Tuesday, April 26, 2011

Unhappy with bearings.

I have been playing with the bearing idea that I had. I am not really happy with them. I could make some adjusters for the bearings but I don't want them that complicated. I am going to have to give this some more thought. When I have a better plan, I will post.

Thursday, April 21, 2011

Test fitting the bearings and guide tube

My general overall impression is that this is going to be a good solution. By no means is this sample good enough to use but I was a bit anxious to see how things fit together. I just drilled the holes freehand because I really didn't know how things would fit together. There is a lot of stuff to potentially interfere. There are several clearance factors:
  • Clearing the fillet on the inside of the angle
  • The nut's clearance on the inside of the angle
  • The interference of the nuts and screw heads
 I was also concerned that the lengths of the screws were right. I had tested with some smaller screws for rough measurements but I knew that many things can go wrong once I started shimming things. The 25mm screws give me enough room for two shims and I could probably handle 4 flat washers and some lock washers on the 40mm screws. Right now, I didn't put any shims so it doesn't line up that well with the centerline of the tube but no point in messing with it until I have the spacing right. There is still lots of room for adjusting things.  I will be mounting the center bearing closer to the edge so I really won't need that many spacers on the short screws. I will probably need to counter-bore the the angle to seat the screw heads squarely. No big problem. It would be simpler to use the architectural angle that has a square inside corner but I will work with what I already have. 


I will now measure how far I need to space them to leave room for wrenches and make a fixture to hold the pieces for drilling on the drill press. It is also going to give a lot of space to mount other stuff to the complete carriages. I was also concerned that it would not be rigid enough but once I tighten up the screws, it stiffens up nicely. I could even make it more rigid by adding another bearing and screw in the middle but it would complicate getting things aligned. I could alternatively go with just some spacers.

Wednesday, April 13, 2011

Metric Rant

With the current world connection created by the Internet, I frequently hear people complain about the US measurement system. Let me let you in on the real problem with metric adoption: We really can't get metric stuff.

I ordered some screws for mounting my bearings. Regular skate bearings (608) have an 8mm bore. I can't just go up to the store and say, "I'm looking for some M8 screws. Let me have a box of 25mm and a box of 40mm." If they have any at all, they will be maybe one length and in little plastic bags of 4 screws for $3 or $4 as opposed to $0.06 for a SAE equivalent.

Instead, my only option is to order them online. Ok, first, I have to find a vendor that will sell less than 50 lb. boxes. Once I find some, I have to go ahead and order many more than I need or end up paying the same amount of money for less screws. I ordered a bag of 100 of the 25mm and another bag of 100 40mm. Well, I also needed washers and nuts. On top of that, I pay almost an additional 20% for shipping.

I had someone that was really surprised when I was talking about building my CNC mostly out of wood and gluing things together rather than all bolted together being a money saver. My fastener purchase was the single most expensive component of my CNC build. This includes things that I have had shipped half way around the world from Hong Kong to the US. The fasteners cost more than any other single component.

As long as we cannot just go down to a local store and things that are really metric, we will not convert to metric. We have this fake metric adoption where you get things in some funky measure like my coke can sitting next to me has a metric measure: 355ml.  It is the same 12 ounce size as it was before, they just put a conversion on it and say "*Poof*, it's metric." Same as when I work on my bike. Some things are really metric, some things are fake metric. We have things that are 25.4mm and 31.8mm which are the same size as when we were using non-metric measure 1", 1-1/4", 1-1/2" and such. So, not only do we have that, we also have them with other dimensions where the measurements that really are metric. So, we have a 1" bore on a 80mm shank so much that we just about have to carry around multiple measuring devices at all times. We also have to keep around two sets of tools because some parts are metric, some are not. I have duplicate wrenches, allen keys, measuring tapes, etc. One set in metric and one SAE. Even more fun is when the parts you are working on have a mix.

I can work with both systems and it doesn't really make any differences as they are all arbitrary. They both have advantages and disadvantages. I just wish we would have one or the other. Playing mix and match with both is just an absolute pain in the rear.

Sunday, April 10, 2011

Finalized Linear Bearing Plan



Ok, I have been thinking, and overthinking, the linear bearings. I have been unhappy with the ones that I have tested out before and finally have figured out what I want to do. I have not been happy with the idea of running hardened steel bearings on aluminum. I didn't want to mess with steel angle tracks because it is not as easy to work. I also wanted to keep the cost down. Here is the cheapest and easiest to work with plan that I have been able to develop.



This design uses 3/4" steel electrical conduit (EMT) for the rails.. It is straight and not expensive. It will also wear much better than the aluminum. A 10 foot piece was $4.33.  The 5 foot pieces I bought today are under $3 a piece at the borg. I could have gotten cheaper but I didn't want to mess with putting full 10 foot pieces in my car or driving around shopping.  If I were going with a more powerful router, I might consider using a thicker wall pipe or larger diameter tubing but I can't see it for a small router.

The bearings will be mounted on 3/4 in aluminum angle. This could be modified with larger angle to accommodate larger diameter tubing but I already have plenty of 3/4 in.  I ordered the screws today also. For the ones between the angles, I figure that 40mm will be plenty long and 25mm for the ones that stick out on top and bottom. I went with M8 screws. They are the right size for the 608 skate bearings without slop and will be stronger than smaller screws.  I also ordered lots of washers so I can shim the bearings to ride centered on the rails. I might put more bearings on each carriage mainly to make it more solid in case the angle starts flexing. I will see how it works once it is put together. I am also going to think about adding a shield around the bearings to keep it clean. Maybe some wipers on the ends.

The rails here are drawn to scale. The 3/4 in conduit is 0.931 in. OD. I plan on attaching it to the conduit with screws every 3 or 4 inches to tie the rails and support together. I might notch the support rail for the conduit to rest in but I will probably just add an epoxy fillet to secure and support the rail.  This rail assembly will have a wider backing plate behind it for adjustment.

If you would like the model for SketchUp, here is a link.