Why do designers do it?

[jonesie]

i too am dam good at what i do as my pay will reflect . you said it all when you said just what i said you can change your setting for the decimal point. i have also worked with good and bad engineers and toolmakers. in a job shop time is money, but as a hobby the non pro can find it hard to know what is fussy and what is not. enough said by me. jonesie

 

[mu38&Bg#]

In my opinion model engine plans are guidelines. This is the reason model engine designers leave tolerances OFF the prints. In a shop environment today, any dimension is acceptable, like others say, it's the tolerance that kills you. Producing anything that must have interchangeable parts leaves you with little option. Just don't be surprised when quotes come back at a ridiculous price.

When I'm building a part for myself, I decide what will and won't do for dimensions and tolerances. When I'm designing a part for production, I must consider many requirements, production cost, how the assembly works together, the requirements of a particular component, etc. Usually there is some aspect of the design that constrains a dimension, and rarely is it what tools are int he machinist's tool box. If he doesn't have a 0.1330" reamer, he can pick up a catalog to get what he needs. If cost is top priority certainly the availability of tooling should be considered.

I think that the ISO system for shafts and fits is a wonderful thing. You can buy shafts and reamers in the tolerance you need to make what you want to make without much effort. That is, if you work in metric.

If you don't like a dimension on a model print you're building, change it to suit your style. It's not a big deal. Part of being able to build a model engine is understanding how the parts fit together to do the job intended. The actual dimension usually isn't critical, just the fit.

Reamers for fractional sizes 1/8-1/2", .001" over/under sets are very common. I don't have a set, but I don't do much with shafts and reamers. But have purchased a few single pieces that were sized for the fit I needed on the shaft.

Greg

 

[GWRdriver]

To Greg's point . . . Early-on, in model engineering (and often by the Rt Rev LBSC), and also in full size engineering, fractional drawing dimensions were often noted "Full" or "Bare" indicating to the machinist or fitter, or model builder, the required or recommended fit. A Full dimension required (or allowed) a looser fit, such as for bolts and screws, and a Bare dimension was to be held close to dimension, such as running bearings and bores. This was primarily indicated as a "heads-up" and left the actual performance fit to the fitter. It's been decades since Full and Bare have been used on an ME drawing, or the terms used in print, but that's still the way many things work for model builders.

 

[steamer]

I have also found it difficult to explain some model engine tolerances in the context of a drawing.

for example

Most model diesel engines have a slight interference fit with the piston and cylinder at just before top dead center. The interference is described as a tightness. I would maybe say it's about half a tenth ( .000050") but only at the top oh 5-10 percent of the stroke before TDC. I have read the description of how it's put there with laps and patience....but I haven't the foggiest idea how I would notate that on a print as a stand alone document. I suspect that the only way would be a lengthy note and fitting drawing of some sort. An lots of explaining to the person doing the job....much like the build articles we often read.

Clearly there is some interpretation required...

Dave

 

[Tin Falcon]

"Why do designers do it?"

Why indeed!

Well, Communication is the key isn't it.

That sums it up well. communication is bases on some common knowledge between the person producing the drawing and the one reading it. This knowledge base changes with time.
Personalty I believe all machinist need to learn a bit about mechanical drawing cad and blue print reading. and designers need to understand basic machining functions. How else is a common knowledge base established.
Lets try to focus on the original question and help the original poster fill in his knowledge base and help him understand.
Tin

 

[mu38&Bg#]

Most model diesel engines have a slight interference fit with the piston and cylinder at just before top dead center. The interference is described as a tightness. I would maybe say it's about half a tenth ( .000050") but only at the top oh 5-10 percent of the stroke before TDC. I have read the description of how it's put there with laps and patience....but I haven't the foggiest idea how I would notate that on a print as a stand alone document. I suspect that the only way would be a lengthy note and fitting drawing of some sort. An lots of explaining to the person doing the job....much like the build articles we often read.

This would be defined as a taper of the bore. Something on the order of .0002-.0005"/1", it's been a while since I measured a production engine so don't quote me. The interference is achieved by selection of mating parts. In production the, machine is setup to produce the required taper in the cylinder and parts sorted by size. Pistons are also sorted by size. Then the two are hand fit, looking for that interference a certain distance from TDC, or whatever is convenient. The actual taper is important, not only the amount of interference.

The the average home shop machinist doesn't have a bore gauge to verify bore taper directly. This can be done with plug gauges etc. In the home shop many functional engines have been built with little thought put into the actual taper or dimensions so don't let the details of the optimum design stop you. Had I thought about whether or not I could achieve the fits and tolerances I needed on my spindle project, I would have never started. As it turns out things are looking pretty good.

Greg

 

[steamer]

Hi Greg,

I understand the tolerances required and I too think I can achieve them. But how would the information be put on a print.

I think a profile tolerance zone probably....but there would still be a rather complicated way to do it.

Dave

 

[mu38&Bg#]

Profile is appropriate. By using profile of a line you could further refine it. I'd hate to ever run into this on a print especially in to tolerance range we're talking about. My GD&T books make it look like the most logical call out.

 

[steamer]

Yup thats the way.....talk about ugly though.

Dave

 

[Runner]

When I read the title "Why do designers do it?" I knew it would be an emotive topic. I think that there will always be some difference in understanding in what the designer was attempting to convey and how the builder interpreted the drawings. Even when drawings were prepared iaw national standards this difference will be there, because for one the designer has in many instances no control on who will be using them and the knowledge base of the user. I worked for an Australian Defence Contractor of UK affiliation that was selected to be a co-producer for a USA Defence Contractor on a USA designed project. The drawings produced where in UK terms short on a lot of detail. The reasons for this was that the USA company had what they termed a "Workmanship Manual" which covered a lot of the detail that was normally annotated on UK drawings.

I realise that this is slightly different argument to what was meant by the initial post, but as been stated in a few replies some review of the drawings is required to productionize them to the skills, tooling and knowledge base of the builder.

 

[steamer]

When I read the title "Why do designers do it?" I knew it would be an emotive topic. I think that there will always be some difference in understanding in what the designer was attempting to convey and how the builder interpreted the drawings. Even when drawings were prepared iaw national standards this difference will be there, because for one the designer has in many instances no control on who will be using them and the knowledge base of the user. I worked for an Australian Defence Contractor of UK affiliation that was selected to be a co-producer for a USA Defence Contractor on a USA designed project. The drawings produced where in UK terms short on a lot of detail. The reasons for this was that the USA company had what they termed a "Workmanship Manual" which covered a lot of the detail that was normally annotated on UK drawings.

I realise that this is slightly different argument to what was meant by the initial post, but as been stated in a few replies some review of the drawings is required to productionize them to the skills, tooling and knowledge base of the builder.

Communication...Communication,,,Communication....if it stops ...no one wins.

 

[kf2qd]

Designers do it because of time constraints - they want to get the drawing to production so some work can be done. Designers do it because the guys in the shop know what they are doing - the machine room guys will look at the related parts and figure out the best way to do it using past practice as a guide. Designers do it because it ias a one off part and they know that the shop will figure out the best way to fit things together, and they need to get the drawings to the shop because there is a big rush to get this thing made. Designers do it because there is a budget for the drawing and not much time is being paid for, so not much time will be invested.

And from my own experience - I drew it that way because that gave me the necessary information to get the pieces made and the engine running, the shaft has to fit the hole in the front housing and the easiest way to finish the front housing was to ream it and polish the shaft to get a nice running fit in the housing...

Besides - maybe your 1/4 reamer is still brand new and cuts 0.0015 or is a bit dull and cuts 0.0005 under over in aluminum. you can adjust the other dimensions and carry on. Remember - its a one off part, mass production reproducibility is not required, unless you personally cannot live with the results...