Bench-top or welded frame

[Doddy]

I'm thinking about investing some time in designing a small (~800x600) router to cut upto and including aluminium. My existing work area supports a bench-top design, sat on a DIY pine workbench. But, is there a genuine advantage in knocking up a free-standing steel frame for this. Would a steel frame, say 100x50x3 box (it's a good price, and the online deflection calculators suggest it's a good geometry) offer advantage over a floating bed made from the same, sat on top of a bench top.

I do have a tig welder that's collecting dust.

Sorry - my background is in the soft-skills of sparks and computers, I don't understand this 'ere mechanical stuff.

[EddyCurrent]

My machine is a 900x600 but it's footprint is about 1200x1250 so you would need a large bench for a start.
Without some extra mass the thing would bounce about all over the place because the forces involved are a lot more than anticipated.
I would always build a frame on which to place the machine but it needs to be designed correctly.

[Doddy]

Eddy - aha, I was preoccupied with rigidity, not mass - something of an oversight of mine.

[Doddy]

So, I've been playing with Fusion 360, as much to try to get some familiarity with the software, using the idea of a frame as a starter project...

23785

Design based around 50x50x4 and 100x50x4 (shamelessly stolen from Neale's AVOR design), external dimensions of 1200 x 700 x 600(h). I'm not afraid of cutting / welding steel (or at least gluing steel together with a Tig torch), but there must come a point where you say enough is enough. I'm looking at this in cold light of day and thinking that I'm creating a design for a somewhat top-heavy frame, be a bit of a git to weld up (and use a hell of a lot of argon up in the process). I've found myself randomly adding bits to the design - the braced bed, the bracing for the internal space to house a small compressor, and maybe a suds sump, partial bracing for the frame - but it's at this point I decided to take a step back and question my sensibilities.

(To do - add the top rails - similar to Neale's, as a raised set of rails on the top of the frame)

Basically... when do you stop adding steel? Is a design like this remotely sensible?, or is it steel-work for steel-work's sake? Are there any pointers or directions into what the requirement for a frame is, and how to best approach a design?


...but it needs to be designed correctly.

I suppose that's really my question - are there any sensible guidelines out there that can be used as a frame-bible?

[Neale]

(shamelessly stolen from Neale's AVOR design)

Come on, now - you should at least feel a little bit of shame :smile:

Personally, I reckon that's a bit over-built. Mine uses 3mm steel, not 4mm, and although my pictures don't really show all the bracing/triangulation that well, I've used a lot less than yours. You could probably do without all that triangulation under the bed, although it does depend on what the bed is going to look like. Have you thought about how you are going to sort the bed? That's the kind of thing that it's worth planning at this stage, just to make sure that you don't paint yourself into a corner. Also, where are the X rails going to be mounted, and what's the gantry going to look like?

Unfortunately, there doesn't seem to be any magic formula, and the best thing to go on is other people's designs and their experience with them. Mine could be a little heavier, although I really haven't seen any structural weakness in it that worries me, even when doing machine-shaking detailed 3D work.

[Doddy]

Come on, now - you should at least feel a little bit of shame :smile:

Nah, not a bit :p

My preference for 4mm over 3mm was partly due to deflection calcs (which I don't understand but can sense that one number is better than another), cost (not much in it) and more chance to hold a thread (although nothing at this stage is tapped). The bit I hadn't done was add the final two horizontal members on top of the bed - very much in keeping with your design to mount the rail. My intent was to measure and order the steel, and spend the next few weeks welding on warm days, and designing the gantry on cold days... but you might guess that much of the gantry would be somewhat familiar (I do like your design).

The bed, and the bracing beneath the bed... the bracing from the maxim that more is good (but you can see that it's at this point I take a step back and wonder if I need to buy shares in a welding or steel supplier) and providing some bed rigidity and also some support for the bed itself. The bed - I haven't thought too much above but imagine a 15mm Ali sheet bolted to the bed frame, shimmed or skimmed, or both.

Plans for more bracing under the centre - top to bottom but I have a strong desire to keep the right bay open for storage - controllers/computer/services and just tool storage.

Painting into a corner?, that's kind of where I was losing confidence with the number of acute angles for bracing - I can see it being a bit of a git to weld all that and it's starting to feel heavy just on screen. And, at the end of the day how much is really necessary. That's when I tried to google about the effects of triangulation and bracing, but I think I'm using the wrong search strings.

Yours, I think you are targeting hard woods, from memory?, or have you done any aluminium? My areas of interest are light alloys and laminate, plastics etc. I'd be interested if you have had success with your design on similar.

[Neale]

Sorry - I just haven't done anything with aluminium apart from one very small job where the problems were more to do with using a small cutter and getting clogged cutter flutes than machine loading. Most of my work to date has been with ply/mdf/hardwood/plastic - that's just what happens to have come along!

I want to do a bit of work on steel, but it might just be limited to spotting hole centres accurately to finish on a drilling machine. One day I shall have time to experiment with these things!

One thing to bear in mind, perhaps, is that because the tubes you are using are fairly chunky, there is quite a lot of stiffening in those welded joints without explicit diagonal braces. My own experience (again, anecdotal not rigorous!) is that most of the "trying to shake itself apart" loads are trying to move the whole bed plane fore and aft, needing the side frame triangulation. There are much smaller "lozenging" loads in the plane of the bed. I doubt if you need quite so much bracing across the bed, therefore. And it's difficult enough to get the bed support components flat as it is.

[Doddy]

All good points

[Neale]

Thinking about it, I cannot remember seeing a single aluminium extrusion-based machine using diagonal bracing under the bed (although that might be a faulty memory!). And some of those machines have certainly been capable of machining aluminium. Additionally, the biggest "shaking" loads are involved in accelerating and decelerating the gantry, followed by accelerating and decelerating the Z assembly along the gantry. That is much less than the first, obviously - the mass of the gantry is the biggest load. I've reduced the effect of the second load by raising the side rails so that the Z assembly is not as high above the X rails - moving mass is more in line with the support points - although that means that the rails need good support. I think I've achieved that. I instinctively feel concern about the designs that use a single tall plate each end of the gantry, which I can imagine bending under dynamic loads. Clearly I'm wrong there as there are plenty of successful machines that are built like that although they need pretty heavy plates to take the bending loads. But engineering is all swings and roundabouts and there are always trade-offs to be made. My gantry design is more fiddly to make, for example, and probably needs a vertical mill as a minimum (which I have available, hence the design).

I do have a single diagonal corner to corner across the end (shorter) sides of the main frame. I have two more-or-less diagonals across each long side, but these are arranged to take the load of the intermediate X rail tube supports down to the bottom of the legs and do not run corner-to-corner. That does leave space to get the control cabinet in (although to be honest I had forgotten that when I built the frame, and I was lucky that the cabinet fitted the space available with a few millimetres to spare). Occasionally the gods smile on us.

I don't have a support leg in the middle of the bed, and I happily crawled over the bed structure when I was building the machine. My bed is all 50x50x3; if you used your 100x50x4 as cross-rails and braced it with intermediate 50x50 segments between them, I'm sure that would be stiff enough.

I would advise having a suitable device for cutting the box section to length accurately and squarely; I used an angle grinder in a small pivoting stand and struggled to achieve accuracy. This was a bad decision on my part, born out of lack of experience. One of those cut-off saws, maybe, with a decent size blade? Personally I would go for a metal-cutting bandsaw but that's because I have other work I do that would make use of it.

[Doddy]

Analogy is always a good reference. Yes, you're right, plenty of aluminium framed designs without so much bracing. As I said, part of this was to provide a strong, rigid base for the bed (to avoid sag), but I've finally clicked why you used 100x50 horizontally - to provide a base for the bed down each side of the X axis (for info, my very first router was a 2nd-hand MD affair and I could measure 0.7mm deflection on rails over a 400mm span... I don't want to go back there). I've got to be careful not to go back to my first post here - I could design and build a desk-top router and simply bolt this to the existing timber bench surface which would provide some mass if only limited rigidity. For me space is the constraining factor here, more so than cost, and the existing timber bench offers me storage and a little work area.

But, I do like the portable table idea, and steel is cheap (argon, on the other hand...). I might revise/simplify the table base (which may start to look even more like AVOR with the 100x50 on the flat). There is a lot of deflection across the 1200mm span with 50x50x4 (and 50x100x4) which was the intent with the support leg to transfer some of the load into the second, lower 50x50, though I don't know if that is better implemented with a deeper beam at the bed (elsewhere on here I did ponder whether transferring a load into a lower, unsupported rail is sensible or not)... but yes, some diagonals to transfer load into the legs.

Yes, I have a metal-cutting chop saw. It was one of those machine-mart moments when you go in with £30 for a hand-held angle grinder and walk out £180 poorer with a Makita chop saw. I don't fancy balancing 6m lengths of steel box on a band-saw table :) (partly tongue in cheek - I'm hoping to source from somewhere that will cut down to a standardised "usable" length - most places do)

[JAZZCNC]

Again can't be arsed to read all previous posts but I get the gist you want to aluminum but there are other areas of design that could be better worked on than building a base which could hold up blackpool tower without flexing.
Also, do you realize just how much heat will go into those joints so closely connected and how much distortion will occur from that heat.?

The frame needs to be strong but doesn't have to be massively built because the cutting forces are mostly lateral. If Was using cutting 250KG blocks of Aluminium then yes go big to stop defelction but doubt you will be.

The best thing you could do to give best cutting capability in hard materials is to build strong gantry and think about how can keep cutter deflection to a minimum.?
Cutter deflection comes from a mixture of Z-axis/spindle and Gantry vibration/deflection. Gantry deflection is easy dealt with by building strong. Z axis/Spindle is not so easy because this impacts on other areas of machine like travels, tool lengths, clearences etc.

If want high clearance for thicker material or long tools then Long z-axis required which deflects more.
Likewise if only cutting thin material then a lower clearance allowing shorter Z axis is possible, but still got to account for tool lengths, clamps etc.
One option is to lift material to the tool and keep Z extension to a minimum but this isn't practical if wanting to cut range of materials and soon gets pain in arse lifting and surfacing bed each time moved.

But with little out the box thinking, there is a better way which allows Minimum tool deflection of any machine setup while still having high clearance. In fact, can have very high clearence and still only have delection equal to tool stickout.?

This is done by Getting rid of the Z axis altogether and raise/lower the whole Y axis. This way the only deflection is from the tool stickout and any deflection in the gantry beam and supporting frame.
Build the support frame and Gantry beam strong enough and then the only deflection can come from tool it's self regardless of clearence and material thickness.
Because your lifting the Gantry beam which spindle runs along fixed to solid back plate you remove the Z axis from the equation altogether so then can have very high clearence and still have low deflection because there isn't a Z axis to deflect only the length of tool sticking out the spindle.

[Doddy]

This is done by Getting rid of the Z axis altogether and raise/lower the whole Y axis. This way the only deflection is from the tool stickout and any deflection in the gantry beam and supporting frame.
Build the support frame and Gantry beam strong enough and then the only deflection can come from tool it's self regardless of clearence and material thickness.
Because your lifting the Gantry beam which spindle runs along fixed to solid back plate you remove the Z axis from the equation altogether so then can have very high clearence and still have low deflection because there isn't a Z axis to deflect only the length of tool sticking out the spindle.

This is going off-thread, but you have got me thinking. Particularly as I've committed to buy a Chinese pack of rails and screws, delivered in the next week or so (as much to get me motivated to get off my arse and do something). I'm not a fan of spending a weekend playing with F360, but, and considering this is a fragment of a design (until I get the bits and validate the generic CAD models I've been using are broadly appropriate), is this the sort of thing that you're talking about, and is this a viable starting point for a design.

238272382823829

Basic design centred around the 400/700/1000 lengths sold cheaply. Steel box for the end cheeks (okay, only one in the images, but you can imagine the mirrored cheek), as well as the gantry. 100x50x4 mild steel box section. Spindle mount on 15mm aluminium tool plate. Some poetic license at this time with the mounting of the rail direct to the box section - but I don't think I need to model the epoxy at this time. Clearly I'd need to slave the Z-Axis drives and source a second 400mm screw but this isn't the end of the world.

I don't want to waste more time in F360 on this design if it's a non-starter, so would appreciate your view if this addresses some of the rigidity issues.

[JAZZCNC]

This is going off-thread, but you have got me thinking. Particularly as I've committed to buy a Chinese pack of rails and screws, delivered in the next week or so (as much to get me motivated to get off my arse and do something).

Well I'm surprised at that.? Thought you'd been around long enough to know better than that.! . . . . Design first then buy later is golden rule.


I'm not a fan of spending a weekend playing with F360, but, and considering this is a fragment of a design (until I get the bits and validate the generic CAD models I've been using are broadly appropriate), is this the sort of thing that you're talking about, and is this a viable starting point for a design.

Yes that's the idea thou wouldn't fasten between those two gantry uprights as it will make setting up difficult with little room for error.
Fasten rails on front, ballscrews on side and put brace between gantry uprights at top. Connect two ballscrews with belt and single motor running along top of brace.
Put rails on front of gantry and ballscrew on top.

[Doddy]

Well I'm surprised at that.? Thought you'd been around long enough to know better than that.! . . . . Design first then buy later is golden rule.

I seriously need to commit or I'll end up chasing squirrels. It's not big, not clever, but it works for me. Also, this is entirely hobby work for me which gives me a slightly different outlook than some others.

Steel is cheap (although my time is precious - to me at least)



Yes that's the idea thou wouldn't fasten between those two gantry uprights as it will make setting up difficult with little room for error.
Fasten rails on front, ballscrews on side and put brace between gantry uprights at top. Connect two ballscrews with belt and single motor running along top of brace.
Put rails on front of gantry and ballscrew on top.

Yeah, was already thinking about the top brace. Wasn't worried about two steppers, slaved, but I suppose rotate the screws 180 and belt across the top - yeah, that'd work and save some complication later on.

Ballscrews on (in)side - I lose a lot of Y travel - struck me during this the depth of the pillar blocks is pretty significant. I'll draw something up and have a ponder. Rails on the front sounds a lot easier.

Cheers - I appreciate the input.

[Doddy]

Yes that's the idea thou wouldn't fasten between those two gantry uprights as it will make setting up difficult with little room for error.
Fasten rails on front, ballscrews on side and put brace between gantry uprights at top. Connect two ballscrews with belt and single motor running along top of brace.
Put rails on front of gantry and ballscrew on top.

Dean,

I've been thinking about this a bit, and been through different iterations of a design for the gantry, as always trying to work out the best compromise. Most recently I've been measuring the width of the shed door opening - I don't want a machine that I have to dismantle to get in/out of the shed.

Can I pick your brains on the following - it does, I think, have problems but I simply don't know if they are problems that I need to be worried about.

23872

My concern is the projection of the spindle substantially away from the rails.

For info: Y-Axis beam is 80x120x5 box steel. Uprights are 100x100x5. Braces and bottom skids 100x50x4 . Sizes chosen on the "big is good" rule of thumb but also with a mind towards availability and ease of mounting stuff. Shiny stuff is 15mm alu tool plate. There's imaginary bracketry involved tbd. Of course there'd be a brace across the top, made from similar big-steel.

The obvious solution for the projection is to place the spindle assembly on the other side of the Y-gantry (this was my previous design), but I have a real-life constraint (the shed door width, and sensibly the amount of real-estate that this can take up) that means I'd seriously compromise the available space in the Y-plane (this current design obviously impacts the X-axis a bit - but I can tolerate that more than the Y).

23877

Kind of shows the impact having the ball screws on the inner face gives me. There's also another issue that the (invisible) top-brace then impacts the available Z-height with the spindle impacting the brace (something I can bodge around somewhat inelegantly by offsetting the brace).

I have looked at replacing the Y-Axis with just a big block of tool plate (20mm), but the deflection calculators suggested a pretty terrible performance by comparison to box section.

My real question is one of opinion - of whether the spindle offset will impact the performance enough to make this design impractical - if you have any thoughts I'd appreciate them.

[JAZZCNC]

My real question is one of opinion - of whether the spindle offset will impact the performance enough to make this design impractical - if you have any thoughts I'd appreciate them.

Well think you could afford to go with something like this setup with you having ball screws on inside and not lose to much travel.
On original design with 100mm wide bearing plates and the gantry going to outside edge the spindle centre line will actually be over the linear bearings or very close. So if you used say 80x40 for the diagonal you would still have room for railbearings and only lose probably 40mm or so of travel.

To be honest I don't think the over hang is massive anyway and provided you build it strong then won't have massive affect. End of the day the extension is still less than most Milling machines.

Personally I'd build a wider door in my shed than compromise the machine.

23882

[Doddy]

Much appreciated - I needed a fresh perspective - 40mm loss of travel is affordable (and should save a reasonable chunk on the X axis in the process).

(I'm avoiding going wider on the machine - yes, it's a compromise, but I am somewhat space-limited - too wide a machine is simply impractical for me)

[driftspin]

Dean,

I've been thinking about this a bit, and been through different iterations of a design for the gantry, as always trying to work out the best compromise. Most recently I've been measuring the width of the shed door opening - I don't want a machine that I have to dismantle to get in/out of the shed.

Can I pick your brains on the following - it does, I think, have problems but I simply don't know if they are problems that I need to be worried about.

23872

My concern is the projection of the spindle substantially away from the rails.

For info: Y-Axis beam is 80x120x5 box steel. Uprights are 100x100x5. Braces and bottom skids 100x50x4 . Sizes chosen on the "big is good" rule of thumb but also with a mind towards availability and ease of mounting stuff. Shiny stuff is 15mm alu tool plate. There's imaginary bracketry involved tbd. Of course there'd be a brace across the top, made from similar big-steel.

The obvious solution for the projection is to place the spindle assembly on the other side of the Y-gantry (this was my previous design), but I have a real-life constraint (the shed door width, and sensibly the amount of real-estate that this can take up) that means I'd seriously compromise the available space in the Y-plane (this current design obviously impacts the X-axis a bit - but I can tolerate that more than the Y).

23877

Kind of shows the impact having the ball screws on the inner face gives me. There's also another issue that the (invisible) top-brace then impacts the available Z-height with the spindle impacting the brace (something I can bodge around somewhat inelegantly by offsetting the brace).

I have looked at replacing the Y-Axis with just a big block of tool plate (20mm), but the deflection calculators suggested a pretty terrible performance by comparison to box section.

My real question is one of opinion - of whether the spindle offset will impact the performance enough to make this design impractical - if you have any thoughts I'd appreciate them.Hi doddy,

What work area are you aiming for?

Did you concidder going vertical?
This might solve some of your space problems.
And you could move through doors more easy.

After reading a lot of the comments on peoples 1st designs, i basicly wanted 3 things.

-A simple and heavy bed/frame
- A simple and rigid gantry.
- The spindle within the boundaries of the gantry x axis bearings.

Will there be a rigid fixed connection between your gantry sides?
How do you prevent gantry going out of square?

Your design does not look like it has a rigid gantry now.
Is it made out of 3 separate moving parts?
I think it will be very hard to get a rigid end result that way.

Also the spindle is not in between the linear bearings, this also does not help for rigidity.

Now for me i am no expert.
So please have the more experienced guys here point you in the right direction.

I started my cnc design reading at cncroutersource then came here :-)
Still on a steep learning curve.


My 2cts worth,


Grtz. Bert.






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[Doddy]

Hi doddy,

What work area are you aiming for?


Bigger than my StarMill (160x100!). Height isn't a massive issue for me (at least not as yet), but 200mm would be an aim. X/Y = around 600x400 was my goal.




Did you concidder going vertical?
This might solve some of your space problems.
And you could move through doors more easy.


Only the crazy people go vertical! Actually, it's something I had just started to think about - I know Dean here has done this in the past. That might be the next machine.



Will there be a rigid fixed connection between your gantry sides?
How do you prevent gantry going out of square?
Your design does not look like it has a rigid gantry now.


(assuming you mean square across the Y-Axis) Yes, a beam across the top, with gusset plates to hold square (sorry, the design is obviously incomplete - more to get the concept right in my mind before burning hours on Fusion 360).



Is it made out of 3 separate moving parts?
I think it will be very hard to get a rigid end result that way.


Yes, more steel to be added yet. As above, this is just a basic geometry sanity check.



Also the spindle is not in between the linear bearings, this also does not help for rigidity.

Now for me i am no expert. So please have the more experienced guys here point you in the right direction.
I started my cnc design reading at cncroutersource then came here :-) Still on a steep learning curve.

My 2cts worth,
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That's exactly why I posted this. Placing the spindle where I did I'm very conscious that it's putting a large force on the rails. Having said that, better and brighter people than me have suggested that the rails are more than strong enough for anything a small machine like this is likely to throw at them, and Dean's earlier comment re. most mills - yeah - they'll place a similar cantilever load on the rails as well.

[driftspin]

Bigger than my StarMill (160x100!). Height isn't a massive issue for me (at least not as yet), but 200mm would be an aim. X/Y = around 600x400 was my goal.




Only the crazy people go vertical! Actually, it's something I had just started to think about - I know Dean here has done this in the past. That might be the next machine.



(assuming you mean square across the Y-Axis) Yes, a beam across the top, with gusset plates to hold square (sorry, the design is obviously incomplete - more to get the concept right in my mind before burning hours on Fusion 360).



Yes, more steel to be added yet. As above, this is just a basic geometry sanity check.



That's exactly why I posted this. Placing the spindle where I did I'm very conscious that it's putting a large force on the rails. Having said that, better and brighter people than me have suggested that the rails are more than strong enough for anything a small machine like this is likely to throw at them, and Dean's earlier comment re. most mills - yeah - they'll place a similar cantilever load on the rails as well.Hi doddy,

What is your main problem with exiting 6040 designs?

I feel like the " lean back " gantry design is most space saving.

Do you think 20 mm alu gantry sides will cover your rigidity needs?

http://www.mycncuk.com/showthread.php?t=11610

like that one... but a bit smaller.


Grtz Bert.


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[Doddy]

Hi doddy,
What is your main problem with exiting 6040 designs?


Significantly - I want to do as much of the build myself, and (to an extent) I can work with steel. Having said that there's a recent threads that show some very enterprising use of hand tools (though throwing a woodworking hand held router through 20mm of alu would certainly be different to what I'd normally use it for). The design (once I understand it!) is something that I can build with the equipment that I have. To some extent there's a certain curiosity value as well.

[driftspin]

Significantly - I want to do as much of the build myself, and (to an extent) I can work with steel. Having said that there's a recent threads that show some very enterprising use of hand tools (though throwing a woodworking hand held router through 20mm of alu would certainly be different to what I'd normally use it for). The design (once I understand it!) is something that I can build with the equipment that I have. To some extent there's a certain curiosity value as well.For cost... steel is the cheaper way to build.. there is a catch though.. steel is not easy to get straight/flat enough to mount linear rail directly.

Did you decide on square or supported rail?


Do you have access to tools to get the steel flat level and straight ?

If not.. epoxy is a solution... but not very easy to do.. and not cheap.



Grtz Bert.

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[driftspin]

For cost... steel is the cheaper way to build.. there is a catch though.. steel is not easy to get straight/flat enough to mount linear rail directly.

Did you decide on square or supported rail?


Do you have access to tools to get the steel flat level and straight ?

If not.. epoxy is a solution... but not very easy to do.. and not cheap.



Grtz Bert.

Verstuurd vanaf mijn SM-A320FL met Tapatalkwhat about this one?

http://www.mycncuk.com/showthread.php?t=11152

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