Mixing fractions and decimal units in an imperial drawing

The Cox model engine maker used to make their engines so good that any piston could fit any bore if it was new. They were the exception for a long time. Neil Wyatt has hit upon a very important part in manufacturing. The service fit is far more important than the actual size in most cases, especially with a home built 1 off project. Making a piston to fit a bore is far easier than making a bore to fit a piston. Went through some old drawings from the 1960's for injection plastic moulds. They used both fractions and decimal inches down to 4dec places. On the parts that mattered it had a statement like 0.0002 clearance fit, or 0.0003 air vent etc On fraction dimensions they were to 1/64 inches unless otherwise stated. Nominal 1.0 inch dimensions were to +/- 0.01 inches, 0.01 inches +/- 0.002 0.001 inch +/- 0.001 inches all these were unless stated. So the drawings have notes all over like FIT or 0.0015 clearance etc.

Now days it is all metric with the ISO standard of fits and clearances , but on important parts, there is a note about the Fit or the detail pertaining to that part for it's function.

Neil

Same old question, How many thousand are you making? and are they for NASA? Industry and hobby are worlds apart. It matters not a jot whether you mix fractions, decimals or dogs danglies on the same drawings as long as the corresponding FITTED components match and forget geometric tolerancing and limits and fits. It's a toy engine so treat it as such and FIT the parts to suit. For the metric converters, metric for 1/2" IN TOY ENGINE TERMS is 13mm so don't expect a piece of 1/2" bar to fit without a fair bit of "Clearance"

Fell into the trap of "Company" oddities years ago when told the 1 1/2" "Machined" dimension I'd had some shafts made to at 1.500" dia. was wrong. A list of "Machined" sizes for shafts and bores was produced that bore no resemblance to the fractions stated as these were specific machining sizes for "Our" products, to ensure competitors bits (Chainwheels and Pulleys) couldn't be substituted for ours, thus ensuring an active Spares supply.

Regards Ian.

NASA have been known to confuse metric and imperial before now. And signed and unsigned integers. (Height above planet is what?)

:0)

Martin

If you are going to attempt to get a workable fit between 2 components in the workshop such as an interference fit or a running fit, it's probably helpful to have an idea what tolerances are appropriate. With something like a press fit bearing, you aren't going to get a second chance and with a squishy part such as a sintered bush, the bore will reduce when you press it in to its hole. You need to take that data from the standards / manufacturer's info unless you insist on using trial and error - how long have you got?

In my mind, home workshop practice is more akin to machine fitting than serial production. So you make stuff fit together and work rather than define components that must be made independently and fit flawlessly. Nothing wrong with that but on occasion (such as above), you have to also be an engineer ie design something with a degree of profound knowledge.

Murray