Drawing Units and Precision

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Deciding on the level of precision needed for cabinet and millwork design. October 20, 2004

What is the best way to approach drawing units and precision for cabinets and architectural millwork?

I came to detailing by way of 25 years of on-site work, with stints in cabinet shops. Everything that I've built was detailed by me, either by hand or on the computer.

I'm curious as to how people approach this. My initial inclination was to set Unit Precision to 1/16", and turn Snap On at the same precision. Occasionally I would dial that down to 1/32 for both or turn snap off. But I have since discovered that some detailers prefer to set the Precision to 1/64, or 1/128 and leave snap off. What are the advantages/disadvantages of both? I imagine you still leave the precision of your dimensions set to 1/16.

Anyone with strong opinions out there? I would be interested in hearing them.

Forum Responses
(CAD Forum)
From contributor J:
When I am making conference tables with inlays or radius hardwood edges, or inlay laminate pieces, Ill drop to 1/128 for the precision.

I keep my cabinets set to 1/32 just to be precise because Im used to pulling the parts out and setting them up for CNC programs like AlphaCAM, RouterCIM, and CadCode. With custom cabinets, sometimes its unavoidable to have a unit with 32nd dimensions somewhere but it all depends on the situation of the area that the unit is going to be installed. If you are working with melamine and MDF, you want to be as precise as possible because this stuff grows enough on its own after it is cut, depending on the temperature and moisture conditions in the shop. I also try not to send drawings out to the shop with a lot of 64ths or128ths on it because it's easier for the guy on the bench to make a mistake. But I will put up with the complaints if it is necessary for the job to come out right.

This works for me. Not knowing your situation, its difficult to give proper advice. You should take a good look at what would be a good setting for you and the shop personnel. Everyone has different taste and you should be comfortable and confident with what you are drawing.

From contributor A:

I set my unit's precision as high as possible (1/256) and my dimension precision to 1/16". I don't like to see 32nds on a drawing unless it has to be there. The theory here is to be able to draw to the highest level of precision but have "sensible" dimensions. This will cause a problem if one does crappy drawings.

I had a CAD file from an architect in which - clear as day on his drawing - the dimensions said 2" + 3/4" +2 13/16". I looked into it and it was simply a rounding error because of his precision settings. Makes you look stupid, though. His drawing was "not exactly" 2 inches, and "not exactly" 3/4 inch.

One thing I don't want is to have to check dimensions and make sure they're right.

From contributor V:
I want to expand on an area that is often overlooked, but eventually has to be dealt with. Frequently when you draw something, the material you will be using has not been delivered yet. You might draw it as 3/4, but when it is delivered it may measure .8475 with a micrometer. Sometimes this matters and sometimes it does not. If you have 8 base cabinets going side to side in a run, with the construction of the bottom going in between the sides, you will be longer than you want. If you are placing another cabinet on top of this one, and it only has 5 cabinets, and both the upper and the base have finished ends that should be flush, you will need to be exact.

Parametric software handles this problem quite well. All you need to do is adjust the material thickness in the library and have your drawing updated. If you are drawing your cabinets and deriving your cutlist from the model or 2D drawings, then you need to either adjust your drawing to match exactly what your material measured or you need to adjust your cutlist after you know the exact thickness of your material. I tend to do the latter in Excel with a formula that does an if, then, else based on a value of 0, 1 or 2. I copy and paste the formula and it will add or subtract 0, 1X or 2X the difference as measured compared to what the actual material is. It is not too difficult to do, but you can make a mistake here pretty easy.

Another point on precision is how accurate you can really cut. Whether you are using a P2P, beam saw or table/panel saw you need to be precise, but how precise are you really? Try cutting a piece of hardwood or Plexiglas 10" x 3". Then cut 20 more pieces at 1/2" x 3. Now push the 10" piece up against a fence and put the 20 1/2" pieces on top and see if they match the 10" piece exactly. The only machine I have ever seen that can pass this test is a Mayer beam saw. This was right after turning on the air table and intentionally slamming 4 pieces of stacked 3/4 x 48 x 48 MDF as hard as I could into the computer fence, after the fence was already in position to make the cut. A saw must not only cut exactly right, but it also must fix itself exactly to specs on being slammed with some weight, if you are want to depend on what is coming out of the machine. No other saw comes even close to the precision of a Mayer - generally there is a 1/4 to1/2 or bigger discrepancy between the 1-10 and the 20-1/2 pieces. Being as most shops do not have such an accurate saw, they will have to adjust their assembly techniques to avoid problems later. Sometimes these techniques can even account for discrepancies between what is measured and what the cheated dimensions are, but I do not recommend people take advantage of the assembly techniques at the drawing level (though sometimes it is unavoidable). Sometimes it is important to be able to cut this precise, especially when you have laminated finished ends that need to be exactly flush. A good work-around is to have the bottoms and tops run full length, when you can, and move the partitions where they need to be. The same technique is used in face frame cabinets to match the partitions with the edge of a stile. The point about material sizes changing is well seen here, as your face frame stile can be 1/16" further the following morning than where it was the afternoon before. All and all, you have to take into account the realities of the materials/machines with the drawing process.

Last but not least, if you are sending information to a saw through G-code, the most accurate way to do this is with the imperial measurement system when dealing with most saws. Typically they will take mm measurement to 1/10 or .01mm and inches to .001. .001 inches is smaller than .1 millimeters, but it becomes a moot point if your saw cannot cut accurate enough. At first glance it appears that if your saw is only accurate to 2/1000th (which is very, very good), the difference between using mm and inches becomes a blur. If you are doing repetition cuts that have to be added together to create a run of cabinets, then this slight factor begins to show its head. Most shops would not even notice it and if they did, they might not be aware of where it was introduced. Use a clamp - the wood will compress that much easier and then screw it off and put it in shipping."

From contributor T:
I always keep my print out dimensions set to 1/16", the smallest dimension allowed. I feel this is what scribes are made for. I make the boxes down to 1/16" tolerance and add the fun part to the scribes.

From contributor C:
I do not want to start a war here, but we have to remember that we are dealing with wood products, not the space shuttle. The tools that we employ in the cabinetmaking business now, like Acad, have a precision factor that far exceeds our needs. Anything in the shop can be designed within 0.50mm. Going beyond that starts to become ridiculous and time-consuming. Remember - every good cabinetmaker has a rubber mallet in his tool box.

The only application where I would go beyond 0.50mm in tolerance would be in machining MDF doors or something similar in nature.

Always consider what the customer needs, not what we as engineers would like to achieve (perfection).

From contributor K:
When I am drawing for an architect, I use 1/16" increments. (The only Snaps I ever use are Object Snaps. I offset lines and trim to make sure my dimensions are accurate. The display errors can add up...)

When I break down cabinets for the shop, I switch to mm. There are at least two good reasons for this: 1. It is very hard to find an inch tape graduated in decimal inches, and 2. It is virtually impossible to read such a tape in .ooo's. (Or you could post fraction to decimal charts all over the shop so guys can stare at the chart to find the closest 1/32" or so and then try to find it on their tape.)

I use .1 mm precision; on the floor, they go heavy or light when dealing with the .1's. This gives adequate accuracy for the CNC equipment and a measurement system the shop guys can use for manual cutting and checking of parts.

From contributor D:
Man, what are you guys building? Do the guys out in the shop a favor and stay with 1/16 (better yet, 1/8) or 1mm. Anything less is just slowing things down. Machine tolerances, climatic changes, measuring accuracy, etc. can add up to a millimeter pretty quick. I may have to use 1/128 when I'm trying to fit a piece, but what a nightmare to deal with a set of prints that have tolerances of .001"! If you dimension with 1/8" as a standard and use anything tighter sparingly, the shop will be more likely to pay attention to a 1/128 tolerance when called out. Better yet, design your stuff so it is an even inch dimension. I can come a lot closer to 1.000" than I can to 1.001".

I'm not trying to start a fight, but I've been at this a long time and have no idea why anyone would dimension a set of prints to tolerances of 1/128th of an inch.

From contributor V:
You guys have made more good points and views. I prefer to set my dimension units to 1/16, but I always want it to read the largest number possible. Nothing less than 1/4 is great.

Contributor D, in answer to your question, sometimes I deal with stainless steel cases from Garcy. They are hand made and vary slightly from one to the next. I then have to create individual cabinets that go below them. I do not have the option to change the desired way they want this stuff to work. They want to be able to disassemble the run of cases and use them somewhere else later and sometimes this runs are better then 10 cases long. The cabinets below must exactly match the case above in width, so that all the seams align exactly with what is above and are perfectly square for the face seams of laminate or veneer. This requires high precision and the ability to do it fast to make any money at it. When I can be in full control of everything I avoid the splitting of hairs when ever possible.

Last but not least, I spent 4 dollars on a 4lb. sledge hammer and it was the best 4 dollars I ever spent. :)

From contributor T:
Being a trained draftsman and an independent, I try to give a drawing that is most suited for the person that is going to use it. If it requires that I draw in mm, then I draw in mm. If it requires 1/32" tolerance, then this is what I do and it makes communication much easier than to get everyone to understand something new or as easy as decimal.

From contributor R:
I have a short little anecdote relating to precision. When drawing up plans for Monticello, Thomas Jefferson dragged his dimensions out to nineteen decimal places. How's that for anal? Tell me he had the wherewithal or tools to make that work.

With cabinets, I wouldn't dimension anything tighter than a sixteenth unless a customer demanded it. I understand the issues when using veneer core and other materials that may be under or over thickness. I choose to adjust machining as necessary if a material comes in thin or thick.

If we know that we are going to be laminating material, we try to draw the material at the finished thickness.

As far as precision while drawing, I set my units to decimal inch. I turn precision to three decimal places and that's more than accurate for my needs. Dimensioning is another matter. Some customers would prefer ARCH units. We accommodate them. The big thing is getting everyone who may work on a drawing using consistent methods. Individuality is fine in a one-person operation. With multiple drafters, you have to develop a set of standards and stick to them.