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How to check for parallel?

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Wolfie14/02/2012 21:22:28
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502 forum posts
How do I test whether two sides of a piece of flat bar are parallel to each other.
 
I milled a piece today top and bottom and when I checked it with a square they weren't fully square. I know I had it square to one side in the milling vice so the implication is that the two sides weren't parallel to begin with.
 
How do I check for this?
Tony Pratt 114/02/2012 21:39:17
2319 forum posts
13 photos
A micrometer is your best weapon to check if a part is parallel, take readings in various places to establish if the job is parallel over its length and width. Please note this wont tell you if your part is straight or bowed.
Tony
Andrew Johnston14/02/2012 22:12:05
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Posted by Tony Pratt 1 on 14/02/2012 21:39:17:
......... Please note this wont tell you if your part is straight or bowed.
Tony
 
Agreed, but if the part is bowed then by definition the two sides are not parallel, even if the distance between them is the same. Only straight lines and planes can be parallel.
 
If I wanted to check if a part had parallel sides I'd use a surface plate and a test indicator. If the test indicator does not move as the part is moved around underneath it, for both sides, then the two sides of the part are parallel.
 
Regards,
 
Andrew
jason udall14/02/2012 22:20:36
2032 forum posts
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quiet so Andrew
But without surface plate?
Straight edge and micrometer is next best.


I thought railway tracks were parallel...so how do they make bends?...lol
jason udall14/02/2012 22:24:22
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btw Wolfie mentions "square"
Two faces can be parallel and even if end faces are also parallel ,they might not be square....thick of parallelogram...or rhombus
jason udall14/02/2012 23:27:12
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*think
 
Wolfie15/02/2012 09:29:47
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Aye but I'd used one side as a datum to mill the top and bottom, and now one side isn't square to the top and bottom but one is!
 
Ian S C15/02/2012 09:38:04
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A bit of plate glass is OK as a surface plate, or the table of your mill should be good to, others will come up with other ideas for a makeshift surface plate. Ian S C
David Clark 115/02/2012 10:01:31
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Hi there
A chopping board from Argos makes a great surface plate.
It is made of granite and the ones I have bought appeared to have been ground dead flat.
regards David
 
 

Edited By David Clark 1 on 15/02/2012 10:02:00

maurice bennie15/02/2012 14:11:27
164 forum posts
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Hi Woolfie ,Have you checked that your milling spindle is at right angles to your cross slide I did not and it was out. A very small amount makes a lot of difference .
Best wishes Maurice.
Tony Pratt 115/02/2012 17:12:32
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Andrew, sorry I totally disagree, a plate can have 2 parallel faces but still not be flat. Quote "Only straight lines and planes can be parallel" I would say both sides of a plate are planes?
Tony
fizzy15/02/2012 18:55:37
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one circle in the middle of another has pallel edges, not very straight though
Tel15/02/2012 19:38:05
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Agreed, but if the part is bowed then by definition the two sides are not parallel, even if the distance between them is the same. Only straight lines and planes can be parallel.

Ah, so the Earth is really flat after all then? I knew them crazy 'Round Earthers' were wide of the mark!

Andrew Johnston15/02/2012 20:29:56
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Indeed both sides of a part can be planes. If the planes are a constant distance apart over infinity then the planes are parallel. By definition a plane is flat. If the planes are not a constant distance apart then they are not parallel, although they are both flat. If the surfaces are not planes, but are a constant distance apart, then the surfaces are neither flat nor parallel.
 
To take Nigel's example of two concentric circles; I agree that two points, one on each circle, and on a common radial will be the same distance apart anywhere around the circle. However, they are not parallel. The tangents to the circles at the two points will, however, be parallel.
 
It all boils down to basic Euclidian geometry; which I suspect is a step too far from the OPs original question!
 
Tel: Sadly the earth isn't flat, but is based on spherical geometry, which is a non-Euclidian geometry in the which the 2D surface happens to be a sphere.
 
Regards,
 
Andrew
jason udall15/02/2012 20:56:28
2032 forum posts
41 photos
Rhum ?
lines of latitude are parallel
lines of longitude are parallel BUT MEET
---- spherical geometry
Again we digress.
Simple geometry surfices in workshop...
These digressions don't answer the op.
The Merry Miller15/02/2012 21:32:27
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484 forum posts
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Don't confuse flatness with parallelism!!!
 
If you are going to say two sides are parallel then you must quantify it with a tolerance as in normal geometric tolerancing.
Taking it to extremes, the two faces can be parallel to whatever as long as you specify it.
 
Refer to the extract below.
 

 
Len. P.
 
 
Ian S C16/02/2012 00:11:01
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7468 forum posts
230 photos
One of our first exercises in engineering when I joined the RNZAF was filing, we wre given a bit of hot rolled steel (when Dad was in the RNZAF war time, they used brass), we filed one side flat to the surface plate, the sides at right angles, to within a certain tolerence, ?miniuts, I forget. The other side then filed flat and parallel to within .002". For the test actual dimentions were not required. Proberbly a bit OT. Ian S C
Ady116/02/2012 00:20:52
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parallelism? Three words IMO
 
buy
a
shaper
Tel16/02/2012 06:18:34
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Posted by Ady1 on 16/02/2012 00:20:52:
parallelism? Three words IMO
 
buy
a
shaper

No argument here!

Ian S C16/02/2012 09:23:05
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7468 forum posts
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It'll be flat, unless you are using cold rolled(BMS), and you only do one side, leave it like that and there is a possibility of it bowing because of the internal stress, caused by the rolling, just a little trap that can happen. Ian S C
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