What tolerances do you hold?

[V22]

I am currently machining Jan Ridders Debbie 2 stroke engine. It has struck me a few times that I have no idea what the general tolerance one is expected to hold when machining these parts.

What tolerances do you generally hold when machining shafts and rotating assemblies? Bores? and General parts where size has on overall impact?

I could look up tolerances in Machinery's Handbook, but I wanted to see what other builders are actually holding and aiming for. Think of it as a model engineer specific question.

 

[Cogsy]

As you are making all the mating parts, tolerances are largely your decision, unless you have to fit a purchased bearing or similar. Apart from that, it matters little if your bore size is +/-0.5mm (or more), as long as you make the piston to fit.

So generally I aim to hit the plan dimensions within 0.01mm (for bores and shafts) but I then adjust the mating parts if I miss the dimension.

 

[bazmak]

Same as cogsy.Most things are near enough at 1 thou.If i have a bore with a reamer or even a drill then just get the shaft to a nice fit.From memory most reamers are H7 and a shaft with na good runninig fit should be -1/4 thou to -3/4thou. Used to aim at at -1/2 thou in a well equipt workshop

 

[Swifty]

As Cogsy and Bazmak have said, we are not making things that require spare parts to fit exactly. We make one off items, so just make mating parts to suit, just have to watch out if you buy bearings, then you will have to be careful with sizes.

Paul.

 

[Mechanicboy]

Tolerance will be used: To fit ballbearing to shaft/hole

 

[Ken I]

Make the more difficult / complex / costly / time consuming part first.

Aim to hit the numbers to within what you and your machine(s) are generally capable of.

Then make the easy or "cheap" part to suit - obviously that which is the easiest to machine - also if you do screw up its also the least cost in terms of time, money and effort if you have to remake it.

All of my model drawings that I have posted read....

"All dimensions are untoleranced - make allowances for limits and fits."

That's how you build one off's as opposed to production tolerancing.

Just treat it as a little personal victory every time you hit the size spot on.

Regards,
Ken

 

[V22]

All valid points. I guess my OCD comes out sometimes and I try to get everything right on, even if it means remaking parts. At times it can be hard to just let it be.

Just treat it as a little personal victory every time you hit the size spot on.

It feels good to hit 0.01mm. I like the idea of it being a personal victory. I am going to keep that one..

 

[Hopper]

And don't get mixed up between tolerances and clearance/interference, as many do.

EG: If machining say a piston and bore, or for that matter a shaft and bushing, the actual tolerance on what diameter you make the bore is not overly important. If the plans say 12mm, then anything from say 11.5 to 12.5 mm will probably do the job, providing it is round and parallel.

BUT, when you come to make the piston, it must be fitted to the bore with the correct clearance -- say .01mm.

So you have a wide tolerance on the actual size in many instances, but not on the working clearances between moving parts, or the interference on a bearing fit or press fitted pin etc.

 

[Brian Rupnow]

What tolerances do I hold?--I can sum that one up very quickly.---As close as I can!!! I find on anything critical fitwise, it is SO easy to turn a part undersize. I always put the bore in first. Then I turn whatever is going to fit into the bore to withing about .002 to .003". That is .05 to .08 mm. Then I use 220 grit emery cloth to remove the last .002" to .003" from the piece being turned.--and that means a lot of shutting the lathe of and checking to ensure that I don't go undersize.---Brian

 

[Cogsy]

about .002 to .003". That is .005 to .008 mm.

Oops, a couple extra zeros here Brian - it's 0.05 to 0.08 mm.

 

[SmithDoor]

Any one working in inches this is .002" to .003"
I use on most small work 0.001 to .005 and fitting 0.0002"
For other data look in the down load files under charts and tables

Hope this helps
Dave

Oops, a couple extra zeros here Brian - it's 0.05 to 0.08 mm.

 

[Geartooth]

Like most of you, I like to make parts to the plan dimensions. I like it when the micrometer stops on the exact number I'm looking for. If not, personally I usually accept +/- .002 inch. More than that and I will use the part after complaining to myself.