them to be sure.

Your other question about the 1200 mm ballscrew on Y - this is getting long for screws but I would say still doable. I’ve not built anything with screws that long but my thoughts would be stay with 1610 to avoid having too much inertia and provision for another ballscrew support at the floating end to add some pre- tension. Also I’d think about adding 2 ballnuts on the Y axis unit spaced as far apart as the Y/Z axis allows to hold the ballscrew in 2 places and give slightly less free length when the Y axis is off to one side.
If someone has direct experience of 1200 mm screws then let that be your decider as the above are just my thoughts based build quite a few slightly smaller machines.

As for the letterbox idea it is tempting but as has been mentioned you will loose a lot of stiffness. Mount the screw somewhere on the outside even if you have to space that axis away from the gantry a bit.

Looking great so far by the way �� (and welcome to the site)Thank you for your welcome to the site.

Thank you for the information that's a great help.
I was mainly wanting to fit ball screw inside gantry to save space and keep it compact. Based on comments not a good idea as it will weaken it to much .
I even thought of welding a round tube inside the gantry for the ball screw to run in, still cut the front out and weld it all up so it braced the open parts together.
I don't know if that makes sence. Lol
Also dont know how much that would weaken it.
I will think on how to mount it on the out side.
Thanks for advice on ball screw length , I've been talking with Fred from bst ( I think that's what his company is called )
He has been very good. I've not ordered linear rails yet as I want to order ball screws at the same time but not sure what to order hence questions.
I'm not ordering any electrics until mechanicals are finished.
What do you think to closed loop steppers vs open steppers.
Regards Paul


Sent from my SM-G900F using Tapatalk

[AndyUK]

What do you think to closed loop steppers vs open steppers.

I think the majority of people on here use open loop steppers or servos - don't forget you can get open loop stepper drivers which can very reliably detect missed steps and feed back an alarm signal.

[Crunch]

I think the majority of people on here use open loop steppers or servos - don't forget you can get open loop stepper drivers which can very reliably detect missed steps and feed back an alarm signal.Hi Andy
Thanks for information,
Ive been following your build [emoji106] [emoji106]

Regards Paul

Sent from my SM-G900F using Tapatalk

[AndyUK]

I been following your build [emoji106] [emoji106]


Thanks! :) I too started with the idea of a steel box section gantry, but then temporarily lost access to the Bridgeport and milling the faces became an outside job. Mild steel isn't near straight enough to mount rails to directly.

Once I did the deformation calcs and realised HD aluminium profile could perform just as well, and was an aweful lot easier to build with, it tipped the balance.

[pippin88]

Thank you for advice,
how big is your machine ?
what size pitch and ball screws you using to move gantry, have you used pulleys, if so what ratio.?

Regards PaulGantry span 1300mm.
15mm pitch screws. 15mm diameter I think. They are secondhand ground screws.
I use 16mm pitch 16mm diameter screws on other axes (from BST automation).

No pulleys.

Steppers are best used with high pitch screws (10mm or more) not 5mm pitch screws. Steppers have max torque at 0rpm and it falls as rpm rise.

Servos are different. They are rated for torque at rated rpm (often 3000rpm).

[Crunch]

Thank you for all your information.

I think I will go with
1610 x 900mm x 2 for y axis
1610 x 1200 for x axis ( gantry )
1605 for z axis ( still sorting out the lenght )
Going to get ends machined to suit pulleys.

20mm square linear rails all round.

Does this sound ok ??

Thank you

Sent from my SM-G900F using Tapatalk

[Clive S]

Going to get ends machined to suit pulleys.

The bit where the pulley is fitted is called the f length 25-30mm is usually fine BUT remember that the f length is included in the total length of the screw.

Standard machining for the f length is about 15-20mm.

Make sure you get the better bearings AC type for the screw as some suppliers will just put in simple bearings in the blocks.

[Crunch]

I was going to order from Fred at BST
He has offered machining the ends for pulleys.
He also offered better bk/bf bearings.
Is this the bearings you are talking about?

Thanks for advice

Sent from my SM-G900F using Tapatalk

[pippin88]

I've had very good experience buying from Fred.

Why 1605 on the Z?

[AndyUK]

Thank you for all your information.

I think I will go with
1610 x 900mm x 2 for y axis
1610 x 1200 for x axis ( gantry )
1605 for z axis ( still sorting out the lenght )
Going to get ends machined to suit pulleys.

20mm square linear rails all round.

Does this sound ok ??

Thank you

Sent from my SM-G900F using Tapatalk

Your X axis is quite long for 16mm - You'll get a 12mm root diameter on these, which means assuming you use a BK and a BF, you'll have a max speed of 1000rpm before whipping. With the 10mm pitch you're okay because it sets your max rapids of 10m/min - definitely wouldn't want to go 1605 though! A 20mm screw instead will give you a max rpm of 1260 which bumps your possible rapids up to 12.6m/min, but will take slightly more power from your motors.

The Y axis gives you a max rpm of 1800, which is fine.

[Crunch]

Pippin88

Only because it only travels a short distance. So thought 5mm pitch was fine for that.

And I don't know . Lol

I've read loads of builds and most use 5mm pitch . But I'm open to advice.

Sent from my SM-G900F using Tapatalk

[Crunch]

Your X axis is quite long for 16mm - You'll get a 12mm root diameter on these, which means assuming you use a BK and a BF, you'll have a max speed of 1000rpm before whipping. With the 10mm pitch you're okay because it sets your max rapids of 10m/min - definitely wouldn't want to go 1605 though! A 20mm screw instead will give you a max rpm of 1260 which bumps your possible rapids up to 12.6m/min, but will take slightly more power from your motors.

The Y axis gives you a max rpm of 1800, which is fine.That is where I started with 2010 on x axis
To stop whip as I had read so much.
The more I read the harder the decisions. [emoji22]

Sent from my SM-G900F using Tapatalk

[AndyUK]

Only because it only travels a short distance. So thought 5mm pitch was fine for that.

And I don't know . Lol

I've read loads of builds and most use 5mm pitch . But I'm open to advice.

Sent from my SM-G900F using Tapatalk

I used 1605 on my Z. Thoughts were rapids are less of an issue as you're travelling the smaller distance, and theoretically the fine pitch gives you twice the height resolution.

At a stretch, maybe thats better for surface finishes?

[AndyUK]

That is where I started with 2010 on x axis
To stop whip as I had read so much.
The more I read the harder the decisions.

Well it was routercnc who said stick with the 16mm due to the added intertia of the 20mm. The man has built four of these things, he knows what hes doing.

I'm saying they'll work, but realise that they'll limit your rapids more than 20mm will. Personally I'd be more than happy with 10m/min rapids - You'd need some pretty strong motors to get up there anyway with a 100kg gantry.

[Clive S]

I was going to order from Fred at BST
He has offered machining the ends for pulleys.
He also offered better bk/bf bearings.
Is this the bearings you are talking about?

Thanks for advice

Sent from my SM-G900F using Tapatalk

Yes those are correct bearings. Just tell Fred the f length you require but as I have said the ball screw length includes the f length . Its an easy mistake to make.

So just order say ball screw machined both ends 1610 x 1200 with f length of 30mm

(where 1200 is the total length)

For the avoidance of doubt my router has twin screws twin motors and the screws are 1450mm overall. Motors nema 23 . without any problems.

[Crunch]

Yes those are correct bearings. Just tell Fred the f length you require but as I have said the ball screw length includes the f length . Its an easy mistake to make.

So just order say ball screw machined both ends 1610 x 1200 with f length of 30mm

(where 1200 is the total length)

For the avoidance of doubt my router has twin screws twin motors and the screws are 1450mm overall. Motors nema 23 . without any problems.

Are your ball screws 1610 or 2010 ??
How heavy is your gantry, roughly ??

[Clive S]

Are your ball screws 1610 or 2010 ??
How heavy is your gantry, roughly ??

1610 and about 70kg

25747

there are two 1000x60x60 x5mm box section behind the front plate on the gantry.

Also you don't need the screw to be the full length of the machine

[Crunch]

I like the ajustable bed.

How wide are the plates top and bottom of your gantry. As you have slotted your gantry for ball screw.
I like the look of your gantry.

The slot in your gantry is what I had planned for my box steel,so I could fit the ball screw inside. But I didn't realise how much strength I was going to lose. I ordered 8 mm thick to allow for it. After posting here and getting the information realise it will weaken it to much.



Sent from my SM-G900F using Tapatalk

[Clive S]

If I was doing it again I probably would have used heavy duty ali profile formed in a L shape. But that was my first router.. Gantry front plate 1000x210mm
Some pics here:- https://photos.app.goo.gl/cfbXGLBdxYvB9s536

[mattnedgus]

Don't cut the tube. Add diagonal braces and end caps (welded, glued, or bolted).

Don't worry about weight. My dual drive 381 Oz in steppers will throw around my 100kg gantry system at 20metres/min.


Pippin88,

How do you get 20m/min out of those motors direct-driving your screws? Is it something to do with the quality of screws you're using?

I can only get 7m/min direct-driving 1530mm of 2010 which (I thought) agreed with the theoretical max speed of the calculations I did.

On a 16mm pitch that same calculation says I could maybe get 12144mm/min on the same motors but that still seems way off what you're managing (and my gantry is light - made-from-plywood-light!).

Matthew

[Max Maker]

Your X axis is quite long for 16mm - You'll get a 12mm root diameter on these, which means assuming you use a BK and a BF, you'll have a max speed of 1000rpm before whipping. With the 10mm pitch you're okay because it sets your max rapids of 10m/min - definitely wouldn't want to go 1605 though! A 20mm screw instead will give you a max rpm of 1260 which bumps your possible rapids up to 12.6m/min, but will take slightly more power from your motors.

The Y axis gives you a max rpm of 1800, which is fine.
Could you make that calculation for a 2.2m long ballscrew? I only have 2010 available so far.

[Neale]

Could you make that calculation for a 2.2m long ballscrew? I only have 2010 available so far.

If you Google for "critical speed calculator", you will find a number of sites where you can plug in your dimensions and check for max speed. They all give slightly different answers but you will get a good idea of what might be possible. Then you can play with different lengths, motor/screw gearing, etc, to meet your needs. For example, my machine is configured more for acceleration than speed as I do a lot of fine work where I seldom hit maximum speed anyway.

[Max Maker]

Thanks. Looks easy enough.