|Home » Forums » CNC » Message||Login|
You are not logged in. Consider these WOODWEB Member advantages:
Economic lot size10/28
I'm basically a CNC job shop. Primarily carcasses and MDF doors but also a lot of random stuff. My batch size is typically limited to 14 cabinets but that's an arbitrary number based only on some racking that I built that will hold the parts for 14 cabinets.
While people keep suggesting that the highest efficiency is attained with a batch size of 1, the reality is that that is not reasonable with a nested base CNC.
Has anyone done any work on what is the smallest lot size of mixed cabinets that makes sense in this application?
First thoughts are that all all parts that need some sort of primary edgebanding (bottoms of upper cabinets for instance) could be done in one batch. All cabinets that involve drawers in another etc.
Being a CNC job shop I don't think that line of thinking applies in the way you are going about it.
A batch size of one cabinet makes sense if you are building the cabinets because additional processes can happen as the second is being cut and flow will be improved.
Your job size might be the customer order, or the type of order. If what you are cutting varies in my experience the bigger the nest the better the yield. If you looked at your production it terms of flow what makes product flow through the shop the quickest, the smoothest with no piles of parts sitting around for days, limit the work in process to what makes sense.
Have you read The Goal?
Should have mentioned that I do assemble carcasses as well as cut them.
I did read The Goal. A lot to think about.
It is a lot to think about and it is a moving target. When you fix one constraint another pops up. We are currently struggling to fix our latest constraint. We added additional CNC capability a couple years ago and it really changed the playing field. It was for the better but it is a lot of work to get every bit of efficiency you can.
The easiest way to look at it is where are parts piling up? Why is that? Fix it and move to the next one.
We do CNC batches, nested is not a one cabinet batch. Our software can be set to try and nest things sequentially by cabinet so you can effectively lesson the batch size. That does not make sense for us but it may for you. I love this kind of stuff, it really effects the bottom line and it is a puzzle that constantly needs to be solved.
We have software that allows us to specify the maximum amount of sheets to finish cutting a cabinet. I've found on really large jobs (100 sheets) I'll use maybe three extra sheets when specifying a 6 sheet run. But being able to start assembling after the sixth sheet is cut is well worth the extra three sheets. Plus it keeps parts from piling up and really speeds up production.
I subscribe to the flow model. Our software can be set to try and complete all the parts for one cabinet as soon in the nests as reasonable. Most of the time, if the job is large enough to stack cut, (parts with no detailing) selves are taken out of the nest and cut on the panel saw, Since doors are normally cut from a different material they are nested after the cases. What limits the batch size is the conveyor space available ahead of the machines. As it turns out everything down stream from the router can process parts quicker than the router. A new more productive router is next on the wish list. There is 40' of conveyor @ the router, 2 lines of 20' each. When the first 20' fills the bander starts. Since not all the parts through the bander require bore & insert that station can be manned by one but a 2nd is required @ the case clamp. They help each other. It is really easy to see where a bottle neck is forming and shift labor. The employees do that without the need to be told. They can see it is needed. At the end of the shift there are normally parts for each station to start on the next day. Our conveyor system is not a straight through line. There are transfer cars that can move piles to temporary holding locations or to the assembly benches for curved work. Parts are sorted to the conveyors as they come off each machine so by the time they get to the case clamp complete cases are in batches of one. Same with the conveyors to the assembly benches. The conveyors solved many issues, not just moving parts.
It sounds like your primary justification for batch size is material yield. That's hard to argue with.
According to the lean guys the root cause of most manufacturing problems is building something before you need it. This is similar to efficiently collecting all the water you will need for your three day hike in the woods just to save the resources it would take to go back to the stream. You may have been efficient at water collection (material yield) but now you have the extra costs associated with hauling it around.
You might take a look at a book by Mark Woeppel called 'Manufacturer's Guide to Implementing the Theory of Constraints'.
Building before we need it is a rare possibility here. We always seem to be scrambling to get it done on time. The conveyor system has done far more than move piles of parts. It limits batch size, it provides visual feed back so simple that employees can change functions w/o having to be told. There is enough space to sort as work comes off each machine. The sort can be for next operation & for parts by case. As parts come off each machine they are grouped closer to the final sort of one case in the order needed for assembly. The final sort happens after bore & insert machining when the parts go into a rack holding all the parts for a case in one slot. Up to that point the parts have been in flat piles on the conveyors. When they are put into the rack they are now on edge so they can be handled in the order needed for assembly. As much as possible the sort involves where to put down the part that you are taking off a machine. No separate operation/handling.