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How accurate and reliable is Inventor’s Flat Pattern?

67 REPLIES 67
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Message 1 of 68
Breeze104
1832 Views, 67 Replies

How accurate and reliable is Inventor’s Flat Pattern?

How accurate and reliable is Inventor’s Flat Pattern?  We have clamping bands that we form in a press and then put a final sharp break in one end as a spacer once they are bolted together.  I am using the contoured flange tool to create the part.

 

The reason for the question is that I can’t make these numbers work together to produce a product that resembles this 2D dwg.  What ever you can tell me sure would be helpful.

 

See doc for pic of part

67 REPLIES 67
Message 61 of 68
Chassuer
in reply to: jletcher

After reading all of these responses in this thread, I believe the initial question was never really answered. How accurate are the flat patterns? Let me explain:

 

Everyone here says to get and use the machinery's handbook. I happen to have Handbook 25.

On page 1250, they explain the formulations to figure out what the beginning metal size should be before any bending takes place. (Figure 4). I made this part in Inventor, and did all the hand calculations to find what the piece of material should be before ALL bends take place (flat pattern). My hand calculations match the book completely (5.545 inches long). When I click the "Create Flat Pattern” button and then measure the overall length of the part, it is longer than it is supposed to be. Using the handbook the overall length is 5.545 inches "BEFORE BENDS". Using the K-factory as my unfold rule the overall is 5.563. Using the Bend Compression rule the overall length is 5.572. In all cases the overall lengths that inventor gives you are incorrect.

 

Flat patterns are supposed to be what the part looks like before the metal is stretched and compressed. I believe Autodesk has the formula in the creation of the flat patterns backwards. They need to be subtracting the stretching and compressing amounts off of the size, not adding to it. Adding to the part is for the actual part after they are bent and compressed to make them to actual size.

 

I'm using Inventor 2015

Message 62 of 68
mrattray
in reply to: Chassuer

The answer: they're as accurate as you make them.
Inventor doesn't know your manufacturing process, so you really need to tailor the unfold rules to match your processes.
Inventor doesn't "have it backwards", it' just making assumptions. If those assumptions are not correct than you need to feed it the correct information.
Mike (not Matt) Rattray

Message 63 of 68
JDMather
in reply to: Chassuer


Chassuer wrote:

 My hand calculations match the book completely (5.545 inches long). When I click the "Create Flat Pattern” button and then measure the overall length of the part, it is longer than it is supposed to be. ... 

I'm using Inventor 2015



Attach your file here - you have made a mistake somewhere.


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Message 64 of 68
jyager
in reply to: JDMather

It obviously varies on your equipment, but I've been using the generic Inventor K factor settings for sheet metal parts for about 18 mos now.

 

We have a 30 year old monster hydraulic press.

 

Everything we have formed using the flat patterns with generic Inventor settings has come out w/n 0 to 1/16 of it's inspectable dimension. That discrepancy could easily be from where the die strikes the metal and have nothing to do with the flat pattern.

 

This is in material ranges 16ga to 1/4" plate, mild steel, galve sheet, stainless and aluminum...so I'd say it's pretty **** accurate right out of the box. If you need a tighter tolerance than that, you can obviously tweak things.

Jason Yager
Inventor Professional 2023.2
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Message 65 of 68
Chassuer
in reply to: jyager

Here is the part I made from the handbook. I made the part exactly like the profile. I had the guys on the floor cut a piece of metal exactly 5.545 and bend it like the drawings shows. I printed the part out 1 to 1 on paper and laid the finished part on top of the drawing and it matched exactly.

 

Next, I had them take the Inventor flat pattern Print and cut the metal to the dimension showed (5.563 long) and bend the part. We did the same thing and the part is larger than the print.

 

Inventor shows the flat pattern in the stretched out version of the metal, not the before bending version as it is supposed to.

 

My guys have been doing this for over 60 years and they are telling me the Inventor is wrong.

 

Also math is math. If you take the handbook information and do the math (long hand) it is still 5.545 not 5.563.

 

If you feel I am incorrect, please make a part based on the information given on page 1250 of the machinist handbook 25 and make it so the flat pattern shows a finished dimension of 5.545 long. Dimensions have to be exactly like the handbook. Please post the finished part on here so I can see where I went wrong. If you us something other than the K-factor or the Bend compression please include the formula you used so I can modify mine.

 

Thank you

Message 66 of 68
jalger
in reply to: Chassuer

Hi Chassuer,

 

Inventor is using a default K-factor of .440 (more accurate for thicker materials).

You are likely using a k-factor of .4 for your calcs.(thats what is typically in the machine hand books)

So the simple solution is to change the K-Factor to .4 to match what your doing. (thinner material's require a different value)

when you change to .4 you get 5.542 (its off by .003, but your press blade will likely cause more deviation then that)

 

It part of the reason the k-factor is flawed it is a guessetimation (guess / estimation) to start with.

Your machine handbook should tell you that its a approximation where the material has no change in size.

Having worked on the shop floor I know it changes from material to material and for different thicknesses.

humidity, and air temp also change the size of the material, making accurate measurements beyond .0001 impossible (except in a controlled lab...LOL)

 

sorry I don't have my machine handbook handy otherwise I would point out the page that it states its an assumption of .4 for the k-factor value.

I truth many places have special bend charts or equations that they use for over bending, or for specific materials.

 

Just change your K-factor default to see the results (if you are unsure where to make the change please let me know I'll post a screenshot).

 

Regards,

 

James

 

James Alger
(I'm on several hundred posts as "algerj")

Work:
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Message 67 of 68
JDMather
in reply to: Chassuer


Chassuer wrote: 

If you feel I am incorrect, please make a part based on the information given on page 1250 of the machinist handbook 25 and .....

 


There are three formulas depending on material.

Brass and soft copper

Half-hard copper and brass, soft steel, and aluminum.

Bronze, hard copper, cold-rolled steel and spring steel.

 

Which of these formulas did you use in your hand calculations (Note: you did not use any of these in the file you attached, therefore of course, since math is math - the numbers will be different)?


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Message 68 of 68
JDMather
in reply to: Chassuer


@Chassuer wrote:
On page 1250, they explain the formulations to figure out what the beginning metal size should be before any bending takes place. (Figure 4).  My hand calculations match the book completely (5.545 inches long). When I click the "Create Flat Pattern” button and then measure the overall length of the part, it is longer than it is supposed to be. Using the handbook the overall length is 5.545 inches "BEFORE BENDS".

Ah, I see.  You used the middle formula in your hand calculations (although this is not what you used in Inventor).
L=(0.64xT)+(1.57xR)

 

You might also notice that I modeled the part a little differently (although this doesn't have any bearing on the results, but I prefer simple, fully constrained sketches).


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Certified SolidWorks Professional


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