Indexing head in issue 198.

I am thinking of making the indexing head starting in issue 198.

could someone explain why the main body of this is stated as being 4.562" long when surely 4 1/2 will be perfectly adequate?

as far as I can see the spindle is sized to suit the body.

This happens a lot with plans, parts that do not have to fit to anything else are in sizes to 4 decimal places.

Roy.

That's alright then, this one is only to 3 decimal places.

Probably a misprint; it should be 4.5625", then it'd be 4-9/16".

Andrew

This is one of the the problems that arise quite often when the drawings haven't been created by qualified engineering draughtsmen.

Len. P.

.

Probably the standard design method:

1. Make a prototype
2. Measure what you made
3. Draw that

MichaelG.

P.S. I remember a design in Model Engineer [many years ago; one of Ian Bradley's I think] where a component was "made from" a bicycle crank arm ... but it was beautifully drawn-up and dimensioned.

...and quite often when they have!

Russell.

What's the poor designer to do? If he specifies the dimensions in fractions someone will complain that's its old fashioned and it needs to be in decimal so that his DRO will understand. Someone else, of course, will want it in metric. The designer's CAD package will kindly convert everything to 3 places of decimals because some of the dimensions need to be that accurate but it makes some other measurements look a bit ridiculous. One convention is that the number of decimal places quoted should depend on the accuracy required but, frankly, 1.12" looks no more sensible than 1.125" for a dimension that is really 1 1/8" measured with a 6 inch rule. If the reader is not capable of deciding which dimension need to be to the nearest thou and which don't, should he be trying to make the item in the first place?

Rod

4 1/2 it is then .

Roy

Let's call it 115mm, that's a nice roundish figure.

Rod

Which is why most (all?) CAD programs let you set a default (say 3) decimal places but give you the option to override it for specific dimensions. Often this is as simple as right-clicking on the dimension and selecting the approriate number of decimal places. It's a normal part of the "tidying up" process before the drawing is released.

That's what I do in Autocad but I'm not sure it's quite so easy in some of the cheaper packages. All this just adds grist to something I witter on about from time-to time - a consistent house style for the magazines.

Rod

Same in Draftsight, one of the cheapest package as it's free. Oh yes, it's left click not right.

Russell

This is where the challenge should come in where you can read a drawing and interpret it for your own use.

Ok if you are a complete newbie then stick to the drawing until you have some mileage / scrap / skinned knuckles [ delete as required ] under your belt.

Some years ago I read a post on a forum where this guy had made one of the tradition 4 tool tool posts, you know the one, block of steel, slot in each side, bolts in from the top.

5 people posted saying have you got the drawing ?

WTF ?

Even if he had posted the drawings chances are it would not have fitted the intended lathes.

Is it rocket science to work out the block need to be as big as the bit it sits on, the bottom to the slot id worked out from centre height and the size of tool steel used, leave a bit on top for strength and bung a hole at each corner and 4 extra ones in between.

It's not about drawing, it's about concept.

Curious. In the Windows gui, left-click is normally used to activate a default; right-click to select and activate an option.

Roy,

Not aimed at you or anyone in particular.

Just a general observation.

I rarely stick rigidly to dimensions unless it matters. There's nearly always a tweak to take into account that the pieces of raw stock I have might be slightly too small or I'm deliberately modifying to suit my personal use. As John says, the design concept is important rather than sticking rigidly to arbitrary sizes.

Edited By Chris Trice on 14/03/2013 00:30:04

ah the joys of the imperial system, converting between fractions and decimals and other rubbish, who'd have thought the far neater metric system was only adopted in the early 1970's - a mere four decades ago.

.

Give it a few more years ... "five decades" is more recognisably "metric"

MichaelG.