Reducing Diameter of Aluminum Rod for Linear Bearing

I've purchased a set of 3/4 in OD aluminum rods and a set of 3/4 in ID linear bearings to mount on those rods. As luck would have it, they're both within tolerances, but the rod is slightly too thick - .753 inches according to the calipers. The bearing can be pushed onto the rod, but it won't move without hitting it with a mallet. To be honest, it probably can be put on the rod due to the softness of the aluminum (I do see some scratches left after removing it).

Question - How can I reduce the diameter of the rods to enable the bearings to slide appropriately? I don't own a metal lathe. In addition, I asked a maker space with a metal lathe about this use case and was told that using the lathe to reduce the diameter:

"would be a 0.0015” cut and the surface finish would probably not be appropriate for a guide rod. Typically this would be done by precision grinding"

1. Is precision grinding the right method? If so, how would I go about doing so accurately? What tools do I need?
2. What surface finish do I need for a linear bearing to function properly? Why can't a lathe provide this?

My background is more in woodworking, so this is a new arena for me.

Thanks!

Scott

Hi Scott - Don't use the aluminium for the guides. Buy the correct chrome plated rods. 1) To achieve the required surface finish will be more costly then buying the correct rods 2) to machine the rods will cost more then the correct guides 3) Grinding aluminium is difficult as the al gums up stones 4) aluminium is too soft for a bearing material unless it is hard anodised, again this will cost more than buying the correct rods.

In short start again with the correct guides (where you got the bearings will have the correct guides?) and use the al rod for something else... Peter

Hi,
aluminum is 70GPa whereas steel is 210GPa, ie steel is three times stiffer than the same size aluminum.

Aluminum rods will be as stiff as limp noodles, they are not right for the job. Centreless ground steel is what those bearings are intended for.

Craig

Thanks for the input! Based on the feedback, I'm going to pass on grinding down the rods. I did calculate the deflection for these rods, so I'm not certain that they're too weak for the loads that I'm planning to apply. However, the bearings aren't going to work for sure

Are there other approaches for linear motion besides ball bearings? I've seen some folks use wheels. Anything else that is inexpensive to start with?

Hi Scott - Buy either round supported rails and suitable cars or square rail and suitable cars. By the time you buy bearings, bits of metal and lots of bolts to make your own linear bearing you will be past the cost of a mass produced linear bearing solution & it will be a poor solution. You will be adjusting them every few weeks.

Machine design is not about strength, sure the rods will be strong enough (they will be many times strong enough) its about being stiff and rigid. Deflection is your enemy in machine design... By the time you get all the other stuff together the linear bearing costs are a small fraction of the total cost. Peter

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Originally Posted by sgoldstein527

Are there other approaches for linear motion besides ball bearings? I've seen some folks use wheels. Anything else that is inexpensive to start with?

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v groove bearings are an alternative and they work great if set up right . As mentioned previously - by the time you gather up all your components you'd be better off just buying the proper rails . I used v groove bearings for a large router table I built years ago , and before that I used skate bearings like many others . If we had components available back then as there are now then life would have been a lot easier for many of us .

There are guys who made some great mdf built machines with skate bearings , and they are probably still running them today . It's just a whole lot more work than buying off the shelf , and at the prices it isn't worth the work involved

hi Scott,
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Thanks