Precision washers were causing slop: what I did to fix it

discussion of design changes / improvements / suggestions
ejs
Posts: 84
Joined: Sat Apr 28, 2012 7:42 pm
Location: Pittsburgh, PA, USA
Contact:

Precision washers were causing slop: what I did to fix it

Post by ejs » Tue Oct 23, 2012 2:07 am

There have been a few points of frustration with the kit as I've gone along. Foremost was the electronics, which many of you know I solved by beginning to produce rather than wait for someone else to.

One of the big, big things that got under me skin towards the end of the build is the increasingly talked about play where the Z axis meets the X gantry. It has been reassuring to see it pop up a couple times in the forum. It went like this: everything was together and I was checking for slop and play before mounting my spindle. I noted that the top of the MakerSlide comprising the Z-axis, where the Z motor is mounted, had a lot of to and fro. I'm not talking about a lot of give or springiness. I'm talking about slop. My guess, which my caliper confirmed, was 2 millimeters, or roughly 1 in either direction.

Proceeding to tear the thing down, I discovered the issue was the precision washers--while they have an undersized hole compared with the standard M5 washers (4.9 mm vs ~5.3mm), they are actually *thicker* than the standard, non precision, washers.

I took every V-wheel assembly apart and unsheathed my calipers. The average washer is 3/1000 of an inch oversized. Not a ton, but that puts it at about 7% thicker than the 1 mm ideal it is expected to be. Taking a few more measurements: after about two dozen data points, it was determined that the lip on the inside of the Delrin V-wheel itself is between 0.0385 and 0.0390 inches. That range is slightly undersized compared with the ideal 1 mm which equates to 0.0393 inches.

How exactly are these thick spacers causing this level of play, you may be wondering. The particulars follow. Take a look at the top half of the assembly's cross section. If you imagine the whole of it, Delrin is at the top and bottom. As pictured, there is a bearing to the left and one to the right. A washer is sandwiched the center with the M5 bolt through both it and the bearings. The bolt and bearing have effectively no slop as they are secured to the plate. The outer races of the bearings are tight against the Delrin lip inside the wheel when the washers are the correct thickness.

CORRECTLY SIZED V-WHEEL AND COMPONENTS
Image

Now picture the inner bearing races that are actually clamping the washer. They are 1.07 mm apart on the provided overly thick washers. That means the gap on the outer races is also 1.07 mm. Which means that there is 0.07 mm of play within the gap where the outer rings are not in contact with the wheel's Delrin lip.

Effectively, the bearings and metal bits are one assembly and are secured to the plate, while the wheels are tensioned to the Slide. This imperfect sandwiching of the Delrin lip means that the bearing assembly can wobble forward and backward within the wheel. If we take a look at this mock up, you can see how the angle can diverge from true.

INCORRECTLY SIZED COMPONENTS AND RESULTING ANGLE
Image

Hence, my 2 mm slop.

I found a solution that I'm very happy with. I got a copper alloy and knocked together some modified tooling to cut out correctly sized spacers. The material is soft and words easily, but needs time to get rid of burrs and flatten out the surface. The final pieces ended up being 0.98 to 1.01 mm thick. I replaced the eight spacers on the gantry with my custom solution and can now say that the Z-axis assembly is absolutely married to the gantry. That's not to say that there is not still some twist in the setup (see the second link below) but it is a matter of tension and spring rather than just untensioned play.

I did talk to Edward about this, and, as he points out, this doesn't seem to be a huge deal in the community or the outcry would be much larger. I can't argue--my machine is functional. But I have designs on making it as solid as possible for soon-to-come adventures in metal cutting, so I thought I'd share my perspective and fix. I also have a very clever Z-axis fix I think you all will enjoy. It has been hinted at by another party and endorsed by Edward, independently of my creation, so I expect the community will be excited about it. My approach is novel and efficient.

As for the spacers themselves, I've put them up for sale on my shop. The finished spacers are not inexpensive. They require machining and so labor is the main factor there. I am also selling rough blanks for anyone wanting to finish their own to save the cost. A couple files and some medium to fine grit paper is what you will need to complete the finish work. And a caliper, of course.

Similar notes on the problem can be found on these two topics
http://www.shapeoko.com/forum/viewtopic ... lit=#p5746 and Improbable Construct talks flex
http://www.shapeoko.com/forum/viewtopic ... 0&start=20
Build it better than it was built for you. And give permission for the next guy to do the same. That's how Open Source works.
ShapeOko # 497: http://bit.ly/reactshop producing the Buildlog CNC Stepper Shield

daveczrn
Posts: 42
Joined: Thu Jul 05, 2012 11:06 am

Re: Precision washers were causing slop: what I did to fix i

Post by daveczrn » Tue Oct 23, 2012 4:00 am

I noticed this as well with my v wheels. I also measured .003 to .005" inch in each bearing of side to side movement. There are washers already available in the sizes required.

ejs
Posts: 84
Joined: Sat Apr 28, 2012 7:42 pm
Location: Pittsburgh, PA, USA
Contact:

Re: Precision washers were causing slop: what I did to fix i

Post by ejs » Tue Oct 23, 2012 4:47 am

Really? Would you site a source? I couldn't find anything more exact than ±5%. And that was for shim washers.
Nothing I could find was as exact as I needed.
Build it better than it was built for you. And give permission for the next guy to do the same. That's how Open Source works.
ShapeOko # 497: http://bit.ly/reactshop producing the Buildlog CNC Stepper Shield

cvoinescu
Posts: 4442
Joined: Thu Jul 19, 2012 6:50 pm
Location: Camberley, UK
Contact:

Re: Precision washers were causing slop: what I did to fix i

Post by cvoinescu » Tue Oct 23, 2012 8:53 am

I agree with @ejs's analysis of the V-wheel and spacer problem. I'd add the fact that the bearings themselves (and don't get me wrong, this is not just the ShapeOko bearings, it's all 625 bearings I've had my hands on so far) have a tiny bit of axial slop -- about 0.01 mm -- enough to make this problem occur even with exactly sized washers. Given that the V-wheels tend to have slightly undersized lips, and that plastic is somewhat flexible, I would recommend a slightly undersized washer, say 0.95 mm. Most M5 (form A) washers that I've had, while nominally 1 mm thick, have been in the 0.90 to 0.95 mm range, so it's just a matter of finding a supplier (and batch) that's on the thick side of that range. The hole in a M5 washer isn't as snug a fit as the "precision" washers in the kit, but I haven't found that to be a problem.

There's another mode of movement that @ejs hasn't covered. The bearings can twist a lot (more than one degree). Again, this is all 625 bearings, not just the ones in the ShapeOko kit. If the V-wheel was perfectly rigid, the fact that there are two bearings in it would convert any twist applied to the V-wheel into radial forces on the two bearings (which they resist best). The V-wheel isn't rigid, though: it deforms a little, so the bearings are twisted too. There's nothing to do about that, except that the undersized washer keeps the bearings squished together more tightly so they can't twist as easily. So, again, an undersized washer is a win.

However, the washer must not be too thin, or the bearings will take a lot of axial load from the bolt holding everything together, which will cause large drag and quick wear. With that in mind, I think @ejs's solution, to use actual precision 1.00 mm washers, is probably the best choice.
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

daveczrn
Posts: 42
Joined: Thu Jul 05, 2012 11:06 am

Re: Precision washers were causing slop: what I did to fix i

Post by daveczrn » Tue Oct 23, 2012 9:33 am

hmmm. Mcmaster seems to have changed their website, i looked not 2 weeks ago and was able to view .031" .032" .035" .039" and .040", now they don't even come close to that specs.

maybe i was looking at shims. but i am not able to currently find metric ones with a tight tolerance like the standard sized ones.

deejayspinz
Posts: 96
Joined: Wed Oct 17, 2012 9:33 am
Location: Burlington Ontario, Canada

Re: Precision washers were causing slop: what I did to fix i

Post by deejayspinz » Tue Oct 23, 2012 10:21 am

dont have my Shapeoko yet, so cant verify this, but i think these may be M5. However, their OD may not be tall enough. not cheap either. I have some extras from my XTR brakes that i will try when my Oko comes in.
some initial results - google disc brake shims

http://wheelsmfg.com/rotor-shims.html
http://www.chainreactioncycles.com/Mode ... odelID=356

northbear
Posts: 254
Joined: Mon Apr 09, 2012 9:21 pm

Re: Precision washers were causing slop: what I did to fix i

Post by northbear » Tue Oct 23, 2012 1:53 pm

Something from McMaster # 93574A807 may work to shim the bearing. See below:
Capture.PNG
Capture.PNG (41.74 KiB) Viewed 4270 times
My buildlog is here

northbear
Posts: 254
Joined: Mon Apr 09, 2012 9:21 pm

Re: Precision washers were causing slop: what I did to fix i

Post by northbear » Tue Oct 23, 2012 2:08 pm

Also, I must say I am impressed with your original post EJS! You took the time to work to debug the problem and develop a solution. Not only that, you went on to clearly explain the issue here and offer the solution on your store.

To this I say "Thank you!"
My buildlog is here

Improbable Construct
Posts: 997
Joined: Tue Apr 10, 2012 3:21 am
Location: Fairhope, AL
Contact:

Re: Precision washers were causing slop: what I did to fix i

Post by Improbable Construct » Tue Oct 23, 2012 3:21 pm

I applaud any and all attempts to make the shapeoko better.
Anyone who has been following my posts knows I have been working pretty hard to make the shapeoko as good as it can be.
It is pretty good right out of the box, and nothing in the CNC world can come close to touching it on price.
That said:
There are only three things that I have not modified or replaced on my machine: The idler wheels, the XML pulley and the V-wheels.
I have no problems with the idlers or pulley, but the V-wheels are another story.
They are just not designed to take the kind of axial loads we place on them.
I have tried everything I could think of to make them work better:

1. Washers of various sizes - this works on some of them.
2. Super glue - some of them can be glued to the bearings to reduce wobble
3. Filing - one of mine had a bit of a "ka-thunk" in it so I took a file and smoothed it out.
4. Moving them around - I put all the worst wheels on the Y-axis rails where they mostly have a radial load.

Most of the play I have now, throughout my shapeoko, is in the bearings themselves.
There are limits to the design and this is where we hit one of them.
Until somebody comes up with a better/stronger V-wheel/bearing combo, this is where we are.
Shapeoko #Classified some of the bolts may be original parts.
Shapeoko 1 # ???? Stainless plates, still in the box.
Shapeoko 2 # 3926 not stock
Shapeoko 3 # 0003
Store:
http://ImprobableConstruct.com
Twitter:
https://twitter.com/ImprblConstruct

ejs
Posts: 84
Joined: Sat Apr 28, 2012 7:42 pm
Location: Pittsburgh, PA, USA
Contact:

Re: Precision washers were causing slop: what I did to fix i

Post by ejs » Tue Oct 23, 2012 3:32 pm

northbear wrote:Also, I must say I am impressed with your original post EJS! You took the time to work to debug the problem and develop a solution. Not only that, you went on to clearly explain the issue here and offer the solution on your store.

To this I say "Thank you!"
Hey, thanks, northbear! I'm just trying to do my part for the community that has been so very helpful and responsive to me. Regarding the shop, I don't expect more than a few buyers; it's something that if I were reading this same fix penned by someone else, I would be frustrated if the solution were available but the materials were not. Now those who find this to be an essential have a place to start.

Northbear, I thank you for your encouragement and recognition.

cvoinescu, thank you for taking the analysis a step further and detailing the specifics of twist. And thanks, too, for the endorsement. The main reason I state that the slop is gone but some spring remains is exactly for the reasons you detail. The deformable nature of the Delrin being the main, as far as I can tell, but also the twist between the races of the same bearing, the give in the bolts, and the flex in the slide all contribute. Counteracting those is a matter of ramping in to the material at a slow enough pace.

One thing regarding your point about the dangers of the material being too thin. That is effectively why I sized the spacer opening to be very precise around the bolt. When there is less material contacted by the races, the rigidity suffers. And when that gap is in the 0.4 mm range, as it would be having the non-precision washers hang on the bolt, then the gap would be mostly in one direction (at the bottom), causing a lot of decreased stiffness in that direction. That was my thought anyway.

Your insight is always excellent and appreciated, Cat.

Improbable Construct, your machine certainly pushes the limits of what can be done. I agree that there is nothing that comes close in price for the base machine. In a way it's nice to be able to upgrade as we see fit. Sometimes it is frustrating, as well, to see the problems that are in place out of the box.

I think that additional points of contact will be part of the fix. IC, were you ever part of a discussion of going to steel v-wheels? I know I read through a series of posts arguing whether the anodizing on the Slide could stand up to steel wheels. It may have been over on Buildlog.net.

~EJ
Build it better than it was built for you. And give permission for the next guy to do the same. That's how Open Source works.
ShapeOko # 497: http://bit.ly/reactshop producing the Buildlog CNC Stepper Shield

Post Reply