To plan, or just to fit?

[rleete]

I'm curious as to how the more prolific engine makers make their parts.

When making individual pieces, whether following a set of plans or just making an engine you saw in your own way, do you make parts strictly to plan, or just to work?

Say an engine has a part 2.5 inches long. Do you make it exactly 2.5 inches, or do you get it close, and just make the other parts to accommodate? Assume it's a noncritical piece, like the supports for a vertical single. Do you make them exactly like the plans, or just so that they're all the same?

How about stuff like bores? Do the bore a cylinder to the exact number, and the piston as well, or just get close, and make the piston to fit?

Obviously, stuff like the team builds, you need to make them all work. But for a 1 off, just to build something engine, do you follow the plans or wing it?

 

[chipstractor]

To fit. Precision to run, Fit to look good enough. Have fun.

 

[tel]

To fit for me, more often than not I'm working without plans anyway, usually just a photo or two.

 

[jack404]

Not with models so much ( not that expert yet) but i build off plans and then fit along the way

get it close and redo the numbers.. theres always a gotcha with every build i reckon

just my 2 cents

cheers

jack

 

[Brian Rupnow]

I am probably like a lot of folks out there. I start out working to the plan. Then a little slip here or there, and you don't want to throw the part away, so it gets modified slightly from the plan. Sometimes this small slip, if its a mating surface, has to be carried out to the next part, which also would have been made exactly to plan if you hadn't made that little slip on the first part. But yeah, we try and work as close to the plan as out machining capability lets us.

 

[stevehuckss396]

I always try to get my bore to size but that might not happen. If I go over on a bore I make a piston to fit. Once the piston is run in a bore it should always go back in that bore only. So if you have a 4 cylinder engine, It could have 4 different size pistons in it without any problem. Same goes for clearance on crank journals and all critical parts. So for critical parts i agree with Jack404

When it comes to non-critical (dressup) parts, some guy's in my club believe the challenge is making everything exactly to the drawings drawn by the designer. I think if that is done, you will have an engine that looks just like everybody elses. The Peewee i am doing has many items that are different like a dipstick, front pulley, timing pointer,rocker arm, ect.

If you want to make a round part square or any other shape, do it. It is yours to build any way you want. Take that engine and make it yours.


Steve

 

[gbritnell]

In my opinion the goal is to always try and make it to the drawing size. Sure there are parts that might come out a little this way or that and it wouldn't matter to the final build but then again there are parts that need to be exact. If for example you were building a multi cylinder engine and each of the rod journals was a different size then you would have to make the connecting rods different sizes. If the cylinder bores were all a few thousands apart from each other then you would have to make 4 different sets of piston rings. What if you got one lobe on the camshaft wrong or out of place, that would change the whole operating performance of the engine. Have I ever made a part different from the drawing, sure I have. Should I have gone back and made it to the print, not necessarily. My deciding factor is how much time I have invested in making that part. Most of my work anymore is scratch work and in alot of cases I try and make the part look like a casting which means many hours of whittling from the solid. A little gouge here or a little nick there most times won't affect the operation of the part but I myself don't want someone looking at my model and seeing these missteps. I'm not totally anal about my parts as I know some are but it comes down to what satisfies you.
gbritnell

 

[Tin Falcon]

I know I tend to make adjustments as I go along. Maybe out of laziness. To some extent it it depends on what you are building . If you are doing a historical model you want it to be and look to scale. If it is just a cool little runner you have a little more latitude. One of my first models called for a piece of angle iron for the frame I found a piece about a 1/2 inch longer than the print called for. I could have cut it down or milled it to length but decided to clean up the ends and leave it long. Another engine I inadvertently tapped the "wrong " side of the block for the intake port so the engine is a mirror image of the original plan. As far as critical fit like piston to bore most prints I see give a bore size then on the piston will say machine close sliding fit to bore or similar terms sometimes the dimension for the bore and piston will be the same knowing the builder will take a little off to make them fit.
I do not recall ever seeing a tolerance of say +.0005 - 0.000 for a tolerance on a bore and a corresponding tolerance of +.0000 -.0005 on a piston although on a team build that is what would be needed of course the best thing on a team build is have the same guy do the piston and cylinder.
This is model engineering so design as you go is OK . But it is also good practice to do your best work and shoot for the mark and stay within tolerance. In the Air guard machine shop I was taught to shoot for tight tolerance in all work that way when the day comes that you need precision you have the practice. This was in a shop with no time pressure. So this applies to the home shop. In a job shop time counts so use the tolerances to there fullest. The boss would not want to pay you to get a part to +- .0005 when +-.005 or +-.015 will do. This also applies to the home shop if you want to get the project done and move to the next.
What are your goals?
Skill building.
Models built with a deadline
Ie a show coming up or a Christmas gift.
Building high quality show pieces
teaching concepts

Remember YOU are the layout guy, shop foreman , engineer, assembler, painter and QC inspector. So it is you shop you can set the rules.
When you show an engine no one will be able to tell the difference between a mistake and a design change unless you tell them.

These goals may dictate how you work. And these goals can change from day to day or engine to engine.

Here is my philosophy:
Work Safe
Have fun
Learn something new
build a few engines
share your knowledge.
Tin

 

[Mike N]

Tin,

I could not have said it any better myself, I agree with your post 100% :bow:

I have been in the Tool&Die business for 36 yrs. The best ToolMakers are real problem solvers, how can I make things work even if I didn't hit the perfect dimensions! How can I make a plug or an insert & hide it, when a mistake was made. (IT Happens!)

Mike N

 

[Metal Mickey]

I have found this thread very reassuring. There seems to be a lot of experience out there that have a real life attitude and if its good enough for them then it is fine for me!

 

[Maryak]

Spot on - Tin :bow:

Best Regards
Bob

 

[rickharris]

Just to add my 2P worth

All manufacturing goes through 5 main phases:

Mark out
Cut out
Finish to size
Assemble
Test

All engineering drawings as made will have a tolerance marked as appropriate on the drawing (or should have).

In general at the prototype stage (which is to say hobbyest i.e. one off ) you are making to work, (fit).

The standard approach of cut out and the finish to size allows that to be a finish to fit another part. As opposed to a CAD CAM approach where the accurate measurements would be established and entered as the finished product to save time finishing to size.


The reality is the product in general should "work" (what ever that means to you) - So you need to produce parts that meet that requirement. If I am makeing a wood model then +_ .5 mm isn't too bad!!!

If it will be run on air/steam the it needs to fit better. (My models often leak a lot but I still like them )

If you expect to do 170 MPH + then I guess your tolerances are a lot closer.

http://www.steamcar.co.uk/

 

[rleete]

Thanks for the insight. The question arose because I am making my first engine (vertical wobbler), and I screwed up the bore. The piston fit poorly, so I made the bore 3/8", and turned a new piston. Since I don't have any reamers yet, it was a matter of either hitting 3/8" dead nuts, or fudging things a bit to get a decent fit. I'm happy to report that the piston now fits, but isn't exactly .375".

 

[Tin Falcon]

I'm happy to report that the piston now fits, but isn't exactly .375".

So you punted and made a design change you are human like the rest or us here.
Tin

 

[stevehuckss396]

Thanks for the insight. The question arose because I am making my first engine (vertical wobbler), and I screwed up the bore. The piston fit poorly, so I made the bore 3/8", and turned a new piston. Since I don't have any reamers yet, it was a matter of either hitting 3/8" dead nuts, or fudging things a bit to get a decent fit. I'm happy to report that the piston now fits, but isn't exactly .375".

Beats making a new part. Nobody will ever know.
What wobbler are you making? The one in the first issue of MEB magazine?

 

[rleete]

My own design, taking features from several I saw on the web. I'll post pics when I finish, promise.

 

[rake60]

Tin your first post here was priceless!

I pay the bills by making machine parts.
I don't know if those parts will end up in Nashville Tennessee or Beijing China.
What I DO know is there is documentation attached to them that leads right
back to ME! If those parts do not fit I'm in deep trouble.

In my home hobby shop the rules change.
Of course I'm unhappy when if I miss a print size, but if the mating piece can be
made to correct that error, no damage done to anything but my ego.
A very wise man once said "If it can be fixed, it was never wrong in the first place."

Rick

 

[te_gui]

Its been said that the difference between a "good" and "great" machinist is one who can screw up and yet no one will know or had no effect on outcome. This can take mutiple forms, whether its making a new part, or modifying mating peice, or sometimes the dimension doesnt need to be to that tolerance after all. In my work I am constantly getting after design engineers for putting unreasonable tolerances on things. Precision costs money and sometimes it doesnt buy much.

Just my opinions
Brian

 

[joeby]

Everyone has their own degree of "fussiness", some people I know would throw a part out if it wasn't exactly to print. I think that's a waste. You are doing the next part, so make it work. Nobody is going to care if the piston you've made is .007 oversize because the bore finished a little big. The issue is whether the finished project works as intended, or not. Besides, you are not likely to have to stock replacement parts, so there's no need to worry about holding a particular size.

I can remember rebuilding car engines having the oil pan rail stamped with a letter under each bore. That was so the worker fitting the pistons knew what size piston to drop in that hole. I've seen automobile engines that had the lifter bores plugged and rebored to correct a problem with oversize or bad locations.

Don't constrain yourself to making sure everything meets the print dimensions and tolerance. The whole idea is to build a running engine. You could fuss over every detail to make sure it's perfect, and get frustrated, discouraged, maybe never even finish it and if you do finish it, it may not run. On the other hand, you could learn a little about what it actually takes to make an engine run reliably, have a finished model that looks great, and be on to the next project. I don't think the criteria for Engine of the Month includes points for meeting print specs.

Just my $.02,

Kevin

 

[Kludge]

My 2 cents worth ...

The advice offered has been extremely good ... not that anything else would be found here. What it comes down to is that it's your shop. You set the rules. The only person who has to be satisfied is you.

Pretty much says it all, doesn't it.

Best regards,

Kludge