Medium sized CNC Mill/Router - Some specific questions


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Thread: Medium sized CNC Mill/Router - Some specific questions

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    Question Medium sized CNC Mill/Router - Some specific questions

    Hello everybody,

    I'm Alex, nice to be part of this forum. First thread for me here after reading and reading and reading many other ones. Lots of valuable information that already helped me figure out a couple different things for my design, thank you!
    I decided to start this thread for some specific questions where I could not make a safe choice yet but also some design advice. I would like to know if anybody could guide me in the good direction please.

    So here we are with another router build!

    First, what do I want to do?
    I am designing a medium-sized CNC Router table to machine some Renshape boards, wood, but also aluminum (apologies for talking about this material in the wood forum ). No steel machining involved here.
    The machine will have a work area of Z 900mm x Y 850mm x Z 150mm (machine itself is 1100mm x 1070mm).
    I attached screenshots of my 3D design to this thread.

    I read a lot of threads suggesting that the gantry assy should be the most rigid part of your build. As you can see, I am using a mix of aluminum extrusions and steel laser cut plates. I use one 4080 extrusion at the top and one 8080 at the bottom for the gantry beams. Also, linear rails with ball screws for linear motion (ordered from Fred at BST Automation based on recommendations on this forum).

    My first question is regarding the movement of the gantry assy, the X-Axis. This whole gantry assembly, including the 2.2k water cooled spindle that I plan on using, should be around 40-45kgs (~100lbs) based on the current design.
    I initially planned on using two linear rails with 2 carriages each + one central ball screw located under the table. But I am currently re-thinking this choice.
    Based on what I could read on the forum, many people go with a dual ball screw/dual stepper design for larger routers. Some say that it is worth the extra investment to get two steppers controlling the X-Axis in order to fine tune the machine and that you will not have any moving parts under the table. Others (not as many!) seem to suggest that one stepper controlling one axis is the way to go to avoid any problems like missed steps on one side of the gantry, plus this is also what "commercial" machines use. However, you need a stronger stepper motor which is why I integrated a Nema 34 stepper motor for this specific axis in my design.
    Now, I am undecided as to what solution I should go with for this build.
    I am not aiming at building the fastest machine as it will only be used to make prototypes or composite molds, no production involved here. I am a little concerned about the inertia the gantry will have when moving and I was wondering if you would recommend going with two ball screws instead of one? The ball screws that I will be using are 1605 for X and Y (as I said, I don't mind slower speeds, hence the 5mm pitch) and 1204 for Z. They are already ordered but I am wondering if I should ask Fred to throw in an extra one in the package before it ships.
    Is there anyone here who could guide me in the good direction considering the specifications that I am aiming for? Many of the threads that I read were started 7-8 or 15 years ago, what would you recommend in 2020?

    Second question is regarding Z Axis height. I am aware that the higher you go, the stiffer and sturdier the gantry assy should be. I designed it with this in mind. The two aluminum extrusions will offer a good rigidity to support the spindle movements and the steel plates on the sides with 3 vertical stiffeners each should be able to do a good job transferring the moves from the ball screw/linear rails with minimal flexion. The goal is to make it stiff but also light. Looking at this design, do you think that I might be aiming for too much with 150mm considering the materials that I plan on machining?

    Finally, my last question concerns the brain of the machine. There is a lot of threads about Mach3 and many users are using it. This being a new build, I was thinking about maybe using the newer Mach4. I planned on using one of PMDX motion controller.
    Any reason why this would not be a good idea?
    On a more specific point regarding wiring, I noticed that PMDX also offers an isolated speed controller board (PMDX-407) that works with their motion controllers boards. My question is: do you need one of these speed controller to control your spindle with a VFD and Mach4 or can you directly connect your VFD to the motion controller board?

    That's pretty much it to start. Any help will be greatly appreciated.

    Thank you in advance!
    Cheers,
    Alex

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    Default Re: Medium sized CNC Mill/Router - Some specific questions

    your gantry is pretty light compared to what i built. mine is around 300 lbs and i have the central drive with a Nema 34 stepper currently driving it and i don't see any issues with what i have but i an running at a 4 to 1 ratio with a timing belt rather than a direct drive. the stepper motor is running pretty slow and it's in a better spot on the torque speed curve. i was able to get up to 400 ipm at the extreme side before lost steps come into play. direct drive on a 5 mm pitch screw i would expect to see fall around 100 to 200 IMP you may get closer to 200 since the gantry is light but the stepper is not going to do well at a higher RPM. and the higher the voltage you can feed it the better.

    your ball screws are a little shorter than what i have on the gantry cross axis of what i built and it does ok up to about 400 IMP then the the screw starts to whip. there are ways around screw whip so it's not the end of the world just a little more complex mechanical end of things as a result. but since yours are shorter than 1500 mm they will do better.

    one thing that might help is some cross bracing on the frame under the table. if you can get some decent acceleration it works on the frame more than you think at first. one other thing is the drive beam under the table should probably be more substantial. with the center drive you need to be pretty solid so you don't get into problems. at your machine size the central drive is not to bad. on a 8' axis it's a completely different animal. the central drives need the entire gantry to be pretty solid.

    anything other than Mach 3 is a step in the right direction. it was been a corner stone for a long time but is getting closer and closer to it's end of days. Mach 4 seems over priced. acorn centroid looks like a good plug and play solution but i have not tried it myself. seems like these days there are a lot more choices as far as control software is concerned.



  3. #3

    Default Re: Medium sized CNC Mill/Router - Some specific questions

    Thanks for your reply machinedude.
    300lbs? Here I was thinking that my ~100lbs gantry was maybe on the heavy side for a machine this size. Good to read that.
    400ipm is definitely a higher speed than I was expecting my router to operate at. I was looking at speeds around 50 to max 100ipm when machining aluminum based on what I could read and the fact that it's sort of a budget build. I guess I could step up my game then.

    The ball screws that I ordered for X and Y have a 1000mm threaded section and I was not that concerned about encountering any whip at the speeds that I was aiming for. Which is also why a direct mount seemed to be the easiest way to go so that I would not have to deal with pulleys or belts. I will do more calculation for 200ipm then. Maybe a Nema 23 would be enough?

    I think that you are right regarding the drive beam. I was thinking about getting something bigger, although my first thought was that the top beams would provide the main rigidity for the gantry and a 1/2'' aluminum plate underneath should be rigid enough to appropriately connect the ball screw nut to the rest and make the whole thing move. I will look into adding another extrusion underneath, like the ones at the top. Thanks for the tip.

    Regarding the frame itself, I will try to add some cross bracing or extra horizontal extrusions like you recommended. It is worth noting that the table is a metal table made of a 3/8'' thick steel plate bolted to the frame, plus a 3/8'' replaceable aluminum plate bolted to the steel plate with threaded holes for clamping purposes. I was thinking that this metal table would help keeping the frame sturdy. I think that the easiest thing that I could do is add another horizontal beam to divide the frame in 6 smaller rectangles instead of 4 as shown in the pictures that I previously sent. What do you think about that?

    Didn't know about Acorn centroid. Plug and play indeed! I will look more into this, my mind is not set on Mach4 anyway.
    Thanks!



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    Default Re: Medium sized CNC Mill/Router - Some specific questions

    Always go dual drive for the gantry, unless you have a really beefy machine. You'll get a lot of twisting with a single screw.

    You do NOT want 5mm pitch screws. I'd recommend 2010 screws for the X and Y, or at least 1610. The machine will be painfully slow with 1605 screws. You'll likely find yourself wanting to cut at 250-300ipm. And you'll wish your rapids were much faster.

    I'd go with 2010 screws, with a small Nema 34 on each screw, in the 450oz range. With a 60-72V power supply, these give the most power at higher speeds.

    For the gantry, you want a single large beam. The two small ones will allow the Z axis to twist from front to back.

    Gerry

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  5. #5

    Default Re: Medium sized CNC Mill/Router - Some specific questions

    Hello Gerry,
    Thanks for your help as well.

    Unfortunately, I won't be able to change the ball screws specs. They were already machined and Fred is waiting for my GO to ship them. I'll have to learn to live with a 5mm pitch for now and if needed, I'll update to 10mm screws later, it won't be a difficult modification.

    Yes, dual drive has been in my head for a while now. Considering the fact that this machine will be built with steel BLH linear guide rails and not the cheaper SBR alternative, do you think that a single centered ball screw would still get a lot of twisting? If so, I will definitely update my design to dual screws and Nema34 like you advised.

    Regarding the gantry assembly, I hear you. A single large beam was my first thought as well. However, going through the design phase, I decided to put two beams instead of one, but I kept one heavy duty square 80x80mm extrusion at the bottom (that I first planned on using alone), and added a lighter duty 40x80mm vertical extrusion at the top. Why? In order to create some clearance for the ball screw in-between them so that the nut housing would end flush with the linear rails carriages mating surfaces.
    Based on my calculation, I get a deflection of ~0.05mm on the 80x80mm extrusion alone when fixing it on both end and applying a 100kg centered load. I thought that this would be acceptable considering the fact that the horizontal load applied to that gantry beam will never even get close to that value. Torsion is also minimal on this beam and adding the other one at the top should just get this assembly a little sturdier but allows me to create a relatively good amount of space in-between the rails.
    Does that make sense? I could definitely throw in a 120x80mm beam in there instead of the 80x80mm.



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    Community Moderator ger21's Avatar
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    Default Re: Medium sized CNC Mill/Router - Some specific questions

    You could add a 1/4" plate across both of the beams to tie them together. That would help a lot, and you can still keep your screw clearance.

    Yes, I'd still go with two drives. If you were to push on one of the gantry uprights, would it move? I think so. Dual drive makes it much stiffer.

    Gerry

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  7. #7

    Default Re: Medium sized CNC Mill/Router - Some specific questions

    Quote Originally Posted by ger21 View Post
    You could add a 1/4" plate across both of the beams to tie them together. That would help a lot, and you can still keep your screw clearance.
    Indeed, that is an easy fix that can easily be implemented to my design.

    Quote Originally Posted by ger21 View Post
    Yes, I'd still go with two drives. If you were to push on one of the gantry uprights, would it move? I think so. Dual drive makes it much stiffer.
    Understood, that's enough to convince me! I was on the fence but now it's pretty clear. Dual drive it is.

    Thanks Gerry!



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    Default Re: Medium sized CNC Mill/Router - Some specific questions

    yeah my gantry is pretty beefy and my uprights started at 1.25 thick but they ended up at 1 3/16 after i machined them flat. i don't have trouble with things moving around to much. the biggest thing was a twisting type of deflection with the extrusion i used. i took care of most of it by filling the voids with epoxy granite which made a really big difference. i even filled most of the t-slots with E.G. if i had to do it over again i would probably just go with a solid cross beam to deal with the twisting all together.



  9. #9

    Default Re: Medium sized CNC Mill/Router - Some specific questions

    Hello,

    So, I made a couple changes to my design based on Machinedude and Gerry's comments.
    Attached are pictures of the updated 3D model.

    Everything is a little more "beefy" (i.e. two 80x80mm beams bolted together with a 1/4'' plate in the back for the gantry). I still decided to keep both X Axis ball screws under the table. I'm hoping this would protect them from any direct wood chips and dust that would not be picked up by the vacuum hose that I plan on installing near the spindle, but also from coolant drops used to cut aluminum. Also harder for anyone to get their fingers stuck in there hehe!
    In the end, the gantry is now closer to 60kg (~135lbs).

    As you can see, the frame now has four X beams connected to four Y beams (all beams are 40x80mm extrusions). I am wondering, do you think this is maybe a little overkill? I've been looking at other designs with aluminum extrusion and they seem to have more space between the extrusions. This table is only 1100mm x 950mm. I definitely plan on keeping the two X beams that follow the ball screws though. Maybe one of the Y beams could be removed? As I said yesterday, the table itself is also bolted to the frame and will act as an extra connector on the top for the frame. It is made of a 3/8'' thick steel plate with a 3/8'' thick replaceable aluminum plate on top, with threaded holes for clamping purposes.

    Attached Thumbnails Attached Thumbnails Medium sized CNC Mill/Router - Some specific questions-cnc-mill-v2-3-jpg   Medium sized CNC Mill/Router - Some specific questions-cnc-mill-v2-2-jpg   Medium sized CNC Mill/Router - Some specific questions-cnc-mill-v2-jpg  


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    Default Re: Medium sized CNC Mill/Router - Some specific questions

    your table is not so big like you said i was thinking more support on the legs since the tale looks tall. if you cut aluminum the vibrations will resonate through out. and the lighter the frame the worse it will be. this kind of machine design is not best suited for cutting metals unless you build it like a tank. this kind of design keeps the foot print small but takes some beef to do any good with aluminum. the high speed spindles have a place in high speed machining with the right kind of tool paths but i doubt they can do anything tougher to machine just because they can't do well running slower.

    if you have any kind of acceleration i think you will notice it on the legs of the frame. if you start getting at or above .1 G it works pretty good on the frame work if your running slow not so much the faster these machine run the better the frame work they need. just some things i have noticed with my build.



  11. #11

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    Quote Originally Posted by machinedude View Post
    your table is not so big like you said i was thinking more support on the legs since the tale looks tall. if you cut aluminum the vibrations will resonate through out. and the lighter the frame the worse it will be. this kind of machine design is not best suited for cutting metals unless you build it like a tank. this kind of design keeps the foot print small but takes some beef to do any good with aluminum. the high speed spindles have a place in high speed machining with the right kind of tool paths but i doubt they can do anything tougher to machine just because they can't do well running slower.

    if you have any kind of acceleration i think you will notice it on the legs of the frame. if you start getting at or above .1 G it works pretty good on the frame work if your running slow not so much the faster these machine run the better the frame work they need. just some things i have noticed with my build.
    That actually makes a lot of sense. The heavier the base, the easier it will be to prevent any unwanted movements and machine jerks when the gantry accelerates and stops.
    I guess I could look into the granite epoxy that you were referring to in order to fill some of the frame extrusions and make them heavier. The table is currently 750mm above ground, but I can definitely reduce that height a little.
    I will look into this these days. I want to make sure I know where I'm headed before ordering the extrusions and laser cut plates.
    I will try to update this thread as I go further into my build.
    Thanks again for your help!



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    Default Re: Medium sized CNC Mill/Router - Some specific questions

    Its good to see that somebody has taken the time to model the whole spindle and cutter.I also see that you have used a high spindle clamp and backplate to increase the chances of the machine reaching into corners and recesses.I take it that the ballscrews are located under the table in order to keep them out of the dust and chippings.This means you have to use the links beneath the table to transmit the drive to the gantry.Would it simplify matters if you turned the stepper mounting plates 90 degrees and installed the ballscrews on the outside of the main rail extrusions?If you used a light alloy angle extrusion beneath the hiwin rails and let the other flange project out over the ballscrews you would still be shielding them and saving quite a bit of metal beneath the table.Two slaved steppers should be more than enough to prevent the gantry skewing.



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    Default Re: Medium sized CNC Mill/Router - Some specific questions

    Quote Originally Posted by AlexFComp View Post
    That actually makes a lot of sense. The heavier the base, the easier it will be to prevent any unwanted movements and machine jerks when the gantry accelerates and stops.
    I guess I could look into the granite epoxy that you were referring to in order to fill some of the frame extrusions and make them heavier. The table is currently 750mm above ground, but I can definitely reduce that height a little.
    I will look into this these days. I want to make sure I know where I'm headed before ordering the extrusions and laser cut plates.
    I will try to update this thread as I go further into my build.
    Thanks again for your help!
    typically metal working machines are built from cast iron because it dampens vibrations. steel tubing is rigid but does not dampen much and the same goes for aluminum extrusion in dampening aspect of things. epoxy granite does wonders for dampening vibrations and the heavier the frame the harder it is to get to vibrate. if you do decide to fill the extrusion voids this will stiffen the frame up quite a bit and dampen things. if your filling tight spaces do yourself a favor and just use sand in the mix so you can get it to pack tight in a confined space and get a epoxy with a long pot life so it has time to settle before setting up.the more E.G. you use can add up fast in price so keep this in mind as well.



  14. #14

    Default Re: Medium sized CNC Mill/Router - Some specific questions

    Quote Originally Posted by routalot View Post
    Its good to see that somebody has taken the time to model the whole spindle and cutter.I also see that you have used a high spindle clamp and backplate to increase the chances of the machine reaching into corners and recesses.I take it that the ballscrews are located under the table in order to keep them out of the dust and chippings.This means you have to use the links beneath the table to transmit the drive to the gantry.Would it simplify matters if you turned the stepper mounting plates 90 degrees and installed the ballscrews on the outside of the main rail extrusions?If you used a light alloy angle extrusion beneath the hiwin rails and let the other flange project out over the ballscrews you would still be shielding them and saving quite a bit of metal beneath the table.Two slaved steppers should be more than enough to prevent the gantry skewing.
    Hello Routalot,
    Thanks for your suggestions!
    Honestly yes, I thought about going with a more conventional setup with both ball screws placed on the sides of the table. I even started 3D modeling it but I decided to turn this idea down for a couple of different reasons.

    First, I may be a little biased but I like the fact that I could build a gantry sort of like a closed loop assembly: one big solid beam at the top connected to two stiff side plates + an extra smaller beam below also connected to the side plates. I may be wrong, but it felt like a sturdier design to me that would induce less stress on the linear rails carriages.
    The second thing, and probably the most convincing one for me to keep the ball screws underneath the table is that I ordered ball screws with BK12/BF12 end support attachments. I was planning on using these in the first design with only one ball screw in the middle - design that has now changed. Placing the ball screws on the sides of the frame with these end supports and keeping the same table size seemed to be more of a headache. The distance between the bottom of the rail/face of the extrusion and the inner surface of the side plate is 30mm, whereas the distance betwen the base of BK12 and the ball screw nut mating face is 45mm. If I were to attach these end support on the sides of the extrusion, I would have to add a 15mm shim between the carriage's mating face and the inner surface of the side plate (which I did not want to do to avoid stressing the rails with more torsion), or find a way to "burry" the end support in the extrusion with some sort of attachment plate coming from the back to account for this 15mm difference. However, this is what I decided to do for the Z axis as you can see on one of the pictures below. Adding a little shim between the carriage and the plate in this particular load case was not a problem considering the loads will be in line with the rails.
    In the end, using slightly longer ball screws with traditional custom laser cut plates placed on both ends of the extrusions and attach FK12/FF12 end support like the ones I have on the Y axis would have been the way to go for this modification I think. Many machines I have seen are built this way.
    I was faced with a dilemma: keep the ball screws underneath the table the same way that I started designing the router, or shorten the table in the X- Axis in order to use FK12/FF12 end supports with plates on both ends of the extrusions (longer ball screws were not an option anymore since they were already ordered). I guess my mistake was to probably order some of the parts before finalizing the design and asking for question on the forum... my bad!
    What I like about the current configuration, is that I am actually using the full threaded length of the ball screws when the gantry is moving at the extreme front and back of the table though!


    Quote Originally Posted by machinedude View Post
    typically metal working machines are built from cast iron because it dampens vibrations. steel tubing is rigid but does not dampen much and the same goes for aluminum extrusion in dampening aspect of things. epoxy granite does wonders for dampening vibrations and the heavier the frame the harder it is to get to vibrate. if you do decide to fill the extrusion voids this will stiffen the frame up quite a bit and dampen things. if your filling tight spaces do yourself a favor and just use sand in the mix so you can get it to pack tight in a confined space and get a epoxy with a long pot life so it has time to settle before setting up.the more E.G. you use can add up fast in price so keep this in mind as well.
    The frame currently have a weight of ~155kg (~350lbs) with a ~60kg (~135lbs) gantry. I have been reading about these Epoxy-granite/sand mixes in the past hour and this material's ability to dampen vibrations. It would be pretty easy to fill the center cavities of some of the frame extrusions. I work with carbon fiber composite materials every day (we design and manufacture high performance carbon fiber road bicycle wheels here in Canada), hence my need for a cheap CNC router to manufacture small light molds. I have a 4gal of a medium performance epoxy lying around in the workshop that I could use for this. I am liking what I see! A mix with sand alone would definitely help with filling these tight spaces. Would you get sort of similar results by using an Epoxy-sand mix only though? That would be easier to source as these are cheaper and available at local hardware stores.

    Attached Thumbnails Attached Thumbnails Medium sized CNC Mill/Router - Some specific questions-cnc-mill-distance-extrusion-side-plate-jpg   Medium sized CNC Mill/Router - Some specific questions-cnc-mill-distance-bk12-base-nut-mating   Medium sized CNC Mill/Router - Some specific questions-cnc-mill-fk12-end-support-jpg   Medium sized CNC Mill/Router - Some specific questions-cnc-mill-ball-screw-full-length-jpg  

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    Default Re: Medium sized CNC Mill/Router - Some specific questions

    personally i think and kind of sand mix will do fine. some have got fancy with their fillers but any cheap solution will do well. the good thing about the epoxy is it only shrinks about .001 and has about the same compressive properties as concrete. early 1900's they did beds for lathes with concrete and from what i read they did well. so if you have epoxy already that would be a cheap solution. 350 lbs is not very heavy at all so you will need some extra help at vibration dampening. your machine could cut as is possibly but i would not want to sit next to it very long will the frame sings to me

    i ended up casting about 2100 lbs of E.G. into my frame and once i do my table top i will have a machine that weighs somewhere around 4250 to 4500 lbs. after 30 years of metal working i have no love for chatter or things vibrating obnoxiously



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    Default Re: Medium sized CNC Mill/Router - Some specific questions

    I was going to tell you to move the screws to the outside, but see that you really want them under the table.
    If you do that, I'd move them as close to the sides as possible.


    You need some triangular bracing on the legs, it'll wobble like crazy without them.

    Gerry

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    Default Re: Medium sized CNC Mill/Router - Some specific questions

    Quote Originally Posted by ger21 View Post
    I was going to tell you to move the screws to the outside, but see that you really want them under the table.
    If you do that, I'd move them as close to the sides as possible.


    You need some triangular bracing on the legs, it'll wobble like crazy without them.
    just a thought but how well does the squaring of the gantry work when the gantry is solid enough to stall a stepper. i have zero experience with that part and was curious ? usually you don't see the bottom cross brace used to drive the gantry with a double screw or rack drive.



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    Community Moderator ger21's Avatar
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    Default Re: Medium sized CNC Mill/Router - Some specific questions

    just a thought but how well does the squaring of the gantry work when the gantry is solid enough to stall a stepper
    The gantry should be installed as square as possible. If it's out of square, and the motors aren't strong enough to push/pull it square, then it's not going to work.

    Squaring the gantry imo is not the main reason to use dual drives. Dual drives keep it from racking. Most larger DIY gantries will rack rather easily, until you get into the big all steel ones that weight 300 lbs or more.

    Gerry

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  19. #19

    Default Re: Medium sized CNC Mill/Router - Some specific questions

    Quote Originally Posted by ger21 View Post
    I was going to tell you to move the screws to the outside, but see that you really want them under the table.
    If you do that, I'd move them as close to the sides as possible.


    You need some triangular bracing on the legs, it'll wobble like crazy without them.
    You are right, I am currently working on this.

    Honestly, after re-reading everybody's advice on the forum today, I went ahead and decided to rethink my design. I ran some more tests and noticed that I had a way bigger deflection than expected on the side 40x80mm beams with the bigger and heavier gantry (when it seats in the middle of the table). I needed to add some support beams under both extrusions, which in the end prevented me from keeping the ball screws underneath the table.

    Some might think that I am a little too undecided, but I am definitely not trying to re-invent the wheel here and want to gather information to prevent myself from making errors that others already had to fix in the past.
    So I searched for more ideas and found a design that I liked and that could help me fix some of the issues that I am dealing with. It was posted on another forum where I think some of you guys might also be members (Building My First CNC Router, Looking for Advice & Other Info).
    In the end, I will be able to put the X Axis ball screws with BK12/BF12 end supports on the sides while keeping it simple and more sturdy.

    I will update the thread once I come up with a design that I think is worth sharing.

    Thanks all for your suggestions. I am so glad that I didn't order the extrusions from Framing-tech yet!



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    Default Re: Medium sized CNC Mill/Router - Some specific questions

    Have you checked out the price of epoxy?It isn't the cheapest substance and some shrinks more on curing that others.



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Medium sized CNC Mill/Router - Some specific questions

Medium sized CNC Mill/Router - Some specific questions

Medium sized CNC Mill/Router - Some specific questions