Reading a drawing – Radius

[Bruce EdneyParticipant @bruceedney59949]

[Bruce EdneyParticipant @bruceedney59949]

Hi

As a newbie I am having a little trouble marking out this part.

I can work out from the top down to the lower datum line (I think that is what it called) but I am wondering if the drawing is missing some dimensions for the lower section.

How does one work out the centre points of the R3, R10 & R5 radii?

I have emailed the author of the drawings but I am just wondering if I am missing something.

Bruce

[Ian PParticipant @ianp]

Depending on how you are going to make the part you might not need to know those particular dimensions. If you have any CAD drawing software you could redraw the part using all the shown dimension and then you could measure off all the missing coordinates.

Is the part visible at the top of the picture the same item? if so then its 59mm to the centres of the two bottom features, knowing thet you can work back to find the missing radii.

The bit that intrigues me is the oil hole at the top! supposedly drilled through the edge of 2mm thick material it would take strength away (presumably) just where its needed, not that I the faintest idea what the part does.

Ian P

[JasonBModerator @jasonb]

Looks like you really need a distance from the datum to the bottom of the bends.

To place the centers of the arcs you can either draw parallel lines to the sides that are the radius away, where the lines meet will be your ctr point. Or they can be done using dividers set to the radius to bisect the angle and then measure bnack from there. First method will be easiest.

I would suggest the legs are left long until after you have bent them.

 

Edited By JasonB on 21/04/2015 10:20:45

[Bruce EdneyParticipant @bruceedney59949]

That's what I thought Jason

Thanks for your advice Jason & Ian

[CirclipParticipant @circlip]

Looks like the drawing was done using either AutoCAD or one of its clones. Pity it wasn't checked before publication. A failing of the drawing programmes, they get everything in the right place but still need telling where to put dimension lines.

Regards Ian

[Capstan SpeakingParticipant @capstanspeaking95294]

While there is no excuse, many times I have seen drawings where the critical fits were specified and the cosmetic areas were left to discretion.

If this was given to commercial machinists they would decline to take it on.

[Paul LousickParticipant @paullousick59116]

Not a failing of the drawing program but one of operator error.

CAD is only another tool and still requires the input from the drafter. Some packages can automatically dimension a part but it is normally left to the operator to correctly layout the drawing and dimensions. (Missing dimensions can also exist on hand made drawings.) This is one of the common problems with drawings, especially for model engines which are made by un-experienced operators.

You may be able to obtain the missing dimensions by using the mating parts.

Paul.

[CirclipParticipant @circlip]

Can't think of any CAD programme that doesn't "Automatically" know the dimension, but does rely on "Someone" to tell it where to start and where to finish between two points.

Wonder if the originator or seller of the design will issue an amendment sheet or modify the plans? Ooops, been down this road before.

Regards Ian.

( Many years as a Draughty)

[Nick WheelerParticipant @nickwheeler]

Perhaps it's my inexperience showing, but where are any of the dimensions for the legs? Not having the centres for the largely decorative radii is pretty trivial by comparison.

[John Bell 2Participant @johnbell2]

I think R3 means 3mm Radius, R10 means 10mm radius etc..

[Paul LousickParticipant @paullousick59116]

Hi Nicholas,

That is what we are saying. The dimensions for the legs are missing and have not been dimensioned by whoever made the drawing. In Autocad I have to click on all of points than need dimensioning. In Solidworks I can automatically add all of the dimensions but the software does not always position the dimensions where I would like them and therefore prefer to do this manually.

Yes, R3 = 3mm radius

Paul.

Edited By Paul Lousick on 21/04/2015 13:23:28

[JasonBModerator @jasonb]

I thought it looked like one of Julius' designs and indeed it is. He does have a tendancy to try and cram a lot of parts onto the sheet

It would be possible to calculate where the bottom of the legs should be in relation to the datum but a bit of a pain to do so, If he does not come back to you let me know and I'll work it out.

Probably 17mm, if you take the crank at 39mm plus say 1mm as he seems to have the pins above the middle of the 4mm long slot you get 40mm. Take away distance from cyl centre line to top of cylinder (20mm) and the thickness of the top cover (3mm) thats 23 from 40 =17mm

Looks an interesting engine, please keep us posted with updates of your progress.

 

J

Edited By JasonB on 21/04/2015 13:39:58

[Neil WyattModerator @neilwyatt]

On a paper drawing a set of circlce templates would be used to draw in the curves and the actual centres would not be calculated. You'd be expected to use templates and scribe them in tangential to the straight lines. Its the lack of detail of the lengths or angle of the lower legs that would worry me – does it match with another, dimensioned, part?

Neil

[David Griffiths 5Participant @davidgriffiths5]

Looks maybe to me like the R3 rad centre is 52.67 off the vertical centre line and the R10 and R5 rad centre is 48.92 off the vertical centre line and 15 down.

[Capstan SpeakingParticipant @capstanspeaking95294]

I've just had a dabble and from that elevation alone it cannot be reconstructed let alone made.

The geometry doesn't match those dimensions either so I expect that having no decimal places is hiding a multitude of sins.

Perhaps it's an initiative test

[Gordon WParticipant @gordonw]

It does look as if the cut-off drg. at the top is another view and gives what I assume is the critical dimns. If so it should be stated somewhere. But as shown on this forum it should not be made, critical dimns. should not need caculating.

[JasonBModerator @jasonb]

The bit of the drawing at the top goes no further, all it gives is the spacing between the two feet, armed with the 17mm I calculated the part can be made.

Its a free plan so you can't really complain, at least Julius has been kind enough to produce this engine drawing and a whole bunch of others so maybe go easy on the critisisum unless you want to offer some engine designs of your own

[Gordon WParticipant @gordonw]

I'm not complaining! Just saying.

[JasonBModerator @jasonb]

Here you go, having had a closer look I have decided 17.5mm would looks more liek the assembled drawing but really anywhere between 17 or 18 would work.

If it can be drawn it can be made

And a few setting out dimensions, click to get a larger image

If you don't fancy doing the bends, then a couple of blocks the same length as the round spacers may be easier – something like this maybe

 

Edited By JasonB on 21/04/2015 17:04:20

[Ed DuffnerParticipant @edduffner79357]

I just had a play with this in CorelDraw and got a measurement of 20.98 (21mm) where your 17.5mm is Jason. I struck a line between the centre of the 4mm hole and the tangent of R3 to give the angle and then more tangents off that line to determine the vertical position of the 2mm foot. The centre of R3 was taken as being the centre line through both M4 holes.  I'm probably wrong!

Ed.

 

 

 

Edited By Ed Duffner on 21/04/2015 17:16:31

[JasonBModerator @jasonb]

Ed I don't think that will work with the mating parts, did you just take the angle from bruce's drawing?

Centre of crank (light green) pivot to middle of the slot for the piston rod is 41mm.

Cylinder ctr line to top of cylinder 20mm

Cylinder top cover 3mm thick

41-23 = 18mm but the piston rod is slightly above ctr of the slot so say 17.5mm

 

Edited By JasonB on 21/04/2015 17:35:22

[Tony Pratt 1Participant @tonypratt1]

Posted by JasonB on 21/04/2015 16:42:48:

Here you go, having had a closer look I have decided 17.5mm would looks more liek the assembled drawing but really anywhere between 17 or 18 would work.

If it can be drawn it can be made

'If it can be drawn it can be made', not strictly true, I have seen a few drawn examples which couldn't be made!

Tony

Edited By JasonB on 21/04/2015 17:04:20

[Ed DuffnerParticipant @edduffner79357]

Posted by JasonB on 21/04/2015 17:28:21:

Ed I don't think that will work with the mating parts, did you just take the angle from bruce's drawing?

Hi Jason, yes just some assumptions based on what was visible.

Regards,
Ed.

[JasonBModerator @jasonb]

Hence the wink Tony

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