How Much Stock Is Required?

Maybe I'm inverting a meaning here but why would fabricating small pieces require less extra material than large piece fabrication. I would expect more kerf due to the larger number of cuts of the initial stock length. Is it because the remainder of the stock piece can be a larger (but still undersized for the next part) piece of stock.

Hi Redfred. It would be related to the mass of the remainder and the Greater significance of any lost parts due to nonconformances while setting up and getting to first piece. Milo

It's probably me but I still don't get it. Allow me to demonstrate my confusion with an example. All dimensions I'm providing are approximate but this is intended to be a set of precise components to be fabricated.

Two designs of custom parts must be fabricated out of 2m long, 1 and 2 cm diameter round bar stock. (Like your picture.) The 2 cm diameter material will be fabricated into ten intricate widgets from rods about 80 cm long. The 1 cm stock will be fabricated into forty doodads from rods about 13 cm long. The saw that cuts the raw bar stock has a kerf of 1 mm. Assuming that only full length bar stock is available then only two widgets per stock unit and fourteen doodads per stock unit can be made. So if no spares are required or fabrication mishaps occur then five 2 cm bars and three 1 cm bars are required for this fabrication. Five 39.8 cm widget remains will be put into the scrap bin. The doodad remains will be two 16.6 cm pieces and one 42.8 cm piece. The widget remains at 19.9%. The doodad remains at 12.7%.

All of my fabrication numbers come to a higher percentage than you posted but the smaller part has a smaller remain than the larger part. Now if the smaller part total order was an exact multiple of the pieces per unit stock length then a smaller 8.4% remain would be possible.

Further precise machining of these rough cut rod segments will only increase the amount of excess material and thus raise these percentages.

I suspect that I'm looking at this in the wrong way but obviously I do not know where.

Bit late in the day for my brain, but when I read Milo's blog entry earlier I was also a bit lost. I'd been playing chicken and waiting for somebodey else to ask (). I'm sure all will become clear in time, but I'm still scratching my head for now.

Can you break it down a bit, Milo ? It's maybe very clear to those who work in specific areas, alas not me. Perhaps an example at either end of that rule of thumb range ? Without reference to the length of bar, I'm not clear on where these figures are coming from. It should be very basic stuff, but I can't grasp it on the info given.

Hi again Redfred. I'm on some pain meds right now so i won't be doing any arithmetic.., so in the situation that you describe, of course you can calculate the material needed. My rule of thumb is based on production of volumes of parts -for instance on automatic screwmachines. I'll do some back of the envelope calculations when I get back on my feet. Milo

Take your time. Get better my friend.

I think where your looking at this is using a specific length of stock to make the pieces. Instead of totaling up all the parts plus all what was talked about then finding the best match of stock in whatever lengths or multiples of available that will minimize waste.

The closer you get to using the exact amount plus the extra to fit stock lengths the less waste you will have.

example: using a "stud length" 2X4 instead of using a normal 8'X2X4 and having to cut the excess off all the 8'X2X4's to allow for top/bottom plates. This saves materials and labor. Even if the stud length cost a few cents more.

I know this is a little different example, it was just off the top of my head example.

Charles

Exactly. The bars loaded into the machine will be if uniform dead length. Thanks. Milo

I do machine work and the way I calculate for stock length (lathe work), is to add together the number of same items. I then add a minimum of 1/4" for cut-off between each two items and then add at least 2" for chucking. Depending om my proficiency at the lathe, I may also add for one or more items for mistake making.