Backlash

[vivek16]

yeah, our machine does atleast .003" (better from what our mentor tells me). We are trying out a waterjet from our sponsors for the first time. They list the tolerance as .005" on their website but my mentor assures me that they can do much better than .003". I made my previous designs for .002" of backlash but it is no big deal to change.

Ok, I will look into flanged bearings. They certainly would make life a lot easier. (Come to think of it, I have half the bearings I need from a toughbox I [slash]demolished[/slash] err... modified )

thanks, Vivek

[IKE]

If you really want to learn what the right demensions are use a machine design book (norton or shigley) or you can buy an old machinist Handbook. Since you are using spur gears any old addition should have the right info.

As far as tolerance goes, almost any mill that has functional lock on it can hold a tolerance tighter than 0.005 learn how to work against slop and practice.

That being said our team made AL side-plates for an AndyMark gearbox. It took two tries. The first shot I merely explained what to do with the students. They made a couple big set-up errors and the plates wouldn't work. I worked closely with them to make the second set and it worked out better.

Rule of thumb on press fits 0.001" per 1" diameter. This has almost always worked for me in the past. Gearboxes are a good test of skill. They hate being too tight and will destroy themselves if too loose.

[vivek16]

I hope to learn more about machining over the summer. Our mechanical team knows how to run the mills but to different extents (and we use a digital readout that holds .0005" of tolerance, although not sure how accurate it is).

I will probably switch to using those flanged bearings.

I am using hex key turned down at the ends for all of my shafts and was wondering how good of an idea it is to turn down hex shaft like that. I would think that all those edges banging against the bit can't be good for it (although if I came in from the side that would be solved. This is what I have done before but the AM hex shafts already had an end turned down onto them.).

thanks,

-Vivek

EDIT: Is an arbor press absolutely necessary for press fit bearings? Could the same effect be achieved without too much trouble with a vice or some blocks of wood and a hammer? I am not against getting one, it seems like they would be quite useful. It is just that we can't really afford luxuries right now .

[sanddrag]

A suitable vice, a couple pairs of hands and eyes, and some aluminum plates can make a suitable press.

Turning hex shaft into round is common and acceptable. I've done it many times without issue. Just feed slowly.

[DonRotolo]

You can make a pocket in your gears on a lathe, and tolerances to .001 or better should be easy with just a little care. .001 interference in aluminum is a pretty tight fit, .0005 for a 1.5" OD bearing would be plenty. You can fiddle this a bit with sandpaper or a file.

For clearance between teeth, you must be sure there is no negative clearance (too close together). The higher the speed, the closer the tolerance, the harder the material, the closer they can be. For moderate speeds (1000 RPM-ish) in aluminum, .0005-.002 is fine, be sure to run the gears in for several hours with light lubrication, in both directions and light load.

When making intermittent cuts (like turning hex to round) don't use carbide bits, regular tool steel will manage that better. Carbide tends to shatter, especially the ones with the little triangular inserts.

An arbor press is convenient, but not required. The reason to avoid using a hammer is the shock loads being transmitted. Better use a vice or some clamps with metal blocks than a hammer, but if unavoidable, go slowly with the hammer.

Cheap bearings are OK for low speeds (600 RPM-ish) and sloppy tolerances. ABEC bearings are great for higher speeds (10,000 RPM) and longevity, as well as tighter tolerances. I buy bearings on eBay all the time, but if you have a bunch of useable bearings, use them.

Old trick if the pockets are accidentally a loose fit: Make a plug for the hole from scrap metal, then drill three (or more) evenly-spaced 1/8" or so holes in the edge of the bearing pocket, with the holes protruding maybe 0.010 into the pocket (and hole plug). Fill the 1/8" holes with metal rod, file them down on the inside a little bit to get your press fit.

Don

[vivek16]

I believe the only bit we have ever used for our lathe is a carbide. There must be some tool steel ones laying around somewhere...

That is a pretty nifty trick about the hole plugging! Never would have thought of that.

Our mill has a digital readout that goes to .0005 (with the last digit being either 5 or 0). Now that is 25 times what I estimated the tolerance to be (I believe the digital readout is a Bausch and laumb). I will have to find out the actual tolerance next time I go to our workshop.

thanks, Vivek

[Andy Brockway]

If you want an alternative to flanged bearings you can make your own support for the bearing out of button head screws. Here is a picture of how I do it. http://www.chiefdelphi.com/media/photos/16362.

[vivek16]

Originally Posted by Andy Brockway

If you want an alternative to flanged bearings you can make your own support for the bearing out of button head screws. Here is a picture of how I do it. http://www.chiefdelphi.com/media/photos/16362.

That sounds like a pretty good idea. Would you recommend threading the aluminum itself or doing a through bolt? It seems like just threading the aluminum should be plenty strong.

thanks, Vivek

[NickE]

Originally Posted by vivek16

Would you recommend threading the aluminum itself or doing a through bolt? It seems like just threading the aluminum should be plenty strong.

I can't imagine that you'd have any problem with just threading the aluminum, if the button-head screws are just for holding the bearings in. Furthermore, it could be difficult to do through bolts unless you are using insanely small gears (smaller gear diameter than outside bearing diameter)

[Andy Brockway]

Originally Posted by vivek16

That sounds like a pretty good idea. Would you recommend threading the aluminum itself or doing a through bolt? It seems like just threading the aluminum should be plenty strong.

thanks, Vivek

I thread the aluminum and use #10-24 button head screws. Center of tapped hole is (radius of bearing plus 1/8") from center of bearing.

I have built all my gearboxes at nominal center to center for the gears. I also run for 5-10 minutes with the wheels off the ground for break in without lube, than lube with lubriplate and run on the ground.