Help!! - bushes, rods

[andy_ck87028]

Hi All

I have a problem or perhaps many! The bottom line is I don't know the answer to even that first question!

I have opted to design my z axis through 4 * 8 mm rods placed in a rectangle around the perimeter of the router. The rods will be attached to the y gantry via bushes in drilled blocks and the whole z assembly will be screw driven.

I have bought bushes which appear to have a teflon inner coating and they are designed to take a 8mm rod. I have created soft metal blocks with a hole of 9.9mm diameter which is the outside diameter of the bush (after being gently squeezed as it is split down one side). I gently hammered the bush into the hole. It was a very snug fit. It did not appear to damage the bush. I have bought some 8mm silver steel rod which I have been assured is of good precision. The rod could not go through the bush freely and even resisted a hammer tap. Net effect ... inside coating of bush is now damaged.

This is a situation that is recoverable as I can drill the bush out (have already doen it once) and start again. If necessary, I can re-manufacture the mild steel block that the bush goes into.

My inclination is to now buy a 10mm drill bit and repeat the exercise on the premise that the bush will go in easier and the rod will slide easier. I might introduce issues of precision by doing this as the router will be in a frame that is less rigid than if I were to persevere with 9.9mm holes.

Frankly I am surprised as I thought an 8mm bush would have tolerance to take an 8mm rod. I understand I might have assumed too many things. I also might have damaged the bush by tapping it into the 9.9mm hole.

For those who have been down this path ....... what would you do now?

Cheers

Andy

[ger21]

Probably get an 8mm reamer. And ream out the other hole to 10mm, and if you need to, shim the bushes until they are snug, maybe even with a piece of paper wrapped around it.

[DareBee]

My first thought would be to try a loose bushing on the rod before assembly. If it doesn't fit someone sold you the wrong stuff.
With your split-clamp style of bush holder, the bush should slide in easy, best open up the holes.

[ynneb]

What I am about to propose is probably heresy to most purists. Get the new 10mm rods and drill out the bushes to 9.5mm. Get a pencil and wrap some sandpaper or emery paper on the pencil, place the bushes on the pencil and roll them back and forth untill they are a perfect fit for your rods.

[mxtras]

Originally Posted by ynneb What I am about to propose is probably heresy to most purists. Get the new 10mm rods and drill out the bushes to 9.5mm. Get a pencil and wrap some sandpaper or emery paper on the pencil, place the bushes on the pencil and roll them back and forth untill they are a perfect fit for your rods.

If the bushings are teflon coated ( and are still in good shape) - you don't want to do this. Maybe ynneb is suggesting this for the ruined bushing??

No doubt that the bushing ID is being closed up because of the snug fit. I would suggest reaming or boring the hole so that the bushing slips in relatively easily, then use a retaining compound like Loctite RC-609 and/or an additional mechanical method. I would not rely on friction alone to hold it.

I have been bashed (not here!) for recommending using 'glue' before, but more than likely the person doing the bashing has NEVER used Loctite products properly. They are awesome for this type of situation.

I would not rely on friction (interference fit) to keep the bushing in place unless it has a steel shell. If it is bronze or other bearing material on the OD (sounds like it is otherwise it would likely not be collapsing) then it is going to migrate out over time - lots of variables, of course, but not good engineering practice. Retain it mechanically with a ring, a pin, a set screw or something. It's a bearing - it's going to do it's job on the OD just as well as on the ID.

If you don't want to use Loctite - make the hole a bit bigger to reduce the insertion force which will keep the ID from compressing as much.

Scott

[ViperTX]

Since the bushings are 'split' when you installed them into a hole smaller then they needed they reduced the inner diameter by some amount. As some suggested..."dry fit" one bushing on the rod to determine if you have the right bushing. If you install the bushing and the rod won't go through then the hole the bushing is mounted into is too small a diameter.

[mxtras]

Originally Posted by ViperTX Since the bushings are 'split' when you installed them into a hole smaller then they needed they reduced the inner diameter by some amount. As some suggested..."dry fit" one bushing on the rod to determine if you have the right bushing. If you install the bushing and the rod won't go through then the hole the bushing is mounted into is too small a diameter.

I wish I could be that brief and accurate! Great communication skills, ViperTX!! Straight and to the point....

I forgot the darn things were split, too! Good catch.

Scott

[Mcgyver]

I don’t know what equipment you to help solve this problem but it would be a good to introduce some measuring instruments. You may be surprised how small a difference creates an interference fit, or that it may be the shafting not the bushing that is off. Regardless, identify and quantifying the problem will require less digit crossing in the solution. If you have a lathe and a micrometer you can turn a plug so it just enters the bushing and measure both it and the shaft to know where you are.

bushings generally come oversized so that when you press fit them, the end up with the desired ID. as others have said you can control fit on the shaft by changing the hole size, machinery's handbook has tables and formulas for this. You'd be better off quantifying the interference before opening up hole.

Silver steel is not necessarily round or accurate. Someone's assurance is meaningless - accuracy to what tolerance? A tenth of a thou can be the difference between a nice fit and slight interference whereas to the guy behind the counter that sold it to you, within a thou sounds pretty accurate. It is NOT good enough if its in the wrong direction (interference). Precision shaft has no over tolerance - i.e. drill rod might be -0002 to +.0002, precision shafting is like -.0002 to 0.0000. I have had this same problem and switching to precision ground shafting solved it. Even if you don’t have the stuff to quantify the problem, trying this my avoid building in .1mm of clearance (think its called slop at that level). Also, the bushes will perform much better with a hardened shaft.

[andy_ck87028]

Thanks guys for the advice.

As a result, I went back and drilled out the original hole to 10mm only to find there was a lot of slop.

I therefore thought I would try a completely new hole at 10mm and the bush still went in tight but the rod does now goes through ok although it is a tight fit. There is absolutely no slop.

So, I now have a way forward. I'm assuming that ebony paper and lots of elbow grease on the rod will help me create smooth and free movement.

I'll also follow advice by putting a top and bottom washer over both ends of the hole to stop the bush from coming out.

I'll definitely swap to hardened ground rod when I have to. Being in Australia, access to materials can sometimes take more effort than building!!

So thanks once more and I must admit to feeling quite overwhelmed by the response.

Andy_ck87028