mass of solid bar

newby.204

not an engineer but thought someone here might be able to help i need to find the mass of a solid steel bar. diameter 80mm height/length 100mm
if any further info is needed let me know because im not sure what info is needed to do the calculations

cant just throm it on a scales, as the variables above will change

godtabh

Do you know its density?

Its parameters should change if its solid

newby.204

no i dont know the density of the metal, cock thought i might need that alright

godtabh

Best you can do is estimate it's weight. check google out

Gurgle

newby.204 wrote: »

diameter 80mm height/length 100mm
...cant just throm it on a scales, as the variables above will change

How will the height / length change ?

Alright, google could have told you this: Density of plain mild steel is 7.85 g/cm3.

Change the dimensions to cm, calculate the volume (Volume of a cylinder, I'm not going to do it all for you) multiply volume by density and you have the mass.

newby.204

making collars, lengths(heights) will change as the weights of the collars vary... cheers lads

mawk

its about 15.75kg at 80*100

but what do you mean you cant just weigh it as it changes the variables?

Gurgle

newby.204 wrote: »

making collars, lengths(heights) will change as the weights of the collars vary... cheers lads

Ah, that explains...... well, nothing.

What?

Turbulent Bill

Gurgle wrote: »

Ah, that explains...... well, nothing.

What?

I'll make a stab and say that the OP is trying to design 'weights' for a barbell or something similar. Therefore the size of the 'weight' is driven by the required mass, the material density and whatever geometry is constrained (the inner diameter of the 'collar', for example).

Could be completely wrong though, who knows...

newby.204

Turbulent Bill wrote: »

I'll make a stab and say that the OP is trying to design 'weights' for a barbell or something similar. Therefore the size of the 'weight' is driven by the required mass, the material density and whatever geometry is constrained (the inner diameter of the 'collar', for example).

Could be completely wrong though, who knows...

correct and right , micro plates to be exact, weighing 125g, 250g, 500g, 750g and 1kg!! the shaft they have fo fit on is 55mm and the largest diameter i can machine is 75mm, therefore they become collars as appose to plates with the length varying with the weight, however with the variance in densities between 7.715g/m3 and 8.05g/m3 its hard to be exact as they have to be within .05g+/- of the nominal weight

Mellor

Why don't you just weigh the piece of metal you will machine them from, and get it's exact density, it becomes quite easy then.

Machine to with a few grams, and remove the last bit by hand, checking each piece as you approach its goal weight

newby.204

thats one way of doing it mellor but i have only so much metal on hand that is the required OD, and i dont want to waste it!!!!

mawk

you doing this in a shed type environment?

im going to go out on an unconstructive limb here and say that you are quite simply NOT going to get it +/- .05g

thats.. awfully precise. in fact, youre not going to get to between .05mm

I make SUPER regulated medical devices for a living, and even they vary quite a bit. if youre not using a hideously well calibrated cnc mill... its not going to be close enough for your aims.

Gonzales

newby.204 wrote: »

correct and right , micro plates to be exact, weighing 125g, 250g, 500g, 750g and 1kg!! the shaft they have fo fit on is 55mm and the largest diameter i can machine is 75mm, therefore they become collars as appose to plates with the length varying with the weight, however with the variance in densities between 7.715g/m3 and 8.05g/m3 its hard to be exact as they have to be within .05g+/- of the nominal weight

You could probably buy these standards weights off the ebay or from NSA Ireland. Save yourself a lot of headaches.

Gonzales

Mellor wrote: »

Why don't you just weigh the piece of metal you will machine them from, and get it's exact density, it becomes quite easy then.

Machine to with a few grams, and remove the last bit by hand, checking each piece as you approach its goal weight

Mellor is bang on there.

Density = Mass/ volume

Kg/Cubic meter = Mass of your bar / 0.0005024 cubic meters

If you need to maintain the OD then you could fine tune the weight using a drill bit or file to remove metal from the flat surfaces.

Next question : do you have a scale that is capable of these accuracies & calibrated?

good luck with this, I've I funny feeling, that this has not helped at all at all.:)

Turbulent Bill

newby.204 wrote: »

correct and right , micro plates to be exact, weighing 125g, 250g, 500g, 750g and 1kg!! the shaft they have fo fit on is 55mm and the largest diameter i can machine is 75mm, therefore they become collars as appose to plates with the length varying with the weight, however with the variance in densities between 7.715g/m3 and 8.05g/m3 its hard to be exact as they have to be within .05g+/- of the nominal weight

If you can't buy standard weights that will fit on the shaft (Gonzales' suggestion is good), then machining custom weights to this spec is going to be very difficult (if not impossible). Weight tolerances are usually given as a percentage of the nominal weight (e.g. 0.1% or 0.05%) rather than an absolute value as the manufacturing and measurement tolerances scale with weight. Find the weight range you will have to measure (e.g., 200-1700g) and the measurement accuracy of your scales in this range, then tailor the manufacturing tolerances to suit these.

If the density variation is still a problem then I'd consider making semi-solid weights. Machine the weight to a lower mass than that required, and incorporate a 'pocket' in the design for holding lead shot, sand etc. That way you could gradually top-up the weight to bring it to the required mass.