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    Member JoanTheSpark's Avatar
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    Default Why aren't portal mills more popular?

    What is the reason, that I don't seem to find a lot (actually none) designs/builds that try to scale down those large portal mills to shop format?
    IMHO the design should be sturdier than a gantry design (moving weight can go into rigidity of essential parts) and the rails/drives are higher up and away from chips.
    What am I missing?



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    Default Re: Why aren't portal mills more popular?

    Quote Originally Posted by JoanTheSpark View Post
    What is the reason, that I don't seem to find a lot (actually none) designs/builds that try to scale down those large portal mills to shop format?
    IMHO the design should be sturdier than a gantry design (moving weight can go into rigidity of essential parts) and the rails/drives are higher up and away from chips.
    What am I missing?

    The dust will be much the same, If the moving gantry rails are under the table they are less prone to dust

    There are lots that build them like this, the only problem you have is less access to the table, and loading material is not as easy

    A well designed moving Gantry does not have any rigidity problems, and will work just as well, and you will have full table access

    Mactec54


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    Default Re: Why aren't portal mills more popular?

    Hi.....by posting the sketch of "that" design, what are you trying to say?.......it's a monstrosity.....my opinion in case Mac wants to fire a broadside.

    BTW.....what is a portal mill????.....the sketch you posted is of a gantry router.....there is no comparison between a router and a mill..
    Ian.



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    Default Re: Why aren't portal mills more popular?

    I just got that from one of the threads here, where someone posted an image of a big one and they call it a portal milling machine - I mean I can find them via that with google.
    I assumed that would be the difference between a gantry (legs/columns connected with the beam, rolling on the floor) and this design with the high walls (beam moving alone, no legs).. probably wrong then?



    I checked wikipedia and came across this (https://en.wikipedia.org/wiki/Gantry_crane):
    "They are also called portal cranes, the "portal" being the empty space straddled by the gantry. The terms gantry crane and overhead crane (or bridge crane) are often used interchangeably, as both types of crane straddle their workload. The usual distinction drawn between the two is that with gantry cranes, the entire structure (including gantry) is usually wheeled (often on rails). By contrast, the supporting structure of an overhead crane is fixed in location, often in the form of the walls or ceiling of a building, to which is attached a movable hoist running overhead along a rail or beam (which may itself move)."
    Not the kind of distinction I'm after really, but it conveys the thoughts I had about gantry/portal mills before I read the article about cranes.
    I don't think one would want to call this design an overhead mill/router or even a bridge mill/router - the last one already being taken by 'bridge mills' from what I have learned so far?

    As for 'my monstrosity', what's wrong with it?
    From my experience the flat pieces for the walls and bed can be welded without skew (SHS) and should come out pretty flat, just being bolted together to get the box. I've done stuff like this before, just not that 'fat' or for such an application.
    Don't you guys usually suggest that bigger, heavier and stiffer is better when people rock up with those router designs that sport thin uprights for the gantry / thin cross sections /etc.?
    And I kinda want to have the dirt contained from the get go if possible. Some sheet metal on the walls from the 'inside' and a door at the front and it's closed for the worst of it, not messing up the shop.


    @mactec45
    Loading/unloading shouldn't be worse than for one of those vertical machining centers then I guess? I somehow expected a more 'sinister' reason that I didn't recognize.. cool. Thanks.


    Last edited by JoanTheSpark; 10-22-2017 at 07:40 AM. Reason: image change, added more info


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    Default Re: Why aren't portal mills more popular?

    Quote Originally Posted by JoanTheSpark View Post
    I just got that from one of the threads here, where someone posted an image of a big one and they call it a portal milling machine,]
    Don't bother with what Handlewanker says, he is very miss informed, on machine building, that's no router Handlewanker it's a 5 axes Milling machine

    There is nothing wrong with your design, others have made machines just like this, and there is no reason not to build them like this, the only thing like I said is loading large pieces, there are some large commercial routers also manufactured just like your design


    Quote Originally Posted by JoanTheSpark View Post
    Loading/unloading shouldn't be worse than for one of those vertical machining centers then I guess? I somehow expected a more 'sinister' reason that I didn't recognize.. cool. Thanks.
    You can't compare your router, to a machining center, it would be a very large machining center, that has a 1.6m Y axes, front loading machines like the Tormach are very easy to load as you park the table at the front, with your design the table does not move, the big portal milling machines like your photo you can walk inside if you have to

    Mactec54


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    Default Re: Why aren't portal mills more popular?

    Well, I was expecting that once pieces get heavier than 15-20 kgs I need to use a mobile crane anyway.
    But anything smaller I should be able to put in the front there, doesn't matter for the mill where it is located on the table.
    And for larger lighter stuff it should be easy to put it in by hand.
    If I have to clamp down somewhere I don't reach I can always climb into the machine (if it wouldn't support me without budging I did something wrong anyway) - walking into would be nicer naturally - but I don't have that kind of money ;-)

    Anyhow, thanks for the clarifications.
    Just one more question that remains, as I wouldn't have started this thread if I'd found any examples for something like this in this size.
    Do you have any links to builds like this - large vertical/z travel as I'm especially interested how stiffness/rigidity has been solved before?
    I'm sure I can learn a lot by looking at others peoples work, at least that's how it usually works for me. Still enough that goes wrong even with that kind of help :-)

    I'm currently doodeling around in my cad tool with the x/z node, trying to get it stiff and manufacturable (by the means that are available to me locally).
    I can get parts lasercut up to 20mm mild steel and machining of some faces of smaller parts is also cared for.
    So yeah, nothing carved in stone. I'm still open to input and options and will have more questions as time goes by.



    Attached Thumbnails Attached Thumbnails Why aren't portal mills more popular?-portalmil_y-z-node-jpg  
    Last edited by JoanTheSpark; 10-22-2017 at 11:34 PM. Reason: had wrong axis convention


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    Default Re: Why aren't portal mills more popular?

    Quote Originally Posted by JoanTheSpark View Post
    Well, I was expecting that once pieces get heavier than 15-20 kgs I need to use a mobile crane anyway.
    But anything smaller I should be able to put in the front there, doesn't matter for the mill where it is located on the table.
    And for larger lighter stuff it should be easy to put it in by hand.
    If I have to clamp down somewhere I don't reach I can always climb into the machine (if it wouldn't support me without budging I did something wrong anyway) - walking into would be nicer naturally - but I don't have that kind of money ;-)

    Anyhow, thanks for the clarifications.
    Just one more question that remains, as I wouldn't have started this thread if I'd found any examples for something like this in this size.
    Do you have any links to builds like this - large vertical/z travel as I'm especially interested how stiffness/rigidity has been solved before?
    I'm sure I can learn a lot by looking at others peoples work, at least that's how it usually works for me. Still enough that goes wrong even with that kind of help :-)

    I'm currently doodeling around in my cad tool with the y/z node, trying to get it stiff and manufacturable (by the means that are available to me locally).
    I can get parts lasercut up to 20mm mild steel and machining of some faces of smaller parts is also cared for.
    So yeah, nothing carved in stone. I'm still open to input and options and will have more questions as time goes by.

    That's a good configuration for that axes support, some will say you don't need 3 linear rails, but I say more the better, something for you to look at, these do not all look like yours, but have the same concept

    This is one of many of similar concept, https://www.finelineautomation.com/p...-4?taxon_id=54

    This is made of wood and metal, a lot of these have been made, same concept, not what you have in mind, but the same idea is there http://www.cnczone.com/forums/momus-...176-forum.html

    There is another very good build that is on the Zone somewhere and was built for a 5 axes build

    Mactec54


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    Default Re: Why aren't portal mills more popular?

    Quote Originally Posted by JoanTheSpark View Post
    I just got that from one of the threads here, where someone posted an image of a big one and they call it a portal milling machine - I mean I can find them via that with google.
    I assumed that would be the difference between a gantry (legs/columns connected with the beam, rolling on the floor) and this design with the high walls (beam moving alone, no legs).. probably wrong then?



    I checked wikipedia and came across this (https://en.wikipedia.org/wiki/Gantry_crane):
    "They are also called portal cranes, the "portal" being the empty space straddled by the gantry. The terms gantry crane and overhead crane (or bridge crane) are often used interchangeably, as both types of crane straddle their workload. The usual distinction drawn between the two is that with gantry cranes, the entire structure (including gantry) is usually wheeled (often on rails). By contrast, the supporting structure of an overhead crane is fixed in location, often in the form of the walls or ceiling of a building, to which is attached a movable hoist running overhead along a rail or beam (which may itself move)."
    Not the kind of distinction I'm after really, but it conveys the thoughts I had about gantry/portal mills before I read the article about cranes.
    I don't think one would want to call this design an overhead mill/router or even a bridge mill/router - the last one already being taken by 'bridge mills' from what I have learned so far?

    As for 'my monstrosity', what's wrong with it?
    From my experience the flat pieces for the walls and bed can be welded without skew (SHS) and should come out pretty flat, just being bolted together to get the box. I've done stuff like this before, just not that 'fat' or for such an application.
    Don't you guys usually suggest that bigger, heavier and stiffer is better when people rock up with those router designs that sport thin uprights for the gantry / thin cross sections /etc.?
    And I kinda want to have the dirt contained from the get go if possible. Some sheet metal on the walls from the 'inside' and a door at the front and it's closed for the worst of it, not messing up the shop.


    @mactec45
    Loading/unloading shouldn't be worse than for one of those vertical machining centers then I guess? I somehow expected a more 'sinister' reason that I didn't recognize.. cool. Thanks.
    As I said....it's just my opinion.....you either like it or not.

    One thing you come to realise when you work with machinery all your working life........the hands on experience which Mac doesn't have......you cannot scale nature and attempting to scale a full size machine down is also not practical.......something Mac doesn't understand as he's not a skilled engineering person, just an armchair engineer who likes blowing off now and again

    So, we're talking about a moving gantry router not a mill....portal or otherwise......that limits your range of materials.

    Attached is a pic of my favourite "portal"??? mill design made in Hindustan......which in actual fact is commonly known as a moving table CNC gantry router.....call it what you like......a rose by any other name is still a rose.

    If I were to build a serious CNC router that is what I would base my design on.

    At the end of the day you have to work with what you are capable of achieving.

    What was your working envelope or table size for a......machine......to satisfy your needs, and what range of materials are you going to use on it?
    Ian..

    Attached Thumbnails Attached Thumbnails Why aren't portal mills more popular?-toolcrafter-fixed-gantry-router-jpg  


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    Default Re: Why aren't portal mills more popular?





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    Default Re: Why aren't portal mills more popular?

    So, Iain, Mactec, 7 hours on is there any clear picture of who has the biggest e-wang yet?

    One of the first videos (and I'm annoyed I can't find it anymore, gave up 20 pages into the search) I saw of a 5 axis machine was essentially a box with the front and top removed. Y rails on the top of the left and right sides, gantry for X sitting directly on the rails with no moving pillars to flex or add torque load to the linear bearings, Z on a carriage same as with a typical gantry machine and an A+C trunnion table fixed to the table beneath.

    Aside from the access and ability to overhang large workpieces off the edge of the machine issues which, already as identified, is just as much a PITA with any enclosed machine anyway, I can't see any disadvantages to compete with the advantages of getting the Y rails out of the swarf (not dust, I don't care about dust) path and eliminating the flex of those side walls which can be framed or whatever as heavy as you like.

    But I've always been too scared to ask, so thanks JoanTheSpark

    Also: Ian, I think it might be a bit silly to automatically write off any gantry machine as "just a router". As a simple, straight off the top of the head example I look at the Datron machines... not at all what I'd think of as a router.



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    Default Re: Why aren't portal mills more popular?

    Quote Originally Posted by handlewanker View Post
    Attached is a pic of my favourite "portal"??? mill design made in Hindustan.
    {...}
    If I were to build a serious CNC router that is what I would base my design on.
    I think I've run across your current machine and it is of that design, a moving table fixed gantry mill.

    Space wise you need longer y-rails and ball screws to move the part past your milling head (rail length = 2x table length).
    Might be OK for small designs, but the y-dimension of your gantry envelope vs. the y-dimension of the work piece length (or table) make this not economical for longer tables (strength of ballscrew vs diameter, flex, etc.).
    Also the y-drive then needs to work for your heaviest work piece, while for the moving gantry design the moving weight is relatively small/constant.
    The only pro I can see is that you get a single drive for the y-axis which means your gantry doesn't skew and that you can build the gantry very heavy to counter flex.

    Also, per common wisdom (I'm lazy and will just follow the herd here ;-) ):
    https://www.reddit.com/r/CNC/comment...outer_vs_mill/
    "I believe the most succinct differentiation is that a mill makes cuts using torque, while a router makes cuts using rotational speed.
    A mill will have a heavy structure relative to the working area and will use a spindle with finely grained speed control.

    A router will be relatively lighter for the same working area and will mount a high speed spindle, trim router, or full-size router or similar tool."

    TL;DR: mill/router doesn't differentiate between the positional arrangement of the axis drives, but only concerns with the cutting force/speed/materials involved.

    And portal vs. gantry for cranes doesn't exist, it's the same, thus same should be true for mills/routers, with the caveat that there are no overhead mills/routers which would distinguish the moving column from the fixed column versions (both moving gantry). This might be a reason some manufacturers refer to the ones with fixed columns/moving gantry as portal instead of gantry.

    Quote Originally Posted by handlewanker View Post
    What was your working envelope or table size for a......machine......to satisfy your needs, and what range of materials are you going to use on it?
    wood, plastic, etc - 1200x800x300
    aluminium, mild steel, etc - 1000x300x300

    I'm still torn between building a single machine or making two - space available vs money vs reality.
    I suppose there is no spindle for hobby kind of money that can cover both, otherwise I'd have come across one by now on this forum I guess.
    A BT30 milling spindle with motor in the 1.5-2kW size as a single item/unit seems not to exist either.

    This is the example I'm mostly hanging onto for the size/power/capabilities of the machine:
    http://www.cnczone.com/forums/uncate...aluminium.html

    Last edited by JoanTheSpark; 10-23-2017 at 12:19 AM.


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    Default Re: Why aren't portal mills more popular?

    @iwaskevin
    Cool, thanks for posting. The y-direction bending/torsional stability of that gantry/portal would be worrisome to me in that design though.
    I'd expect some struts and wire-strapping could make it stiffer bending wise.
    Same as they use around here for those agricultural watering sprinkler cranes:


    Torsional-ly not much that can be done, besides beefing up the gantry.
    Worst case is full z-depth and in the middle of the gantry and a force in x-direction.

    Last edited by JoanTheSpark; 10-22-2017 at 11:32 PM.


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    Default Re: Why aren't portal mills more popular?

    Quote Originally Posted by dharmic View Post
    ..I've always been too scared to ask, so thanks..
    I would have asked earlier, but never had time to sit down and draw something up, as I couldn't find examples ;-)

    Quote Originally Posted by mactec54 View Post
    some will say you don't need 3 linear rails, but I say more the better, something for you to look at, these do not all look like yours, but have the same concept
    Yeah, as short torsional lengths as possible, while the carriages as far apart as possible.
    The biggest worry is the parallelism of the opposite mounting surfaces for the rails there.
    And being able to get everything together, piece by piece, aligned and tight, while still being able to reach the bolts.. hehe.
    The weight of the z-ram and the shorter x-directional spacing of the bottom runners on the gantry begged for an additional rail on the top.
    A nightmare making those planes parallel though.

    Last edited by JoanTheSpark; 10-23-2017 at 12:13 AM.


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    Default Re: Why aren't portal mills more popular?

    Quote Originally Posted by JoanTheSpark View Post
    The biggest worry is the parallelism of the opposite mounting surfaces for the rails there.
    A nightmare making those planes parallel though.
    Not to bad on a large Mill, when I do them like that I pocket all the rails in one operation, the other way, if you have your beam Blanchard ground on both faces, you can mill the rail pockets 1 face at a time

    Mactec54


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    Default Re: Why aren't portal mills more popular?

    Quote Originally Posted by JoanTheSpark View Post
    I think I've run across your current machine and it is of that design, a moving table fixed gantry mill.

    Space wise you need longer y-rails and ball screws to move the part past your milling head (rail length = 2x table length).
    Might be OK for small designs, but the y-dimension of your gantry envelope vs. the y-dimension of the work piece length (or table) make this not economical for longer tables (strength of ballscrew vs diameter, flex, etc.).
    Also the y-drive then needs to work for your heaviest work piece, while for the moving gantry design the moving weight is relatively small/constant.
    The only pro I can see is that you get a single drive for the y-axis which means your gantry doesn't skew and that you can build the gantry very heavy to counter flex.

    Also, per common wisdom (I'm lazy and will just follow the herd here ;-) ):
    https://www.reddit.com/r/CNC/comment...outer_vs_mill/
    "I believe the most succinct differentiation is that a mill makes cuts using torque, while a router makes cuts using rotational speed.
    A mill will have a heavy structure relative to the working area and will use a spindle with finely grained speed control.

    A router will be relatively lighter for the same working area and will mount a high speed spindle, trim router, or full-size router or similar tool."

    TL;DR: mill/router doesn't differentiate between the positional arrangement of the axis drives, but only concerns with the cutting force/speed/materials involved.

    And portal vs. gantry for cranes doesn't exist, it's the same, thus same should be true for mills/routers, with the caveat that there are no overhead mills/routers which would distinguish the moving column from the fixed column versions (both moving gantry). This might be a reason some manufacturers refer to the ones with fixed columns/moving gantry as portal instead of gantry.


    wood, plastic, etc - 1200x800x300
    aluminium, mild steel, etc - 1000x300x300

    I'm still torn between building a single machine or making two - space available vs money vs reality.
    I suppose there is no spindle for hobby kind of money that can cover both, otherwise I'd have come across one by now on this forum I guess.
    A BT30 milling spindle with motor in the 1.5-2kW size as a single item/unit seems not to exist either.

    This is the example I'm mostly hanging onto for the size/power/capabilities of the machine:
    http://www.cnczone.com/forums/uncate...aluminium.html
    Hi.....steel?......you would need to know your stuff to design for that beastie and have a good workshop inventory to do it.

    CNC routers, no matter how you are gifted in the design or manufacturing department, just aren't up to it......wood, plastics, maybe aluminium, but steel......forget it.

    You "could" over engineer anything to make it work, but a practical design to make it work so far has not eventuated in any of the builds, that had steel as a desirable material to cut, on any I've looked at on the forum.

    One reason gantry type routers are not easy to build is the mass required to absorb cutting forces and deflective loads.........a .02mm deflection at the cutter due to a flimsy build means the cutter is on a self destruct course from vibration.

    The big problem for all machines that go as benchtop or floor standing is having enough grunt in the spindle to make cutting a reality instead of a joke.

    Mills tend to have a better configuration as they can have a separate spindle and more powerful belt driven motor to get the low down torque that you must have for steel hacking while at the same time stepped pulleys or a high speed spindle can be attached to do whatever you need in the higher rpm area.

    It's all a matter of personal preference.....we could debate the attributes of many designs for ever, but in the end it can turn out to be just another common or garden CNC router of one description or other.....and the Earth will never move beneath your feet.......everyone plays that game......no two designs are exactly the same, but they all do the same thing more or less.
    Ian.



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    Default Re: Why aren't portal mills more popular?

    Quote Originally Posted by mactec54 View Post
    Not to bad on a large Mill, when I do them like that I pocket all the rails in one operation, the other way, if you have your beam Blanchard ground on both faces, you can mill the rail pockets 1 face at a time
    Thanks for using the word pocket - made me think twice ;-)

    A question in regards to this - the mounting reference for the rails - does it have to be one solid piece, or can this reference also be a bolted-on piece of metal (base only needs to be ground/milled flat, not also pocketed)?
    Also how important is this - as most DIY builds seem to omit this as far as I can see?



    Just leaving this interesting essay on guide rails here - Picking the Right Linear Rails for Your Application

    Attached Thumbnails Attached Thumbnails Why aren't portal mills more popular?-portalmil_rail-mounting-ref-jpg  
    Last edited by JoanTheSpark; 10-30-2017 at 11:32 AM.


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    Default Re: Why aren't portal mills more popular?

    DMS has all these configurations.



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    Default Re: Why aren't portal mills more popular?

    Quote Originally Posted by JoanTheSpark View Post
    Thanks for using the word pocket - made me think twice ;-)

    A question in regards to this - the mounting reference for the rails - does it have to be one solid piece, or can this reference also be a bolted-on piece of metal (base only needs to be ground/milled flat, not also pocketed)?
    Also how important is this - as most DIY builds seem to omit this as far as I can see?



    Just leaving this interesting essay on guide rails here - Picking the Right Linear Rails for Your Application
    You only have to do that to the master rail and master blocks - provided you bought your LM bearings as a matched set.



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    Default Re: Why aren't portal mills more popular?

    Quote Originally Posted by louieatienza View Post
    You only have to do that to the master rail and master blocks - provided you bought your LM bearings as a matched set.
    Should be done with all rails, it is mostly done to make the rails straight

    Mactec54


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    Default Re: Why aren't portal mills more popular?

    Quote Originally Posted by JoanTheSpark View Post
    Thanks for using the word pocket - made me think twice ;-)

    A question in regards to this - the mounting reference for the rails - does it have to be one solid piece, or can this reference also be a bolted-on piece of metal (base only needs to be ground/milled flat, not also pocketed)?
    Also how important is this - as most DIY builds seem to omit this as far as I can see?
    You could bolt it on and dowel it in place, it just depends how accurate you want the rail to be, I always pocket the rails, But a milled shoulder is better than nothing at all, this is mostly used to straighten the rail, plus align the rail parallel, the rails are not straight so you need some way to straighten them

    They all have a mounting tolerance depending on the Grade of Rail / Bearing being used

    Mactec54


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