One of my clients is doing a short run production of an RC motorcycle, and he needs some bronze bushings. All of the dimensions are metric. He hasn't provided any tolerances.
We've agreed that SAE 660 is adequate for the task.
I've looked over Machinery's Handbook and can't locate anything on how to specify tolerance for such a bushing. I assume there is a ratio of tolerance to diameter -- " X-thousandths per D-inch of diameter". Is there a "rule of thumb" for this?
His drawing shows a 7mm (.2756") bushing running on a 6.98mm shaft. That's about .0007", which seems tight to me. In fact, it seems impossible to assemble.
So, regarding reamers: there don't seem to be many metric sizes, and certainly no over/nominal/under sizing groups like the imperial sizes. Should I just use a "close enuf for gubmint wurk" imperial reamer?
How are fits specified in the metric system? Are they always done to the outsize surface? In plain English (pardon the measuring system pun [img]smile.gif[/img] ) does metric system design always make the hole a nice even number, then make the shaft smaller for the running fit?
That seems good design practice because it's far easier to adjust the outside that to hit some obscure number on the inside.
Metric tolerances for bores and shafts are spec'ed with an 'H' (for shaft) or 'h' (for bore) number. The info is in Machinery's Handbook, and, I'm sure someone will find a table online.
I don't see any problem with the .0007 running fit...
Standard metric reamers are typically H7. 7mmH7 means a size between 7.000 mm and 7.015 mm.
0.02mm clearance on a 7 dia is not tight unless the bush is some 30-40mm long - that's where shaft straightness comes into play. It's a common size for valve guides and you should be able to find high quality reamers ( ask a BMW dealer...). Normal metric reamers will cut SUBSTANTIALY oversize as much as 0.1mm. They seem to be of abismal quality lately. What I do is run a 3mm carbide rod over the cutting edges while applying some pressure, This helps. The right way to do this would be with an adjustable reamer , or MUCH better with a diamond burnishing tool in the lathe. I suggest to simply look at the function performed by the bushing and the shaft. Might be less to worry about than meets the eye.
Perfect! Thanks for your help.
RAS - you're right, I'm sure that the information *is* in M.H., but in 25 minutes of looking, I couldn't find it.
Ok, so I had it backwards...
metric and multistandard components
the metric and multistandard lefthand spiral metric reamers make a real nice hole. for inbetween sizes i use mcaster-carr for their decimal reamers.
personaly metric makes me crazy.
i recently figured out that us calls out threads per inch. metric calls out pitch in metric. it also helps to get some metric drills.
class of fit h7 H7 still is foreign to me. i charge extra for parts with metric dimensions to convert to inches.