Design Question

[ynnek]

I have a build coming, I have been donated some nema34 motors & drives, along with power supplies & 2 96" lengths of 25mm supported linear rail, which I will cut in have to build a 48 x 48 or so router. Aluminum extrusion & 6061 aluminum plate, 6" or less gantry clearance. Is R&P recommended for a machine of this size, or is a lead screw a good choice? Just looking for a bit of advice. I have no concrete design as of yet, But I do have these parts in hand along with some lengths of R&P & gears. I was undecided on the drive train. Ball screws are more than I will be willing to pay for, though I may put one in the Z. If I go with lead screws, would I need one on each side of the gantry or one in the middle, what are the advantages & disadvantages? I hope to have all of my design finished soon to start ordering the extrusion & the plate. I will have the plates water cut from a local supplier.

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[awerby]

At 48", you can go either way - screws or rack-and-pinion drive. If it were me, I'd keep those rails long and buy some other ones for the Y axis; short rails aren't hard to find, and you'll find the extra work envelope comes in very handy for projects you haven't thought up yet. For a 96" span, rack-and-pinion makes more sense than a screw. Many people like to drive the longest axis from both sides to avoid the "stuck drawer effect", but it's not strictly necessary. You only have to use aluminum for moving parts like the gantry (and not necessarily even there); it's a waste of money for stationary parts, where steel not only is cheaper but works better. That t-slotted extrusion that's so popular is especially expensive; it's light and fairly stiff but hard to justify on a budget build, unless you don't have the equipment to deal with other materials.

[ynnek]

Thanks for the reply. These rails ar from my 48 x 96 router at work. My co-worker & I upgraded them from the round supported rail to the profile rail. Thomson I think. The parts I am getting weere left over from a complete remodel of the Vortech router. The spindle died & we upgraded to an ATC spindle from Blurry customs with a 16 position rack completely full of tools, & the steppers that came on it were replaced with a servo kit from CNC4pc, as well as the c32 bob. Also, as upgrades go, a home brew rotary axis with 48" between centers, which I build variable pitch feed screws for packaging equipment. I do not think I will need 4x8 at home, but one never knows,It is something to think about. I pretty much have to do extrusion as I am no welder, nor do I own one. I do not think I could get the fab work done at work, but once again, you never know. It will be a while to buy all of the materials. I have some design done but nothing to show as of yet.

[wizard]

andrew gave you good advice but there is one thing that is worth a warning. Make a bigger machine and it becomes really heavy if you work to maintain the same stiffness. It becomes something like a bolted down bench, there is a lot of work involved in moving it around. This is especially important in a home shop where you have limited material handling equipment.

I only mention this due to building a work bench in my shop from re tasked metal frames I got from work. Great bench but it is very heavy and with a lathe on it impossible to move without a floor jack. So size is an issue even a 48" square machine takes up a lot of floor space.

Also I agree on the issue with Aluminum, unless you can get stuff very cheap, second hand or whatever, it is very hard to justify it on cost. The only way it might be justified if you are someone that wants to build a precut kit and want t avoid the cost of shop equipment to deal with aluminum or steel processing.

[hanermo]

The "best" choice for driving a gantry is one motor/leadscrew, in the middle.

It has complications in motion, mounting, height, etc.

Usually, many people mention mass re moving parts, which in the real world has zero effect.

Example:
I used tiny nema 23 steppers, at 1:3 belt drive, to move a 200 kg moving table, with 100-200 kg of stuff on it.

Probably multiple of your gantry.

Another typical "error" is speed.
You dont really care about speed if you are using steppers.

If You want/need speed it´s due to desire to make "more stuff faster" aka commercial use.
Just use servos, done.

Another, controversial (here), issue is alu in machines.
Just dont.

99.+% of all machines in the world don´t use alu.
There is a Reason.
Why pay 3x more for stuff vs steel or ci ?

My current up-down bridge will be about 700 kg in mass (bridge only).
I have a small nema 23 servo - more than sufficient.

[ynnek]

The "best" choice for driving a gantry is one motor/leadscrew, in the middle.

It has complications in motion, mounting, height, etc.

Usually, many people mention mass re moving parts, which in the real world has zero effect.

Example:
I used tiny nema 23 steppers, at 1:3 belt drive, to move a 200 kg moving table, with 100-200 kg of stuff on it.

Probably multiple of your gantry.

Another typical "error" is speed.
You dont really care about speed if you are using steppers.

If You want/need speed it´s due to desire to make "more stuff faster" aka commercial use.
Just use servos, done.

Another, controversial (here), issue is alu in machines.
Just dont.

99.+% of all machines in the world don´t use alu.
There is a Reason.
Why pay 3x more for stuff vs steel or ci ?

My current up-down bridge will be about 700 kg in mass (bridge only).
I have a small nema 23 servo - more than sufficient.

Again, thanks for the advice. The motors & drives might be overkill, but they are free, so I will stick with the nema34 steppers. The choice for aluminum extrusion is I can work with it at my house. I am no welder, though I may be able to get some done where I work, but I do not want to push my luck.you have me thinking though. :-)

[ynnek]

Well it looks as though I will be driving the gantry from the sides. First thoughts are to use a 1/2" 5 start screw, but I may go with R&P. My design, on paper will have a 48 x 48 cut area. I will use the nema 34 motors I currently have with a C32 board from CNC4PC, the same board in my router at work. The linear guides on the Y & X axis are 25mm & the guides on the Z axis will be 20mm. I considered some v-groove bearings, because I have them on hand, but I have opted to purchase the 20mm units instead. I will go with a 2.2kw chinese, water cooled spindle. This will be an aluminum extrusion & aluminum plate build, onto a wood table constructed from 4 x 4 or 6 x 6 with a plywood top. Gantry clearance will be less than 4 inches, which will be plenty for what I will be making. 8020 3030 beams to mount the y axis rails to & a 3060 beam for the gantry. Once I gather 70% of my parts I will start my build, It can be seen here on my Fusion360 Page - Home CNC Router

I am open to any and all constructive criticism, so comments are welcome.

[Al_The_Man]

As a general rule, the type of cutting/machining on a gantry type cutting table does not require the precision of a ball screw.
So R&P is usually the more economical,especially when sizing 48"x48" and up.
To avoid using a method on both sides in order to maintain tracking accuracy, only side is made compliant.
Pacific Bearing make some useful linear bearings, such as their Simplicity series.
Al.

[ynnek]

As a general rule, the type of cutting/machining on a gantry type cutting table does not require the precision of a ball screw.
So R&P is usually the more economical,especially when sizing 48"x48" and up.
To avoid using a method on both sides in order to maintain tracking accuracy, only side is made compliant.
Pacific Bearing make some useful linear bearings, such as their Simplicity series.
Al.

Sorry for the confusion, I was referring to an acme lead screw. But thank you for the input & the lead on the pacific bearing products.

[ynnek]

I do have access to some R&P. it is 20 pitch, 14 1/2 degree pressure angle. Is there a draw back to using this versus 20 degree?

[Al_The_Man]

This Boston gear PDF shows the differences.
Al.

[ynnek]

This Boston gear PDF shows the differences.
Al.

Thanks again, AL. BTW, I love your avatar. So, do you think is 48 inches too long of a span to use a belt drive?

[Al_The_Man]

If you mean endless timing belt drive system I have used the products from Breco Belts for a 8ft span.
BRECOflex CO., L.L.C. - High Precision Drive Components
Al.

[ynnek]

Yes, exactly. I may go that route. How is the accuracy with this method?

[Al_The_Man]

These belts are very accurate, and are non stretch etc. Support is needed to avoid sag etc.
Al.

[ynnek]

I like the idea of using belts but after pondering it, I will be going with the R&P. Upon further investigation, the R&P I have is actually, 20 pitch, 20°PA. I have matching pinions. Just more stuff I do not have to buy. :-)