Voicecoil
10-10-2018, 10:21 PM
So, I'd worked out roughly what sort of machine I need to build and had done some basic sums using both the excellent spreadsheet @routercnc provided and some other calculations and was just about to order in some great big lumps of profile. It seemed clear that for the gantry, torsion (twisting moment) was going to be the dominant source of cutting tool movement, kind of obvious when the tool tip is down near the bed as the length of the Z axis acts a a lever. I'd assumed that the performance of a piece of rectangular box section and a profile of roughly the same dimensions would be fairly similar as the moments of area (Ix, Iy) for normal flexing do seem to roughly match up weight for weight, so had happily gone ahead assuming I could plug values into the spreadsheet and get meaningful tool deflection figures for profiles.
THEN, I happened to come across the Item24 website, those guys are possibly the Nirvana of aluminium profiles, they have something to solve pretty well every problem ina very efficient german way. And, uniquely they state the torsion constant (J) for most of their profiles (basically how hard it is to twist it). AND the figures are very different (i.e. a LOT smaller) than the equivalent sized rectangular tube. Thinking about it now, it kind of makes sense, as playing around with anti-roll bars taught me that to get a stiff tube, you need to get as much thick, continuous metal as far away from the centre of rotation as possible. Whilst all the slots might add or certainly won't detract much from flexural stiffness, profiles only have a continuous "tube" at the bottom of the slots, a fair way in from the outer periphery, and the bits at each corner (where the stress is concentrated in a rectangular tube) do practically nothing to add torsional strength.
Anyway, here's a little comparative table of profiles and rectangular tubes: the profiles are all Item/KJN/Motedis patterns (they all are the same sort of shape and have similar Ix,Iy within a few %), stated prices are from whoever was cheapest when I looked. Prices for box are from my local stockholder, there's obviously money to be saved as well as gaining torsional stiffness :beer:
Profile
Moment of Area x
Moment of area y
Torsion constant
weight
cost per metre
Item 80 x 120 lite
201
422
165
8Kg
£55
Item 80 x 120
273
575
261
11Kg
£81
Item 80 x 160 Lite
270
919
250
11Kg
£77
Item 80 x 160
362
1232
397
13.5Kg
£101
Item 120 x 120
799
799
578
12.4Kg
£158
80x120x6.75 tube
251
482
498
6.9Kg
£48
2"x6"x1/4" tube
104
642
287
6.7Kg
£43
3"x6"x3/8" tube
352
1152
853
10.9Kg
£58
70x80x4.4 tube
191
860
493
5.9Kg
£39
80x200x6mm tube
360
1570
922
8.8Kg
£56
100x200x5mm tube
515
1522
1183
7.8Kg
£56
You can probably guess what I'm going to be using for my gantry
THEN, I happened to come across the Item24 website, those guys are possibly the Nirvana of aluminium profiles, they have something to solve pretty well every problem ina very efficient german way. And, uniquely they state the torsion constant (J) for most of their profiles (basically how hard it is to twist it). AND the figures are very different (i.e. a LOT smaller) than the equivalent sized rectangular tube. Thinking about it now, it kind of makes sense, as playing around with anti-roll bars taught me that to get a stiff tube, you need to get as much thick, continuous metal as far away from the centre of rotation as possible. Whilst all the slots might add or certainly won't detract much from flexural stiffness, profiles only have a continuous "tube" at the bottom of the slots, a fair way in from the outer periphery, and the bits at each corner (where the stress is concentrated in a rectangular tube) do practically nothing to add torsional strength.
Anyway, here's a little comparative table of profiles and rectangular tubes: the profiles are all Item/KJN/Motedis patterns (they all are the same sort of shape and have similar Ix,Iy within a few %), stated prices are from whoever was cheapest when I looked. Prices for box are from my local stockholder, there's obviously money to be saved as well as gaining torsional stiffness :beer:
Profile
Moment of Area x
Moment of area y
Torsion constant
weight
cost per metre
Item 80 x 120 lite
201
422
165
8Kg
£55
Item 80 x 120
273
575
261
11Kg
£81
Item 80 x 160 Lite
270
919
250
11Kg
£77
Item 80 x 160
362
1232
397
13.5Kg
£101
Item 120 x 120
799
799
578
12.4Kg
£158
80x120x6.75 tube
251
482
498
6.9Kg
£48
2"x6"x1/4" tube
104
642
287
6.7Kg
£43
3"x6"x3/8" tube
352
1152
853
10.9Kg
£58
70x80x4.4 tube
191
860
493
5.9Kg
£39
80x200x6mm tube
360
1570
922
8.8Kg
£56
100x200x5mm tube
515
1522
1183
7.8Kg
£56
You can probably guess what I'm going to be using for my gantry