Design Engineering is a language. So that we understand what we're wanting to convey, we must all be speaking the same language.
DESIGNING AROUND STANDARDS
Standards, standards, standards…
Standards were created to make lives simpler, to establish commonalities which people can relate with and use a means to compare, measure, judge capacity, weight, extent, etc. For a designer, standards are a must for a design's economics once it goes to prototyping and especially manufacturing.
Standards in dimensioning allow for ease of finding locations of features by those who are familiar with drafting protocol. It's much simpler to calculate features on large integer centers like 10.00 millimeters or 3.000 inches than numbers like 13.345 of any scale.
Standards in material selection allows the shop to use material sizes which they may have in stock or can readily obtain without having to spend time machining stock to a non-standard thickness or size. Purchasing catalog items such as bushings, precision shafting and gears are invariably less expensive than making them or having custom gears hobbed.
An ambiguous standard for beginner designers to get their heads around is hole sizes for fasteners. That's because not everyone uses the same standard. Interestingly, these are different on the West Coast than on the East Coast. Take a clearance hole for a #10 screw. It's larger on the West Coast than it is on the East Coast. The western standard diameter established by the aerospace industry is .221" for a free fit and .207" for a close fit. On the East Coast it's .201" for a free fit and .196" for a close fit. It may be different for some companies, but this is the recommended hole size in the Machinery's Handbook. For metric screws there are typically three standards, namely a close fit, medium fit and a free fit. What's a bit ironic about the aerospace's .207" diameter hole is that there's no standard inch drill size this diameter but a 5.25mm drill comes darned close. Industries that use large sheet metal parts in their products will often have their own standard holes that even larger than these standards. In any case, the standard to use is what your particular shop is familiar with and has the tools for. The point is to use them consistently.
Adding lists of standards to a book or binder for reference is a good idea. I copied the page of standard tap drill holes and clearance holes out of the Machinery's Handbook and stuck it in my reference book of standards.
Let's design something round standards. We'll begin a design with a need to use some bar stock, some sheet metal, a few blind holes and a piece of aluminum extrusion. Our design requires two precision bars that linear bearings will ride on and a couple of gears on shafts driven by a motor. Remember, it doesn't matter what this thing is, the point is that we will be using standards in our design.
First we're going to design a frame with some bar stock. This frame will be made of 1/2" by 2" aluminum. Yes, the inch system is still alive and well in the United States and most material suppliers here only supply material in the inch scale. If you are in Europe, just replace the inches with some close equivalent in millimeters. It should be obvious why a size of 17/32" by 2-1/16" should not be used in this design even though it might fit better or look cosmetically better in your CAD model. Aluminum just doesn't come in that size. It's always a good idea to check into a supplier's web site catalog for available sizes. What one assumes should be a standard size may in fact not be available. This can be especially true of aluminum extrusions. For example, from a popular supplier in southern California, a 2" X 1-1/4" rectangular shape in a .120" wall is available. However, a 2" X 2-1/4" rectangular extrusion is not available in any wall thickness. Often material is shown in a certain size but requires a large mill run to obtain it.
Next we'll chose our sheet metal thickness for a cover. One place to look for available gauge thickness is your Machinery's Handbook - a great place for all sorts of things a mechanical designer may need. You may want to bookmark a website for many standards as I have for convenience. This sheet doesn't need to be very strong, so let choose to make it around .040 thick, which looks about right in our model. So, just in case no such thickness is available, we'll just check what's available. Well, no material .040 thick here in the US, but 19 gauge comes close at .0418". What really happens when you use 19 gauge in your design? You will probably get a call from your purchasing department telling you to use something that's more common since 19 gauge is a standard, but it's rarely used and they're having trouble getting it. They tell you that 18 gauge is readily available. So, you redesign your part to make it .0478" thick. Wow, these standards can be a pain, but in no time you will learn what to use and what to avoid.
Next we are going to bolt this extruded aluminum frame together, so let's choose some fasteners. How about some socket head cap screws in a 1/4-20 unified national coarse, 2-A thread. It's important to know classes of threads, what UNC and UNF mean and even the designation on taps such as GH3 and so on. More detail on this subject that will be covered in future articles.
So, continuing onto holes for our 1/4-20 UNC-2A screw. We designate its tapped hole as a 1/4-20 UNC-2B - the "A" is used for external threads and the "B" internal threads. UNC stands for unified national coarse and a 1/4-28 thread would be designated as UNF, or unified national fine.
When choosing a counterbore for our socket head cap screw, it's a good idea to specify a standard counterbore diameter. Most parametric CAD programs will choose this for you. In some parametric programs such as SolidWorks, changing this diameter makes like going to the dentist a fun experience. So, if your machinist's standard counterbore doesn't match your drawing, put a screw in the hole, of it clears the head and doesn't look too goofy, go out of your way to make sure the part isn't rejected by the inspection department, or have your machinist make himself a counterboring tool that matches your drawing. Oh, the challenges for using CAD! The standard counterbore diameter for a 1/4-20 socket head cap screw is .422"
(as non-standard as it looks) and the depth can depth can vary from the head height of .250" to around .375" (3/8"). I recommend that we use this screw throughout the frame design. It's inefficient to use different size screws within an assembly even though it may seem a bit overkill in one area and under-kill in another. The point is that it is very cost effective to use the same screw in as many places as possible within any particular assembly. These cost savings show up in the form of fewer tool changes during machining and assembly, and simplify purchasing and stocking of hardware wherever possible. Some very complex products have been deigned with only a few screw sizes. An early Apple computer boasted only one screw in the entire assembly!
We've laid out our frame with 1/2" X 2" aluminum rectangle and put in several screw holes with counterbores and tapped holes. We've chosen or sheet metal thickness. Now let's crew the sheet metal to the frame. We’ll make it easy for the fabrication shop to attach these sheets by having them transfer drill and tap the holes at assembly. We could use 1/4-20 screws here, but let's decide that this would in fact be overkill. Therefore, let's tone it down and use some #8-32 screws. We will then make it standard for all the outer skin pieces.
The inside stuff is what we'll add next. We need to pick a motor, some bearings, a shaft and some gears. We also said we would need an aluminum extrusion somewhere.
For power, we'll need a fractional horepower motor. Several manufacturers make motors and exactly which motor to use is not important for this exercise. The issue is how we plan to utilize this motor's output by using standards. We'll assume that this motor was chosen by someone else. Mot motors usually have locating shoulders at their output shaft ends. They have a bold pattern, or a bold circle of some sort. This is pretty standard with motors which are mounted to a plate. Shaded pole motors are one exception. Most hole patterns are an even dimension in either inches or millimeters, but not all. To mount the motor, we'll match its hole pattern and make a hole for the motor's locating should to fit into.
This motor has a nominal output shaft diameter of 1/4". The end of this shaft will drive another shaft via a standard coupling. There are several standard couplings on the market. Let's locate one with a 1/4" nominal hole off a website. Although there are many things to consider about a motor coupling such as backlash, shaft offset, torque value and so on, but for now we are just going to rely on the fact that couplings come in standard hole diameters that match motor shaft diameters. This is usually within a couple of tenths of thousandths of an inch!
Picking the driven shaft will also require looking into shafting online or in a catalog. I think people still use catalogs. I normally use precision ground shafting since I an order it with the standard tolerances I need. Mosst precision ground shafting comes below nominal with a tolerance of plus zero, .0000, minus two tenths, .0002". Doweling comes in a plus or minus one tenth, .0001, and drill rod which comes in at a sliding scale depending on the diameter. Then there's pump shafting and even suppliers that have their own tolerance standard. So, it's always prudent to check with your supplier on tolerances, shaft material, surface hardness and finish. By the way, one "tenth" in the mechanical world refers to .0001" and when you speak with electronics engineers it's .1".
The reason that drill rod and precision shafting have different tolerance standards, one at nominal and the other on a sliding scale and pump shafting is again different is because of their use. Shafting, depending the tolerances can slip or light press into bearings while others must slip fit into bushings and couplings. This is why shafting is a little smaller and rod or dowel is a little bigger to match with the purpose of standard components. Keep in mind that there is also plain cold rolled rod and bar stock that have much looser tolerances. These come from the mill with varying diameter and straightness tolerances.
The ends of the shaft in our design need to be supported by bearings. Let's use a standard ball bearing. These have a nominal 1/4" ID, 3/8" OD and a width of 1/8". This bearing is easy to get from most every bearing supplier.
For the bearing support block, it's common to make the bearing bore's tolerance line to line, .2500" to .2502", for a slip fit. If you want a tight, or press fit, make the bore .2498" to .2500". Depending on the size of the bearing, a press interference of as much as .0004" is used. Interestingly, the larger the bearing, the tighter the tolerance must be. That's because it takes much more pressure to press a large bearing into a hole with the same interference as it does a smaller one. For very large bearings, the bearing itself is often cooled in a refrigeration box or dipped onto liquid, or the part with the hole is heated to expand the metal. After it's inserted, the hole shrinks or the bearing expands and it's really in there to stay!
To get the full scoop on tolerances on shafts ad bores, again refer to the Machinery's Handbook in the ball and roller bearing section. There's an elaborate chard called the AFBMA Stand Shaft and Housing Bearing Seat Diameters for Inch Dimension Radial Ball Bearings of ABEC-1 Tolerances (what a mouthful).
Now for the gears. We'll use plastic gears for our design. These come I n standard tooth profiles, pitch diameters, pressure angles, diametral pitch, etc. With all the standard gears out there, it’s rare to need a custom gear made to your specific design. I've had to design custom gears and supply their specifications on several occasions. These too must follow specific standards of tooth profiles, diameters and specific number of teeth even though they are considered "custom". Let's establish the standard pressure angle at 20 degrees. Now all the gears will have a pressure angle of 20 degrees because you can't have two different pressure angles running together. The gears will have a 1/4" nominal hole, of course, to fit our precision ground shaft within a few tenths.
Gearing is a vast subject. Knowing a few things about them will open up an interesting understanding of how they work and why they can last for long periods of time. Therefore…
1. Gears fall within the constraint of standard size teeth.
2. Gears come in standard pitch diameters relative to their number of teeth.
3. Gears have standard center hole diameters toleranced to match precision ground shaft.
4. Gears come in standard tooth widths.
5. They fall into categories, namely, spur, bull, bevel, helical, pinion, cluster, worm, rack, etc.
They are available with hubs and set screws while others are available with standard keyways for anti-rotation.
Gears run on what's called pitch circles. The combined radii of the pitch circles of any two gears running together determines their center spacing. For example, a gear with a pitch diameter of 4 inches running on a gear with a pitch diameter of 2 inches will have a center spacing of 3 inches. There are a few instances where this is violated such as with enlarged pinions, but this is more specialized and rarely needed.
Let's go get the gear. We'll use a common stock gear supplier to choose our gears. We already chose a common 20 degree pressure angle and a nominal 1/4" center hole. Let's choose a ratio of 2:1. Let's use a diametral pitch of 48. This is the number of teeth to the number of inches of pitch diameter - a standard. The choices start with a 27 tooth gear, so we'll need a mating gear with 54 teeth for our 2:1 ratio. But, guess what? There is no 54 tooth gear with a diametral pitch of 48! This means we'll have to keep looking. A next choice is a 28 tooth; is there a 56? Yup, there is. Our gears are a set of 20 degree pressure angle, 28 tooth, .5833" pitch diameter, .125" face width, 1/4" bore running on a 56 tooth, 1.1666" pitch diameter, .125" face with a center to center distance of .8749". Watch your tolerance here. Check with your engineer, or look up the recommended center to center tolerance. Fine pitch gears such as a 96 pitch gear can require a tolerance of a couple of tenths of an inch for minimum backlash. On the other hand, coarse gear trains can have quite a bit of backlash especially if they are not reversing a lot, and still do the job. Our specific geartrain will have a tolerance of plus .002", minus .000. The American Gear Manufacturer's Association, AGMA, has prepared a gear classification manual, AGMA390.02, that covers gear quality and tolerances.
Now for that linear bearing. Our deign requires a slide or sliding table for some side to side motion. We could do this by deigning our table on rollers top and bottom, with adjustment to take p the slop. But a more elegant way to do this is to use linear bearings.
These bearings run directly over THEIR standard precision ground shafts. I must caution here that here is a difference between precision ground shafting and THEIR precision ground shafting. The caution is that you should use the linear bearing manufacturer's shafts. If you chose to use some other, they recommend that the shaft be on the low side of the of the standard industry tolerance, even a couple of tenths under. If yours isn't, then, you may need to design a new bearing around the shaft you just chose. If you need your slide to be extremely high precision, you may want a floating rod end mounting block and run the carriage back and forth, slowly tightening the screws so that it finds the best center to center distance for least binding or bending of the shafts during travel.
Our table will be made from that aluminum extrusion we mentioned. The reason that we're using an aluminum extrusion is that there's a literal plethora of standard aluminum extrusions to choose from. It's relatively inexpensive and usually comes in standard 20-foot lengths. If you can't find a standard shape that will suit your design, then, it's probably a pretty unique cross section. One supplier here in Southern California has over 17,000 unique shapes and over 450 standard shapes such as angles, tees, channels, zees, "H" beams and bars. If you search "Extruded Shapes" on the net, it will blow you away with about 420,000 results.
It's complete! I have no idea what we've just designed, and it doesn't matter. What we accomplished was the use of standard holes for screw clearances, standard material thicknesses, and use of available materials. We learned that standard counterboring tools for socket head cap screws exist, and if your machinist doesn't have the one your CAD program chose for you, then modify the program or buy/make them. Or have your inspector ignore the discrepancy - something they just hate to do. We tandardized on fasteners throughout our design and mounted our motor by standard methods. We used standard precision ground shafting and an off-the-shelf coupling that fits our precision ground shaft - no problem. We learned about standard shafting versus dowel and drill rod and their particular uses. You'd be amazed at how many people buy cold rolled stock and attempt to hammer gears and couplings onto it, or turn it down to fit. We learned a little about gears and some of their standard callouts. If you found the gears portion a bit complicated, don't worry. Soon I'll be devoting an entire article specifically on gears.
Learning and complying with existing standards will enhance and advance your career dramatically. Learning a CAD program is a step in a person's career within the creative mechanical design world. The trend over the past 5 to 10 years is that many CAD draftepersons are being either phased out or moving up to design status. My challenge to you is to remember and use as many facets of the industry, materials, machine processes, assembly techniques and outside processes as possible. Designers that commit these to memory will outshine those who guess, fudge and waste time looking them up. They say the devil is in the details. I say each detail is like a little angel providing you with knowledge that equals brightness.