Allowances for Coatings : Machinist or Drafter?
Allowances for Coatings : Machinist or Drafter?
(OP)
I have a question, it is pretty basic - but I would like to know what the consensus of opinion is.
We make a lot of parts, through job-shops that need to be hard coat anodized, which increases the size of the part by 0.015" per surface. People having work plated will also have the same problems.
When making detail drawings for the machined part, should the drawing give finished part dimensions (after anodizing/plating) - or the sizes that the part needs to be machined to, so that after machining the required size is reached?
e.g.
Case in point: 0.750" Bore required for bushing.
1. Should the detail drawing call out a 0.753" Bore, so after anodizing the finished size is 0.750" OR
2. The detail drawing state a 0.750" Bore with a Finish notation for Hard Coat Anodizing - putting the onus on the machinist to make allowance for plating?
It's just a little detail, but makes a big difference to the final assembly!
Thanks for any hints on how to deal with this, or is there a standard convention?
We make a lot of parts, through job-shops that need to be hard coat anodized, which increases the size of the part by 0.015" per surface. People having work plated will also have the same problems.
When making detail drawings for the machined part, should the drawing give finished part dimensions (after anodizing/plating) - or the sizes that the part needs to be machined to, so that after machining the required size is reached?
e.g.
Case in point: 0.750" Bore required for bushing.
1. Should the detail drawing call out a 0.753" Bore, so after anodizing the finished size is 0.750" OR
2. The detail drawing state a 0.750" Bore with a Finish notation for Hard Coat Anodizing - putting the onus on the machinist to make allowance for plating?
It's just a little detail, but makes a big difference to the final assembly!
Thanks for any hints on how to deal with this, or is there a standard convention?






RE: Allowances for Coatings : Machinist or Drafter?
When I was a machinist, my credo was "Ask for what you want, because I will give you what you ask for, not what you want."
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RE: Allowances for Coatings : Machinist or Drafter?
If a finished part was needed in the assembly, due to tolerance build up, a second configuration of the part was created for that purpose.
As a side note, (you may do things differently) we always detailed reamed holes to finished size, and had the anodizer mask the holes for us. We did not want to have to account for "build up" in a hole.
RE: Allowances for Coatings : Machinist or Drafter?
Thanks for the quick response!
To clarify:
"We design finish allowances into the part"
You would dimension the example bore 0.753" so that the machinist does not need to make allowances?
(Just so you know - this is how I normally do my drawings)
Sorry if I appear thick, I just need to get this stuff right!
RE: Allowances for Coatings : Machinist or Drafter?
I'm in the aerospace industry and it's normal to put finished dimensions and surface finishes on the print and then put a note stating dimensions and surface finish apply after anodize. Then the machine shop makes a process print to account for the coating dimensional stack. But I've noticed with some "job shops" we use that they tend overlook those small details called notes. So we end up scrapping parts....then we started doing the process print for them and that seems to work out better. So all they have to do is turn a handle or punch a button. But to answer you question: Their is a difference between a engineering print and a process print.
Best Regards,
Heckler
"Never underestimate the power of very stupid people in large groups" John Kenneth Galbraith
RE: Allowances for Coatings : Machinist or Drafter?
•You are the one with quick(er) access to information.
•You are the one who knows the processes the part will go through.
•You are the one that sits next to a phone and in front of a computer and with a calculator in arm's reach.
•You are the one who can best make the decision about how much allowance is needed.
Again, I say, ask for what you want.
Some drawings have hole callouts for machined size and post-finish size. Minimize the math that machinists have to do. They are quite busy enough.
RE: Allowances for Coatings : Machinist or Drafter?
Chris
Sr. Mechanical Designer, CAD
SolidWorks 2005 SP0.1
RE: Allowances for Coatings : Machinist or Drafter?
My recommendation is to add a note that states “Dimensions apply after finishing”. Do not anodize holes. Model your parts at nominal or MMC. As a side note .015 thickness for hard anodize is really thick. Are you really calling out .015 thickness on your drawings?
Bradley
RE: Allowances for Coatings : Machinist or Drafter?
The coating thikness is separeted information, according to relevant standards.
Although the drawing dimensions refer only to machined parts, these dimensions and tolerances take into account the coating thikness.
Regards
RE: Allowances for Coatings : Machinist or Drafter?
For our Engineering prints, the dimensions are for a finished part - how could your inspectors do their job otherwise?
For Process prints you would indicate the machine dimensions and note an allowance for 0.015" anodize per surface is included...
RE: Allowances for Coatings : Machinist or Drafter?
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RE: Allowances for Coatings : Machinist or Drafter?
Bradley:
You are correct 0.015" is VERY thick, I mean't 0.0015"
The situation started when I sent a heap of work to a jobbing shop to have a bunch of parts made. I "took the liberty" of making the drawings exactly as they should have been machined, so after plating/anodizing the parts would be the correct sizes.
I never expected the machinist to make allowances for the post machining anodizing. As much of a good machine shop as they are - they always stick to the drawings, right wrong or indifferent, except this time...London to a brick, if I didn't make allowances for the anodizing - they wouldn't have either! However, as this is a prototype - not too much damage done, as I have extra clearance - if it was 0.003 of interference I would have a much bigger problem!
We are a small company - I will discuss this with the machine shop and note the drawings as to finished or machined dimensions. I like the concept of Engineering and Process Drawings - I was not aware of that until now.
Best Wishes & Thank You!
Anyone Else?
RE: Allowances for Coatings : Machinist or Drafter?
RE: Allowances for Coatings : Machinist or Drafter?
Machine to .xxx/.xxx
.xxx/.xxx after coating
In some cases, say a .750 shaft running in a bushing with .001-.003 clearance, if the coating is only a couple of tenths thick, you could disregard the effects of the coating and just call it a .750 shaft.
RE: Allowances for Coatings : Machinist or Drafter?
A perfect example of what I was trying to explain. Thanks.
RE: Allowances for Coatings : Machinist or Drafter?
Chris
Sr. Mechanical Designer, CAD
SolidWorks 2005 SP0.1
RE: Allowances for Coatings : Machinist or Drafter?
If you look at the MIL-A-8625 spec you will find that it says “As specified on drawing”, for thickness.
Bradley
RE: Allowances for Coatings : Machinist or Drafter?
thanks
Chris
Sr. Mechanical Designer, CAD
SolidWorks 2005 SP0.1
RE: Allowances for Coatings : Machinist or Drafter?
Best Regards,
Heckler
"Never underestimate the power of very stupid people in large groups" John Kenneth Galbraith
RE: Allowances for Coatings : Machinist or Drafter?
If we wanted to be purists about drawings, then I agree that you would want to split the single drawing into 2 drawings. Some might argue that they should even be separate part numbers, such as your casting and machining example. We actually do that here - casting one part number and one drawing, machining another part number and another drawing. I would imagine that is pretty common and it would definitely be crazy IMO to combine them on one drawing. For the example(s) I gave above - it is more practical to have the dimensions on one drawing if it is just a few or even one critical dimension. I say practical, because when you look at the large amount of drawings that this could pertain to in a company like ours, it would be much more time efficient to not have to create and manage tons of other drawings just for a few or even one dimension on each. There is an inherant cost associated with drawings - paper, ink, time to create, time to check, time reserve part numbers, time to enter MRP information, time to file hard copies, etc...That is the reason why tabulated drawings are often used for certain circumstances. I guess for us its just not something that is a clean cut, hard and fast rule as to when we do this, but rather a combination of a lot of factors. I'm sure every company has different thoughts on this and thats why we see in this discussion a good spectrum of differing views.
RE: Allowances for Coatings : Machinist or Drafter?
Chris
Sr. Mechanical Designer, CAD
SolidWorks 2005 SP0.1
RE: Allowances for Coatings : Machinist or Drafter?
I think much of this was not done years ago, but computer tracking led to it.
RE: Allowances for Coatings : Machinist or Drafter?
I have a condensed print from Treffers Precision, Inc. in Phoenix, Arizona 602-258-1554. That has a list of processes with the Mil Spec number and thickness and Comments. So I guess I cannot point right to the page number.
Bradley
RE: Allowances for Coatings : Machinist or Drafter?
Chris
Sr. Mechanical Designer, CAD
SolidWorks 2005 SP0.1