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Terryd22/01/2010 22:53:06
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I am considering building a ball turning tool similat to Steve Bedair's.  For those who aren't familiar with the device it can be seen here:
 
http://bedair.org/Ball/ball.html
 
I am thinking about using thin sheet PTFE as a low friction washer (perhaps 0.5mm thick) between the base and the rotating toolpost.
 
My question is about the compressibility of the material.  Will it stand up to the rigours of such an application or will it compress too much?
Terryd22/01/2010 22:54:41
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P.S.
 
The washer will be full size, i.e. the same diameter as the toolpost.
mgj22/01/2010 23:11:30
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Looking at that, is there any advantage to PTFE over a roller thrust washer full width or nearly so?
 
Just as easy to machine a recess for a proper roller washer and you can be certain it won't compress.
 
Will PTFE compress too much?
 
Well you'll get a lot of opinion, but at the end of the day you are going to have to do the sums and calculate the downforce (assume max torque for the lathe to give a safety factor and the cancelling leverage to convert back to a force is the tool length) Then, because you have a tip down/ tip up load, use half the bearing area as the area for the stress. Look at a web site or two to get Poissons Ratio for PTFE  - that will give you stress by strain. You will know the stress , and hence the strain and change in dimension.
 
Then you'll know for certain.
 
You may find figures for sections which will help you out if you are not into the various moduli, but you will still need to work out the load.
 

Martin W23/01/2010 00:00:48
940 forum posts
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Terryd
 
Personally, and I am ready to stand corrected on this, I would avoid any form of plastic or potentially compressible material under the tool post. If put under load it could allow the tip of the tool to dip and dig in and then release. This I believe could cause chatter at best and at worst could damage a carbide insert.
 
If the cutting load is kept fairly light then metal to metal surface, if lubricated say with a molybdenum loaded grease, would probably be fine. If you really want to fit a bearing of some sort then Meyrick's solution using a roller thrust washer of suitable size is probably the best option.
 
Cheers
 
Martin W
Circlip23/01/2010 10:51:23
1723 forum posts
To ask and answer your own question Terryd is double indemnity
 
  If the load was centralised over the whole diameter, you would probably get away with a THIN PTFE washer, same stresses apply to a thrust race, but they are more resistant to deflection although localised contact.
 
   Martin W's answer of face to face is the best one (See MEW Issue 1 pull out plan for the rounding over rotary table). Got any Kn*ckered taper roller wheel bearings??
 
  Face to face contact fot the table part and taper roller spindle. You KNOW you can machine to microns.
 
  Regards  Ian.
Martin W23/01/2010 12:45:14
940 forum posts
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Terryd
 
Having had a more detailed look at the drawings I would be surprised if you could buy a roller washer to fit the major bearing/load surfaces at a sensible price, that said I have not looked to see what is, or is not, available.
 
Another option, though I am unsure what advantage you would gain with regard to lowering the friction. is to use a thin brass washer machined to size to fit between the main bearing surfaces. This would require that the depth of the registration spigot would have to be increased by a similar amount in order to retain enough engagement with the base. Again I would suggest sparingly lubrication using a loaded grease.
 
One concern I have about the device as shown on the drawings is that there is no way to lock the securing bolt, item 7 on the assembly diagram, to the tool holder. I would have thought that unless a locking compound was used on assembly  then there is a real risk of this slackening.
 
There may be something to be gained by mounting a thrust washer assembly with, or possibly without, the existing belleville washer under the head of the securing bolt (item 7).
 
Just a few ramblings to keep my grey matter active(ish)
 
Cheers
 
Martin W
 
mgj23/01/2010 16:30:22
1017 forum posts
14 photos
Nachi make ball thrusts from 24 to 230mm od.
 
Personally I wouldn't bother - if I was feeling really keen I'd shove a couple of oilite bushes onto the vertical pivot. Not as good or adjustable as a pair of taper rollers or angular contact ball races, but quite good enough.
 
If I had some cast iron about I'd knock up a couple of bushes out of that - they like to run on steel.
 
But no, I wouldn't use a PTFE washer.
 
How often is thing going to get used? 100 - 200 times in a life -time? A few drops of oil would be as good.
 
As for loadings, the trouble is that the load isn't centralised. You have a downforce on the tool side and a tendency to tip up on the far side, which is generating a bending moment in the vertical pivot which translates into a shear stress. But I do agree with Circlip - one is much better off putting the bearings into the vertical, because they  are in the direct line of the forces shearing the pivot.

Edited By meyrick griffith-jones on 23/01/2010 16:36:31

JasonB23/01/2010 17:07:50
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25215 forum posts
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Having made a similar ball turner  there is no real need for any form of washer, infact I ended up using the small spanner from my QCTP as the lever which is quite short. Lets face it out cross and top slides are just metal on metal and they run OK under the same loads.
 
As Meyrick says a drop of way oil under the body will do the job.
 
Jason
 
 

Edited By JasonB on 23/01/2010 17:10:33

Ian S C24/01/2010 08:23:56
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7468 forum posts
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Hi TerryD,I looked in Google-ptfe for machine bearing surfaces.The fitting you are making would be better, if you must have a bearing, would be better with a shim steel washer,but you are proberbly best to have nothing other than (not sure how to word it),turn a circular base on the ball turning tool.There was some interesting stuff about ptfe bonded to steel as bearings, plus lots more.Ian S C

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