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Threads in fixtures?

trochoidalpath

Cast Iron
Joined
Jan 17, 2016
Thanks for the feedback everybody.

I think my plan is to use 4140PH for the non-sacrificial risers. It's roughly breakeven with aluminum after I deal with getting it to and from anodizing and getting inserts in there.

Now to figure out how much I can safely pocket the underside and lighten things up!
 

Bobw

Diamond
Joined
Feb 8, 2005
Location
Hatch, NM Chile capital of the WORLD
Lightweighting like that doesn't increase stiffness, it actually reduces it. It's generally advantageous because it reduces weight much faster than it reduces stiffness, if done well.

Exactly. Its a trade off. The solid is always going to be best if weight is not a consideration.

A bar doesn't get stiffer when you drill a hole down the middle.
 

boosted

Stainless
Joined
Jan 4, 2014
Location
Portland, OR
What? A tube can be much stiffer than the same diameter and material solid.

Is there a shop equivalent of an "old wives' tale"?

It just seems that way because the tube has a much higher strength/weight ratio. Like trochoid said, if mass is the same, then the tube is much stiffer.
 

Nagol

Aluminum
Joined
May 21, 2020
I considered getting fancy like that with mine for weight reduction but I decided to not over think it. 4" thick 20" square table riser. 20201130_171726[1].jpg20201125_100718[1].jpg Drilled 1" holes with an indexable insert drill 3" deep between the 2" grid of holes on the opposite side. I figure it took about 80 lbs out of a 400 or so pound fixture. Think of it as a 12' long bar of 1" stock.

(note, not actually the same part. Made some 2" thick ones for our Variaxis at the same time as the 4" thick ones for our horizontal. Same grid pattern on the fixture side of things. Needed 4" thick on the horizontal to bring the table surface up to the bottom of travel on the Y axis. No blocks needed for table work..)
 

Wsurfer

Cast Iron
Joined
Apr 1, 2013
Location
Austin Texas
Image for comparison..

Steel 3" bar 24" long - 48.1 lb
Steel 3" tube 24" long, .25 wall thickness - 14.7 lb


FEA on steel bar Vs Tube

barstress.JPG
 

Rick Finsta

Stainless
Joined
Sep 27, 2017
I did some digging and it looks like I may have been confusing "stiffness" with "resistance?" I'm not convinced that I wasn't just 100% wrong since I haven't dealt with any of those measurements since physics classes about twenty years ago! Thanks for the corrections.

I may play around with FEA over some beers tonight since I'm totally unsupervised for a few days (i.e. the wife is out of town).
 

Garwood

Diamond
Joined
Oct 10, 2009
Location
Oregon
I've had to remake a few fixtures where I thought a weldment would work, but it was too flimsy and went to solid.
 

Madis Reivik

Hot Rolled
Joined
Jan 23, 2011
Location
Estonia
We usually do it like

1) Very few parts - aluminium fixture
2) Then customers wants some more - dammit, Helicoils to replace worn threads
3) Afterthoughts - should have made fixture from steel

But for rigid fixtures, aluminium is better - its 3 times lighter and the plate with 3x thickness is MUCH more rigid compared to steel 1x thickness. Also, it saves tools when you ran into fixture with aluminium cutting speed :)
 








 
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