Experiments with Bearings

Skate, 608, bearings have served well on DIY CNC machines.
They are inexpensive, and are available from sports shops and online suppliers.

Experience here has been that bearings with any ABEC rating will work. Bearings that are sold as only "608 Bearings" can be low quality.

Bearings can be shielded with metal, or sealed with plastic. They seem to hold up similarly. Both types may need to be cleaned and lubricated over time.

The plastic seals can be easier to remove and re-install than metal shields.
Bearing seals
Plastic seals can be pried off.

The plastic is resilient and will re-seat better than a bent metal shield.

Different lubricants, including motor oil, sewing machine oil, axle grease, etc., have served well.
This unit was designed to tension against the rails, and to allow clearance for a rail support.

It uses an offset on one bearing that is twisted to tighten the bearing against the rail.

It is  made by tapping a threaded rod off-center into the head of another bolt.
This works well, but it takes a while to build, and requires welding.
Welded bearing truck
Welded truck with an offset.
This was an attempt to keep things as simple as possible.

It gains its tension by flexing the aluminum supports.

Unfortunately the aluminum is too soft to hold tension well.

When one leg is solidly fixed, with the other left as 1/8 inch aluminum, the system holds tension well.
The single flexible leg always keeps the bearings tensioned without the risk of overloading.
Bearing truck from aluminum strap
Bent aluminum supports bearings.
This truck is a wooden version of the above idea. It is made from a section of 2x4.

The 2x4 is ripped at 30° (it is 60° from the horizontal) as shown below.

Cut 2 x 4 at angle
2x4 cut at 30°.

One piece is flipped end to end so it mirrors the other. See photos.

Holes A, B and C are drilled.

A is for a threaded rod that holds the top bearing. It also ties the blocks together.

Hole B is drilled small enough for the bearing's bolt to be tightly screwed into the wood; the wood acts as a nut.

Hole C is oversized so the bolt can wobble in it. This allows for tensioning adjustment against the pipe-rail.
The head is recessed to clear the top of the wood block so the block can be attached to its axis. Bottom image.

All bearings are held fast by nuts tightened firmly on each side.

In the top image a small bolt head protrudes from the side of the block.
This bolt is threaded into the wood. It serves as a tensioner that presses against the bearing's axle (bolt C), and tensions the bearing against the rail.
This bolt makes it much easier to adjust the bearing before tightening bolt C.

Truck made of 2 x 4
Wooden bearing truck.
Drill wood blocks
Orientation of drilled holes.

bolts in block
Recessed bolt head.
This truck is made of 3/4 inch plastic.
The purpose of this exercise was to see what design modifications were required when a thinner material was used.
The only difference is threading the right block for the A axle since there is no room for the axle bolts to cross.
Truck made of plastic
Plastic bearing truck.
This is an aluminum version that is used on our 4x8 steel and aluminum machine.

These trucks are solid and work well, but they take a fair amount of time to build.
Truck made of 1/2 inch aluminum
A routed 2x4 served as a rail support on the trial machine.
Pipe on 2 x 4
Routed 2x4 serves as cradle.
Pipe on channel
Aluminum channel pipe cradle.

Metal channel was used for the rail supports on the 4x8 machine. It works well, but small channel can be challenging to find.
Another bearing system was a pipe that supported channels, which held the bearings.

The bearings were placed 120° apart around the pipe; in this image only two bearing channels are visible.
Play was removed by tightening the nuts on the studs. The studs were soldered into the copper pipe.

The system does not permit supported rails, which is a deal breaker for aggressive cutting.

Truck made of copper pipe
Pipe with bearings in channels.

These slides were made of PVC male adapters and conduit.

The threaded sections of the male adapters were cut so the caps, which were cut down to rings, could tension the adapters around the conduit.

The plastic has to be plumbing fittings rather than electrical fittings. Plumbing fittings have tapered threads, which permit the tension adjustment.

The adapters were glued to PVC pipe and held in place with conduit straps.

The ¾ and ½ inch EMT conduits just happened to fit inside the plumbing fittings.

This system worked remarkably well, but the parts' price added up quickly.
Surprisingly, skate bearings are cheaper.

Also, the units would freeze when left dormant, and the tension of the threaded rings would have to be readjusted.

This system gave a satisfactory quality of work after adjustments were made. However, the rails could not be supported, which limited the table size.
Slides made of pipe fittings
Male adapters ride rails.

Slides made of plastic pipe fittings
PVC pipe ties adapters together.
Linear bearing blocks on supported rails were tried on one of the prototypes.

They were a disappointment. The rails arrived bent, the travel felt gritty and the blocks were challenging to align.

These bearings require hardened rails, which are costly. Mild steel (hardware store) rod will deform.

Steel pipe and flat bar are significantly less expensive than even the small proprietary rails, and the small proprietary rails will require cradles, which add to the price.
Bearing blocks
Linear bearing blocks.

These bearings are frequently used, but when price is considered, I prefer other systems.

V bearings are ten times the price of skate bearings.
They are simple and perform well when firmly tensioned against their rails.

Wooden supports do not carry this high tension as well as metal does. The wood will flex and deform.

The rails on which the V bearings ride also need to be rigid, and aluminum rails are too soft for good long term performance.

The edges of steel flat bar and angle will mushroom from the pressure required for these bearings. The deformation will stabilize with time, so costly proprietary rails are not mandatory for DIY machines.
V bearing
V bearing.

Patio door V bearings were tested as a low cost substitute. Their loose tolerances made them unsuitable.
Sleeve bearings, also called bronze bushings, are inexpensive and simple, but their rails cannot be supported along the span. Therefore, these are not suitable for more aggressive work. Though they can carry a trim router, the rails will flex well before the trim router is worked to its potential.
These match the capabilities of a small Dremel type roto-tool.

Chrome plated rails are costly, top rail in photo. They are smoother than hardware store steel rod, bottom rail, but they still flex. The less costly rod can be filed and sanded for smooth operation.
 Sleeve Bearings
Half inch sleeve bearings.
Half inch copper couplings on copper pipe make a serviceable rail-bearing system, though they are not able to carry much of a load. They permit more chatter than the above bronze bushings, but they are easily found and inexpensive when purchased by the bag from homecenters. However, bronze sleeve bearings can be ordered from Use-enco.com for a slightly higher price.

The center stops inside the couplings have to be ground or filed off. Couplings with full circumference stops are hard to file smooth.

I have found these useful for low cost, quick-to-build teaching machines.

Copper Fittings
Half inch copper fittings.