Mechanical Drafting questionssssss.

[hobby]

Hi everyone.

Any time I did a mechanical (machining) or woodworking project, I always did the drafting of the parts in a CAD program (turbocad).

Using the snaps features and all gives the preciseness I need to machine to close tolerances for mechanical parts to fit properly.

The scale I use is decimal to three places and the precision is in the thousands of an inch.

With my woodworking projects the lowest figure is to a 1/16th of an inch, mostly.


Here is my question,

There are times when I would like to draft out a mechanical part by hand, instead of getting out my computer, I took drafting class in high school, 1980, so I have a basic understanding of using the instruments.

But we only did mechanical drawings in the fractional inch measurements.

How would I do a mechanical drawing of a part that is in the decimal range such as steam engine parts.

What scale to use?
With enough practice is it possible to get the accuracy needed to design parts with close tolerances, with the same precision that I can get with a cad program?

I'm planning on purchasing a staightedge drawingboard, (20" x 26")
and a few triangles and a scale.

What scale is best for mechanical drawings with decimal precision to .001"
Is it possible to go that low in decimal inches without a CAD program?

Are there any hints tips or tricks to designing a mechanical device with CAD precision,
by classic drafting methods, and instruments.

Thanks for anyones help....

 

[GWRdriver]

is it possible to get the accuracy needed to design parts with close tolerances, with the same precision that I can get with a cad program?

Hobby,
Let me qualify myself by saying that I began drafting with a ruling pen and india ink on linen. That was a long time ago. The answer to your question is yes, but the difference will be that the computer is doing the math for you. You can arrive at the same results with at least as much accuracy but you have to do the computations on paper (or in your head) yourself. It really isn't practical to think that you can track .001"s or even .010"s on paper. In the course of a normal day the variations in humidity will shrink or expand your paper more than that. In pencil and paper drafting, and in computer plots, the image is only a close represenation of the actual dimensions, hence the old addage "Never scale the print." The numbers that are entered on the drawing are (or are assumed to be) proofed and the result of mathematical computation, whether done in pencil or by the computer.

 

[Kermit]

I must confess, the question has me a little confused.

Math would be responsible for making measurements come out to whatever exactness you need. Are you suggesting using a scaled drawing for taking measurements FROM?

In that case it would be very very very hard to get three decimal places of accuracy. Scaling up a drawing by a factor of 10 would only bring you ONE decimal place closer. Meaning going from 1/16 to about 1/128 inch resolution. Three decimal places with pencil and paper? I'll say no, it's not possible to take measurement that close from such a drawing.

Is that what you mean? ???
Kermit

 

[terrywerm]

Hobby,

Before I had any CAD software, I did all of my drawings on paper the "old fashioned way" and still do from time to time, though very seldom. I always found that it worked best to draw each part to where its drawing and dimensions would nearly fill the sheet of paper, which in most cases was letter size (8.5" x 11"). Regardless of the scale, I would draw it as close as possible to the necessary dimensions, but the dimensions that I drew on the paper would always reflect the desired values. A dimension that was supposed to be 0.550" would get drawn as 35/64" (0.546"), but the dimension line would show 0.550". Since it's on paper, you can pencil in any figure that you wish.

In a pinch I've done simple drawings in MS-Word (yes! it can be done!) though the item drawn would not be exact to scale. Despite that, it was a close enough representation of the part to do the job.

Terry

 

[kcmillin]

Hello hobby. Itsd nice to see soomebody else uses a drafting table and drawing out plans. cool man.

I think I can provide a little help. What I do when I need to get precise with my drawing I use graph paper with 10 divisions per inch. This way you can scale things verry easy and accuratly. When you need to get down to .001 you have to write down reference measurements on the part you are drawing and using math (or a calculater) you can get exactly what you are looking for. Just double check all your measurements and make sure they are all coming out the way you want them to on paper, and then all you have to do is make it. Thats the fun part. not to mention the great feeling you get when you finish a precision part exactly the way you invisioned it. Remember as long as you have a reference to take a measurement from you will have no problem desinging mechanicle parts. So keep all your mesurements written down to .001.

(sorry, my spelling in neglegable at best)

You shoulld be able to find the graph paper at the store you are getting the drafting equiptment from. They should also have rulers that have divisions down to .010 wich is pretty small. (usually the lines on them are .010, so it dont get any smaller than that. Unless you want to use a magnifying glass.)
Hope this helps.

Kel

 

[hobby]

Thankyou,
GWRdriver, kermit, terrywerm, and kcmillin,

Thankyou everyone for the replies, I realize my posted question was a bit vague, but to put it to example,

On my cad drawing, I measured all my hardware, and drew up each piece to exact scale, then I would draw up my project pieces, as of my latest endeaver with my engine I'm currently building, I needed to drill a compound angle, so on paper I made a quick sketch, and using trig. was able to calculate the angles needed.

However the block that will be drilled was set a certain distance from the benchmark.

By plotting on my cad program the exact angle, and drew a centerline at that angle, I then placed the block the specific distance where it needed to be, and then using my cad I snapped a dimension from the bottom to the intersecting of the angles line, to give me the exact height to make a mark on the block for drilling.

That dimension was givien by the program all I had to do was draw everything to scale, then move the components to find the intersecting of the centerlines.

If I did that by hand drafting, would I have been able to get that same dimension figure by measuring with a scale.

kcmillin, I'm going to look at your post more carefully because what you shared seems to be the answer to my many questions concerning this...

I appreciate everyones help in this thankyou very much....

I may have some more questions, later on so please any one feel free to reply.

Thanks again for everyones quick replies....

 

[Kermit]

Yes. the answer is in Trig AND in the fact that translated edges of a triangle keep the same angle and the edge will change in direct proportion.

You just use your base and angle to get the opposite side and then substitute the longer measurement and use the same angle to get the 'new' opposite side.

That may only make sense to me, ;D
Kermit

 

[hobby]

Kermit,
Thankyou ,
Your absolutely right,
when it comes to tasks like that, it is not so much of drawing it out and measuring, but just using the math, trig.

Looks like there is a lot of math involved in getting the parts to match up just as the computer uses math to calculate the dimensions. Like GRWdriver mentioned.

Thanks....

I am looking forward to doing classic mechanical drawings for my projects, like kcmillin said it is a nice feeling of accomplishment to design it on paper by your own math and calculations, anf getting the parts right.

 

[ieezitin]

Hi.
A great question.

Drawings are art. Art is in the eye of the beholder. The rules you have, you have been taught. Use them. I maybe subject to ridicule here but rules are just pointers to a full picture. Draft drawings are just what a engineers sees at that time, rules are just a format for a language. The way you express them is your prerogative. Paying attention to detail and other facts will be expressed in your drawings. Using mathematical language and description is your paint box.
I have worked in nuclear power stations and seen drawings that a crack addict has drawn, so don’t think it’s a science

 

[hobby]

Ieezitin,
Thankyou for your reply.

 

[kcmillin]

unrelated to thread

Ieezitin,
I hope the crack head doesent work at the nucluler plant.

kel

 

[lee9966]

There are a number of free programs that can print graph paper. I can't remember the one I used, but I printed dots every .010 X and Y

 

[Brian Rupnow]

Finally, a subject that I can speak with some authority on-----. I worked on a drafting board from 1965 up untill 1997, doing mechanical design and drafting. Rule #1 is-NEVER, EVER, EVER scale a drawing to build/draw mechanical parts. You calculate every single thing that you do. When I started, and for the first 5 or 6 years, calculations were done either longhand or with a sliderule, or using logarithmic tables, and using sine, cosine, and tangent math. You calulated everything you needed to put on paper, BEFORE you even started to draw lines on paper. When you did put the drawing on paper, you put down every possible thing that a machinist would have to know to build the part without scaling the drawing---EVER. In fact, any machinist that was even seen scaling a drawing was quickly shuffled out the door to the unemployment line. And woe betide the draftsman who got too many "call ups" from the shop floor looking for dimensions and tolerances that were left off the drawing. The computer world has changed things dramatically. Now the accuracy that a computer can generate when making mechanical drawings far exceeds the tolerances that can be kept by most machine shops. As a general "rule of thumb" when making mechanical drawings on a computer, most overall "material shape" dimensions can live comfortably with two decimal places. Location of "clearanced holes" (as in bolt holes with a standard clearance) and dimensions to mating surfaces should be taken to 3 decimal places. Locational holes for dowel pins should be dimensioned to 4 decimal places.---As far as "scale" goes, draw it to the largest practical size that will fit on the paper you have and still have room for a front, top, and right hand view. (sometimes you need more views, sometimes less, depending on the complexity of the part.-- Something small, less than 8" long, draw to "true size"---Something huge, as in the main driveshaft for a ship---draw to a "scale" that fits your paper. Something ridiculously small that has to be machined, then draw it to a "larger than real" scale---i.e. 5:1.----Brian

 

[Tin Falcon]

How would I do a mechanical drawing of a part that is in the decimal range such as steam engine parts.

You have a choice here :
You can draw using fractional measurements and use a regular architectural scale.

Draw decimal inches with a engineers scale

they also make metric drafting scales for the metric folk.

What scale to use?

Drafting styles differ greatly from person to person and design to design . I have seen some engines that pack all the views for all the parts on one page . Elmer Versburg tended to do one page with an assembly drawing then cram as many parts on a page as he could. others put one part per page. Find a style you like and emulate it.
as far as precision this is all in the dimensions and tolerance not in line placement. Yes you want the dimensions on the drawing close not an 1/8th or .100 in off but I have never heard of anyone drawing within a couple of thousands it is not needed or practical.

Draw for readability : a part drawn too small is hard to dimension and hard to understand. make the parts to large and they start getting crammed together or too large for the paper.
Hope this helps.
Tin

 

[hobby]

Hi everyone,
I didn't get a chance to read the last posts, but I intend to , cause there is a lot to learn here,
However I'ver been experimenting this evening,

I used my turbo cad program to draw out a graph paper (81/2 x 11) with 0.10" scale,
and scaled it up to "10:1" ratio, so now I have 0.100" increments, that I can comfortably work with using a magnifying glass, so I made a graph using .010" increments.
Then in the top bottom and side margins, I made small marks in between, at .010" to designate the 0.005" increments.

I drew with a compass a crankshaft end view with a 1/4" rod and a 1/8" hole, and checked it with my calipers and got a good reading acurately.

What I've learned is on a graph this scale, I can acurately draw and measure objects up to around 0.800".

Anything larger than that goes off the paper.

So my idea in this is, I can draw up on turbo cad all my hardware to different scales, and cut them out on cardboard, then what ever scale Im using for drawing a part I would use those scaled cardboard hardware to test for fitting.

If I want to check clearance on the piston in the cylinder I would draw all the components, in question that would fit that scale, to see what hardware and where to put the placement of parts for best fits, and stuff, just as I do with my cad.

I 'm going to go and read all these other posts. Now..

Thanks again..

 

[hobby]

I think I know what my next machining project is going to be, I might try to design my own parrallel straight edge drawing board, instead of spending the money for a board, using instead of rollers, maybe a 3/8" drill rod, as a guide and the runner could be a 3/4" block of some sort witha lapped sliding fit with the 3/*" rod guide.

We'll see I like designing things like that...
Fun machining project, and for the locking mechanism, I wont use any kind of screw to tight against the rod, that would damage the rod, I would make a clamping system to cinch down on the guide rod....

Thanks again...

 

[GWRdriver]

I think I know what my next machining project is going to be, I might try to design my own parrallel straight edge

Hmmm . . . keep in mind that the advent of Cad made a million Mayline ball-bearing parallel bars and a few less Vemco drafting machines obsolete and those all went somewhere. Vemco machines which originally cost $400-$600 are on eBay for $50 or so. An orphaned Mayline bar might bring $15-20. How much of your time and materials will that buy? Just for laughs call up a local engineering office and see if they have one in a closet they need to get rid of.

 

[hobby]

Hmmm . . . keep in mind the Cad made a million Mayline ball-bearing parallel bars and a few less Vemco drafting machines obsolete and those all went somewhere. Vemco machines which originally cost $400-$600 are on eBay for $50 or so. An orphaned Mayline bar might bring $15-20. How much of your time and materials will that buy? Just for laughs call up a local engineering office and see if they have one in a closet they need to get rid of.

Yeh this is very true,
those poor machines are being put out to pasture, :big:

I probably eventually get one of those portable drawing boards I see are very well affordable $60.00 at different stores, I sen.

But this is mostly just for a design challenge,
I'm the same person who got a oscilloscope, with my electronics course from NRI, and after that, thought it would be neat to design one using all transistors and LED arrays.

So off I went and a few months later had my own designed osciloscope, from scratch.
Never used it for anything, but just the enjoyment of a design challenge.

That's pretty much what this would be a design challenge to try to make something usefull, this time, and if not I'll learn what not to do, in sucha endeaver.

Thankyou GRwdriver, and Brian, and I'll be back need to see who else posted lately...
and IEE9966 and Tin Falcon.

Great stuff you guys all helped me out with...

Here is an example of why I want to get the dimensions close.

mostly for testing fits, and clearances to know what hardware to use and where...you get the picture.

hope the attachment comes in below. First time using the attachment thingy..

It worked, I'll start using the attachments more often...

 

[ieezitin]

Hobby.

First things first.

Try to get a copy of a book called : Engineering Drawing & Design. Written by Cecil Jensen & Jay D Helsel. ISBN 0-07-826611-4

I believe they have around 8 editions this one I have mentioned is the 6Th this edition explains both conventional and cad drawing instructions. I own this and an earlier copy which is strictly hand drawing techniques I prefer the latter.

This book is a bible for this subject it’s the only reference you will ever need.

Go to your local goodwill stores and book shops to see if you get a copy cheap. They are sold on ebay anything from $20 - $50.

Now I do not use cad, my skills on using it are very poor. I believe cad is a phenomenal invention and has its place in the world. But cad takes away your ability to really think about what you are designing and drawing.

Your scaling to make model templates for visuals and fits purposes absolutely superb keep it up. People have been doing exactly what you have for over 6 thousand years since the age of Babylon.

Your drafting table, knock-yourself-out kid, do it everything you make and create your learning something knew. A little advice here, while making your table your going to need to screw and bolt things together, think about ways on how to make them of high quality and presentation, what I am getting here is think of a 17 century hand made cabinet, look at the details and woodworking skills necessary to make such fine pieces.
Be an artist, making something of quality takes time and experience, a great investment for the life of the piece created.

Good luck kid! All the best. Anthony.

 

[GWRdriver]

Now I do not use cad, . . . But cad takes away your ability to really think about what you are designing and drawing.

Millions of people use Cad every day to design the things we use, ride in, fly in, etc, so Cad could not possibly obstruct the thinking process, otherwise there would be no Cad. Most users, including myself, know that Cad can enhance the thinking process. However, there are always a few who focus on the system (software & hardware) rather than to the design objective, because that's easier and appears to be important. They want to allow Cad to dictate to them (and me) how the ideas will appear in print, which often isn't best for clarity and accuracy, and they become so bogged down in Cad "issues" they are no longer viable as designers or draftsmen. I've had several young draftsmen tell me "Cad doesn't want me to do that and we need to do what it wants to do", and to that I would say that Cad has no say in the matter whatsoever. Cad is YOUR pencil, you tell IT what to do, not the other way around.