CNC Machine Build Quality

      A shop owner's considering a CNC purchase wonders why one machine weighs more than a similar competitor. But as he learns, quality construction depends on more than weight. October 18, 2005

Question
I am interested in opinions on the weight of CNCs and how this relates to productivity. Running typical nested based cabinet parts, is there any real advantage to a machine that weighs 10,000 lbs versus 6500 lbs? Both machines are all steel. The biggest difference I see is about $35K or a buck a pound for more steel. I'm cutting .750 IPB and MDF. At the end of the day, am I going to have more parts? What's your guess on a percentage basis?

Forum Responses
(CNC Forum)
From contributor O:
There is more to the machines then just the weight. For MDF and particleboard, the power of the spindle and rapid travel will have a greater effect on the number of parts you can put out. What manufacturers are you considering?



From contributor R:
I'm confused. What are the mechanical differences? Are you saying hp/imp capabilities are identical, support, software, tool changing abilities, etc. - all the same? I've never had my production limited because of the weight of my machine. Operator, software, capabilities - these factors affect the bottom line. A heavier machine would cost you more in shipping, but more parts at the end of the day? If you are manually pushing the head, you might get more done with the lighter machine.


From the original questioner:
The machines I am referring to are both fully equipped. The spindle HP, cutting speeds and rapids are very similar. Both machines use very good components. I have done my homework as far as comparing the specs. I prefer to leave the name of manufacturers out of the discussion. I fully understand the importance of the support, training, and especially the software.


From contributor R:
I have never heard of anyone having a problem with a machine due to weight. I would exclude that from your decision. The difference just may be due to the manufacturer's quality. Look at product reliability. (I'm sure you already did, having done your homework.) The price may be due to a solid history of support and performance. If all these things have been considered, I think you should just pick the one that seems to give you the right balance of what you need. I have never heard of weight being a factor, either good or bad, but a very interesting question…


From contributor T:
My common sense tells me that a heavier machine will vibrate less, and give a better cut quality.


From the original questioner:
I agree with your thought, but we're talking about 35K. Don't forget I'm cutting MDF and IPB, .750" and thinner.


From contributor W:
A heavier machine will vibrate less, and generally have less tolerance issues. This will affect you even with only running 3/4" material. Many lighter machines are not designed to operate 24/7, while heavier machines are.


From contributor C:
The weight difference can be due to moving tables versus moving gantry. Moving tables are more rigid. Lightweights can have some issues over time with sag in the gantry. Under high load, a light duty gantry can have torsional movement, leading to poor accuracy. Have an operator tip over a sheet of 3/4" MDF and let it drop 60 or 70 times a day. You'll be glad there is some steel under the table.


From contributor J:
Typically, the weight issue comes from a salesman of a machine that is not feature rich or well designed.

A machine with hollow steel tube construction will weigh less and flex, vibrate and move more than a machine with a well designed gusseted steel frame. The machine with the heavier, gusseted steel frame is also probably stress relieved and CNC milled. This is a much more durable and stable platform.

Even though you are only cutting .750 IPB and MDF, the benefit of an accurate and fast CNC is that all parts are cut to the exact size so that they assemble much more efficiently than parts that are not square, out of size tolerance, etc.

A well built CNC should have a well designed (typically using finite element analysis) frame made to support the table and gantry. Some manufacturers will stretch or shorten one machine's frame to build another model and cover the gaps with plastic. Others use hollow steel tubes and cover the ends with plastic plugs.

A particular machine may be heavier than another because of additional gusseting for added strength or from using a heavier gauge hollow steel tube to dampen vibration and movement. You need to look at the design of the frame to really know.



From the original questioner:
Thanks for your input. Good stuff. The machines I am referring to are both moving gantries. I did some more research on the 6600 lb machine (the one for less money). The frame is 1/2" thick tube, welded into one complete piece, stress relieved, then CNC machined in one operation. They don't bolt anything together. The steel tubing does have gussets welded on the ends of all of the tubes. The heavier machine can only claim 3 of the above features. It may be obvious which way I am leaning.


From contributor J:
A couple of other things to put a lot of your decision weight on:
1. Helical rack and pinion on the X-axis - ball screws do not perform well in the 10' to 12' lengths needed in the x-axis, suffering from whip, chatter, vibration, noise. Precision ground ball screws are acceptable for the Y-axis (approximately 5' long).

2. Vacuum pump - for a nested based flat table, carbon ring pumps are not a good choice (they're the best choice for P2P pod machines!). Nested based machines pull a lot of dust through the vacuum system, which can damage the carbon rings, leading to higher maintenance costs. Select a liquid ring vacuum pump for a nesting machine (change the oil every 5 years).

3. Choose a bakelite table over aluminum. Bakelite is easier to repair, stays flat during temperature changes (compared to aluminum) and is probably cheaper for the manufacturer to produce.

Regarding gusseting in the steel tubes, I recommend you evaluate how the manufacturer designed and analyzed the structure. Did they use finite element analysis? Was the frame modified from another model? Is there a lot of plastic covering the frame (especially where it supports the table)?



From the original questioner:
If I run a solid carbide .375" tool cutting .750" thick IPB and MDF (typical cabinet parts) at say 800IPM, what is your guess as to how many sheets I would get out of a tool before resharpening? Is resharpening a good idea? What configuration of tool would you use?
Thanks again for everyone's help. You have helped make this tough decision an easier one.


From contributor J:
Most shops run a 1/2" mortise compression for cutting 3/4" melamine. The 1/2" tool seems to have the longest life (minimal breakage). Smaller diameter bits tend to break at higher speeds or limit the cutting speed. The cost difference between a .375 and .5 bit is small compared to cycle time and yield. Yield does not suffer using a 1/2" bit compared to a .375 bit for most cabinet parts.

Most folks cutting cabinet parts will load a couple of 1/2" mortise compression bits (for cutting parts) and a 1/2" downshear bit (for cutting rabbets and dados), a 1/4" downshear (for cutting dados for backs), a .375 or .5 ball nose (for flutes, pocketing and engraving) and a flycutter for surfacing the spoilboard.

Depending on the speed of machine you select and the HP of the electrospindle (make sure the electrospindle is S1 rated for continuous duty; a cartridge-type electrospindle is also recommended for easy replacement), I would recommend a 3 or 4 flute 1/2" mortise compression bit for the majority of the cutting. The salesman should be able to calculate the speeds (based on number of flutes, material being cut, RPM, IPM) to verify that you will get the throughput you need.

You should be able to sharpen a good quality bit about 3 times, cutting between 20 to 40 4x8 sheets between sharpenings (your mileage may vary, depending on material composition, carbide quality, tool geometry, operating speeds - too slow is worse than too fast).



From contributor R:
800IPM is way fast for me to be cutting. Do you other guys machine sheet goods that fast? I've cut holes at 700IPM and they are not always round. I usually cut around 350IPM, but I am cutting for prototype work, not production.


From contributor A:
I cut 1000ipm .75 plywood, nesting cabinet parts with 2+2 compression bit and was wondering if I should change to 1250ipm with a 3+3 compression. I have tried this and it works okay, but I don't want to abuse the machine. I don't know if it will hurt or not, but I have never cut slower than 1000ipm.


From contributor D:
I cut 3/4" sheets at around 950. Higher speeds are not unheard of. A good, heavy, high horsepower machine will do it easy. I use 1/2" 2 flute compression bits for cut outs. No problem at those speeds.


From contributor M:
Depending on the core and part sizes, I cut 800-1200ipm with a 3 flute 3/8" diameter mortise compression and see a life of 30-60 sheets before changing tools. Sometimes there is slight tool deflection, depending on the core, which requires a slower feed or 1/2" diameter tool.

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