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  1. #61
    Gold Member LeeWay's Avatar
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    Default Re: Need help - Fixed gantry design

    Dowel pins can take some skills and proper tools depending on how they are used. I don't have great machining skills like a tool and die maker might, so...I kinda use them my way. I hate hidden dowel pins. For several reasons. Not so bad when they are tapered. When I install them myself, I usually make through holes when possible. Much easier to deal with if you have to take them apart again. I haven't used any tapered pins yet, but I do like that idea.

    Lee


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    Default Re: Need help - Fixed gantry design

    Incidentally, while tapered dowel pins probably give the highest degree of alignment, ordinary parallel roll pins can be used to good advantage in most cases. They do have the advantage that you only need simple jobber drill bits. You can get them out with a drift quite easily too.

    Cheers
    Roger



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    Default Re: Need help - Fixed gantry design

    That's fine, we don't always need to agree .

    I respect that there are lots of things I don't know, and all of your opinions too.

    IMO, the point of using a bolt together design is that it's adjustable. If you use alignment pins, well, now it's not.

    For the machine I'm making, I've made it so I can move it in and out of a basement, and I used 1/2 inch holes through the plates with the idea that if I needed adjustability I could use M10 bolts. It's a larger moving gantry style machine. For the most part I got lucky and the alignment of what I made is good enough that I am just using 1/2" bolts in the 1/2" holes.

    There are no alignment pins for the vice on my mini mill, but whenever I put it on, I manage to get it aligned with the axis (although I admit it can be a PITA and sometimes I wish there were alignment pins).

    I'm not sure if I'm right or wrong on this, it's just one man's opinion, and it could change. I've never used the pins you describe, so still lots I don't know.



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    Default Re: Need help - Fixed gantry design

    The bigger problem is if the mating surfaces aren't machined, you'll introduce twist into the joining beams when you bolt them together.



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    Default Re: Need help - Fixed gantry design

    Quote Originally Posted by rocketflier View Post
    The bigger problem is if the mating surfaces aren't machined, you'll introduce twist into the joining beams when you bolt them together.
    That's why you bolt the plates to the opposite side, then clamp everything together then tack weld everything in place really well. This creates a custom joint between the plates / tubes without any strange twists in the frame when assembled.



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    Default Re: Need help - Fixed gantry design

    Quote Originally Posted by NIC 77 View Post
    That's why you bolt the plates to the opposite side, then clamp everything together then tack weld everything in place really well. This creates a custom joint between the plates / tubes without any strange twists in the frame when assembled.
    They're talking about bolting and pinning so things can be removable.



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    Default Re: Need help - Fixed gantry design

    Quote Originally Posted by rocketflier View Post
    They're talking about bolting and pinning so things can be removable.
    I am aware. Perhaps I didn't explain it well.

    Check out my posts in this thread #19 and #34, perhaps that will clarify what I'm trying to say.

    Essentially you clamp the entire thing together before you weld the plates to the frame. You don't weld the plates to each other, because obviously, you wouldn't be able to disassemble it afterwards.



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    Default Re: Need help - Fixed gantry design

    Quote Originally Posted by NIC 77 View Post
    I am aware. Perhaps I didn't explain it well.

    Check out my posts in this thread #19 and #34, perhaps that will clarify what I'm trying to say.

    Essentially you clamp the entire thing together before you weld the plates to the frame. You don't weld the plates to each other, because obviously, you wouldn't be able to disassemble it afterwards.
    Boltzzz latest (post #58) is no welding, bolting it together with alignment pins.



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    Gold Member LeeWay's Avatar
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    Default Re: Need help - Fixed gantry design

    Right. I only install the dowel pins at the end when I know alignment is correct. Just extra insurance initially and helps when you have to disassemble and reassemble multiple times.That dowel pin and a single bolt snugged up will give the same effect for fitting requirements and save lots of fiddly time in the end. Again not for every joint. Just where reasonable. Blind pins I think are used for heavier or more critical tolerance objects like machine columns, cylinder heads, etc. A little more than an alignment aid in those cases. More of an absolute placement position.

    Lee


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    Default Re: Need help - Fixed gantry design

    All of the above.
    I would only put dowels in after I was quite sure that the alignment was RIGHT. Until then the bolts will hold fine.

    I still hanker after the idea of using a reject granite or basalt tombstone as the FLAT base for the whole machine. Sure, seriously heavy, but stable.

    Now, where do I find a large machine to mill the surfaces? Um. I wonder whether a very large radial drill might work, just for light surfacing?

    Cheers
    Roger



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    Default Re: Need help - Fixed gantry design

    Quote Originally Posted by RCaffin View Post
    Now, where do I find a large machine to mill the surfaces? Um. I wonder whether a very large radial drill might work, just for light surfacing?

    Cheers
    Roger
    That's actually something I've been kicking around. Build what amounts to a portable surface grinder. Cup wheel on grinder that you can transverse the entire surface.



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    Default Re: Need help - Fixed gantry design

    Quote Originally Posted by rocketflier View Post
    Boltzzz latest (post #58) is no welding, bolting it together with alignment pins.
    To be perfectly honest, I don't actually know how he plans to do that from reading the post. A drawing would help me understand. Perhaps it's my ego, I just thought you were responding to what I said.

    Quote Originally Posted by RCaffin View Post

    I still hanker after the idea of using a reject granite or basalt tombstone as the FLAT base for the whole machine. Sure, seriously heavy, but stable.
    Yes,skrubol is doing something like that, and a flat base would simplify things.

    http://www.cnczone.com/forums/vertic...48456-cnc.html

    I am thinking that might be hard for the OP as the point of making this disassemble is so he can move it?

    Last edited by NIC 77; 11-17-2017 at 06:41 PM. Reason: spelling


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    Default Re: Need help - Fixed gantry design

    Hi, on the topic of dowel pins........they are a pain to insert and once placed make any adjustment due to slight variances impossible to achieve.

    From long experience in the field and on structure building and assembling too, I would not dowel any interfaces as once the structures are aligned the bolting will more than suffice to hold any structure rigid without it moving around.

    With a CNC router build the base framework is not all that subject to forces that would throw it out of alignment.

    It's quite easy to render two surfaces flat to one another simply by lightly touching the faces with a flexible grinding disc to remove the high spots........but first you have to ensure the entities that are to be bolted together are square and aligned to one another before you make the bolting interfaces true too.

    My personal gauge for accurate surface mating was 4 spots of contact to the inch.....this will give you more than adequate accuracy for bolting two faces together in intimate contact.......and I use chalk to indicate the contact points.......you can build an entire machine without any machining at all using that process.

    It would help a million fold to have an accurate surface to work from and an off cut of granite as is used in kitchen worktops would be more than adequate for that purpose.

    That is one reason I like working with an all steel welded construction.......you can add bits on and cut them off without having to resort to bolts etc.
    Ian.



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    Default Re: Need help - Fixed gantry design

    Hi Ian

    dowel pins........they are a pain to insert and once placed make any adjustment due to slight variances impossible to achieve.
    Well, yes, but that IS the point of dowel pins after all.

    Cheers
    Roger



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    Default Re: Need help - Fixed gantry design

    Hi.....well no, I mostly used dowel pins for tool making and press tool work.....with structural work I never doweled any entity due to so many cases where settling forces and subsequent re-alignment issues needed to be catered for

    BTW.....roll pins are NOT dowel pins, they are the cheap and cheerful location methods used in industry by unskilled people to keep parts in close proximity........NEVER for ensuring perfect replacement of components on assembly.....drilling and reaming a hole to fit a dowel pin is an exact process.

    When parts that are doweled need to be disassembled the dowels need to be first driven out......anyone who fits dowels in blind holes needs to be strung up on barbed wire for the crows to eat.
    Ian.



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    Default Re: Need help - Fixed gantry design

    Hi Ian

    settling forces and subsequent re-alignment issues needed to be catered for
    Well, yes, but if you build the machine on a seriously stable heavy flat plate (a basalt tombstone?), then there should be no settling to cater for. That would be my goal.

    Yes, I know roll pins are not dowels, but if they are used only for location, leaving the clamping to be done by the bolts, then they can be very functional. And while I mentioned tapered drill bits for dowels, I have to confess i can't find any matching, say, DIN 1A right now. Taper drills for wood-working, sure, but their angle is far to much. Tapered end mills are available of course, but do you know of any tapered twist drill bits?

    Cheers
    Roger



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    Default Re: Need help - Fixed gantry design

    Well i dunno if you can get tapered drills for drilling tapered pin holes.........drills cut on their ends and so you would drill to the smallest size and then ream to fit the tapered pin.

    Having worked on equipment that had collars with tapered pins I can say that they are the most difficult bits of tackle to work with..........I once saw a guy trying to hit a tapered pin out with a pin punch from the small end.

    Re-assembling a collar to a shaft that has a tapered pin means you need to carefully mark the position on the shaft and collar to make sure you assemble the collar or whatever with the tapers in line not 180 deg out of wack.

    I've used roll pins for a job I once supplied but the application was to ensure the parts only stayed together as an assembly not really a location fit per se.

    I've got a few small tapered pin reamers and I think I also might have a tapered drill bit too........as far as I can recollect the drill has some tiny slots down it's length so it might well be a tapered drill bit.
    Ian.. .



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    Default Re: Need help - Fixed gantry design

    I must have been think of tapered end mills. They ARE available at any angle you want: much used for mold-making.

    One can get DIN dowel pins with tapped holes in the end for (sensible) extraction.

    Academic right now. :-) I have other things to build.

    Cheers
    Roger



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    Default Re: Need help - Fixed gantry design

    Roll pins can be used instead of dowel pins in certain cases. They are just as good at helping align assemblies. Especially those that need constant or at least several assemble and removal operations. They are also more forgiving to temperature changes and do not require as much engineering as dowel pins might.
    They can also have different shear ratings which can be a big benefit to an overall design and usage. Roll pins have their place. They are not dowel pins though. They can do the same job sometimes and are much more forgiving to inaccurate machining of it's receiving bores. They are in effect, a spring which has some tolerance allowance when inserting.

    Lee


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    Default Re: Need help - Fixed gantry design

    Yep, I quite agree.........rolls pins are the answer to an engineers prayer when it comes to simple location and multiple removal.

    The fact that they can have a drilled hole and then be inserted to take up the drilled tolerance is quite handy for many cases I've had to use them on.......i wouldn't be without them.
    Ian.



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