[a3dcreator]

I'm building a new CNC machine.

It is a fixed Gantry. the moving table X is around 50 pnds.
the Z axis assembly also weighs about 50 pnds. Which rides on the Y axis
They are riding on HIWIN rails. with blocks EGR20CA. the rail is about 20mm or .78 inches wide
I have 690oz Clearpath Servo motors with 200oz continuous torque. They can turn at 1748 rpm

The only thing left to do is buy the proper leadscrew and gearing. It will be belt driven.
Im thinking of keeping a 1:1 ratio with the gearing. so ill basically just be transferring the motion with the gears

so far this is what im calculating and want to know if im doing something wrong.

I could get a leadscrew, 1 start 0.2 inch lead, 5 TPI.
0.75 Inch in diameter 0.563 minor diameter. (length of leadscrew is around 42 inches)
its critical rotation speed is around 1500rpm (thats with 2 ends supported simple. with one bearing on each side.)
is it ok to machine my ends to .5 inches so they fit my pulley clamp bushings?

Which would theoretically give me if I understand it correctly:
right around 300 inches per minute @ 1500 rpm. Or 240 inches per minute @ 1200 rpm.
1500/5 turns per inch = 300 or 1200/5 turns per inch = 240

Im thinking of getting 2 pulleys the same size overall diameter of 2.39 inches one on the motor and one on the leadscrew. link here : McMaster-Carr
To simply transfer the motion to the leadscrew.
Is that size diameter for the pulley good? i cant find any smaller ones that have a clamping Hub. will my motor be able to turn the pulley successfully?
if i understand it correctly smaller gears are better?? what about slippage if i go small on both of the gears?

Im thinking my servos would be able to turn them since my leadscrew would have 1 start at 5 TPI. i would have a bit of mechanical advantage. wouldnt I?

One thing I don't understand as well is how much force is required to move each axis. if the table weighs 50 pnds but its mounted on the rails does this make it lighter?
Probably not right? since i still have to push it around sideways along the rail? I would like to know how much force is required to move each axis.

Ive include some pictures of my machine build so far as well as an image showing motor specs.
thank you so much!

[diepchess]

Posting over month old with lots of questions. Note i'm no expert at all on steppermotors.

As for the forces. Things depend of course upon acceleration a lot and the milling spindels forces you intend to use.

I assume you built it well and that it is very straight. Leadscrew would be fine. I counted number of holes so i guess it's about a 1000mm+ spanning over Y axis.
That is quite a lot for this constructions stiffness.

Without having a running spindelmotor and small acceleration basically something very lightweight would be able to move it.
The spindelmotor will make the difference of course. If you'd be using a watercooled 5 kilo el cheapo chinese spindel of say 2.2 kilowatt,
which doesn't have too much torque. Maybe a kilo or 40 in force it would add. That could be any direction yet especially Z direction.

Will go through wood very easily and mill it nicely. Whereas i like myself a moving X-axis workplate a lot, when it's over a meter length that's quite a lot for such a thin plate, though i can't see whether you reinforced it a lot underneath.

Let's do some crabble note type point calculation for a 30 mm thick plate over 1 meter with in the center 1000 newtons pushing it down (say 500N from the spindle and your workpiece and clamps and weight itself).

First of all all these tubes are working like a kind of a spring as it's just 3 mm thick aluminium tubing, so any crabblenote calculation would fail. Yet even if it would be solid aluminium of 30 mm, you look at a deflection of:

strength of a solid aluminium plate (1000 mm in X length and 30 mm height) : Iyy (mm^4) = (1000 * 30 ^ 3) / 12 = 2.25 mln
deflection = (1000N * 1000mm ^ 3) / (48 * 70k * 2.25 mln) = 0.13 mm

Now this is factors less stiff as it's not a solid aluminium plate and it'll act as a spring because of thin 3 mm aluminium walls, so it's a multiple of 0.13mm worst case.
You wouldn't see this problem with wood though only when milling aluminium not to mention steel.
So quick milling of aluminium or steel with this is probably not a good idea on this.

In short a much stronger moving X plate would be interesting for your mill. that would require some thick tube reinforcement underneath the bed and those tubes you put onto the hiwin rails.
When that makes your X-bed a 50 or even 100 kilo heavier - no big deal of course.

8x8 cm tube structure underneath X axis plate to reinforce it would be massive improvement. Also may be from steel optional. Steel is 3x stiffer than aluminium, so that 70k would be 210k then in the above calculation yet the material is less relevant here than the shape of it that gives the stiffness.

I'm not so experienced in motorcalculations yet it really will depend upon which spindel you intend to use.

The sort of motors you drew seem to me like total expensive overkill in terms of accuracy compared to the stiffness of the gantry.
Cheap nema23 stepper will do at most spots and after reinforcements a nema34 for the X axis should be more than enough as well.

Stiffness is a function of spanning distance to the power 3 in this case so with this seemingly meter length it's a tough one to judge whether the structure is stiff enough with just reinforcing the X-bed and one additional tip below.
I really like the strong standers and the 45 degrees reinforcement. Yet i'm not sure what their size is and i'm guessing it is 8x8 CM which if it is true is again disappointing yet they really are so much better
in combination with the 45 degree reinforcement..

For this huge distance overspanning the Y axis i would've preferred using the 8cm * 16 cm profile as a stander and a single 8x16 as the 1 meter span with both rails of the Y axis on that 8x16.
Then you can just simply adjust the entire 8x16 profile with Z axis and Y axis and leadscrew on it if you want to mill one time in your life higher in Z axis and by default use at most 1 foot of clearance for the Z axis reach to the bed. For such weak aluminium structures maybe much better idea is Z clearance of 200 mm so 2/3 of a foot.

You can always attach the entire Y-axis higher later in life...

Gives a much stiffer Z axis. Right now the Z axis is going to be junk probably as the distance from the rails to the bed is gonna be huge.
Obviously that's why you put so much room between the 2 extrusions of 4x8 CM i suppose, yet that just enlarged the stiffness problem of the entire machine as that moved up your gantry a lot in the air.

What i do not understand exactly is how you want to run the leadscrew. Seems to me that you want a very complicated construction of a multifunction al plate that holds a bearing where the leadscrew rotates and a pulley that is on the leadscrew on the inside of the leadscrew. Aren't you afraid in this setup that the pulley will bend the leadscrew?

Yet the complicated thing in the setup seems to me how to attach and adjust it in a good manner. Seems complicated spot for the leadscrew to me.

Yeah distance between the 2 y-axis rails really is too much i suspect. Will be very complicated to get those 2 running very parallel to each other i suspect -
but who knows - some guys are good in adjusting that. Myself i really would want the rails to be on the same tube (or reinforced beam).

In this case you might get away with a lot of the above by attaching say a 5mm aluminium or 3mm steel plate on the back of the 2 extrusions of 4x8 cm on which the Y-rails get attached upon.
That plate ideally would be fastened on the 8x8 standers as well as the 4x8 cm extrusions. So what is it like 4 feet plate x 1.5 feet or something?

And then fasten it with really a lot of t-slot nuts.

Yet the leadscrew you might want to move a bit more forwards towards the rails. As i suppose those rails with carriages is ging to add roughly 4 centimeter anyway.

And i really would move the entire Y-rails a lot lower and closer to each other as well. Limit that Z-clearance as much as possible i'd say (of course if you reinforce the X bed it gets higher).

With some minor improvements as mentionned above i feel you can get structure a lot stiffer which is the most important thing to fix. The other choices are going to be way less relevant.

[a3dcreator]

Thanks for the reply and the help. and sorry for my late reply. I've been finishing the machine up.
i agree with some of your comments. i wish i wouldve used 3 x 3 for the gamtry horizontals. Ive been running some tests on the machine its looking very promising so far. here is a video.
Sorry about the quality. this website wont let me upload anything more than 50 kb.

[diepchess]

Measure stiffness of it with a dial gauge each time on another part of the gantry. From a video you can't see much anyway - you need to measure it.