Thomson vs. ground drill rod

[TommyB]

I am in the process of gathering material for my cnc. I just got 1" thomson linear bearings and now i need to get the rods. I have access to 1" precision ground drill rod for next to nothing. I was wondering if it would be a good substitute to thomson rod. The surface is a bit rough compared to thomson rod, but i could clean that up on a lathe by polishing them. I know thomson rod is case 60 hardened, but would that really make a difference since the load on the bearings from the gantry will be about 50 pound, or 25 pounds per rail? Nowhere near their load ratings. I also have access to a surface hardness testing machine so I will run a test on the drill rod on monday and see what i get.
One more thing, has anyone tried to take a thomson bearing and cut it open to make it an open style bearing? I was thinking of doing that to add support to the shaft to minimize deflection. I looked up the deflection of 1" rod and wasn't happy with it, but could live with it.

[pack rat]

I am in same boat. I did take thomson bearings out of twin 16 block. I did not cut housing because when I inserted open type thomson bearing, it had far too much play in it and was loose. I am still contemplating of using bronze bearings that have been cut for supported rails. I also assambled gantry on two non-hardened shafts with end support 43" apart. This setup I put on 6 foot granite table and measured the deflection. It came out 0.025+ with 50 pounds of weight on the gantry. I took an additional shaft bented it on the hydraulic press to offset the deflection and installed it between the other shafts. Never measured again the deflection, I just want to get it done. Maybe today I'll try cutting one of the bronze bearings.

[Mr.Chips]

Pack rat,
Looks like the twin 16 bearing has a steel insert between the alum housing and the plastic bearing retainer.
How did you get the plastic bearing retainer out with the steel insert?
Thanks
Hager

[Hobbiest]

The open style bearings have a different pillow block which has a slot cut lengthwise. This slot allows for a set screw to take up the play in the bearings.

[pack rat]

Mr. Chips, I took a small flathead screwdriver and pry it between the wiper seal and retaining ring. Did that around circumsference in few places and it came off. Year ago or more I did see retaining springs and wipers sold individually at graingers.com. 3 or 4 dollars Mcmaster has pillow withou the bearing 30$. By the way twin 16 does NOT have a single diameter hole through it, the middle of the block is 1/16" smaller and 1" long.

[HuFlungDung]

As has been pointed out in other threads, the contact area of a ball bearing ball on a flat surface is practically zero. Since pressure per square inch is based on the amount of surface in contact, a load of 25 lbs on a surface area of 0 square inches is infinite psi! Well, I am exaggerating, but you get the point

Balls rolling on a non hardened surface will "roll their own raceway" for a while. Unfortunately, this process introduces extra slack in your rail system after a while.

This is why you should use hardened shafting. You could also search in the larger size steel warehouses for induction hardened chrome plated shafting. This is a prehardened commercial product, and is not excessively expensive.

If I were building a machine, I would not use recirulating ball bearings if I was going to go cheap on the rail rods. Its a case of not spending enough money to do the job right and is a waste. Go for a plain bronze bushing sliding on a chrome plated steel rail. For this, you can use the cheaper CPO "chrome plated only" shafting and forget the induction hardened requirement. This provides very low "sticktion", eliminates the problem of sawdust getting into a rolling ball rail, and is cheap, and will support a very high load without any difficulty.

[TimW]

Hello,
HuFlungDung said in a previous post "....If I were building a machine, I would not use recirulating ball bearings if I was going to go cheap on the rail rods. Its a case of not spending enough money to do the job right and is a waste. Go for a plain bronze bushing sliding on a chrome plated steel rail. For this, you can use the cheaper CPO "chrome plated only" shafting and forget the induction hardened requirement. This provides very low "sticktion", eliminates the problem of sawdust getting into a rolling ball rail, and is cheap, and will support a very high load without any difficulty."

I have been thinking along those lines, ie bronze bushings on CPO shafting. What are the disadvantages to this approach ?

Are sintered bronze bushings available, and are they suitable in CNC guide rail applications ?

Thanks, Tim.

[vacpress]

i have been considering the same problem : get thk style rails and be done with it($) or try and go cheap. i have considered:

roller bearings in self machines block riding on the cheapest reasonable steel rod - for a 4x4 or 3x3 router. - very cheap. ive got lots of high quality new abec-5 rollerblade bearings.

igus slides that are cheap, but dont look very good (the N27) - 1.02/Inch rail - $5.00 carriage - prolly too weak

Used THK off ebay - 100-300 for 2 of the 48" rails i would prefer with 2 blocks each rail- not bad?

i have also got some half-baked ideas about using rollerblade bearings riding on some sort of CRS square tubing Linear Rail style system - with CNCed bearing blocks.. i do have access to a very nice 4x4 cnc at school.

any advice? cost breakdowns?

Thanks!

[HuFlungDung]

Originally posted by TimW [I have been thinking along those lines, ie bronze bushings on CPO shafting. What are the disadvantages to this approach ? Are sintered bronze bushings available, and are they suitable in CNC guide rail applications ? Thanks, Tim. [/B]

Many machines use plain bearings: a lathe carriage slides on the bed, the quill in a mill slides up and down in the cast iron housing. Dovetail slides can be found in many places on many machines, such as knee type mills that provide years of service.

Yes, sintered bronze bushings are available in many standard sizes. Probably the most difficult part is that you would have to get the bearing blocks custom made by someone with machine shop facilities, or at least a good sized mill or drill capable of drilling and reaming or drilling and boring the holes for the bushing. The fit of the commercial bushing on the rail can be controlled (to a degree) by the amount of press fit required to put the bushing into its block.

While you are planning it, I would also add some inexpensive single lip wiper seals to both ends of each bushing. You can even grease the bushings if you like. The grease will still come out past the seals, but the seals will scrape the shaft as it slides back into the bushing, to keep from drawing the dirt back in.

Commercially available sintered bushings are quite inexpensive to replace, when the need arises. They will last a very long time, especially when running on a clean chromeplated surface.

When designing the slides, try to come up with a solution that gives you a wide seperation of the bushings on any one rail. This is the surest way to avoid jamming of the rail when using a single, centrally located feedscrew, yet having the tool working hard at the extreme right or left end. The closer the bushing fits the shaft, the less will be the jamming tendency as well. I would strive for .001" to .002" diametral clearance.

[TommyB]

Well, did the hardness test the other day on ground drill rod, got 15.6 avg. on the rockwell c scale. Don't know if thats something to worry about, but leaning towards just buying thomson shafting. If I'm gonna put the time into making the darn thing(and $$$), might as well do it right and not cut corners.

[Hobbiest]

rc 15 isn't great! Get something harder.