Speeding Up Production Cutting Plywood Parts

      Thoughts on ways to make production faster on a CNC and on the limits to that. June 22, 2010

I have hired a guy to do a production run of plywood parts on his CNC. The parts are cut from 3/4 unfinished birch ply. He is new to CNC, so it is going a bit slower than I would like. We are looking for tips to speed things up. What are the best bits to use for cutting fast and clean? Can we stack the plywood and cut two sheets at once? All of the parts are identical and are just cut, no other machining. We want to cut 100 sheets per day and are currently getting half that. The router is an SCM with an 18hp spindle.

Forum Responses
(CNC Forum)
From contributor O:
Are the parts just squares/rectangles or do they have radius and other irregular shapes? If they are just squares, find someone with a beam saw and you will easily hit your target per day.

From the original questioner:
We are using the CNC to cut irregular shapes. There are some square pieces and we have a beam saw for that.

From contributor O:
You could try a second spoilboard and just slide the first one off and unload it while the second one is cutting. But depending on how your matrix table is gasketed, this can be more of a pain than a help. As far as stacking sheets, that could be asking for trouble. Can the vacuum provide enough hold down? The last thing you want is moving parts.

From the original questioner:
I was thinking we could leave a little tab on each corner, but wouldn't that slow down the cutting? We have an offload table and a rake, which helps. I think the answer is using the right tooling and feed rates.

From contributor C:
A shuttle board would work great as long as you have enough area to run out the parts in long cuts. If these parts are comprised of many arcs, your machine will never get up to maximum speed before it will slow down due to the curves. You can modify the settings to allow for shorter curves without slowing down the acceleration or deceleration, but be very careful as you do not want to mess up the breaking resisters by reducing the acceleration and deceleration of the router. A shuttle board is the way to go for change over - just use a piece of 1/4 inch MDF. That will work, as we do this all the time and attain anywhere from 100 to 150 sheets per 8 hour shift.

From contributor B:
Are you cutting these in one pass? If so, what is your feed rate and spindle speed?

From the original questioner:
Yes, we are cutting in one pass. I will have to check on the feed rate and speed.

From contributor M:
I would address the issue from a few angles, with my primary focus on machine idle time.

First, are the programs written in a manner that takes into account machine configuration in order to maximize its utilization? How much time is spent unloading and loading and is the machine milling while this is happening? For example, we have a Shoda that has 2 heads and 2 - 60" x 60" tables. If one sheet of plywood is loaded, the tables are run tandem, and when the program is finished, the machine sits until the tables are loaded and unloaded. If your parts allow, consider first cutting the sheet so one table can be loaded and unloaded while parts on the other table are being milled. Also, we run both heads at the same time, and produce two parts in the time it takes to mill one.

Second, what is the limiting factor in regard to feed speed? My guess is vacuum. Consider increasing feed speed at the start of the program and reducing at the end (when hold down is more compromised). Cutting that much plywood also must take a toll on the tooling. We use a 1/2" upshear/downshear carbide tool when through-cutting on our CNC equipment, and would probably have to change/sharpen the cutter at least every third day. Be sure additional tools are set up - if a tooling change is required during the shift, there is no down time. Also consider using 5/8" plywood. We are a fairly large furniture manufacturer using 5/8" birch and oak veneer plywood. We switched over from 3/4" to 5/8" about 5 years ago, improving stack heights on our panel saws, and providing opportunity for greater feed speeds on our CNC equipment.

How well are the parts nested? Consider using a panel locator position (we used dowel pins) on the table in order to eliminate straight cuts down the edge of the plywood (assuming the plywood is coming in with clean edges).

From contributor P:
I don't think the guy being a newbie is the problem. It sounds to me like he is doing pretty good. Let's say it takes 10 minutes per sheet to load and unload - that's 500 minutes for 50 sheets. There are 480 minutes in 8 hours. And he's doing 50 sheets in 8 hours. You might be able to shave a minute here or there, but there's no way to get 100 sheets in an 8 hour day unless you can get 1 sheet cut every 5 minutes. At that rate, the CNC operator will not last very long. You will simply need to run 16 hour days until you meet your needs.

From contributor I:
If you want 100 sheets a day from a router, you'll either need two machines/operators or to run two shifts. Tweak all you want, you'll never do better than 50 sheets a day in 8 hours. In fact, that's moving pretty quick.

From contributor N:
Since we have no idea what size the parts are, or the geometry, guessing is all that is being accomplished here. How many parts are coming out of a sheet and what size is the sheet material?

From contributor J:
You could stack 2 sheets on a router that has a roller hold down system. There are 4 large pneumatic rubber coated rollers that rush the stack down with standard vacuum hold down on the bottom. Unless you run 16 hours a day, that's the only other way on one machine.

From the original questioner:
They are 4x8 sheets of 3/4" birch plywood. There are two different types of parts. Some are shaped like an A and are nested in three rows, 20 pieces per sheet. The other parts are shaped like a U with three square sides, nested into 10 parts per sheet. There is an ogee shape on the inside of the U.

From contributor R:
For pure speed, try an Onsrud 60-053. It leaves a very slight ridge to the edge. For fast and clean, try a 60 353. Both of these are full upcut 3 flute. I use them very successfully on shop grade ply for diewall ribs and plates, for example.

They are also available in compression configurations and 4 flute, if your machine can push and accelerate fast enough to make 4 flutes work. The compression style will slow you down slightly but will not chip the top of the sheet.

For what it's worth, 50 sheets a day is not terrible if your run times are in the 8 minute range. That leaves a 2 minute switch. A one minute switch is very doable with some thought and effort.

From contributor N:
Based on your last post, I am guessing 50-52 sheets in 8 hours is the most you can generally achieve, cutting 1 sheet at a time. I would guess the average in 8 hours would be closer to 45.

From contributor E:
We use Router-CIM advanced nesting and it makes a stay down nest that eliminates the rapid travel from part to part. This can cut the cycle time up to 20% depending on the number of parts in the nest.

From contributor I:
I agree with contributor J. Cut multiple sheets at a time, or if the machine can't do it, ask the guy you hired to buy another router with twin tables so the machine will run non-stop between the sheets.

From contributor Y:
I'm having the same kind of troubles cutting 1/2 inch Baltic birch. I have a Thermwood 40c runs at 18,000 rpm and has 10 hp. I'm using 3/8 inch compression bits to rough out the shape and a 1/4 compression to clean up and cut into the corners.

I would like to go faster than the 200 ipm that Thermwood recommends, but I have been breaking bits left and right. I finally figured to cut the part leaving the finished piece on the left, and that has helped with getting cleaner edges.

My question to the group is
1) Is 200 ipm all I should expect?
2) Should I just use a 3 flute 3/8 inch up cut for the roughing, or is one of those chipper bits a better tool?
3) Should I expect more that 24 hours of wear per bit?

From contributor G:
If you have enough business that requires you to do 100 sheets per day, why not lease another machine? It's just simple logic, if you want to make (more) money you need to spend (more) money. Nothing comes for free.

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