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Drawing Quality

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Jeff Dayman23/10/2011 14:08:08
1914 forum posts
45 photos
In MEW 181 I read the article on pgs 24-24 by Mr. Santafe for his micrometer holder. A nice little tool. However the drawing for the micrometer holder part shows dimensions which will result in a zero wall thickness. The 8mm dia hole is shown at 21 mm from centre on a 25 radius part. (Also, the hole should probably be 8,1 or 8,2 dia to pass an M8 screw. ) There are several simple ways to remedy the wall thickness before cutting metal, the 21 mm dim could be 20, or 50.8 mm stock could be used, or a smaller M6 screw and 6.2 hole could be used. A minor point but a notable one for draughtsmen is that a round part should have the OD called out as a diameter, not a radius. (He makes the same mistake again on pg 57 in fig 6 for the clamp screw detail of the topslide article).
 
In the same issue there is an article on pgs 30-31 for a tangential flycutter. Again, a nice little tool and cleverly made. The drawing of the base plate however is poor. The chosen scale results in views far too small to plainly show the details, and clamp screw hole location dimensions and the slot angle dimension are not shown.
 
A poor job in MEW 181 by the illustrator.
 
JD
Phil P23/10/2011 14:30:08
668 forum posts
168 photos
That's the reason I NEVER work to other peoples drawings.
 
I am a mechanical design engineer by profession. So I either design the part myself or re-draw it myself before actually making anything.
 
That way if there are any errors on the actual working drawings, I have only myself to take outside and give a good kicking.
 
Phil
 
Mark P.23/10/2011 16:25:01
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616 forum posts
8 photos
You should have seen the drawings we used to get from Boeing they were full of errors!!
 
Regards Pailo.
Laurence B23/10/2011 22:13:03
58 forum posts
There was an interesting article in October's edition of Engineering in Miniature by a Mr Hawkins who has built some impressive 5" gauge locos (a King,2 Castles and a Star).
He was quite rightly,in my view, not too complimentary about the standards of model engineering drawings.They usually fall short of any drawing standards that I'm familiar with.And in some designs that I've worked to,individual components have only some of the dimensions added,the rest usually appearing on a mating part that appears on another drawing sheet!
It's no wonder errors creep into some model engineering drawings,that never seem to be corrected.All too often an unsatisfactory 'back of a fag packet' approach is deemed good enough for model engineers.
Anyone contemplating publishing any design in the model press should familiarise themselves with BS 8888 (or even the old BS 308 would do) or buy a copy of Tubal Cain's book on technical drawing.Fortunately there are just a few people writing in 'our' magazines who can draw properly.
John Stevenson23/10/2011 22:25:20
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I think it boils down to the fact that fag packets are not what they used to be.
Lets face it we won two world wars on fag packets and now some erks can't build a model chu-chu without NASA spec drawings.
 
John S.
Andrew Johnston23/10/2011 22:39:17
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5718 forum posts
659 photos
Posted by John Stevenson on 23/10/2011 22:25:20:
 
I think it boils down to the fact that fag packets are not what they used to be.

 
Quite right; now that they've got government warnings all over them that severely limits the space available for the all important sketches.
 
Regards,
 
Andrew
Nicholas Farr23/10/2011 22:47:28
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2477 forum posts
1198 photos
Hi, I don't know what fag pakets are like now, but there used to be plenty of room on the inside, but you had to chain smoke or give everyone a fag so you could turn them inside out to use them.
 
regards Nick.
Gordon W24/10/2011 09:42:30
2011 forum posts
Split beer mats,
The Merry Miller24/10/2011 10:42:28
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484 forum posts
97 photos

Nick, I shouldn't be telling you this because somebody might be watching.
 
Ordinary English people use the back of fag packets but the inside is reserved strictly for spies.
 
Len.P.
 

Baldric24/10/2011 10:49:58
154 forum posts
11 photos
And for those of you across the pond a fag packet over here is the common term for a cigarette packet, not anything else!
Oldpeter24/10/2011 10:56:00
5 forum posts
OK so you have redrawn your project accurately on your favorite CAD program. How do you go about machining to the specification on your drawing? Let me start with a shaft and a bearing of lets say 6 mm. (Remember I am a beginner and leave the fag packets out of it.)
Do you do the bearing first and then the shaft and then making the shaft slightly over- size before starting to use emery paper to get to the exact size before doing trial fits between shaft and bearing or do you use measuring tools and if so which measuring tools? I believe if you can machine a shaft exactly 6mm and drill the bearing is exactly
6 mm you have an interference fit or press fit. In this case you want the shaft very slightly smaller than the bearing - how much - by being just able to push it through or by it just falling out on it's own weight??
I do not know how to machine or drill to exactly 6 mm!! ( the whole length )
What is the standard practice? Don't tell me you can do it with machining on a lathe with sharp tools - you end up either under or over size. It is quite difficult for me doing exact measuring as well with a vernier callipre - do you take the average reading of several different places on your work - it seems to me you get different thicknesses at the start and the end of the cut. Side cutting or face cutting? How do you do buffing or lapping and what exactly is it and how do you do it accurately? I did buy a new mini lathe as my old reconditioned 1921 Packard lathe is fit for rough machining only. Help advice and tips on starting out will be welcomed. I am interested to eventually build a small steam engine.
Phil P24/10/2011 13:09:35
668 forum posts
168 photos
Gosh......Where to start ?
 
As a rule of thumb if I am making a hole and shaft combination of parts I generally make the part with hole first.
Forget using only drills and emery paper, they are for farmers who are repairing ploughs etc.
 
Make the hole either by drilling then reaming to final size, or by boring it on the lathe or milling machine.
Once you have the part with the finished hole you can then turn the part to the correct outside diameter so it has the type of fit you require.
It is usually much easier to make the shaft fit the hole rather than the other way round.
 
I will not go into detailed information about tolerances here, but that is what we are really talking about. Rarely do you see any tolerances on average model engineering drawings, usually the article writer will explain what type of fits you are trying to achieve in the text.
 
From your questions it sounds like you could do with reading up on some basic techniques to get you started.
Dont be put off, it will all become clear...... honest.
 
Phil
John Stevenson24/10/2011 13:24:18
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5068 forum posts
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Try to machine the part you have least control over first.
Holes are hard to get exact to size so do the hole then the shaft which is easier to control dimensionally.
 
Even when repairing a plough it's best to aim for less than a field tolerance.
 
John S.

Edited By John Stevenson on 24/10/2011 13:24:37

Oldpeter25/10/2011 11:05:58
5 forum posts
Thank you for the advice and encouragement - My problem is not that I have too little information - I have too much info and I need practical advice - I do NOT have a CNC lathe, only a minilathe. I have read Bradbury's workshop technology part one and they are on about surface finish 1 to 2 RA and better, but although it is there you can't measure it so it is pure theory - In practice you get feed marks which you can see but can't actually measure. With a travelling steady I hope to reduce the middle hump. With a dial micrometer I can measure differences but not the thickness - I am also reminded to be thinking 3 times, measure twice and cut once. I can't justify getting go - no go gauges as I am not mass producing.
So what must I be looking at - how far do I need to go to get a practical solution to accurate cutting? I don't want to cut ten times and throw it all away. Is lapping or honing
the answer once you are close and then trial fitting? What equipment do you need for lapping and honing?
In practice do I have to aim for slightly tight and then run it in - or do I rely on steam oil or graphite rings to close the gaps? The books do not tell you these things!
 
Richard Parsons25/10/2011 15:07:13
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645 forum posts
33 photos

Oldpeter


Mini lathes are I am afraid very light in their construction. They are really clock maker’s lathes. I after I bolted and gluer (with Araldite) my Unimat SL to a length of girder I found I could ‘put on and achieve up to 3mm cut in mild steel. I always regard new machines as being an assembled kit of parts. They will need not only cleaning but a lot of work fitting back together. This will include scraping the bed ways, getting rid of ‘backlash’, truing up and surfacing the ‘gibs’ and you will get a lathe which is accurate. And remember the ‘Headstock beatings’!


Well thought out tools with very sharp edges and fine cuts are the order of the day. Do not select tools with chip breakers these take power which you can ill afford with a mini lathe.


The three rules are reinforce the rigidity, sharp tools and light cuts.


I always measure after each cut and if the resulting diameter is not correct repeat the cut at the same settings..

Oldpeter25/10/2011 16:50:11
5 forum posts
I got hold of a brand new "mini-lathe" with 2/3 HP motor and 18" between centres - 70 mm radius swing - Was that too small for steam engines of 1" bore? (Still have worn out 1921- Packard lathe for rough work.)
 
Posted by Richard Parsons on 25/10/2011 15:07:13:

Oldpeter
 
Mini lathes are I am afraid very light in their construction. They are really clock maker’s lathes. I after I bolted and gluer (with Araldite) my Unimat SL The three rules are reinforce the rigidity, sharp tools and light cuts.
I always measure after each cut and if the resulting diameter is not correct repeat the cut at the same settings..

Stub Mandrel25/10/2011 21:20:48
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4311 forum posts
291 photos
1 articles
Richard,
 
I can take a 3mm cut on my mini lathe, ARC euro used to have a video of me plunging a parting tool into a 2" diameter MS bar, without chatter on their website. Yes I /have/ done the bearing swap, but nothing more than careful setting up , not the rebuild you describe.
 
Neil
 

Steve Withnell25/10/2011 21:40:59
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821 forum posts
217 photos
OldPeter,
 
You certainly do have too much information!
 
Suggest you find the local model engineering club and see if you get get some demonstrations off the old hands. Or even better college classes - if any exist nearby these days.
 
Failing that - empty your head of all that stuff thats bothering you and get a copy of Harold Halls book "Lathe work - a complete course". Let Mr Hall walk you through tools and techniques.
 
Don't forget Turners used to serve a seven year apprenticeship, it's not quite read the book and switch on...
 
 
 
 
former member 128/10/2011 12:54:59
2 forum posts
Hi All
 
Im new to this game. Was wondering if any one could tell me if auto cad 2009 is compatable with microsoft paint as i have some drawings on my computer that i want to try and load into my hurco CNC machine.
 
FM
John Stevenson28/10/2011 13:18:23
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Moderator
5068 forum posts
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Posted by former member 1 on 28/10/2011 12:54:59:
Hi All
 
Im new to this game. Was wondering if any one could tell me if auto cad 2009 is compatable with microsoft paint as i have some drawings on my computer that i want to try and load into my hurco CNC machine.
 
FM
 
 
 
Sure, those splodges of paint seen on the back of the Tiger tank in Kelly's Hero's were done in Autocad 2009 .
 
John S.

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