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  #1  
Old Fri 08 June 2007, 08:15
Atifeh
Just call me: Nader
 
Tehran
Iran
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Alternative ideas on V-roller shapes & materials

Hello all,

I had 2 problems making Gerald’s “home made V-rollers” which were:
The CNC shop could not deliver the 28H6 bore and suggested grinding.
Heat treating imposed distortions that needed grinding on both the inner bore and the groove.
Of course heat treating in vacuum will eliminate the oxidation but heating will always impose distortion on the machined part, mostly due to internal stresses both during the initial production method, machining and cross section differences; also due to phase change of steel and grain growth during heat treatment.

To overcome the distortion and avoiding the grinding process which increases the price I came out with this idea. Please comment on it to see whether it is worth elaborating/modifying or should be left behind.

The idea is to use thin cross sections that will hopefully be pressed on the ball bearings even if it was distorted during the heat treatment process and take the shape of the bearings’ outer race. I believe the production process will also be easier.

I am suggesting one 6201 (ODxB 32x10 mm) deep groove ball bearing.
The combined basic static load rating for Gerald original design with two 6001 bearings was given previously to be 2x2.36x0.5=2.36 kN. For one 6201 bearing it will be about 1.55 kN, still above the load bearing capacity of BWC bearings.

Instead of heat treating, I thought of selective Chromium electroplating on the rim of the rollers and subsequent polishing with grit 600 sandpaper on a lathe. I was advised not to. The reason stated was that it is prone to spalling under minor shocks. Please also comment on this idea.

Thanks
Nader
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  #2  
Old Fri 08 June 2007, 09:29
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Hi Nader

Some good lateral thinking!

28H6 bore is very common - I am surprised that a machine shop had problems with that. (The H6 in this case indicates 28.000 to 28.013mm or 1/2 thou inch over).

In practice, the vacuum hardening done here gave us no noticeable distortion. The guys who do it are careful that the orientation in the oven is correct, and they will only harden from steel that they have supplied themselves. However, I can see that there may be problems with hardening.

A single bearing "rocks" too easily. If you put a shaft through a single bearing, you will see that the point of the shaft can be moved up and down quite easily.

Thin rings can not be machined inside and outside with re-chucking (re-clamping) and there is a risk that the inside and outside are not concentric to each other.

Three months ago I made some wheels out of: cast iron, bronze, mild steel and non-hardened tool steel. I plan to put one of each on the 4 corners of the gantry for a test. I am sure that one of those cheap & easy processes/materials will run for a year at least. My feeling is that hard rollers (on soft rails) are not needed - we need "soft" rollers because they are much cheaper to replace than the rails??
  #3  
Old Fri 08 June 2007, 16:22
Atifeh
Just call me: Nader
 
Tehran
Iran
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Hi Gerald,

Your point about single bearing “rocking” is correct. With deep groove ball bearings, axial play is also high enough to wear the rail. Therefore I think that single bearing is not a good choice.

I am considering to make the V rollers out of 42CrMo4 (4140) steel. The material is sold in annealed state which has a hardness of about 220HB. A 50 mm round stock, If normalized will provide a hardness of about 290-300 HB, both machineable and harder than the rail. The material is tough and withstands deformation to workshop conditions.

I also believe that the rails are more valuable than the rollers, considering the efforts in making and then mounting and adjusting them, and it would be better to look at the rollers as the consumeable parts. I think ductile cast iron would also be a good choice, contributing its damping capacity, graphite bearing structure that acts as a friction reducer, dimensional stability and machineabilty.
  #4  
Old Fri 08 June 2007, 23:26
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Hi Nader, glad to see someone who shares my "unconventional" thinking. The hardened rollers come purely from everyone shouting that one must use BishopWisecarver rollers. . . .

I did indeed make the cast iron roller from ductile iron (GGG 50) because I have had a lot of joy with this stuff in other applications. But it isn't easy to find here and I never know if I can trust the supplier to be supplying the right stuff without using a microscope and checking for the black spots.

But you still have to find a machine shop that can hold 28H6
  #5  
Old Sat 09 June 2007, 17:14
Atifeh
Just call me: Nader
 
Tehran
Iran
Send a message via Yahoo to Atifeh
Hi Gerald,

You know, I would only praise the BWC for their rails. As has been discussed before Ytong makes the bearings but not the rails. I have even seen Russian brands of these bearings locally. Hard contacting surfaces in contact with each other endure better abrasive conditions but we mostly produce wood dust and the only existing abrasive might be the dust in the air, which actually is useful to polish the contact surfaces along time. Therefore I assume 60Rc hardness is redundant; I would vote for just enough hardness to withstand blows and drops in a workshop condition.

A simple way to verify the quality of a ductile iron is by macrography. Polish a piece under running water with grit 100 220 400 and 600 sandpapers, then polish it on a piece of wet cloth sprinkle with a little amount of 1 micron abrasive powder (most of the time I use toothpaste successfully!). Check it under a 4x magnifier, you can clearly distinguish the round graphite globules vs thin lamellar graphites in case of plain grey cast iron. You can hit a hammer on and edged end, if it bends or indents, it is definitely ductile iron, otherwise it will break, showing it was a degraded ductile iron or even gray cast iron. This might seem a joke but it is not; you can make distinction between a gray cast iron and a nodular iron by smelling the freshly broken part (needs some practice). Ductile iron has a stronger odour. I really don’t know the reason but it might have something to do with the Mg-Ce elements that are used for nodularization.

In addition, the composition and strength can be calculated if you could get the chemical analysis from the supplier. Thus you can verify the GGG class.

A good supplier for ductile iron could be water pipe spinning foundries that also make the connectors (Tees, 90 and 45 angles,…). They have to use the best material (low Sulphur and low Manganese) but I am not sure if they could provide GGG50.

The machine shop still insists on the grinding and as you said I have to find another machinist, but I would like to know your opinion about this:
Getting the parts with +0.04 to +0.02 and using an H6 reamer and ream it on a lathe. Do you think this is a good idea?

After I get a suitable machinist I will make a hollow pattern (45/25 dia. x 400 mm) to submit to a foundry to try ductile iron too material.

Another research I made was that Chromium electroplating needs a very smooth surface to adhere properly, otherwise chipping and spalling will happen. I intended to use it on the rails but it seems not applicable.
  #6  
Old Sat 09 June 2007, 22:47
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
The smell of broken cast iron. . . . . now that is a foundryman's insider tip. Thanks!

The H6 tolerance for the bearing fit is straight out of the bearing producer's books. Every half decent machine shop I know will produce a nice fit for a bearing, but they won't exactly know what tolerance they produced. Just give them a sample bearing and I am sure they will be fine. Remember, we are still going use Loctite to hold it together.

Before discarding your favourite machine shop, let them make a sample mild steel wheel for you?
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