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  #1  
Old Mon 30 October 2006, 13:03
Sheldon Dingwall
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Thoughts on the angle iron rails of the MechMate

Any thoughts on making the V-rails out of heavier angle. Maybe 3/8" or 1/2"? Something that would resist twisting a little better.
  #2  
Old Mon 30 October 2006, 20:28
Gerald_D
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Hi Sheldon.

On the moving gantry, the weight needs to be kept down (that's the one reason I trim the vertical part of the angle rail so short). But, the x-rails could easily be heavier. My approach is to stick to a standard profile for both rails and beef up the underlying support if twisting is a problem. However, I'd need to be convinced by a dial gauge that there really is twisting. On our old SB, the bolts in the rails were too far apart for the support to be doing a proper job.
  #3  
Old Mon 30 October 2006, 21:55
Sheldon Dingwall
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How are you using a dial guage to measure twist in the rails? Clamp it to the gantry or car?
  #4  
Old Mon 30 October 2006, 22:24
Gerald_D
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Place the gauge under the gantry and run the car over it? (Havn't done it myself):


Differences in readings between those 3 points is probably what one is looking for....
  #5  
Old Mon 30 October 2006, 22:30
Gerald_D
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Or, more practical, only 2 points......

  #6  
Old Mon 30 October 2006, 22:37
Gerald_D
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Something for the weekend....


  #7  
Old Mon 30 October 2006, 23:25
Gerald_D
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Lost - Grinding Vprofile on rail edges

Took a quick breakfast break and went to visit the MechMate upstairs armed with a dial gauge....

Parked the y-car to one end of the 1.9m [6 ft]gantry, laid a plank over the middle of the gantry, put the dial gauge under the middle of a cross-tube, Applied and removed a 12.3kg [27 lbs] weight a couple of times. The dial would fluctuate by between 6 and 7 increments of 0.01mm. The flex being average of 0.065mm [0.0026"]

Moved the gauge to under the tip of the angle rail (outside the rack) and the readings mostly fluctuated 6 increments, with a couple of 7 increments sometimes.

Conclusion:
1. Instruments not accurate enough to discern the rail twist factor.
2. The rail twist factor with a 12.3kg load variation [27 lb] is less than 1 marking of 0.01mm [0.0004"]
  #8  
Old Tue 31 October 2006, 01:35
Dick van Randen
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I did a similar measurement on the Shopbot a while ago. I did my measurement between the gantry and the table and discovered something interesting. You get the expected bowing effect as the car reaches the middle and you measure in the middle. However, it all seems to be interacting as the y car mass moves to its extremes.

Y car position
close..........middle.........far
x flex max....xflex min......x flex max
y flex min....yflex max......y flex min

if the maxs and mins cancel out it would seem you could get a flat surface!
  #9  
Old Wed 20 December 2006, 19:31
Rob Smith
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would this work for rails it would allow for some adjustment as well as the use of flat stock which might be easier to put a nice edge on?
  #10  
Old Tue 26 December 2006, 23:43
Gerald_D
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I don't think that flat stock is much easier to put an edge on. In any case, you are illustrating the problems of mounting flat rails and my approach is rather to spend that extra effort toward getting the edge directly on the angle iron.
  #11  
Old Sun 04 March 2007, 14:47
bugmenot dillbert
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How about this?



The end plates of the gantry would have to be modified, the shaft of the motor may not be long enough and the support structure would have to be modified.
  #12  
Old Sun 04 March 2007, 17:03
DocTanner
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The Mechmate as drawn works extremely well. PERIOD
I didn't think the grinding of the angle iron was a worthwhile effort. I was wrong.
The grinding of the angles is not that difficult. I cut mine down to an inch high with a thin abrasive blade.
Then mounted an 8 inch grinder on a 45 degree angle. Passing the angle through like a tablesaw.
Yes, I spent the day grinding away.
My rails are less than perfect!!! But I am pleased with the results.
I am strictly a hobbiest. I was caught by suprise by how many people want things cut.

Use the "Microsoft Method"
Functional ------------------------> proceed
Will it make money as it is? ---> proceed
Can it be upgraded?-------------> proceed

Main goal is to get a functioning machine that will make you money.
Every part of the MechMate can be changed with a little effort.

Perfection should be strived for but it is not mandatory,
DocTanner
  #13  
Old Sun 04 March 2007, 22:25
Gerald_D
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Thanks for that post Doc. You have perfectly captured the essence of the MechMate project.
  #14  
Old Wed 07 March 2007, 09:47
bugmenot dillbert
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Doc,

would you mind posting a pic or two of the setup you used to grind the rails?

I don't know but I think it would be interesting if there was a way of using the 'proper' linear rails with the mechmate allowing choice for the builders. Secondly, I hate any form of cutting (of anything) that involves cutting it lengthwise.
  #15  
Old Wed 07 March 2007, 10:15
Gerald_D
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"Bugmenot", there IS an interesting choice for the the builders.....they can go and build something else!

Seriously though, have you thought of what 12 meters [40 feet] might cost you, what the shipping cost will be, what's the chance of those long slender rails being damaged in transit, what will you fix them to, with screws at what intervals, and is that "precision" really necessary when cutting unstable wood or plastic? Have you ever picked up a 4m [12 ft] long top brand name linear bearing in the middle and seen how the $1000 ends droop by 6mm [1/4"]? Have you tightened a heavy linear bearing to a bed with a bit of dirt under it and seen how the rail bends? Okay, I am going too far now....

I also hate splitting long items, but I bit the bullet and got over it.
  #16  
Old Wed 07 March 2007, 14:07
DocTanner
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Bugmenot,
I only took a couple pics of the whole build, none of the rails. Will try to get a sketch of what I did.
Gerald's tablesaw method would have been my first choice, but I have a brand new tablesaw that I wouldn't let myself use.
My hobby can make money. My Mechmate has already paid for 1/2 of itself, so it must be close enough.

DocTanner
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