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  #1  
Old Wed 09 July 2008, 17:02
chopper
Just call me: chopper
 
Big Lake Minnesota
United States of America
Why does the angle iron have to be cut shorter for the rails?

Hi,
I was wondering why the rails are being cut in the first place?
is it because they add to much height to the main beams?
or are they considered to be to flexible?
I would think if one used a 2.5 inch X 1.5 inch angle for the rail that it is only
about 3/8th of an inch taller than cutting it down to the 1.10 size
or is it an accuracy thing with the rail?
I would like to know what the reasoning is for this
thanks, chopper
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  #2  
Old Wed 09 July 2008, 19:26
smreish
Just call me: Sean - #5, 28, 58 and others
 
Orlando, Florida
United States of America
Chopper,
It is all about motor-to-rack in pinion differential.
Study the drawings and you will see that all revolves around this vertical height issue for the x and y axis.

Sean
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  #3  
Old Wed 09 July 2008, 21:40
chopper
Just call me: chopper
 
Big Lake Minnesota
United States of America
thanks I understand what you are saying I guess I didnt think of that
thanks for the help
//chopper
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  #4  
Old Thu 10 July 2008, 00:40
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
There is some history, and some technical reasons . . . . . . .

Our MechMate project started as replacement parts for our original ShopBot, where the rails were only 25mm [1"] tall. (They used a rare 2 x 1 x 3/16 angle with a little bit of grinding).

All the lasered profiles for the motor carriers and swivel plates, plus the tensioning springs were designed for the 25mm [1"] height, to retrofit onto the ShopBot rails at the time.

Then the time came to make my own rails. A 2 X 1 X 3/16" angle is completely unheard of in most places, especially out here. Besides, the 3/16" was a bit too thin and was prone to wear with the small rollers that ShopBot used. Plus, I wanted the long horizontal leg of the rail to easily reach the center of the box tube of the gantry - a 2" long leg is a bit too narrow.

For a 2.5" long leg, the lowest angle iron easily available internationally, is 50mm [2"] tall. That would cause a major jump in the motor/rack/gearing design, as well as add far too much mass to the moving gantry. So I cut that back to 25" [1"]

The cutting is a day's work, and I never expected everyone to have such huge issues with it, 99% of the issues from people who had never tried it . . . .

Anyway, a year ago there was much pressure on me to use the V-Rail "caps" in the design. A simple, accurate solution for that was developed, which put the rail height at 28mm [1.1"] and which needed a revision to the lasered motor mounting system. Because that revision could be confused with the earlier 25mm [1"] system, it even got its own model name; Mamba. (Now that the 28mm [1.1"] rail is entrenched, the Mamba name is being dropped)

So today we have a set of lasered parts that will work equally well with the V-Caps, as with DIY-ground rails. However, everyone seems to have forgotten they wanted to use the V-Caps as the alternative, easier method. . . . .

Also today, we seem to accept extra mass on the gantry as being "okay", or even "good" as it makes a stronger gantry without a noticeable penalty on speed performance. We seem to have won our argument against the promoters of alu.

Having said all that, I am comfortable with people wanting to use 1.5" tall rails. But I am nervous of the confusion another set of optional drawings will cause - especially when:
- there is an easy alu / V-Cap option available
- 2.5 x 1.5 angle is not that commonly available
- cutting rails down is not that difficult.
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  #5  
Old Thu 10 July 2008, 11:08
orotemo
Just call me: orotemo
 
Warburg
Israel
Common problems?

Common problems, or is it just me?

Started building my MM, Got the laser parts, beautiful work done on them (only it cost over 1000 USD ), all happy.

I'll build a 6X2.5 meter MM ( I bought them two 200mm C channels, it's a waste to cut them down to my originally planned 4 meter long MM).

Cutting the rails: had a 70X70X7.5 mm angle from scrap yard, so I used it, it proved quite a doable task cutting it down to 26 mm (it was easy for me to cut to that height since I have bolts that when fastened all the way, make the cut exactly 26mm high). unfortunately, the damn person who operated the grinder (myself) managed somehow to have variable cut heights varying from 26.4 to 25.3mm. I find this very disturbing, as I'll probably do some 3d work on my MM.
Finally, the question: can I put a v cap on this height of rail? I was hoping the v cap will 'iron' the mistakes done by the poor grinder operator (me again)?

Will my solution be okay with the motor mounts? any wiser suggestions for solution? if I finally decide to try and grind the rails as they are, will this rail height be okay (25mm minimum)?

Thanks in advance.
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  #6  
Old Thu 10 July 2008, 11:28
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Does your rail height change suddenly from 25.3 to 26.4 mm? If that type of change happens over a distance of 0.5 meter, you can use shims under the rail to compensate for it.

V-cap will not improve anything like that - it bends more than that under its own weight. Even V-Cap requires careful shimming.

Suggest you try and grind some more with a thick disk this time.

The motor mounts should still be okay. Are you using metric (15mm) or inch (12.7mm) racks?
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  #7  
Old Thu 10 July 2008, 17:34
orotemo
Just call me: orotemo
 
Warburg
Israel
Thanks Gerald, shimming it is.

I'll use 15 metric. Here in Israel we use metric, although you could find inches for just about anything, but from what I hear, there is a tendency now to go metric everywhere. I'll try to use all metric.
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  #8  
Old Thu 10 July 2008, 21:27
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
If you use the thicker metric racks, you can cut/gind the rails a bit lower down, because we try to keep control of the overall dimension from the bottom of the rack to the top of the rail.
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  #9  
Old Thu 10 July 2008, 22:28
chopper
Just call me: chopper
 
Big Lake Minnesota
United States of America
Thanks Gerald,
that's what I wanted to know, makes sense to me
//chopper
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