Workshop working tolerances

Hi guys

I am currently attempting to build a 3" scale Mclaren traction engine and was wondering what sort of tolerances would be acceptable. Obviously spot on would be fantastic but this is not always possible.

I am currently 80% completed on the machining of the crankshaft which has been a trial but enjoyable and a massive learning curve to boot.

On the two off big end throw diameters I have had to finish 10 thousandths under due to the fact I was struggling with chatter and therefore had to keep going. Sods law but the finish was pretty good until the final passes. The big end bearings are not yet made and so they can be made to suit however I have got all other dimensions to within 2 thousandths after a very long week and am extremely happy.

I had the gears professionally machined and find the tolerances are ( well not what I expected ) Am I being too exact as I find myself getting pretty downhearted when I miss my final exact dimension or am I just aiming too high. It is a hobby after all and I am not a time served toolmaker.

The traction engine is 3" scale and not a pocket watch so what sort of working tolerances would others find acceptable.

Just a very quick side note I am machining on a Harrison M300 and one of the cam locs falls through into the spindle and has since I owned the machine. Is this purely held in situ by a spring and detent / ball bearing ?? any help at all is appreciated.

Dave C

There should be a pin at right angles to the cam to hold it in place. On my Bantam D1-3 spindle there are 3 cams and 3 slotted screws. Remove these to have a closer look. Presumably one of them has been broken.

This thread shows some drawings from the standard that defines the dimensions etc.

Murray

There can't be a hard-and-fast rule because, as you note, you have control over the actual value you reach for dimensions on each part and full interchangeability with other examples of your model isn't a requirement. What is required is to reach an acceptable fit with interacting components.

A very rough rule of thumb is a thousandth of the diameter - clearance for a close running fit and interference for a solid drive fit. But there are other dependencies; for example if you've got 2 bearing bushes about 3/8 diameter and 4 inches apart, a couple of thou clearance is more realistic, because you have to take care of alignment as well.

You're unlikely (AFAIK) to meet them in Model Engineering, but there is also a labyrinthine system of Geometric Tolerancing for defining the limits of dimensions with respect to datum features elsewhere on the drawing. I can remember tackling these as a Tool Designer nearly 40 years ago, but have had little to do with them since.

It really comes down to what "fit" is required for each part.

For example your gears that are fixed to the shaft want to be a closer fit than the ones that will slide along the splines, a solid bearing will need to be made more exact than a split bearing that can be adjusted for fit. Your flywheel wants to be bored so it is a light press fit on the shaft so that when the gib head key is fitted it is not tilted on a loose fit resulting in a wobbly flywheel, and so on.

Add that that everything is bolted to a big bit of steel tube that will move when heated so you don't want fits so tight that it all binds up the first time you put a fire under it. On the otherhand you don't want any noticable slop where it is not wanted.

Edited By JasonB on 02/09/2018 13:28:53

Back when hand work was commen ± 1/64 " / 15 thou / 0.5 mm was considered realistic for ordinary craftsmen doing careful work. Maybe twice that ± 1/32" for ordinary, but not ruff, work.

For machined parts a fairly common seems to be that ± 5 thou / 0.1 mm is a sensible tolerance for non critical dimensions. Should always be able to do better on any half decent machine unless there are serious material or tool issues. Portass "S" , Sooper Adept and other inexpensive, low end machines will probably struggle tho'.

For parts that have to be to size but not to a fit ± 1 thou / 0.02 mm should only need a bit of extra care on any decent machine. Ought to be pretty much routine on industrial style machines if the job warrants that bit of extra effort to properly account for backlash et al. That's about what I normally expect to work to. But my Smart & Brown 1024 is a high end toolroom machine which seriously helps and my normal work practices are mid way between the "get 'er done quick'n close enough" appropriate to ± 5 thou and "extra care". Mill is a Bridgeport with DRO's on all axes which makes it easier too.

For press fits, bearing clearances and the like you have to work to measurements to get paired parts right. Tolerances don't really apply there for home shop folk. If I really concentrate I can get into the ± tents thou range but, frankly dear I'd rather not.

In practice its more important to know what your personal "quick'n close", "careful work" and "really concentrate" capabilities are. The range of skills, machines and simple practice for home workers are so great that its impossible to be dogmatic. The important thing is that you don't waste valuable workshop time trying to hard when routine would be fine or by do-overs when routine wasn't enough.

Worth remembering that in practice a thou is about as close as can be reliably measured using normally casual methods. Comparative measurements between parts of very similar sizes can be much closer, this sort of thing is what the teeth's thou vernier on micrometer is for. Generally helps if you can use the same measuring stick for both but realistically a thou is it.

Geometric Tolerancing is a weird and wonderful world of its own. Strictly its for CNC boys. Even the pro's have problems applying it to manual machines. Common practice is often to convert to normal rectilinear (or polar if rotary table is involved) tolerances and forget all the fancy multidirectional gains.

Clive.

Just make sure everything fits everything else.

You aren't mass-producing them, and no one is going to creep up behind you with a micrometer to check.

Neil

.

I believe that the creepers are more interested in colour-matching and rivet-counting

MichaelG.

I remember "This Old Tony" on youtube saying, "The devil's greatest trick was convincing you to buy cheap tools"

Well, guilty as charged in my case I'm not sure how true it is but it did make me laugh.

Michael W

Edited By Michael-w on 02/09/2018 19:15:15

Wise words indeed.

So why am I always looking over my shoulder? Guilty complex?...

You are quite right tho'.... make everything to fit... and all will be good...

I try to be accurate, within a thou or so... (no wonder the scrap bin is always full!!!) but its a trade off between time and accuracy and materials on hand...

How accurately can you measure. A shaft can be fairly easy to measure whatever the diameter, but a hole is entirely different. Unless you have inspection room gauging equipment just make it fit.

I feel your pain! Been there, done that; I hate SG iron. More chatter than an Islington "liberal".

On the other hand I'm not the best person to comment on tolerances; too pernickety. I'm building a couple of 4" scale Burrell SCC engines; I've been told several times on the TT forum that I'm building traction engines, not a clock.

For general shaft and bush (no relative rotation) I aim for about half a thou. For parts with relative rotation I aim for about a thou. For other parts it depends. I generally aim for 2 to 3 thou give or take. I tend to make parts as the fancy takes me, so parts that fit together may be made weeks, or months, apart. I expect them to fit together, and ideally be interchangeable between engines. Of course some things don't matter, or close tolerance isn't possible, like the smokebox and other sheet metal parts and some casting features.

I've made all my own gears. Generally the spur gears and the steering worm and wheel have 5-10 thou backlash. The differential bevel gears have no backlash, which will need to be sorted with shims as they need at least a thou or two to run smoothly.

Really it's a case of knowing where tolerances are important and where they are not. I like things to be just so. That also means some considerable redesign effort as the drawings are error ridden, are often not true to prototype, and some items will never fit or work as drawn,

Andrew

I grind my own gauges, compared against gauge blocks, and use those to set the fit of internal holes.

Andrew

"I had the gears professionally machined and find the tolerances are ( well not what I expected )". Do not accept the gears if they are outside of the tolerances specified on the drawings. Send them back and have them re-made.

Machined parts to an exact dimension is extremely difficult which is the reason to specify a tolerance on drawings, it is a range of sizes which are acceptable. Drawings should specify a general tolerance for all dimensions and specific tolerances for critical parts. If not shown, you have no come-back from suppliers.

Do not make tolerances smaller than needed because as the tolerance goes down, the cost of machining goes up.

Paul.

Amongst many other things, I used to run an estimating section for a manufacturing division. Obviously we had all sorts of metrics available, but one that used to impress people was "...halve the tolerance, quadruple the time....". Sometimes it would be worse than that, when an "easy" tolerance could be achieved off the mill, whereas for the same feature but with a tolerance that demanded grinding, or even lapping, it could be off the scale......

To make things exciting some of our "draughtsmen" (Engineers on a CAD station) didn't have the faintest idea how any given part would be made. Good fun, I miss it like you miss the flu!!

cheers

Bill

As Bill says above, tolerances are a matter of horses for courses. If boring a cylinder and making matching piston you might want to work to half a thou or so, or at least a thou. Ditto your crankshaft. But many other dimensions can just as successfully made to the closest 10 thou or even more. The trick is deciding when to use which tolerance. Common sense and experience I suppose.

As for measuring holes, a set of good quality ball gauges for half inch and under, and telescopic T gauges for over half inch will give all the accuracy you need in the average home workshop when measured with a good micrometer.

Problem is the drawings were more than likely dimensioned in fractions and no other tolerances given on the drawing so the maker may have used the general rule that a fraction is +/- 0.015"

Judging from sound alone, I suspect the fit on many full size engines is far looser than would be obtained by scaling up the tolerances from a miniature.

Brian

If the drawings were only dimensioned in fractions and no other tolerance you cannot expect them to be machined to a fine tolerance. (+/- 0.015" sound good for general machining). The other general rule for tolerancing that we used to have is +/- half the value of the last digit. (eg 10.0". assumed tolerance = +/- 0.05", 10.00" tol. = +/- 0.005"

Paul.

Edited By Paul Lousick on 03/09/2018 08:26:42