Post By JV
Post By AlexO
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.
Take a look at the second table in this link.
ISO tolerancer aksel/hul : Frylunds Fagteori
When you see an IT followed by a number then the number is the tolerance regardless of the letter preceding it within the relevant scope.
A capital letter is always an internal dimension and a small letter always an external dimension.
H for example always means minus zero (from the dimension given as nominal) and plus the relevant number in the table.
The opposite applies to h. +0 and - something. Sizes are given in Ám (0.001mm)
7H7 is thus max 7.015 (as mentioned by JV) and 7h7 is max 7.00 and min 6.985.
The combination by the letters stated give the various fit qualities. Usually H or h is the base.
Here's an example. Hole And Shaft Basis Limits and Fits,Hole Limits And Fits,Hole And Shaft,Tolerance,
In cases like this I'd suggest sticking to metric rather than converting to inches.
I recommend you learn how to use metric tolerance. You will see more and more of it. Get a copy of DIN/ISO286 and study.
There is a lot of info on this forum. Gordon has good info on the tol. required to achieve a certain fit. Click on his last link.
First you will have to understand how the system works. Mach. Handbook comes in handy.
One thing to keep in mind. DO NOT MIX THE TWO SYSTEMS! If your job calls for metric dim's than use the proper metric reamer designate by a capital letter and a tol. number. That will give you the correct fit.
Jeeeez Gordon (and Jurgen)..... look at the post date guys........
Peter - you are so right! Red face on my side. Thank you.
x2 on rolling the fine edge on a reamer. I use a dull 3/8" B&S reamer and it cuts on size. Some say to use grease as a lubricant when looking for a tighter bore and light oil for a looser bore (with the same reamer). Maybe filling the gullet with material keeps the reamer cutting on size? I don't know and have never tested it. but I thought I would bring it up.