Understanding Mechanical Drawings

By | July 12, 2016


When reading a set of drawings that has an assembly and details, it’s best to begin by a study of the assembly drawing first. Doing this gives you an idea of the whole machine or whatever the object is, so that the details are better understood.

You should start the study of the drawings of the Bench Grinder by first looking at the assembly. See the image below. All the parts of the machine are assembled with each piece in its proper place. Right away you’ll recognize that it’s a common two wheeled type of bench grinder intended to be mounted on a bench or pedestal. As you look it over you’ll soon realize that there is a little difference between the two ends and a closer look shows that different types of rests are used for the different wheels. Maybe the machine is made with either type of rest and the drafter has made one assembly show both arrangements. This matter can be settled when the student gets to studying over the detail drawings, because the detail drawings will show the number of each part required for each machine.

Some details ordinarily on the detail drawings are left out of assembly drawings. On this drawing notice that there’s no dimension lines except three or four over all, giving an idea of the size of the grinder. Drafters will sometimes place these on an assembly drawing that might be of some use to the one assembling it. Apparently this drafter considered that the overall dimensions would be all that’s necessary on the bench grinder. There’s nothing here to indicate the finish or notes about the kind of materials. Very few hidden edges are on the drawing. Some necessary parts that aren’t shown on the detail drawings like stock cap screws, set screws, oil cups, etc., are shown in the assembly only. Many parts can’t be built from the assembly drawing because the drafter left them out of the details to prevent the confusion of too many lines. For instance; there’s almost no information shown about the spindle that the grinding wheels are mounted on.

Now glance over the detail drawings, (the images below) getting a general idea of the individual pieces of the machine. You’ll no doubt turn back occasionally to the assembly in surprise at the many things which had escaped your attention in the maze of lines making up the assembly drawing. You’ll also go back to the assembly to locate the place of a part on the machine as a whole. (click on any of the images to enlarge in a new window and print them out if you like)

After a quick look at the detail drawings, you should go over them one by one for a thorough detailed understanding. It’s usually best to begin with the larger details first because they’re likely to influence other details, so our first stop will be the Grinder Head. The drafter made a three view drawing – top view, front view, and side view. The rectangular boundary line of the top view is fairly easy to recognized, after a glance at the other two views, to show that the base of the grinder head has a rectangular shape and that the base was elliptical, or oval. Study of the front and side views show that the center part of the grinder head is sort of wedge shaped. A note on the front view says that the corners are rounded off. You’ll find this same information given by a radius dimension in the top projection. The side view clearly shows large projecting lugs on the front and back. The top and front views show that there’s four of these lugs, two at each end. A note shows that these lugs are drilled with 3/4″ holes. Reference to the assembly shows that these holes are for the Rest Brackets to go into. The upper part is clearly a double bearing. It is shown with babbitt bearings in place. The small finish marks “f” show that the top and ends of each of the bearings that’s supposed to be finished. The note in the upper right part of the plate gives the number of these Grinder Heads required for the machine and also of what material it’s supposed to be made of.

The next drawing is the Head Cap. In most respects it’s like the top of the Grinder Head. Most of the details shown in this drawing are a duplication of similar parts in the top of the Grinder Head. The babbitt bearing part is shown to be an exact duplicate except for the oil hole drilled through the top and a babbitt vent as an aid in pouring the bearing. Screw threads and a note show that an oil cup is to be screwed into the oil hole. The holes for fastening the Cap onto the Head aren’t threaded as they were in the Grinder Head. A note states that each machine is to have two of these caps and that they are to be made of cast iron.

In the drawing of the Safety Guards, you’ll note a common practice among drafters in combining similar parts so that one view serves for two drawings. This is a drawing of two separate guards, one for the left and one for the right of the machine. The right guard is drawn with a front and right side view but only a front view of the left guard is shown. Evidently the side view of the right guard is to be interpreted as being the same for both guards. The draftsman has made use of an auxiliary view of the bottom omitting from the view all parts already sufficiently illustrated in the other views.

In the drawing of the Spindle, the drafter made use of a convention often used in machine drawings. This is the use of the large crossing lines in the 2 9/16″ lengths of the spindle. Crossing lines used on a machine drawing show a bearing surface and serve to put the machinist on notice to be careful of the size and finish of the surfaces.

Note that there’s a difference in the threading of the two ends. This will explain the reason for two drawings of the hexagonal nuts. The drafter could have combined these similar to the drawing of the guards in the previous drawing using these views, or they could have drawn just one drawing of two views drawing the hidden edge lines representing the threads horizontally and depended upon a note to explain that one was to be made with right hand threads and one with left hand threads. The drafter probably thought that the saving of these few lines hardly justified the risk of error involved. Look at the use of decimals instead of fractions to allow extreme accuracy in those parts involving pressed fits.

In the drawings for the flanges, two view drawings are used. Note however that one view is rendered half in section.

The Spindle Pulley, is easily seen as a solid cast iron pulley, crowned in the center and intended to be held in place by a headless set screw.

In the drawing of the Rest Brackets, note that the drafter made one drawing serve for three different length rest brackets by the simple method of dimensioning the length with these different dimension lines and placing a note stating the members wanted after each dimension. In order not to have their drawing out of scale for two of the dimensions, they resorted to a break. The break also helps to establish the fact that the shaft of the bracket rest is round though the word “turn” used in connection with the 3/4″ dimension also make this clear.

The drawings of the angle rest and T-rest, are so simple that there’s no problems for you to understand what’s meant.

It’s a good practice to go back and forth from drawing to drawing as you study these drawings just like you might read a newspaper by reading headlines and the more interesting news items first then finally the text of the article.

Source Ref: Interpreting Working Drawings by Edwin Wyatt and the Mechanical Drafting Course by Tim Davis.






One thought on “Understanding Mechanical Drawings

  1. Jennifer Wilson

    When it comes to understanding mechanical drawings, it can be daunting. A quick glance at the drawings & some people can – understandably – become overwhelmed and confused. One thing that can help is to treat each drawing like a layered puzzle. One piece builds upon another, and those two pieces together build upon another. Take it layer by layer & the final piece will reveal itself in your mind. This helps me, so I only hope it makes sense to others as well.


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