Bearings don't fit (Drill rod) shafts ?

We have bought 1/4", 5/16" & 3/8" “Drill Rods” to use as shafts. They seem to measure to be exactly to size. We bought EBC bearings for this shafts and none of them fit. For example the 5/16 measured ~ .311 while the drill rod measured 0.3125. Am I supposed to be reaming the bearing up to 0.313 or something or did I buy the wrong rod. Not sure its possible to downsize the hardend Drill Rods with our equipment. Is there some shaft type that is specifically designed to be a few thousand under its nominal diameter ?

Just to make certain, I am not aware that I bought “unground” bearings but I always assumed “unground” referred to the bearing race, nothing to do with the ID of the bearing. Anyone know for certain what “unground” refers to ?

What part numbers did you buy? When selecting bearings and shafts, you always have to check the dimensional tolerances of mating features.

Doing a quick check of the 1600 inch series bearing datasheet here shows that the bore size range is .3125/.3120 . Drill rod from Mcmaster Carr is .3130/.3120 . A tolerance stackup calculation shows your fit could range from a .0005 slip fit to a .0010 press fit.

I have to point out that tolerance stackups are the most basic calculations a mechanical designer needs to do. I will also admit that I didn’t check a single bearing fit for any of the stuff we ordered, or teach my students how to do them. (smack forehead) One more way I need to be a better mentor…

A lathe is really handy…it’s kinda difficult to make rotating stuff work without one.

You can always try putting the shaft in the freezer or using cooling spray and see how much you can shrink it.

Hi Dave. You can also chuck up the offending shaft in a cordless drill and hit it with a strip of emery cloth. Rolled emery cloth in a range of grits is readily available. A bit of oil helps. Cheaper than a lathe.

The water hardened rod we bought
doesn’t state a tolerance. The oil harden version is ± 0.0005. So far my old school vernier has not detected any variance at all from listed size in the few measurements I have done. The problem must be the bearings (which I can only measure with a cheaper digital vernier). I never expected bearings to be materially and consistently on the small size. Not sure it is possible to do anything on the lathe with the surface hardened 5/16 except trash or bend it. Will try the emery cloth to see what happens but I noticed KBC sells oversize reamers. A few key oversize reamers might provide a long term solution. Bearings were 1603 2RS. We ordered by bearing # and BDI never provided a vendor or catalogue when we placed order. Only when delivered was vendor obvious.

If anyone knows for certain that reamers can be used on the bearings successfully, would appreciate the heads up. I know I once tried to use a drill as a reamer on a bearing by clamping the inner rotating assembing in a vice and using a portable drill. I am kind of hoping I can do the same with a reamer for a thousand or so.

I am much more in tune with measuring electrons versus 0.0005’s …:slight_smile:

My guess is that trying to ream the hole in the inner race of a ball bearing is not going to be easy.

If you give more info about how you’re planning to use the shaft, etc, maybe we could offer some useful suggestions.

btw we are using a couple of shafts to run some mechanisms, we’re just using cheap 3/8" hex aluminum shaft which we turned the ends to fit into a standard bearing. This is for moderate torque use, not in the drivetrain.

Thanks for offer. We just want to have the ability to make typical small motor rotating mechanisms with shafts and bearings in short notice (like a shooter this year etc) without worrying about bearing issues etc.

My current thoughts are the bearings are all slightly undersized and there is a fair chance the reamer could easily do it. I don’t think the races are that hardened. The bearings are only a dollar and change.
I suspect all I have to take off is a 0.001.

Fun thing about bearings is they’re usually very hard, in addition to having to hold the inner race still to ream it. I haven’t seen anyone open up a bearing bore with any luck.

I would suggest the drill and piece of really fine sandpaper

I always assume that all values are nominal. A lathe or a drill press and some sand paper usually work when it comes to a necessary fit – though the latter is not precise enough if a press fit is needed.

The easiest way that I can think of to re-size the ID of the bearing would be to broach it since any method involving spinning tools would result in the bearing doing what bearings do. Hardness might be a factor, it would be great if you could anneal only the inner bore without softening the race itself, but that might be beyond your capabilities, especially given the amount of time left to build.

Most bearing races are extremely hard. So hard they are normally ground rather than machined. You are more likely to trash your reamer than to open up the bearing bore.

The drill rod, on the other hand, is supplied in an unhardened condition. Notice that the page says “water hardening”, not “hardened”. It is quite easily worked. It is designed to be worked to shape and then hardened by the user for use in tooling. You should be able to chuck it up and hand work off .0005" without too much trouble, using fresh silicone carbide abrasive paper. It’s not fun, but doable, especially if your bearing is going to be installed near the end of your shaft. I wouldn’t want to have to polish the whole shaft to slide a bearing all the way down it, though.

Notice that the drill blanks, at the bottom of the page, are hardened and ground to size. The tolerances are much tighter. You wouldn’t want to try working the diameter on those.

You can tell the difference by using a fine file and attempting to file the material. On unhardened rod, you can easily file a chamfer on the corner. With hardened material, like a drill bit or bearing race, you will be lucky to scratch it with your file.

Went to run our Cim-Sim’s from andy mark this weekend, and found that the shafts were so oversized that they had broached when forced into the bearings and were causing the bearings to lock up.

I used our arbor to press the shafts out, then chucked them in the drill press and move the belt to the high speed position. Then I took one of our fine files and turned the drill press into a temporary lathe to take off the couple thou required to make it a nice smooth slip fit.

Took me about 20 minutes, but the AM shaft is probably mild steel, so you may want a more aggressive file.

Point of safety: Never use a file this way, or on a lathe, unless you have a handle installed. If the file catches, and it doesn’t have a handle, the tang will pierce whatever body parts are nearby (usually the palm of your hand).

Bearing ID’s are sized to fit SHAFTING, not nominal OD drill rod. Shafting tolerances are ALWAYS slightly undersized from the nominal. You can use shafting or sand down the drill rod a few thousandths, it’s much easier to use the shafting such as this from McMaster Carr:

45 auto has the correct answer. We have when in a pinch used the drill press and emery cloth technique.

Thanks you guys. Its now clear what is going on and where I should have bought the rod from (and will in the future). Will try emery cloth or file (good safety tip) on our lathe for our 2 short term needs. If not a solution (rod too hard?), will buy the correct rod.

We need more mech guys on our team :slight_smile:

Taking a couple thou off an end of the unhardened drill rod shouldn’t be hard if you can hold and spin it properly. We once had to open up two drive sprockets from 5/8" to 16 mm, or make the hole about .005" larger. It took a very long while with a 5/8" stub in the mill-drill and lots of valve grinding compound. Reducing a shaft diameter is a LOT easier. You can glue the emery paper on a stick and use it like a file.

I have had ground shafting come in oversized and even crooked. :mad:

update: We used emery cloth to downsize two drill rod shafts say 8" long and 2 out of 2 shafts appeared to be microscopically “bent” when installed on our shooter between 2 brand new bearings. The center of the shaft moved up and down when hand rotated even though it had never been run under power. All I can think of is the drill rod surface hardness was not consistent and the emery cloth wore down parts of the surface faster than others.

Next year we buy the correct shaft rod from McMaster Carr. And find a machinst mentor :slight_smile: