How To Read Measurments *LINK*

Re: How To Read Measurments

Dick Coers

That is the tolerance you are allowed. For instance, the 21.7 has the 0 with a +0.3 above the 0. This means the hole can range in size from 21.7 to 22.0. In other words that 0 says you can't make the hole smaller than 21.7, but the +0.3 says you can go oversized to 22.

Re: How To Read Measurments

John K Jordan

Tolerances written the way I am familiar with might look like

25.4 +/-0.5 or

16.7 +0.2/-0.3 or

35.0 +0.0/-0.4

I think the crushgrind drawing would be less confusing they included the plus sign with the zero:

44 +0/-0.5

or perhaps better, include the significant digits

44.0 +0.0/-0.5

BTW: For some reason metric numbers often are written without the trailing zeros, such as 0.08 instead of 0.080. The engineering classes I took many decades ago insisted on keeping the trailing zeros as significant digit specifiers. And that's all I remember about that.

Here is a note about writing tolerances:

http://draftingmanuals.tpub.com/14040/css/14040_49.htm

All that said, I usually drill with what I have and enlarge if needed to fit with a box scraper. I have also on occasion used epoxy when a hole was a bit too large, but I would never mention that in public. Oops...

JKJ

Re: How To Read Measurments

Dave Paine

Others have explained the tolerances.

When you mentioned not fitting are the holes not large enough or too large?

Easy to hollow a little to get the hole larger.

If too large check alignment of headstock and tailstock. I had an alignment problem I did not realise which resulted in larger holes than I expected.

I have a NOVA DVR XP. A sloppy tailstock fit between the lathe bed due to bad design, and a sloppy quill fit unless almost locked down.

These days I drill holes on the drill press or my metal lathe.

For deep holes on my pepper mills I like to use the metal lathe. I drill smaller holes and use a boring bar to get to desired dimension. Takes a bit longer, but I get a hole with more consistent dimensions and a much better finish.

Re: How To Read Measurments ... thanks

Stuart Johnson

My fit is usually a bit sloppy. The fit using 44 mm + .5 is 1.73 to 1.751 imperial so drilling a 1.75 hole like Craft Supply says is going to give you a loose fit at best. The same is for the 38mm +.3 or 1.496 to 1.508. These are just to close. As a result I need to mess with epoxy to insure no movement. The price of a couple new drill bits dedicated to one use is high but then so is the frustration and constant fiddling trying to get the right fit.

Drilling

John K Jordan

Stuart, a few things improved my holes. One was a set of carbide forstners - they stay sharp. The second, I start all critical holes with center bit or a spot bit, normally used by machinists for metal work. These are short and stiff and start the hole precisely in the center of the wood. (If not exactly centered the hole can get too big.) This has helped a lot.

Third, I always direct a stream of compressed air into the back of the forstner bit while drilling. This clears chips and cools the bit at the same time.

JKJ

Both are what I do too!