Fixing an egg-shaped hole

[trlvn]

I've got a little project and I think maybe a bearing-bronze bushing is the right answer but I'd like some input.

Background: I bought a tiny table saw--a model maker's saw with a 4 inch blade. It is old and the blade arbor (9/16 in. diameter) has a noticeable amount of play or slop. It is just a steel shaft running in cast iron 'pillow' and the hole has enlarged and become egg shaped. There is a hole in the casting for oiling but it doesn't look like it was regularly lubricated...if at all!

BTW, I believe the arbor needs to turn pretty fast because of the small diameter blade. Maybe 6,000 to 9,000 rpm. I also need a motor to drive this thing but that is a separate problem. I might well try to power it with a corded Dremel tool.

I think I want to bore or ream out the casting and press in a bronze sleeve bearing. I will polish the shaft a bit which should give a few thous of clearance in the bearing. None of this is really ultra high precision--the saw was originally pretty cheaply made. However, given the shaft rpm, I think it deserves a reasonable bearing surface. The outer diameter of the casting is about 1.05 inches so I don't have a lot of 'meat' to work with.

An alternative might be to enlarge and plug the existing hole with a small chunk of cast iron and bore a new hole. Obviously, it survived for years with the steel shaft running in cast iron.

Any thoughts on a good approach?

If I go with a bronze sleeve, what are some good suppliers to look at? I would need 9/16 ID, OD could be 11/16 or 3/4 and length would be close to 2 inches. My local bearing supply place (Ted's Bearing, Oakville) does not have anything close.

Thanks for any input.

Craig
PS I couldn't resist this saw; super cute. Got it for $5 in an auction!!

 

[DavidR8]

I would bore and go with the bearing. If there is no provision for oiling I'd go with Oilite for the bushing.

 

[Brent H]

Hey Craig,

Bore it out on both sides and press in an oilite busing on both sides as @David_R8 suggested. You can get the bushings at Princess Auto for about $15 for 4. The space between the bushings will retain oil and keep things lubbed better than having one solid bushing.

 

[YotaBota]

You could use 2 @ 1" long sleeves, press them in from the opposite ends and trim if required.
Edit - Brent H - we posted at about the same time. lol

 

[YYCHM]

I just finished installing a 1/4" X 3/8" PA bronze bushing in my wobble engine (crank/flywheel shaft). I was a little shocked as to how sloppy a nominal 1/4" bolt shank fit. Maybe I needed to use 1/4" drill rod? It's better but not great.

 

[Brent H]

@YotaBota : great minds thinking alike!

 

[YotaBota]

And I'll leave the second part of that saying alone,,

 

[PeterT]

The bronze sleeve shouldn't be an issue but my gut feel is 6000-9000 rpm is a lot for 2 sliding surfaces unless you have a steady lubrication plan & particularly if the shaft is a bit iffy & unhardened. If your shaft is nominal OD, which it sounds like it is, you might be better off with counterboring either side of the casting for push-in bearings either end. Looks like you have lots of meat but hard to judge dimensions by the pics. It might also be that standard bearing ODs are too big for the boss. But shielded grease sealed bearings would provide lower drag, lubrication, & sealed protection from dust which wood machines make in big quantities.

 

[trlvn]

The bronze sleeve shouldn't be an issue but my gut feel is 6000-9000 rpm is a lot for 2 sliding surfaces unless you have a steady lubrication plan & particularly if the shaft is a bit iffy & unhardened. If your shaft is nominal OD, which it sounds like it is, you might be better off with counterboring either side of the casting for push-in bearings either end. Looks like you have lots of meat but hard to judge dimensions by the pics. It might also be that standard bearing ODs are too big for the boss. But shielded grease sealed bearings would provide lower drag, lubrication, & sealed protection from dust which wood machines make in big quantities.

I wondered about the high rpms. I just did some more searching and found an Oilite document that says 1,200 surface feet per minute is a practical maximum [1]. A 9/16" shaft has a circumference of 1.77" or 0.147 feet. It would hit 1,200 sfpm at 8,149 rpm so I think I'm OK. Please let me know if you think I'm wandering off into the weeds, here.

I don't think I have enough space to install a ball bearing unit. The OD of the casting is just over 1 inch and it necks down to less than that at the very ends. Plus, I don't know how I could create the bearing housings at either end of the casting to be truly concentric. I can't see a way that I could bore both at the same time and it would be very challenging to flip the part and realign the holes.

[1] https://www.bearing.co.il/OILITE.pdf

Craig
(Wonder how far this thing is going to fling oil?!?)

 

[kevin.decelles]

+1 on the concern over 6-9K rpm. But........ if you do this......... I'd be very interested in the temperature readings from the bearing under load. I've been taking heat measurements of my plain bearing surfaces over time. I try to keep them under 105F if possible.

 

[PeterT]

I just finished installing a 1/4" X 3/8" PA bronze bushing in my wobble engine (crank/flywheel shaft). I was a little shocked as to how sloppy a nominal 1/4" bolt shank fit. Maybe I needed to use 1/4" drill rod? It's better but not great.

Assuming its a typical 1/4-20 class 2A bolt, the tolerance range is 0.2489 - 0.2407" so that could be somewhat loose. The shank may be different I'm, not sure. And then there is coating. Bolts are not really the best for running like an axle/shaft.
http://theoreticalmachinist.com/Threads_UnifiedImperial.aspx
Drill rod will be much closer to nominal size, maybe even a thou over depending on the spec.
I'm not sure what ID specs are of typical bushings, assume they would be 1-2 thou over nominal.

 

[PeterT]

https://www.bearing.co.il/OILITE.pdf

That's a useful document, thanks. Let us know how you make out.

 

[YYCHM]

Assuming its a typical 1/4-20 class 2A bolt, the tolerance range is 0.2489 - 0.2407" so that could be somewhat loose. The shank may be different I'm, not sure. And then there is coating. Bolts are not really the best for running like an axle/shaft.
http://theoreticalmachinist.com/Threads_UnifiedImperial.aspx
Drill rod will be much closer to nominal size, maybe even a thou over depending on the spec.
I'm not sure what ID specs are of typical bushings, assume they would be 1-2 thou over nominal.

A 1/4" drill bit shank did fit better, however after pressing in the bushing the ID appeared to have shrunk and the bolt shank was a rather tight fit. Not sure what that was about.