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eccentric spacers

Discussion in 'General Talk' started by Metalguru, Apr 28, 2016.

  1. Metalguru

    Metalguru Master
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    Anyone know of a source for 1/4" thick eccentric spacers with a 6mm or 6.35mm (1/4") bore?
     
  2. Giarc

    Giarc Master
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    The won't be pretty, but could you drill holes in 1/4 inch plate then cut small squares? I did this to test something and it worked.
     
  3. Joe Santarsiero

    Joe Santarsiero OB addict
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    You looking for eccentrics for steel vwheels?
     
  4. Metalguru

    Metalguru Master
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    No, Joe Santarsiero | OpenBuilds, the steel V-wheels use the standard 5mm bolts. AFAIK

    Giarc | OpenBuilds, how would you make the larger diameter circular shoulder?

    I was thinking of increasing the strength of standard v-wheels by changing out the bearings and spacers to 1/4" or 6mm bore. I am of the opinion that the M5's are one of the weak points of the V-wheel setup.

    I can easily get MR616-2RS bearings, 6x16x5mm. Actually, the 1/4" bearings are R4-2RS and they are just a tad out of spec, 6.35 x 15.875 x 4.98, so they might not work. About 5 thou undersize. Thickness difference of only .7 thou should not affect anything...

    Might be able to drill out the existing 5mm eccentric spacers to 6mm...
    Nope, just did some calcs and I would have to increase the bushing side diameter to 8mm, and the hex diameter to just under 9mm.
     
    #4 Metalguru, May 1, 2016
    Last edited: May 1, 2016
  5. Giarc

    Giarc Master
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    D-oh. Brain fart. I didn't read your original post right.
     
  6. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    The easiest solution would be to make your own. DIY Eccentric Spacers for less than 40c each (link originally posted by @jzhvymetal in DIY eccentric spacers even cheaper | OpenBuilds) This design was based on M5 screws so you would need to modify it a bit but would still be fairly simple to make.

    @Colin Russon was looking at manufacturing eccentrics based on M8 screws for one of his designs. Don't know how far along he is with the process or if he will be selling them individually. M8 screws would provide fairly robust axles however and would work nicely with the 688Z bearings.
     
  7. Rob Taylor

    Rob Taylor Veteran
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    It's possible to both increase strength and get press-to-contact eccentricity simultaneously using 688z (8x16x5) bearings. You could even reinforce the axle contact by eccentrically-through-drilling some 8mm steel rod stubs on heavier/more vibration-prone machines. I didn't like using the eccentric spacers, so I've been setting up to do it on my next project, I'll report on how it goes.
     
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  8. Metalguru

    Metalguru Master
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    Hey Rick 2.0 | OpenBuilds. I did see the article somewhere about the M8 eccentrics, but I thought that was a bit of overkill. Something like the guy who had 50 or 60 wheels on his OX-like build. M6 should provide a substantial increase in strength without too much modification. It probably isn't possible to get the low profile head metric bolts in M8 either. I need to have the head recessed completely flush in a 1/4" plate.

    As far as making your own, I would think it would be difficult to make any quantity of these things so that they were all exactly the same size and offset. The offset would not be that important for accuracy, but the height and OD would be critical, and the faces would need to be exactly parallel.

    On a small metal lathe, these things could be made out of hex stock with a 4 jaw chuck to drill the offset hole. I was also thinking of using my existing mill to make them, but I would need to machine both sides of a 3/8" plate and I don't know if I could do that accurately.

    Rob Taylor | OpenBuilds I'm not quite sure what you are suggesting. Can you perhaps throw up a small sketch? Picture is worth 1000 words...:D
     
  9. Rick 2.0

    Rick 2.0 OpenBuilds Team
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    Take a closer look at how they were done in the link. All those problems were solved. He drilled out stock hex nuts and pounded stock dowels into them. The stock nuts provided a constant thickness and parallel faces and the stock dowels provided a constant diameter. Follow that with a simple jig setup in the drill press and all the offsets come out the same.
     
  10. Giarc

    Giarc Master
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    What if you cut slots in the gantry plate where you normally had larger holes for eccentrics. Then, make 1/4 inch spacers with normal diameter holes for the axles that line up with the slots. Using angle at the bottom of the gantry, tap holes and have screws that screw perpendicular to the spacers pushing on the bottom of the spacer forcing it up.
     
  11. Giarc

    Giarc Master
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    Here are a couple screen shots of a VERY crude drawing. The wheels and axles are missing, it was just done to illustrate my lamely described concept above.:)
    [​IMG] [​IMG]
    [​IMG]
     
  12. Metalguru

    Metalguru Master
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    Yeah, I was thinking about that. That's the way they do it on the larger machines that use 6 or more bearings that run on cold rolled steel flat rails, like CNCRouterParts.com, and finelineautomation.com.

    But what's the angle piece for?
     
  13. Giarc

    Giarc Master
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    I was just trying to think up a quick way to make the screw that raises or lowers the spacer perpendicular so it could push on the bottom of the spacer do make it go up or down. It doesn't need to be a threaded angle. There are other ways to do it. That was just three different views of the same thing.
     
  14. Metalguru

    Metalguru Master
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    Craig:

    I gave that some thought. It might work, but there is one flaw: The axle bolt is in a slot that is parallel to the major force on the wheel. This means that there may be a significant twisting force on the wheel/axle, and there is nothing to support it in that direction because of the slot. Also, the setscrew acts as a fulcrum. The only support it gets is from the bolt head and the spacer which are flat surfaces resting on the perpendicular faces of the plate. Might be ok, especially if these flat surfaces are larger.

    I thought of something like a bicycle chain adjuster, where you have a u-shaped bracket with a screw into the edge of the plate. The u-bracket has a hole on each side for the axle bolt to pass through. Plate still has a slot, but the u adjuster keeps the axle bolt aligned. Might be difficult to drill and tap the hole in the edge of the plate... You could extend this so the u channel runs the full width of the gantry plate and all the wheels attach to it. This would also allow you to use any size axle bolt -6mm, 8mm, etc.


    wheel adjuster.jpg


    I just had a look on Mcmaster Carr, they have a 1/2" wide aluminum channel with 1/8" thick walls, and the depth of the channel is 1" outside. That makes the channel 1/4" x 7/8" inside, perfect for this application... Part number 4592T12
     
    #14 Metalguru, May 3, 2016
    Last edited: May 4, 2016
  15. Giarc

    Giarc Master
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    That is a great find, and great idea. Much better final product than my idea.
     
  16. Metalguru

    Metalguru Master
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    Why thank you.

    It's nice to have intelligent people to bounce ideas off of as well. This is one of the big pluses to the Open concept. Keeps the creative juices flowing, as it were.

    I still have to do a full size model of this and see how it works in the real world.

    McMaster Rules. They Have Everything :cool:
     
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  17. Giarc

    Giarc Master
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    I like McMaster. The only thing I dislike is that I don't know what the shipping costs will be until I submit my order. I always get a bit nervous especially when I ordered the 6 foot acme rods. It was very reasonable, though.
     
  18. Dr.Doc

    Dr.Doc Well-Known
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    Necrothreading....

    I also want to use 688 bearings - 8x16x5mm with OB wheels, and have come up with this:
    Shouldered bolts in 8mm diameter shoulder and M6-1.0 thread are common.
    An M10 partially threaded bolt can be drilled off-center through the head, tapped to M6 and then cut a bit shorter than the mount plate's thickness.
    Instant highly-eccentric spacer, and the 8mm shoulder
     
  19. Barry Danks

    Barry Danks Veteran
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  20. GrayUK

    GrayUK Master
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    Barry Danks.
    Looks like an interesting site. I might enquire about their Linear rails and carriages in the near future.
    Quite often these specialised companies make just the right gear, but charge an absolute fortune for it. :nailbite:
    So you end up in China again. :(
    Gray
     
  21. drxlcarfreak

    drxlcarfreak Journeyman
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    That is a great idea by metalguru, but couldn't the same effect be had by having a slot for the bolt to go through that is tight left to right, then drilling and tapping the riser plate vertically into the bottom of the slot and use a setscrew to push the bolt/wheel tight to the v-slot. Looking at the model, it looks like the bolt head radius is 1.75mm bigger than the radius of the shank, so even a 1.5mm slot at either extreme would actually still give the bolt head a bit of bearing area all the way around so the twisting force should be minimized. A 1.5mm slot also gives about twice the adjustment of an eccentric spacer (I think that I saw that they were 0.79mm adjustment). Here is a quick model I made to illustrate what I am saying.

    Eccentric.jpg

    In quickly looking at this, the only downside I see, is the plate needs to be thick enough to accept a set screw, which I would guess the minimum thickness would be 1/4" for there to be enough meat for the set screw to grab onto.
     
  22. GrayUK

    GrayUK Master
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    I've come along in the middle of this idea, but, maybe the problem of the thickness of the plate could be alleviated by affixing an additional cube of material, to the inner side of the plate, and after drilling that, fitting the grub screw in that?
    Gray
     
  23. drxlcarfreak

    drxlcarfreak Journeyman
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    For my build I am contemplating 3/8" plate, so a set screw should fit well I think (hope), but that is a good idea! A simple idea expanding along those lines, make adjusting the wheels easier, and would actually make it pretty quick to pop the gantry straight off the X rails...

    Take your riser plate, and drill a hole large enough for the bolt head to pass completely through and adjust up and down. In between these holes drill adjustment slots. Then take another plate that is just about the height of the wheels by the width of the riser plate. Add your bolt holes for the wheels with no room for adjustment as well as the holes between the wheels. Bolt your wheels to the holes, and line the plate up under the rail, then put bolts through the remaining holes on the wheels plate and into the slots on the riser plate, and you can then adjust the wheels to the proper adjustment and crank the bolts down. The racking issue should be gone now since it essentially is just bolting to a plate, albeit a plate thickness closer to the rail, which actually I would think would help with rigidity since there is a shorter moment arm on the bolt. Hopefully that was decently clear!
     
  24. Metalguru

    Metalguru Master
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    Not a bit... As they say, a picture is worth a thousand words...

    MG
     
  25. drxlcarfreak

    drxlcarfreak Journeyman
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    Clear as mud huh?

    Here is the riser plate see through, you can see the recessed wheel bolts and the "wheel plate" behind it.
    Untitled.jpg

    Here is the riser plate not see through and the wheel plate adjusted all the way tight. For the model I chose 4mm full travel for the bolts.
    s.jpg

    Adjusted back in the middle.
    22.jpg
    Here is the back side, only a 1 mm spacer between the wheel plate and the bearing, so I would think it should help increase the strength of the M5 bolt attachment.
    aa.jpg

    Obviously more bolts could be added to the wheel plate to have a sturdier attachment. I just chose 2 for illustration purposes.

    Only downsides I see here is potential for individual wheel tolerances being out a tiny bit, you cant adjust wheels individually and it may be hard to get enough pressure just pushing the plate up by hand to get the wheels snug to the vslot.
     
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  26. Metalguru

    Metalguru Master
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    Ah, but if you combine this with your setscrew idea you just might have something... This would still allow individual adjustment for each wheel, and it would allow you to use the setscrew to put some preload on the wheels.. I don't think just pushing them by hand while tigtening the mounting screws would provide enough wheel preload... I've seen this arrangement on larger routers using steel ball bearings or steel v-wheels. The only issue with a setscrew is that the wheel would put a torsional force on the axle screw, trying to twist it in the slot. That was why I came up with the u-channel idea in an earlier post, that would keep the axle straight in the slot. Might be able to put the setscrew closer to the wheel side of the axle, thus biasing the pressure closer to the wheel, this would keep twisting force to a minimum.

    If you put some thought into hole layouts, you could do several different wheel combinations of 2, 3, 4, or more wheels on a single wheel plate. And, no need for spacers, just make the wheel mounting plate the right thickness... You may need a 1mm spacer between the wheel and plate just to keep the wheel from rubbing.

    Also, if you put the relief for the screw heads on the wheel plate instead of the large plate, there would be no need for the large slots on the gantry plate...

    MG
     
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  27. Barry Danks

    Barry Danks Veteran
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    Imo stick with an eccentric they are precision made and fit into a socket. I think you may lose that. Nice concept though.
     
  28. drxlcarfreak

    drxlcarfreak Journeyman
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    I think I'm gonna need to see a picture now to see how you'd use a set screw in addition to my plate, or the relief in the wheel plate!
     
  29. drxlcarfreak

    drxlcarfreak Journeyman
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    I'll give you that if I am sticking with M5 bolts. I am aiming to use M8 bolts and bearings though. Not an off the shelf setup available, so I am trying to think of alternative options.
     
  30. Metalguru

    Metalguru Master
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    Just going back to your original concept drawing above, and instead of having slotted holes for mounting the plate, slot the wheel axle holes. Put the reliefs for the heads of the screws in the mounting plate, not the larger plate. The way the C-Beam XL plates have all the mounting screw heads recessed so you can put two plates back to back without interference. wheel block.jpg That way, the axle bolt heads are buried in between the two plates and not visible from the outside. You'd have to leave a small hole in the big plate to get the allen wrench through to tighten the screw. The set screws would come up from the bottom of the mounting plate, as in your original sketch, but biased toward the wheel side as much as possible to keep the twisting moment to a minimum.

    If you only have adjustment by slotting the mounting holes for your wheel plate, the wheels are not independently adjustable. I think this will run into problems as each hole and each wheel has a tolerance that, as Murphy says:

    "Tolerances accumulate unilaterally towards maximum difficulty of assembly."

    MG
     
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