How accurate is bar stock ? Should you order a larger diameter ?

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How accurate is bar stock ? Should you order a larger diameter ?

This topic has 8 replies, 7 voices, and was last updated 10 September 2014 at 20:54 by nigel jones 5.

Viewing 9 posts - 1 through 9 (of 9 total)

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6 September 2014 at 17:56 #162836
Brian John
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@brianjohn93961
If the drawings of a steam engine called for a 1/2 inch stainless steel piston would you order 1/2 inch bar stock or 9/16 inch and turn it down ?

Edited By Brian John on 06/09/2014 17:56:29
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6 September 2014 at 17:56 #3224
Brian John
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@brianjohn93961
6 September 2014 at 18:04 #162837
Tony Pratt 1
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@tonypratt1
Order 9/16" and turn it down, bar stock will have a tolerance on it for size but I wouldn't rely on it and also its not always round.

Tony
6 September 2014 at 18:06 #162838
JasonB
Moderator
@jasonb
For a piston of that size I would turn it from larger stock, preferably doing the final cuts once the piston is fitted to the rod to make sure they are concentric.

Having said that if your drawing call for a bronze piston you will find that bronze usually comes about 1/32" over the nominal size so you will be OK in that case but check with your supplier. If it is brass then you will either need to buy the next size up or machine your bore a fraction undersize. Stainless will be close to 1/" particularly if its ground stock but it won't allow you that final skim to get things true.
6 September 2014 at 18:49 #162845
JA
Participant
@ja
A word of warning. If the bar has been centreless ground it may not be round although it has a constant diameter! Its surface could be made up of three, five etc arcs. If you are confused measure the diameter of a 50p piece. For critical parts it is always worth rolling the bar on surface plate.

JA
6 September 2014 at 21:36 #162861
Boiler Bri
Participant
@boilerbri
I buy centre less ground stainless steel bar for linear motion, in twinned pairs and I have never had a probelm with it, however we do pay a premium for it.

for your piston I would recommend turning your piston from larger stock bar and if possible have it attached to the piston rod. You will have to true up your piston rod and then screw on the piston blank. This will ensure its concentric with the rod.

Bri
7 September 2014 at 12:37 #162902
Ian S C
Participant
@iansc
Brian, bore the cylinder first, then turn the piston down to fit the bore, it's easier to adjust the size of the piston to suit the bore than the other way round, and if the bore is a few thou one way or the other, it's not really the end of the world.

Ian S C

t
10 September 2014 at 20:51 #163311
nigel jones 5
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@nigeljones5
As stated above, but allow me to stress….ensure the piston is on the rod before even getting close to final cuts, ive had many a miserable time trying to true them up by doing it the wrong way!
10 September 2014 at 20:54 #163312
nigel jones 5
Participant
@nigeljones5
If your bar is a nats undersize the method I use is to get it red hot then quench it. Provided its a decent diameter it wont distort and is will 'grow' that precious thou! Very useful for getting wheels tight on axles.
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