Help with 5 sided work fixture solution


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Thread: Help with 5 sided work fixture solution

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    Default Help with 5 sided work fixture solution

    I need help, I am working on a project where I need to carve multiple wood cars on my 3-axis CNC for a corporate client. The issue is that I need to machine all sides of the wood blocks with repeatability and precision in mind. (I tried a one-off test and had opposite sides misaligned by a tiny bit).

    While my plan isn't fixed in stone, so far I intend to machine the underside of the cars first and carve an attachment point for a metal mounting base (perhaps using alignment pins and screws?). Once I have the wood block mounted to the metal base I would then fix the metal base to a metal fixture and machine each of the remaining 5-sides. Carving one side at a time.


    The issue I am having is exactly what the mounting base and associated fixture should look like. The fixture and base would need to be able to secure together in a way that ensured proper alignment. I would prefer finding already made components I could use for this as I can't mill metal with any precision. Thought about 1-2-3 blocks but don't think they would secure together in a way that would work but not sure.

    I found similar cars online that were made for the purpose of pinewood derby, not sure how they were made but provide an example of what my client is looking for. Appears the 2 round holes on the bottom where used to mount it to a base but not sure. Thank you for your help.

    Help with 5 sided work fixture solution-3-jpg
    Help with 5 sided work fixture solution-4-jpg

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    Default Re: Help with 5 sided work fixture solution

    Totally possible,not quick or particularly easy and you need a machine with enough Z axis travel to reach the length of the model when it stands on end-unless you have a hole in the table and some space below it-in which case you need a Z axis travel equal to the width of the blank for the model.You will also need a fixture that will allow the model to be held in a number of positions for the various operations that you need to carry out.As you have learned,each and every tiny error has a cumulative effect and this principle applies to the fixture(s) that you will need.Furthermore,the tiniest of errors relating to the fixture offset on the machine will have an effect.I would think that with some planning you could use patternmakers dowels for location and maybe a screw or two for clamping.My preference would be to machine the bottom first with dowel holes similar to the example I would make a good sized angle bracket type fixture to take dowels that could be spaced such that a vertical row of dowels could hold the blank while the front and back were machined and then a horizontal pair of dowels could locate the blank while the sides were machined in turn.For the top surfaces,I suspect the dowel holes might be getting a bit worn by this stage and the fixture might include partial mating surfaces for the wheel wells.How well it turns out will be totally dependent on the accuracy maintained at each stage of the process and may still need an amount of hand finishing.I note that no mention of overall size has been given.



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    Default Re: Help with 5 sided work fixture solution

    I would offer some general observations; given the detail of the model, this isn't a "quick deal". Your client isn't expecting a Pinewood Derby quality product. I'm also presuming that the finished part is to be rendered in hardwood which would machine more cleanly and show better detail.

    My approach would be to sequence the process such that the transition areas would fall along hard breaks where any small misalignment of surfaces could be addressed thru follow up hand sanding.

    I would start by machining the front and back, then on to the sides as 4 separate operations. The bottom is next with attention given to creating voids that would closely match a negative form milled in aluminum. The holes in your form would allow for positioning dowels to be used in your final fixture. Any fixturing that is made along the way is fashioned as negative forms of your workpiece DURING THE PROCESS OF CREATING TOOLPATHS FOR YOUR PART. It is important to retain the locations of the fixturing via work offsets relative to part creation to minimize transition errors. I would also gasket the last fixture to allow the use of vacuum to aid in hold down (presuming 25" Hg at the part or better).

    The last thing to mention is accuracy at the outset is important. In a "normal" wood shop, it is difficult to accurately size blanks prior to CNC machining. If you are referencing different surfaces that were manually generated at the outset, accuracy will likely be compromised. Start, if you can, by CNC milling reference surfaces at the initial operation.

    Your test model looks great to start. Let us know how the job goes.



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    Default Re: Help with 5 sided work fixture solution

    Just a quick follow up; This could largely be done on a 4th axis. Mill the bottom first and reference that when mounting to the 4th. Then mill front, sides and back. Last operation is done on a reference fixture as in my last post. Wood tends to move as its being milled as stress is relived, so expect there to be some touch up sanding of transition areas.



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    Default Re: Help with 5 sided work fixture solution

    routalot,
    Thank you for your reply. The cars will be approximately 3" x 3" x 9". To do the ends I will indeed need to utilize a hole in my work surface so I can get the ends below my cutter height however I checked and I have sufficient clearance. I also anticipate I will only need to cut to a depth of 1-2" from each surface.

    As for the dowels I was thinking of using metal pins as it wouldn't wear as quickly and be more rigid than a wood dowel but there would be limited room to do 2 pins and 2+ screws. I have also been considering making those 2 holes threaded for hold down bolts that would both align the piece and secure it in one step. Only concern with bolts is that I was worried this could introduce alignment issues between the bottom and the other 5 sides due to the threads. The below sketch is what i was thinking with regard to utilizing bolts combined with 1-2-3 blocks (which I would need to go buy) and I also understand not all 1-2-3 blocks could be used this way. (wish I had the Renzetti blocks I saw online). In the below sketch the dark gray block would be bolted to the table and remain a fixed point. The light gray block would remain fixed to the wood block throughout each step. The red bolts would then be removed and replaced as needed to lock the 2 blocks together in each configuration.

    Help with 5 sided work fixture solution-sketch-jpg


    MARV,
    Thank you for your reply. The final wood has yet to be decided by the client but indeed both soft and hardwoods are being considered (even HDU foam is on the table). Each material has its advantages and disadvantages. If I understand your suggestion correctly it would require machining a negative form for each different car design and thereby require more material and machine time. The large pocket on the underside would be unique to each car design but I would try to have the mounting point (dowels/pins/bolts/screws/etc) be universal to all cars to minimize fixture variations and setup time. I also don't really have the capability to mill metals with any precision. I have cut very limited amounts of soft metals like aluminum on my CNC router but I suspect precision isn't great as its not as rigid as a proper mill.

    Unfortunately I also do not have a 4th axis and this job wouldn't justify adding one. I was thinking if I did have a 4th axis it would speed up the process and improve the alignment between the 4 surfaces, but it would still require the fixture to do both ends. Note that the car pictures I attached in my previous post were found online but are similar to what I am trying to achieve.



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    Default Re: Help with 5 sided work fixture solution

    tallen, I was thinking of an aluminum fixture plate that the steel dowels would be attached to. The plate would be fitted with a gasket to assist hold down with the base plate and dowels serving to maintain alignment. If there are areas on the bottom common to all the cars you'd be milling, the fixture could be a generic device. Could also be used for positioning each of your milled surfaces (in a variety of positions). Milling a fixture plate in aluminum might not be as difficult as you think. Particularly if you are only milling dowel holes (with dowels seated in thread lock) and a gasket cavity 1/8" deep.

    Something worth mentioning is that work is more easily aligned to fixturing when you've started with a CAD template that has verified locations for machine coordinates. Once that is done, fixturing devices placed in the CAD environment for which there is tool pathing written for fixture as well as mating parts, misalignment will be minimized. Hope this helps.



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    Default Re: Help with 5 sided work fixture solution

    I agree with Marv about the transitions.

    If you thought about the car, in a normal car configuration (wheels down) facing the passenger side, I'd machine the bottom 90% normally but stretch the model a hair for the last 10%. That way, when you put the block in a top down view, any mismatches should be blended or, at the very least, can be easily sanded.

    I have to also agree a rotary A axis would be awesome for this. The fixture would be easy as the rotation is controlled by the machine.



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    Default Re: Help with 5 sided work fixture solution

    Machine bottoms first with 2 holes for mounting to alum plate, 2 holes for pin locations. Then machine tops in vise with wood mounted to alum plate, then rear, front n sides on the A axis.
    Alum plate has pins on both sides, one side mates with car body bottom other side has 2 pins to mate with A axis plate, plate has thru holes to mount car body first then mount to indexer with other 2 holes. That is what I do with my cars on my Haas TM-1.
    If I get some time I will see if I can get some pics.

    MM



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    Default Re: Help with 5 sided work fixture solution

    The patternmakers dowels I mentioned are metal,usually brass.





    I would try for a fixture something like the crude model I represent here and you may find it beneficial to use a spacer block to get away from the vertical face.I would still try to keep the dowel separation sufficient to mount the block end on between the dowels used to locate the sides for machining.

    Help with 5 sided work fixture solution-model-car-machining-jpg



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    Default Re: Help with 5 sided work fixture solution

    Agree that a rotary A axis or indexer would help speed up the project since it could then do 4 sides in one go and maintain alignment between each. Unfortunately I do not have one and this project doesn't warrant getting on at this time.

    As for the transitions, if there is a tiny misalignment then I should be able to do minimal sanding to fix edge imperfections. Biggest issue I ran into with my test car was that the wheel holes were slightly misaligned which I hope to fix with a proper fixture and matching zero points.

    As for the patternmaker dowels that gave me an idea. If I take the route of machining an aluminum base plate to hold the block I could perhaps use hollow positioning pegs milled into the aluminum and then run a screw up the middle of the peg into the block of wood to hold it down. I would then use threaded holes in the base plate to bolt it to the angle or fixture square. (see below sketch)

    Help with 5 sided work fixture solution-mount-jpg

    If I go this route I would just need to come up with a fixture that I could use for all remaining 5 sides so I dont have to re-zero my X and Y as I believe this is where my error was introduced last time.



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    Default Re: Help with 5 sided work fixture solution

    G54, G55, G56 X0 Y0 Z0 is bottom center of car for programming. Once bottoms are done then the alum block is put in vise and the 1" center hole is for probing the center of car/block. Once G55 is established attach block to plate to do the top. 1/8" steel dowel pins locate it. Then mount to indexer to set G56 for the front, rear and sides. You could get one of those hand cranked indexers with pattern / increment / 90 degree plates or have the four holes in an angle plate to match the alum car mounting plate. Keeps everything at 90 degree increments. Simple n functional.

    MM

    P.S. Yes indexers are nice.....

    Attached Thumbnails Attached Thumbnails Help with 5 sided work fixture solution-img_0384-jpg   Help with 5 sided work fixture solution-img_0383-jpg  


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    Default Re: Help with 5 sided work fixture solution

    MegaMoog,
    Very impressive, I think I even saw some of your cars while searching for solutions online. I may look more into the option of a manual indexer to see if that is a reasonable option to do the front/back and sides. Little concerned with clearance and achieving proper alignment if I mount to table.

    I am also considering the use of the mounting plate I showed the below in combination with a square fixture like this:
    https://www.mcmaster.com/1392N13/
    Help with 5 sided work fixture solution-fixturing-square-png

    It would give me a place to bolt all directions. The open side would give me easy access to the bolts and the mounting surfaces would be precise with holes to use for positioning.

    Revised mount with 2 alignment pin holes for use with square fixture:
    Help with 5 sided work fixture solution-mount2-jpg


    With this I would:
    Step 1) Carve car underside from stock lumber while resting flat on spoil board with positioning holes for mounting plate.
    Step 2) Secure mounting plate to block underside and secure with screws
    Step 3) Secure block so top is facing up on top of the fixture square to machine car top
    Step 4) Secure to side of fixture square and machine first side
    Step 5) Rotate and repeat

    Last edited by tallen84; 01-27-2021 at 01:24 AM.


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    Default Re: Help with 5 sided work fixture solution

    That sort of thing really needs a 4th axis.
    Yes, it would cost a couple of hundred $ (ebay, China), but once you have it and know how to use it, the world opens up.
    Treat it as an investment for the future.

    Cheers
    Roger



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