[SweetAs]

I currently run an 8 x 4 MDF machine. Its a machine built off of a kit from a company called SheetCNC which uses machined 25mm MDF for the frame, angle iron for the rails and chain for the drive. Not the greatest by far but all in all, its worked pretty well for what I needed and once it was tuned it up. I've earnt far more than I paid for the machine kit and its a few years old now so Im looking to upgrade.

It runs a 6mm cutter at 3mm DOC at approx. 2500 - 3000mm/min feed in MDF but there is some visible flex on the Z and noticeable chatter on some machined pieces. In an ideal world I need to be pushing those speeds and doc's. I would be happy with that speed but zero chatter and bang on accuracy to be honest, on that note, its good to 0.5mm

I run a joinery shop and have lots of 18mm and 12mm Baltic Birch Plywood left over so my thinking was to use that, a bit like the Holtzman? type machine but different. The idea will be to keep the table/base (25mm MDF) and chain drive from the old machine to limit costs at the moment. I've got 3 425oz motors running on ST-M5045s, Ill be getting rid of the dual motor (X) and upgrading to 2 single motors on the Y axis, single on the X and single on the Z.

For the main Gantry beam, Im trying to decide which would be better, a 300mm x 100mm torsion box made from 12mm Birch Ply with a 12mm Birch skin

OR

36mm double laminated piece of Birch ply sandwiched between a couple of pieces of 3mm steel plate. Either using screws or through bolts from side to side. What do you think?

Maybe an torsion box built with 18mm Birch Frame with a 3mm steel skin and each side? Would it need to be 3mm or could it be thinner? Steel plate is relatively cheap at 3mm thickness where I am (UK), the Birch Ply is paid for and on the shelf so that's zero cost, I also have a few sheets of 25mm MDF.

The plan is for the Z axis setup to move along the X on 20mm Vevor HGR20 (Ebay) rails, see pic. Im undecided on how I will fix them, kind of depends on the make up of the gantry. Initially I had thought to made a box, laminate/Dado (Like Biggs build) in a couple of sections of 12.7mm thick ally bar onto the box and then fix the rails to this. I could always skim the ally on the current CNC to get it fairly flat and go from there? Ive also got a large double drum sander which I could feed the finished section into it with the ally bar fixed to it and level the ally up this way.

For the sides of the Gantry down to the rails Im thinking of doing something similar to the above or maybe double/triple layer 18mm Birch with a steel skin? My concern on the Y gantry was to eliminate the twisting forces, not something Im too concerned with on the X so it might not even need the steel skin, what do you think? My thinking is to sit it onto SBR25LUU, 25mm supported round rails (CNC4YOU Ltd)

Would it need a set on top and bottom of the base sheet or would a single set on the top work?

What do you all think? Am I way off on these ideas? Is there anything I should be thinking of or builds it would be a good idea to watch? Would aluminium plate be a better option? 3mm Aluminium plate and 3mm steel are fairly close on price, obviously thinner/cheaper is better lol.

For a bit of background, I run a Joinery/Carpentry shop which specialises in bespoke/weird designs. I build everything from small boats to tricks for magicians to props for stage shows. The build I can do, its all the details that I need a couple of pointers with which is where I hope you clever people can help me with. Ideally Im hoping to get rid of any costly mistakes or redesigns before I start lol.

Thanks in advance for taking the time to read this, its very much appreciated!

[Texasbowhunter]

If that's your machine above that is a beautiful work of art...You got some skills
I have nothing to add to this conversation other than what I stated...
Paul

[peteeng]

Hi Sweet - I would use aluminium vs steel as the steel will rust. If painted it presents issues with change of thickness and paint compression under fasteners. The exception is 2mm or 3mm galvanised sheet. I use alot of that for machines. I would graduate from chains to timing belts and use supported round rails and ball bearing cars. I've built ply and MDF machines and ply is stiffer. Most of your questions can't be answered unless you do FE to compare concepts. But boxes are always better then laminates, make parts as big as possible to maximise stiffness and use good quality thread inserts for parts that need to come apart. Parts that are permanently connected use epoxy or titebond. For laminating metal to timber use adhesive (epoxy or suitable glue) minimise fasteners. If you have a vac bag then use that. Peter

Heres my latest plywood build. It uses 2mm aluminium laminated to ply for the gantry. I'm started designing a full sheet size machine in ply and start building soon. Look up the Lanky build.

[pippin88]

The gantry (and uprights) are often the worst designed big of machines.

The gantry needs to resist torsion. Torsion / twisting is the biggest problem.
The Z axis acts as a big lever arm, increasing the torsion.

So the gantry should be square, or triangular.
Not a tall rectangle.

Your current gantry looks fancy, but there is a lot of loss of stiffness to achieve that lightweight, and weight does not matter nearly as much as most people think.

Torsion box is good, as the name suggests.

You need something to bolt your linear rails etc to. Wood is not great for this as compresses under fasteners. So you need a strategy for dealing with this.

[ger21]

I'm building a wood 4x8 machine, and am using a square torsion box gantry, roughly 200mm x 200mm.
I'm using laminated MDF for the torsion box, because it was free.
Here are a couple links.

https://www.cnczone.com/forums/cnc-w...ml#post1162909

https://www.cnczone.com/forums/cnc-w...ml#post1168468

[jaguar36]

Before you go all in on a new machine have you tried just improving the one you've got? Little hard to tell from the picture but I bet you could significantly increase the stiffness of the gantry by bolting on a couple extra pieces of ply onto the verticals and the gantry. The connection between the vertical and the gantry doesn't look that great either, I found that by running a threaded rod from one end of the gantry to the other and tightening that down on each end I was able to significantly increase the stiffness on my mill.

[awerby]

I like the steel sandwich idea; it would definitely give the Baltic birch more stiffness. Paint it if you're worried about rust, or use galvanized steel. But I'd get rid of the chain drive, if you're going through the hassle of building a new machine. Rack and pinion drive would be a big step up,and eliminate a lot of slop.

[routalot]

Several opinions to choose from! I always question why almost nobody adds an external triangulating web to the ends of the gantry to more rigidly link the gantry beam to the longitudinal rails.In fact I would suggest making this easy addition to the current machine so you can cut the parts for the new machine a bit more quickly.I'm in the same camp as peteeng in that I like supported round rail and I like torsion beams.

[peteeng]

Hi Sweet - for interest calculation shows that the bending stiffness of 17mm birch ply is the same as 3mm of steel. But the birch weighs 11.9kg/m2 and the steel weighs 23.4kg/m2. So keep the moisture out or consistent and its a great product. Birch is about 12GPa modulus whereas MDF is ~4GPa so ply is good. Laminating stops moisture ingress egress. But you have used an mdf machine for some time and you are happy with it... Seal the edges of the ply and it will be more stable then the mdf. I have found MDF to be flatter then ply. Keep at it. Peter

edit - but I would use aluminium. You can cut your ply parts on your current router then rough cut the aluminium skins with a jig saw (can your machine cut aluminium? ) and bond them on. Then use a flush router to trim it up....

[SweetAs]

First off, apologies and thank you for the replies. Work is either non existent or manic and this week has been a manic. Its Friday, nearly 1800 and I think Ill be here for another few hours yet but cant complain, better to be busy than not at all! I hope Ive answered any questions below. So the current thinking is this...

Rails - Look at round rails. They are cheaper and easier for me to get hold of in the lengths I need from CNC4YOU as Im in the Uk. What sizes should I be looking at? This is the page I am looking at if anyone has any suggestions... https://www.cnc4you.co.uk/Linear-Rai...ound-Rail-Kits

Torsion Box - The consensus is to look at some sort of torsion box, square not rectangular, with an aluminium skin on 2 sides or 4?

Chain Drive - I fully understand where you guys are coming from in relation to ditching the chain, it does have its faults that's for sure. I've worked with belts on a small machine, I think they were 9 or 12mm, I've found 20mm timing belts here in the UK, is that enough or would I need to be looking at 25mm? I do like the idea of a R&P, just not really sure how to go about it to be honest. More than happy to learn if anyone can point me in the direction of a few builds.

Sides - It might make loading a little more difficult when using thicker 18 and 25mm sheets but would it better for me to raise the sides up, like in Peteengs build above? I don't seem that being too much of an issue to be honest if we think its worth while doing. I could create another torsion box, made with wood and then fit a steel plate/bar onto the top for me to drill and thread for the round rails. I don't really see it being much of an issue if raising them up will help everything.

Texas - Thank you! For what it is, its a great bit of kit as long as you are aware of its limitations.

Peteeng - Thanks for the info. I like your builds, Im thinking I might rise up the sides on mine to help with the twisting forces, try to keep the spindle around the centre of movement I think. In terms of the ally, Im thinking of sandwiching both sides of the torsion box and gluing plus screwing in the ally sheet. Does that sound like a good option? In terms of rails, how do you have the rails mounted, are they top and bottom? What sort of sizes should I be looking at for the X and Y? Do you have any issues with the long ones trying to lift when the router plunges into the material as that was my main concern? Thanks for the calculation, its very helpful. I think Im definately going to go for the torsion box with the ally skin. I have a full workshop to work with so I can cut the ally panels, no problem.

Pippin - Thanks, so you think I need to go as far as having a 300mm x 300mm torsion box? Maybe 250mm? If that's what it needs to be then that's fine, I can build it to fit. I assumed there was a reason for the rectangular shape as that's what I see other people using but square is fine. I had thought about running a strip of steel or aluminium bar to bolt the rails to, do you think that would work?

Ger - Thanks for the link, I've been reading through your build logs. Really impressive and I like the sound of the 8 x 4!

Jag - I had thought about it but I have limited funds and time so thought maybe this would be the best option. To be fair, I think this build is going to be sort of a Frankenstein version of my original lol. Should be interesting thats for sure.

Awerby - Ill be honest, I've looked at the R&P and I think it would be a good option but I've never built anything using that before so Im a little out of the loop in how it works. Doesn't it need some sort of built in pretensioner to keep the pinion against the rack without creating a problem? I wonder if anyone has made one using wood...

Routalot - In terms of the triangulation, do you mean additional 45 degree type bracing? I hear you with the round rails, they are easier and cheaper for me to get hold of for sure. For a machine this big, what sort of size should I be looking at?


Apologies if I have misread anything, its been a long week. Thank you again for the replies, its a massive help! Have a great weekend!

[peteeng]

Hi Sweet - I have never used metal plate inserts in timber for threads. If its a thread that will be used often like hold downs I use furniture timber threaded inserts. These have worked hard for years in ply. In 16mm MDF with 16mm long inserts M8 I have torn them out after years of abuse using battery drills to do them up and undo. For rails and mechanical parts I use timber screws or std metal thread screws like M6 or M5. I tap the hole with a taper tap so as the M5 goes in it pulls up in the taper. I also set the screw with PVA glue and they have never given any trouble and come out easily with a key. The aluminium support on round rails is ideal for mounting direct to timber.

If you are concerned with the rail connection its no big deal to drill more holes in the extrusion. Use long screws, I like Kreg screws, hate Phillips heads.... they should be banned,

gantry - like all machine parts the bigger the better. Here's a paper that may help. Attached is a recent MDF machine. I feel the high rail design for a 8x4 machine may limit your access, plus you have to front load it. I have contemplated building it the "long" way so the 8' is the front but that makes it a 2.8m gantry which is really long. I'm designing a full sheet machine at the moment. It will use 30mm belts for the long side and a ballscrew for the gantry. I like belts...

I'd use 20mm or 25mm dia rails for a 8x4 machine.

thread inserts
don't get the zinc moulded type they crack often. Get the formed steel ones that are zinc plated. These are 2x the cost but 10x better. Peter

[routalot]

It doesn't have to be a 45 degree bracing web,anything that eliminates the possibility of flex between the height of the bottom edge of the gantry beam and the longitudinal tracks is helpful.We accept the need for bracing the gantry uprights along the machine ,so we need to be as careful to contain the loads across the machine.
You mention the use of a metal plate in the gantry beam and this can be a great help when it comes to attaching the rails as getting inside a long box section to position nuts and washers is a major challenge,peteeng's document contains very useful information.You mention 25mm supported round rails and they are probably adequate,if not a bit large for the spindle power you currently use.

[SweetAs]

Peteeng - Thanks again for your comments and thank you for the link to the document, that was very interesting. Out of interest, which would you use for the gantry beam, laminated aluminum, and plywood or a torsion box design? Would you look to use aluminum and ply for the sides of the gantry? How do the stiffness calculations relate to the twisting of the material do you think? Would a better option to create a torsion box with ply and then laminate aluminum plate onto the outside, would that work if the ply isn't laminated on both sides? Apologies for all of the questions, my company specializes in sheet good work and product development for designers and architects. We create everything from custom bars to furniture to canoes & small boats with plywood, Dibond, foam boards etc and so this peaks my interest!

All received on the supported round rails, to be honest, that makes life a lot easier as I can source them relatively easily and I know the supplier so they will be of good quality. Is there a recommendation on how to mount the rails on the gantry, should one be on top and one underneath, opposites etc? Im sure I researched this years ago but I can for the life of me find the posts where people had figured all that out.

In terms of the loading issue, my machine bed is currently only 600mm high from floor level so it really wouldn't be an issue in terms of loading, only with heavy 25mm+ MDF boards and then there would be two of us. Do you think it would help stiffen up the machine a lot?

Thats really interesting to hear about the screws, I would never have thought of securing the rails with wood screws but Im sure it would work just as well. I will have to have a think then as if I raise the side rails I could create a Birch Ply torsion box and then screw the rails directly to that....interesting!

With the belts, how do you know they are correctly tensioned? The belt machines Ive built in the past were not too difficult to tension as the machines were small but 30mm belts take it to the next level!! Do you use a specific calculation to ensure that there are enough belt teeth connected to the pulley when routing the belt?

Routalot - Thank you for the picture, that makes sense. Ill bare than in mind as I think I am going to rebuild this gantry and machine and go from there as it looks like work might be picking up...I hope!

[peteeng]

Hi Sweet - A ply torsion box plated in aluminium all round is a great start. Just like laminexing furniture. The issue of where to put the rails is a tough one. I prefer to mount them on top and make a 90deg saddle. This separates the Z axis cars and the gantry cars. If you put the rails on the front the cars tend to get into each others way. I have a design rule that all fasteners must be accessible without pulling anything off to get at it. I think you have to start getting busy with CAD....Peter

I think a 150x150mm gantry would do you fine. If space is not an issue 200x200 is better. Belt tension - been trying to establish an easy way to determine that for years. I use screw adjusters that push the mount along. Then tighten it up. Tight is tight it should have a small twang...

[routalot]

For loading the machine with heavy sheets,a high lift pallet truck can be a great help.It really needs to have brakes to hold it in place if you need to slide a sheet on single handed but a 25mm sheet can be moved from a pile to a machine this way.You would still need a helper if you ever ventured into the realms of 50mm MDF and had a pallet truck,or three helpers if Armstrong methods are in use.

[peteeng]

Hi Sweet - I built a gantry model 150x150x16mm mdf (1200mm long) and same with 3mm aluminium plating all round. I applied 1000Nm torque on one end. The timber is MDF as I had that handy. The MDF twisted 2mm the plated version twisted 0.34mm. Quite a delta. The plating technique allows shapes that extrusions can't get and it allows tailoring of thickness. ie you could insert thicker metal at hard points if you wanted to under the plating for instance or on top. I think its a great strategy. Peter

[SweetAs]

Afternoon, been a few months!! Apologies, work went mad, couple that with the summer holidays, kids in the workshop etc and time has been at a minimum. The increase in work has meant that I need to get this machine sorted even more now so I've been thinking and here is my plan...

250mm x 250mm Gantry - Torsion box built using Birch ply for the internals and then Steel/Ally plate which will be fixed with PU glue and screws into the Birch. 12/18mm Birch is what I plan to use. What do we think will make the best option? Previously we discussed making an aluminium laminate, I can get Galvanised Steel plate which is half the cost of the Ally, would 1 or 1.5mm Steel plate match up to the 3mm aluminium do you think? We could go to 300mm if needs be on the size of the cube/tube, Im mindful of trying to keep the Z axis with as little deflection as possible but its not that much of an issue to go with 200/250/300mm if I am rebuilding the X and Y.

Motors - I'm currently looking at getting a set of 2 x 425oz 3NM 4.2A Steppers for the Y axis. I have those on the X and Z. I would love to upgrade all to Nema34s but they are going to cost more than I can spare at the moment and in reality, the machine seems to have enough umpf, just need to get the rigidity where it needs to be. They will be on the future upgrade list I think.

Linear Rails - Initially I was looking at SBR20 for the Y and X, the X will be mounted either top and bottom or one bottom front and one rear of the top plate in an L shape, do you think that will work? The main reason for going with a new gantry "box" is so that I can upgrade the rails on the X as I just don't think there is enough room at the moment. It also means I can increase the Z travel height which it needs as currently it's a 110mm which is too tight. Should I be looking to upgrade the rails to 25mm? With the Y, I think I will add a thin steel plate the length of the MDF machine base on the top and bottom, I can through-bolt the rails to the MDF/steel laminate to ensure it doesn't squash/move in the future. What do we think to that idea?

Side Plates - Would a thin(ish) torsion box type setup with the plate on the faces be a better option then what I had originally said about just laminating the steel onto an 18mm birch slab? Im thinking of maybe a 50-75mm deep box. I could even make it out to 100mm if that would work, rebate all of the internals into the external faces to make it super rigid.

Z Axis - I've not really got many idea for that at the moment. I do have the unsupported round rails which is on the current setup, maybe I can reuse those or need to upgrade? I do have a a 10mm leadscrew but I think this needs changing out as there is room for the axis to move up 3-5mm with the slop in the current setup if I lift the axis which isn't ideal. There is a place in the UK which sells premade Zaxis set ups, although they run to about £280 which I should be able to make, just depends if its worth making or not. Here is a link to the prebuilt ZAxis https://www.cnc4you.co.uk/Z-Axis-Unit?product_id=682

X Gantry - This is the biggest headache if I am honest as I can't really picture it in my head. If I were to go ballscrew on the X, I can get a 16mm Ballscrew for £55 on eBay at around the right length, will that be thick enough or is that going to be trash? Probably looks to be a 1400mm span. Dont forget that Im currentlly running chain and angle iron rails lol so anything is better than what I have.

Y - How easy is R&P to install? The MOD1 rack will only cost around £35 for each side of the Y and then maybe go for a I am thinking of mounting the R&P on the side or underneath the machine to remove the issues with dust from the machine. Ill need to come up with some sort of motor to belt to pulley mount for the motors if I go with R&P. R&P is all a bit new to me so probably just need to wrap my head around it and I will be fine...hopefully.

Glue - I'm thinking of using either a normal PU glue or looking for some sort of construction adhesive which is marketed to bond metal to wood like Soudal Metal Fix.

Design is incoming, just need to make sure I haven't overthought anything or forgotten anything before I spend time getting that sorted.

Ultimately, as nice as it would be, I understand that I am not going to get a machine which will do 18mm birch in a single pass at 5000mm/min BUT Im more than happy to put time into the machine to increase the accuracy, repeatability and hopefully some speed + DOC improvements. Repeatability is key as we found out about 3 days into a big job that the machine was having issues which then meant I lost another 3 days catching up.

Thanks again for any help and apologies to those who helped initially before I got swamped with work.
Gantry Rail set up -

[SweetAs]

Here my thinking on the Gantry rail set up. Not sure which would be easiest to get sorted with the mounting on the rails and which would be the strongest. I guess the left one would be the strongest? I dont need 300mm under the gantry so that can probably be dropped to 200mm of clearance and still be enough, what do we think?

[SweetAs]

Here is a quick sketch up of what I have in my head, as before, the gantry sits far too high for what is going to be needed. What height do people normally build to? Is it 200mm or less? Can't see myself needing any more than that but you never know ????*?? I wonder if it would be better to try and make the side supports more of an equilateral triangle or if this sort of shape will be strong enough. The base at the moment is 400mm wide.

Thanks!

[SweetAs]

Maybe something like this. This gives 220mm clearance under the gantry beam, that's 200 plus 18mm for an MDF spoil board. Not quite sure how I'm going to mount the Z which will probably hang down under that maybe? Not sure....