Need HELP with Design and Parts

[Texasbowhunter]

I have always thought about the ingenuity that is involved with the construction of a CNC machine and just figured It was out of my ability to build that I would have to buy one and move on…That is until recently when I meet a man who had built one a few years back…Well It did not take me long to realize I have to try and build one so here I am with questions I hope to get answered…So presently I’m in the design state…I have watched a lot of videos from a lot of amazing folks with quite an ability from start to finish and all I can do is take my hat off to them…
Kudos to those of you that understand all the little intricate and details that would go into a build like this…For me I’m lacking in the understanding of a lot of that engineering that would go into a build but am most interested in trying to understand…So please bear with me…I probably will ask a lot of seemingly stupid questions for those of you that are talented

So let me start out by stating I’m trying to build a nice machine that will be used as a hobbyist machine for me and family so something that is built for commercial use will not be factored in however we will try and build as much quality into the design and fab…I was thinking as a bonus It would be nice if it was able to cut aluminum say up to ½” would be a huge plus…I have a budget and am hoping to stay in, I figured around $3K-$3500 range…

I’ll be building this in my garage with basic tools…Drill press, table saw to cut the aluminum, although I was thinking that it may be better to fab all up in wood 1st then come back and cut on a CNC router the aluminum…I also have access to a Mill however I’m a bit weak on its use, but have looked for an excuse to improve…Now with this said my thoughts for the wood was to fine tune my drawings for the various parts with the help of a friend and ask him to possibly cut the aluminum plate on his CNC machine I have stashed…
I have ¼”, ½” and 1” and may consider getting the 2” thick I saw in a pile and purchase that…however this would all be part of the budget…

Anyhow
My intentions is to build a machine with a 4’ x 4’ cutting area…
The platform is going to be 2” x 2” tube steel perhaps .185 thick in the wall so from everything above the “X” linear rail I was thinking would be aluminum…Is the .185 an over kill and I could get away with 11 gauge at .120 thick… savings for better electronics later perhaps

My questions from what sizes of linear rails and trucks including ball screws and how to read the pitch of a ball screw when ordering...I'm thinking the heavier duty the better so I was thinking 25mm trucks and linear rails but if 20mm would do the trick then let's go that route and spend the savings on something else...
Then there's the advertised dimensions on the linear rails and ball screws as to where those are measured from...end to end or block to block...I think I can safely say on the rails from end to end but the ball screws are they a different story...
ex: 1000mm is it from end to end or after both blocks are on, is it from the outside of the blocks or perhaps the inside of the blocks...
Trying to visualize this and hand draw it out for the rendering, so I can get the frame welded together
What size of pitch of ball screws should be considered and would be best suited for what I'm shooting for...I was watching the many videos and there was conversation that some thought was to have the x-y axis the same (I think I remember 10 being mentioned for the X-Y) but then the z would be different (and if I recall it was 5 for the Z) ...

Stepper motors...Open or closed loop...Then there's the size of motor... which size would be best for my performance I'm trying to accomplish...Direct drive would be easier for me to design and build into my work, but, should and if so a gear ratio system like 3-1 be a better choice and if so why? Would have to design this into the design...I’m not speaking of the gear rail on the side just the reduction of a gears on the end of the ball screws and motor…I’m leaning towards 2 motors for the “X” and one motor for the “Y” & “Z”

Then we have the Gantry...for cutting aluminum would a piece of 3 x 6 or 80160 heaviest “T” Extrusion I can find do the job or should I try and figure a way for using steel? And if I use steel what size? More involved and the additional weight vs the "T" slotted extrusion...I had thought about only if needed was to fill the extrusion up with perhaps sand or I might try and find a nice piece of rod and put it in the cavities of the extrusion and fill the rest of the cavity with acrylic for additional rigidity...
I have a visual that will allow me to get the biggest part of the table and the X rails pretty darn close and then be able to adjust the “X” linear rails and get them really fine tuned

Then for the “Z” it would also be aluminum plate with perhaps the 16 size used? or at the least one size smaller then I used for the X-Y axis...thoughts?
Reason being that the mass would not be as great as the other mentioned axis...

Then there's the depth from the top of the frame without any spoil board to the bottom of the gantry...How far...What's a general rule of thumb? 12"? 6"? and if I go 12" then what size of linear rail and ball screws should I order for the Z axis? got to keep that as ridged as possible for cutting aluminum but would like to accommodate a nice piece of wood perhaps sometime...

Then there’s another concern I had noticed on allot of the videos I watched
Gantry…would you put both linear rails on the vertical front face or one on top and one on the bottom…My thoughts here one on top and one on bottom was to reduce the amount of weight sticking out beyond the face of the Gantry which should help with rigidity…Also seems to me that the weight would be better dispersed then both hanging on the side? Thoughts?
Then there’s the location of the linear rails for the “X” and along the same thoughts would be mount them on top strictly of the weight distribution…Thoughts?

Hope I did not scare you off with these questions...
Paul

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[peteeng]

Hello Paul - Welcome to the forum. The Q's you ask are typical of a new machine designer. The real Q you should ask is 1) what is the machine required for? You mention aluminium and so far you are way off a good aluminium cutting machine. It tends to be a mill vs a router for aluminium. 2) If its a hobby machine and does not need to make money then the machine can have short comings as you are the Maker & will have to live with it 3) consider a kit machine from Avid or similar they have put 1000's of hrs into design and have things right. At least have a good look at their 4x4 machines and you will see straight away what is required and what cost 4) you may think that you can make a machine for less then Avid but by the time you spend 6 months designing a machine and then 6 months making one the $$$ may look reasonable

$3500USD is about $4900AUD and I build machines that you describe for about that. $800USD is freight component from china for parts (freight has become frightful). You may find local suppliers of stuff thats OK. I suggest you find a thread of a similar machine on the forum and read through. Will save you a year of effort in 3 hours. Beware of welding in fact don't weld for your first one. There are too many grief stories in the forum about welding and distortion (one happened today). 20mm rails are fine, 10mm pitch screws are fine use 5mm on Z for more grunt. Good luck Regards Peter

gantry rails on front or top or diagonal is a big subject I go the top for various reasons... If your serious about design and build then you need to start design in a CAD system and explore the possibilities. But if you want a machine vs a long and serious learning curve then find a machine that looks to your liking and copy it. But develop a specification of what you want then many of the questions become self evident. The more general your concept the harder it will be to realise. again good luck you will be helped via the forum

[Texasbowhunter]

Peter
Thanks for the reply...I feel that this machine will spend its entire existence wood working but for those once in a while, where I ask a bit more out of it to be strong and cut some aluminum on occasion, I would like it to accomplish this...It won't be designed with the understanding that it's going to cut aluminum full time and on occasion wood...Kinda like my Dewalt portable table saw...it will cut wood quite nice but from time to time I ask it to cut aluminum...It does a satisfactory job considering but its design is for wood...The machine I have in mind is a hobbyist machine...nothing that is going into commercial production but just something that will from time to time allow a nice something to be made and put some coin in my purse on occasion...
My $$$ are more then I wanted to budget for, but I figured that with the items required at this time I would have to pay more at this time for this project...
Thanks
Paul

[joeavaerage]

Hi,
the big question is whether you want to cut aluminum or not.

A machine of rigidity 'R', will cut wood and plastics, and maybe engrave the surface of aluminum, but to cut aluminum even with a small diameter tool, say 6mm or less will
still take '5R' rigidity.

If you want to make a rigid machine for aluminum then make it much smaller say 12" x 12". Its much easier and cheaper to build a rigid machine at smaller sizes. Double the size
say to 24"x 24" the price goes up by a factor of five or ten! Do not be fooled a rigid machine is a costly undertaking.

If you restrict yourself to wood and plastics, plus engraving then you should come up with a nice machine for that budget.

Craig

[joeavaerage]

Hi,

Beware of welding in fact don't weld for your first one. There are too many grief stories in the forum about welding and distortion (one happened today).

I'm less afraid of welding than Pete is, although he is certainly correct that any welded structure will have stresses and will move. If you want to weld a steel structure, and when all said and done,
is still probably the cheapest and most rigid material you can use, then allow for having it heat treated after welding and then machined flat. This is what puts people off steel structures, the cost
of heat treating and machining. Having said that both heat treating and machining are widely known and supported processes and can be done and make a superb result, even if it does cost.

For the dollars expended welded steel is still bloody hard to beat.

Craig

[peteeng]

Hi Paul - Its a nice thought saying that you want to cut aluminium sometime but then that sets the bar much higher then it needs to be for your first machine. It has to be a very stiff machine if you cut once or many times in a year. If you accept that it won't cut aluminium then it can be quite cheap and cheerful. eg you can use round rails vs square they are cheaper and easier to line up. You could make an entire machine from plywood (and it could cut aluminium if done right by the way) While you are researching look for a second hand machine they do pop up occasionally or an incomplete kit... Do not weld anything in the hope it will stay true, it won't... and a bolted connection is damp and still stiff enough if done right and you can bolt and epoxy and its better then a welded joint. So lots of ideas to work through. The air height of the gantry is up to you. If you are going to do flat panel or sheet work it does not need to be very high so the machine stays stiff. If you are going to do deep work (I do moulds and bowls and boxes) then a pit or apron maybe good or a high Z. I strongly suggest you consider a timber machine, easy to build, modify and play with. It can do everything you have described and is economical. Dig around for some examples in the forum. Big bamboo, Frankenrouter are a couple. There's a german company that make large plywood machines for cabinet making and I'm sure they could cut Al with it. But put up some ideas here and the "team" will give you feedback especially if your going down the wrong path

If your keen to make a CNC the motors, electronics and non mechanical work is its own universe as well... But the machine is its DNA get that right and your a happy camper. Peter

[luke peter]

I also favour welded steel - but the recent example of warping Peter mentioned is me It could have been avoided If I was a more competent fabricator though. If you have access to decent machining services, then welding a machine shouldn't be an issue, if you don't, then you need to be a top tier welder to make it work.

[peteeng]

Hi Everyone - All welding distorts the structure independent of the competency of the welder. I don't want to labour the point but if you want to make a welded machine and meet tight tolerances you must weld, stress relieve then finish machine. I expect this puts this processing out of Paul's budget. People do muddle through the welding maze (I have done this too. I have worked as a welder and as a welding trainer) but why? Welding is just one of many ways to build a good machine especially a hobby machine. Peter

[Texasbowhunter]

I also favour welded steel - but the recent example of warping Peter mentioned is me It could have been avoided If I was a more competent fabricator though. If you have access to decent machining services, then welding a machine shouldn't be an issue, if you don't, then you need to be a top tier welder to make it work.

So I am self taught welder...I can produce a spark and get what innocently seems like a good satisfactory weld which has seemed to be sufficient in what I have welded...
Now with this said I have a dear friend that I can lean on that is a professional welder by trade and I'm certain he'll assist me with the welding...
That is unless I stumble upon a base that will do the trick like my CNC bud had found...

[Texasbowhunter]

I'll try and find a base that can do the job at perhaps some machinery auctions or the scrap yards...
I can see what you guys are saying that if I want it to cut aluminum build a smaller version...I understand the logic here and perhaps build and plan a smaller version like perhaps a 18 x 18 table
I have watched numerous videos where the software and electrical components were used with another table for his plasma cutting when needed...certainly a valid thought
Help me understand the markings on the various descriptions of linear rails and ball screws...
Example: 25mm ball screw RM2505-1500mm+BK/BF20 end bearing+25mm Linear Guideway 2 Rail CNC
what is the oval length of the ball screw? 1500mm?
The rail would be 1500mm also?
The pitch of the Ball Screw? is that the 05 in the description where it states RM2505? 25 being the diameter of ball screw...If so if I was looking for say a 10 pitch ball screw I would look for RM2510
What does the RM signify
BK/BF 20 does this state that the bearings are 20mm ID
Thanks
Paul

[ardenum]

I wouldn't dabble with welding, if it distorts and it will you'd have to send everything to stress relief and machining, that's over your budget. T's and H's construction steel are very bad against twisting forces so avoid using them 'as is' all together.

If you want to build a router you probably wanna build with the biggest and heaviest aluminum extrusions you can get, something like 80x160 or 90x180 or larger. If they have big holes in the cross section even better, you can pour some local non shrink grout with some gravel for damping.

You want ballscrews(size 20-25), linear rails(size 20-25) and closed loop steppers(nema 23 or 24) with drives running 80-110V DC, or cheap but good ac servo's(60st) like lichuan from aliexpress(you can bypass aliexpress and buy from them directly too). For the controller you have a few options available depending what you want to run, mach, uccnc, acorn etc. I'm going with uccnc and their axbb board for my build, particularly their ready-made electronics box.

Do you have weight constrains? Best scenario would be to get a precision granite plate and build with extrusions on top of it. It would be the easiest, lest prone to error and give you the best results.

[Texasbowhunter]

I hope I did not give the wrong impression on the aluminum extrusion I was considering...when I mentioned "T" I was referring to 80160 Extrusion that has those slots all the way around for attaching stuff to like the rails and such
This is what I was attempting to express for the material for my gantry

[peteeng]

Hi Paul - I would not build a smaller version. A small machine and a 4x4 machine will cost about the same. If you require 4x4 then go for 4x4. Construction extrusions, like welding is convenient but has drawbacks. They are not stiff compared to their size, they are expensive for their weight and all those small free edges can vibrate. All covered in other threads. But many good machines are made from it so if thats the pathway stick to it. All build methods have challenges and limits.

The ballscrew and rail are rarely the same length. Get going in CAD and start putting this puzzle together. Don't CAD at detail level just block geometry level to figure out the major dimensions of everything. Once happy there you can start detailing out... Peter

a 4x4 machine would use 16mm screws. Use 100mm plus long nema23 motors and a high voltage controller say 48V plus and your away... Scoot below uses a 24V system for various reasons and its fine. But as a general statement the higher the voltage the more speed you can get out of your motors.

On the joining topic you can also soft solder, hard solder and braze the machine together. I've done all of these. Use a propane , oxy/prop or oxy/acetylene torch. Zero to miniscule distortion easy to do.... Scoot my current machine has an MDF screwed and glued base and it cuts aluminium fine. Still figuring out tooling and depth of cut limits. Many ways to skin the cat.

Excuse the rough dust plumbing was sorting out a hood concept Have moved on from that now

[davida1234]

Cutting aluminum......... That's a wide ranging call-out.

Do you want to have 0.005" depth of cut which makes every aluminum part at least a 3-hour undertaking or do you want to be able to have 1/4" or more depth of cut at 60 or 80 ipm?

I started out with an all MDF Joe's Hybrid CNC with V-bearings and a 24 volt HobbyCNC chopper board electronics and I was able to cut aluminum with it. At 0.005" depth and 30 ipm. But it did cut all the parts for my next machine. I am now on my 6th machine with rotating ball nut drives and 85 volt servos.

So, I believe almost any machine can cut aluminum to some degree and you need to ask yourself how serious you are about "cutting aluminum".

[peteeng]

Hi Paul - regarding ballscrews. The pitch of a screw determines the amount of force it can convert from torque to force. This is independent of diameter. A 5mm pitch screw of 10mm diam or 50mm diameter will produce the same force. The diameter of the screw determines two things a) the buckling load vs length ie the longer the screw the more chance it has to buckle. (Euler buckling) so a bigger diameter provides a larger buckling load at a given length b) Critical vibration speed or whirling speed. Gravity pulls down the middle of the screw. At slow speeds this curve can be overcome by the elasticity of the steel ie it curves down but straightens as it revolves. At critical speed this cannot happen and the screw behaves like a skip rope maintaining its curved shape and creates havoc with the system. These are easily calculated for. Minimising ballscrew diameter is important because when you do the math the motor torque is more to accelerate the screw then to move the job. In high acceleration ballscrew systems 90% of the torque is there just to turn the screw. So don't use a screw bigger then required just for belt and braces reasons it affects the performance of the system drastically. Peter

You can use a plastic damper near the middle of the screw that gets pushed out of the way as the nut moves past to enable the use of a smaller screw then an unsupported screw. If this is done don't place it in the middle as this is a node and you could end up with a second order vib. So place it asymmetrically or use 2 so the next vib is well away from the natural vibes. Some screws are preloaded to straighten the screw as well.

To follow the vib theme on when you use ribs or secondary members to stiffen a structure its natural to place these at even spacing. But you may have just created a natural amplifier. So if you are using a CAD system that can do a modal analysis like Fusion then do this and you will see what the vib modes are. Plus place the ribs in some sort of uneven progression to stop natural nodes to be made. By the way I have just been informed Fusion is going up in price, seemed too good to be true and I expect it to slowly climb as they get more on board users... still good value at the moment and still can be used for free at hobby level.. Peter

[Texasbowhunter]

Cutting aluminum......... That's a wide ranging call-out.

Do you want to have 0.005" depth of cut which makes every aluminum part at least a 3-hour undertaking or do you want to be able to have 1/4" or more depth of cut at 60 or 80 ipm?

I started out with an all MDF Joe's Hybrid CNC with V-bearings and a 24 volt HobbyCNC chopper board electronics and I was able to cut aluminum with it. At 0.005" depth and 30 ipm. But it did cut all the parts for my next machine. I am now on my 6th machine with rotating ball nut drives and 85 volt servos.

So, I believe almost any machine can cut aluminum to some degree and you need to ask yourself how serious you are about "cutting aluminum".

David,
Since the machine is NOT going to have a life in cutting aluminum but wood, Plastics and such and only on occasion will it cut aluminum I don't need a build that will hog a 1/4" deep it would certainly be nice, but if all I can get is .005 out of the build then so be it...I'd hope for a bit more but hey it will be what it is...If I can design by designing a little something into it in the beginning then lets do it...If that means up grading some of the parts, Motors, gantry, design of the "Z" axis then lets do that while I am constructing, but, at the same time not bleeding the bank dry...Wife would have a fit if I was to spend several 1-2k $$$ more then I have already allotted for...
Paul

[davida1234]

Paul,
The 0.005" was with a MDF machine which is known to not be very rigid. I am sure you will be far better off with your new design, particularly with all the new hardware available to builders which was exorbitantly more expensive when I built my first machine many years ago.

[Texasbowhunter]

Hi Paul - regarding ballscrews. The pitch of a screw determines the amount of force it can convert from torque to force. This is independent of diameter. A 5mm pitch screw of 10mm diam or 50mm diameter will produce the same force. The diameter of the screw determines two things a) the buckling load vs length ie the longer the screw the more chance it has to buckle. (Euler buckling) so a bigger diameter provides a larger buckling load at a given length b) Critical vibration speed or whirling speed. Gravity pulls down the middle of the screw. At slow speeds this curve can be overcome by the elasticity of the steel ie it curves down but straightens as it revolves. At critical speed this cannot happen and the screw behaves like a skip rope maintaining its curved shape and creates havoc with the system. These are easily calculated for. Minimising ballscrew diameter is important because when you do the math the motor torque is more to accelerate the screw then to move the job. In high acceleration ballscrew systems 90% of the torque is there just to turn the screw. So don't use a screw bigger then required just for belt and braces reasons it affects the performance of the system drastically. Peter

You can use a plastic damper near the middle of the screw that gets pushed out of the way as the nut moves past to enable the use of a smaller screw then an unsupported screw. If this is done don't place it in the middle as this is a node and you could end up with a second order vib. So place it asymmetrically or use 2 so the next vib is well away from the natural vibes. Some screws are preloaded to straighten the screw as well.

To follow the vib theme on when you use ribs or secondary members to stiffen a structure its natural to place these at even spacing. But you may have just created a natural amplifier. So if you are using a CAD system that can do a modal analysis like Fusion then do this and you will see what the vib modes are. Plus place the ribs in some sort of uneven progression to stop natural nodes to be made. By the way I have just been informed Fusion is going up in price, seemed too good to be true and I expect it to slowly climb as they get more on board users... still good value at the moment and still can be used for free at hobby level.. Peter

So Pete
After re-reading this many times trying to understand what your explaining I take from what you stated that I don't want to over kill with the screws...The larger dia. can over power the unit and cause more problems then what I was expecting to accomplish with larger dia. screws...So trying to understand this smaller in this usage is better where the force of the rotation will not create as much of a bucking motion...
So to small would be in-appropriate like the 12mm but the 16mm @5/8" will be just the ticket. and the 20mm 3/4" would be approaching the larger side..
Now if I'm understanding your explanation somewhat, this would not apply to the rails would it? 16mm rails or can I jump up to 25mm on the Y(I sure would potentially like to see the distance between the top of the rail and the bottom of the truck to possibly attach a1/2 thick angle for the ball screw attachment block, if I decide to go with that detail) and 20mm or even 25mm on the X gantry
What about the Z would 16mm be to light or stay with the same all the way around...I know that the bigger I go the further away the Z would be on the gantry...Just trying to think ahead with the weight and torque the head is going to create...
Hey guys let me take this moment for all your input and I certainly appreciate you guys guiding me and offering suggestion on this build and especially the beginners novice questions form a very green member...For this a Huge Texas Kudos to each one of you...
Thank You
Paul

[peteeng]

Hi Paul,
No the larger unit does not over power the motor. If you use a larger screw then required you will just accelerate slower (if you have a truck with a full load it can't accel very fast compared to when its empty. The screw is a big inertial load). If you want to do 3D engraving or moulding work you will want a very fast machine so your job gets done in hrs vs days. So you have to decide what "fast" is 20m/min? or 1m/min? Typical cutting speeds are 1-5m/min rapids 5-10 m/min/. High speed machining techniques can get to 100+ m/min and then you will need servos and high pitch screws or belts....

The motor can only put out a certain amount of torque. You can look at motors torque curves from suppliers. To rotate/accelerate a shaft or screw takes torque, to rotate it fast takes even more torque (just spinning it takes torque). If you run this machine at hobby speeds half the torque of the motor will turn it and half the torque will move the gantry or Z. Each issue is separate 1) torque required to do the job 2) will the screw buckle in compression at max length? 3) will the shaft whip? Each has a calculation and if you want to see the math's go the the Hiwin site in the document section and look these up. A compete design manual is published there.

Coupled with this is steppers have a declining torque curve so as you go faster you have less torque available. At 1000rpm generally you have near zero torque so if you have a 10mm pitch screw 10m/min is not possible, a 5mm pitch 5m/min is not possible....

Rails do not move so do not need any powering. 16mm 20mm will be fine for your machine,. Since 16 &20 are about the same price most use 20. 25mm is stiffer and more $$$ you have to figure out your suppliers and costs. The ballscrew nut and the carriage heights do not match. Some people use the 25mm car so the ballscrew is a better height match. All these things that have to be worked thru. You need to start CAD or scaled dwgs NOW.

The Z being short can use a smaller screw, But to keep some things common (eg motor mounts, spacers or C extrusions) I & others usually use the same as the other axis. Cheers Peter

edit - re cutting Al. Small lightweight machines like the ShapeOK and others can cut al. They achieve this by using very small DOC's and fast rpms. They do not cut effectively but they do cut and if you have the time and wear out lots of tools along the way they get there. I'm sure your machine will be better then these once you and the forum kicks it around the CAD workshop a few times....

[Texasbowhunter]

Pete
I am very OLD school...utilizing my Old drafting education with the pencil and graph paper, designing a scaled drawing to get an idea of the various component's fitment for this very reason...For me to get a good representation of the fitment to ensure its going to work the way I have envisioned it...So for example I have a very close scaled drawing by getting the dimensions from the vendor of the various parts for that area that I am concerned with to see just what is involved and whether or not its justifiable and sound before I bite the bullet on that design but at the same time I may redesign to get a visual on that to see if it's a better fit for me and what I'm seeking to accomplish...

We have had extensive conversations about the welding of a frame to support the Machine on...I am looking at other options', perhaps brazed joints perhaps even scraping the entire idea of a welded frame if and only if the frame stress relief is going to be $$$...I've seen many very nice machines on the tube where they had purchased one of the all extruded units and mounted on a wood Bench...Nobody says I have to mount on Metal I just figured it would be nice and more ridged...I'll look around and see what stress relieving a table like this will be but I assure you any type of machining after the table is welded and stress relieved bill be NOT come into play...$$$ I wont have budgeted...

But if we can get around the platform the unit is going to sit on I'm still questioning the size rails and screws diameter along with the pitch of the threads on the various axis...
It seems that suggestions are toward the 20mm linear rails for the X - Y axis...the 25mm are $$$ extra (Would it be better if budget allows to go 25mm on the "Y" axis to carry the gantry and spindle and such??...Ball screws would be 10 pitch
Motors you or someone mentioned the long NEMA 23's... would be fine
The height from the top of sacrificial board to the bottom of the Gantry...for me the 6" that was suggested should be a nice all around number...so I'm leaning towards this dimension
For simplicity I was going to go with the 80160 Extrusion for the gantry and probably fill the cavity's with something to give the gantry a bit more rigidity...it maybe something like Non shrink grout with play sand mixture or a SS rod or solid square pipe and put in there and pour some acrylic/grout to encapsulate inside to help with rigidity...If I do put a rod inside perhaps drill and hole and tap on both ends for a bolt to secure with additional strength to increase rigidity...
While were on the gantry lets discuss the "Z" axis...same 20mm rails or 16mm rails with 16mm screws with a pitch of 5...Thoughts here is to keep as much weight off the gantry as possible and also the distance from the center of the spindle to the gantry will be reduced...also it seems that are various heights of trucks for any given size of rail...should we consider lower profile trucks and rails to achieve the reduced distance from the face of the gantry to the center of the spindle???

Lets start here while I get the platform together...whether it be brazed, fabed out of wood, or bought something that will suffice and add a granite base plate to rest the unit on...Only then will I be able to decide on how my "Y" rail will mount to but in the mean time I can start ordering and mocking up so I can find out exactly what size of what ever I need...
Thanks
Paul