tolerance on formed parts

Discussion in 'Sheet Metal' started by quickcut, Sep 20, 2015.

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  1. Sep 20, 2015 #1

    quickcut

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    Good Day all. Can anyone throw some light on the size tolerance say of a formed rib. My rib formers (for a 750 cruzer) have been cut on a water-jet cutter. I drew the template , but it is drawn by a series of co-ordinates and you have to connect the dots. This I did by joining them with a bezier curve and then re-drawing by means of arcs , as my cam package does not like Beziers. Secondly, what size holes would you use to prevent tearing/ stress relieving when bending the sides up. Would it be dependent on the material thickness via a ratio ie 10 times the thickness or is there a set rate/ formula, thank you. [​IMG]
     
  2. Sep 20, 2015 #2

    dcstrng

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    There is a set formula for the bend radius (I think it is in your Zenith plans package, but is available -- Google -- 6061 bend radius), but 6061 is quite forgiving and you've got far more precession in what you've done than I did in my abortive CH750 wings project. I traced mine out with a Sharpie around the actual rib bending template using the washer trick (have a handful of different sized washers for different "tab/flange" sizes) and then cut them out by hand with snips -- as I recall I simple bored a 0.025 hole [oops, 0.25" not 0.025" -- fat fingers] at each of the four corners... deburred accordingly and never had an issue with cracking, actually was easy and pleasantly therapeutic work... Mine came off pretty nicely -- saved them for a one-off project I hope to tackle once my current one is flying...
     
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  3. Sep 20, 2015 #3

    quickcut

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    Thanks DC, You are correct about the bend radius .it is on the plans but the hole diameter is not.(not that I can see) Any way you have given me a starting point. Thanks
     
  4. Sep 21, 2015 #4

    mcrae0104

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    AC43.13 might have some answers for you.
     
  5. Sep 22, 2015 #5

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    Thanks . Lot's of reading there
     
  6. Oct 14, 2015 #6

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  7. Oct 14, 2015 #7

    mcrae0104

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    0.1 meters? Angstroms? Where is this standard coming from?

    Practically speaking, the tolerances that are achievable in a home shop (and those that are of any measurable difference in performance on the typical homebuilt) are closely related to the width of a Sharpie. YMMV.
     
  8. Oct 14, 2015 #8

    Angusnofangus

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    I should have said 'inches'. These are the standards for manufactured aircraft parts. If you want the parts of your homebuilt to fit together in any sort of decent manner, you would want to adhere to these standards. It has nothing to do with measurable performance, but everything to do with structural integrity. The closer you are to specified dimensions the better your parts fit together. the structure is stronger, and the better your A/C looks in the end. Now I know, that as homebuilders, we are not trying to build space shuttles, but a little quality goes a long way.
     
  9. Oct 14, 2015 #9

    ekimneirbo

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  10. Oct 14, 2015 #10

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  11. Oct 14, 2015 #11

    Dana

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    There is no "standard" tolerance. The appropriate tolerance for, say, a wing rib shape would likely be too loose for main spar attach fitting, and vice versa.

    That said, drawing title blocks often include a standard "unless otherwise specified" tolerance... which as often as not has no relation to reality for the part shown on any particular drawing.

    Tolerance may be specified to guarantee 100% interchangability, or 90% (with some parts needing fitting), or for the requirements of structural integrity or mechanical function. In many homebuilt designs, match drilling on assembly is used as an alternate to close tolerances.

    A center drill is a stepped drill with a small diameter point which allows good accuracy starting the hole and serves as a pilot for the larger portion.

    Dana
     
  12. Oct 14, 2015 #12

    Angusnofangus

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    Those are the standard tolerances. Sometimes there might be a part where the standard doesn't apply, thus the "unless otherwise specified" note, in which case the part in question will be noted. The tolerances are what you live and die by when you manufacture aircraft parts. Out of tolerance parts will be rejected be a QC inspector faster than you can say "Jack Robinson",and you will have scrap on your hands.

    As for the 'centerdrill', same thing as what i call a piloted drill.
     
  13. Oct 14, 2015 #13

    ScaleBirdsScott

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    Seems yall have the same concepts with different nomenclature.

    The key to remember is that tolerance adherence for the homebuilder is really just a guideline for good practice and no means a pass/fail unless the builder themselves holds to that.

    Meanwhile a vendor supplying parts goes out of specified tolerance at their own peril.

    The real challenge here is that of the homebuilder who is trying to leverage big boy toys like water jets or laser cutters as in the OP case, by generating patterns based on what they get from their plans set. The problem being often these shops doing the work are probably used to making signs, sculptures, or architectural decor where tolerance is not a live or die word, and if it is a factor is usually 1/8" or somewhere thereabouts. Unless you are very strict with them and develop a full set of dimensioned drawings for the flats being cut and specify tolerances and make it clear that the parts must meet those specs or you won't pay for the part: you are going to get dimensions that look close but in a few spots may be significantly out. Or you will pay a lot most likely for a shop to actually stand behind the precision.

    Ask me how I know this :p

    There's a few good solutions to that problem that I've seen:

    1. Find a shop that does this type of precision work regularly and who won't shrug or laugh at the idea holes need to be accurate to just a few thousandths. Find one that also doesn't charge extra for this standard, but a fair rate.

    2. Design parts to be cut out such that the tolerances can be wildly out. Ie only have basic alignment holes, if that, and if you insist on predrilling, only drill one of the sheets in a given rivet joint. If you just need the airfoil+flange cut out and can do the holes later it's no problem to be 1/8 in or out: your forming block will make consistent ribs.

    3. DIY a CNC machine and take it upon yourself to learn the particulars of tolerance and transposing a design to an accurate part. It's a lot more work up front but the long term payout is probably worth it if you can make the up front investment of time and space and effort and money.
     
    Last edited: Oct 14, 2015
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  14. Oct 14, 2015 #14

    Angusnofangus

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    Looked at your website, very interesting, indeed. I have a soft spot for Hellcats. In 1972 I worked at a small FBO at Brown Field in San Diego, and we had a Hellcat parked on site. The guy I worked for was going to fly it one day (he was an old Navy pilot), but had brake problems and came back.
     
  15. Oct 15, 2015 #15

    mcrae0104

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    Again I ask, what standard? You're either making this up, or imposing a standard inappropriate to homebuilt aircraft. No disrespect intended; I just want to know your source.

    Baloney. I don't mean to be rude to a new member, but thousandths of an inch in the profile of a wing rib (which is the question at hand) do not affect structural integrity. Unless one works for NASA, Scaled Composites, Boeing, or Airbus, he likely doesn't have the capability of measuring the physical difference of +/- 0.010" let alone the aerodynamic performance difference. The structural difference between a simple beam (which a wing rib is) with a depth of 6.000" and 6.020" is precisely irrelevant.

    Advice: use your Sharpie (ultra-fine tip if you are an engineer by training), cut close to the line with your snips, file to the line, and sleep well at night. It makes no practical difference, structurally or otherwise.*

    *If you are building an XB-70 replica in your garage, YMMV; my garage is not that big, nor is my eyesight that acute.
     
  16. Oct 15, 2015 #16

    ekimneirbo

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    Let me see if I can explain it another way. Instead of calling it a "rule of thumb" its probably better to say "a generally accepted practice". Many eons ago while machining a piece of copper on a surface grinder, I was having trouble holding a .0002 (two tenthousandths) inch tolerance. The copper was soft enough that as the wheel removed the material it would embed itself ever so minutely in the wheel causing the part to vary .0003 or .0004 inches. This was just enough to take the part outside the requirement. I tried
    several different things to correct the problem but nothing worked. I called the engineer and asked for some assistance. I explained the problem to him and asked if the tolerance could be widened. He said "Sure, how much do you need ?" I said if
    he could give me half a thousandth (.0005) it would be easy to stay within the tolerance. I asked him after he unhesitatingly changed the specification, why he had put such a close tolerance on it in the first place if it wasn't really necessary?
    His reply was...."I had to put some tolerance on it, and .0002 seemed like a good one". The point here is that there is no magic wand in establishing a specification. Many engineers would often put excessive tolerances on parts simply because
    they often were challenged by a lack of hands on experience, and they would protect their reputation by erring on the safe side. Before the advent of CNC machines, it was much more time consuming to machine parts to exacting tolerances.
    Gradually, to keep costs down , a +/- .010 tolerance became pretty routine on three place decimal dimensions. I don't believe it was ever officially declared a "standard", but it was commonly used. The thing you have to consider is that in each type of industry,there are things that dictate that the drawings they work from may have more or even less need for precision, and the norm for them could be different. I'm not sure if the companies manufacturing cell phones and cameras have to work to
    tolerances in 4 place decimals or not. (.010 is ten thousandths .001 is one thousandth .0001 is a tenth of one thousandth, and .001 is the same as 10 ten thousandths .0010 ) (.001 = .0010) An easy way for people who are not used to using decimals is to think of having ten thousand dollars in your pocket, or 1,000 dollars in your pocket. If you gave someone 10 dollars out of your $10,000 stash, you would have given them 10/10,000 which is 1 thousandth of what you had.
    In the sheetmetal part of the aviation industry, I would imagine many parts are manufactured with .010 tolerances, but then other things like rivets are held to much closer tolerances, and the holes they go in must be held closer than .010 in diameter in order for the bucked rivet to fill the hole. My point here is that all tolerances are situational rather than standard, but I do agree that there has been a majority of drawings issued with the +/-.010 as a general tolerance for the whole drawing unless something else was specified for a specific need.... in the sheetmetal and machined parts world.

    For those folks not familiar with a centerdrill, they are usually short stubby and extremely rigid bits that have a small drilling tip that quickly changes to a thick body. While sometimes called a pilot drill, pilots are usually not drills per se.
    A pilot (at least where I worked) was usually just a smooth round tip that inserted into an existing hole, and allowing the tool following the pilot to cut circumferentially to the hole...as in the case of a counterbore. I do think there were drills that were custom ground on their tips to form pilots when the depth of the counterbore needed to be deeper than normal. They usually did this on drills called "core drills" which had 3 or 4 flutes instead of the normal 2 flutes on common drills.

    In the pictures below, people can see the basic differences between the tools. I also realize that different tools are often commonly called by similar names in different areas of the country or other countries. Its no big deal as long as you get what you want when doing a job. Centerdrill 1.jpg Note the Centerdrill has a short ridgid body. It doesn't "walk" off location as a two flute drill bit might. Its kinda like a precision centerpunch and the two flute bit will then drill at the correct location with no "walk".
    Core Drillbit 1.jpg The Core Drill has more flutes and is more rigid. It is somewhat like a reamer and usually is used to follow an existing hole. The tip is sometimes ground in the shape of a round pilot to insure it follows the existing hole. It usually doesn't have the ability to initiate drilling a hole from scratch because the tip is blunt.

    counterbore 1.jpg The counterbore tool has a removable pilot shown next to it. The pilot is selected to fit an existing hole size, and the counterbore tool then cuts a perfectly concentric counterbore around it.

    Pilot Drill 1.jpg The pilot drill is similar to the a coredrill with a ground pilot more so than a centerdrill, but the similarities often cause people to interchange the terminology.

    Hope some of the readers out there learned a few helpful things from all this............:)
     
    Last edited: Oct 15, 2015
  17. Oct 15, 2015 #17

    Angusnofangus

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  18. Oct 15, 2015 #18

    Angusnofangus

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    For those folks not familiar with a centerdrill, they are usually short stubby and extremely rigid bits that have a small drilling tip that quickly changes to a thick body. While sometimes called a pilot drill, pilots are usually not drills per se.
    A pilot (at least where I worked) was usually just a smooth round tip that inserted into an existing hole, and allowing the tool following the pilot to cut circumferentially to the hole...as in the case of a counterbore. I do think there were drills that were custom ground on their tips to form pilots when the depth of the counterbore needed to be deeper than normal. They usually did this on drills called "core drills" which had 3 or 4 flutes instead of the normal 2 flutes on common drills.

    In the pictures below, people can see the basic differences between the tools. I also realize that different tools are often commonly called by similar names in different areas of the country or other countries. Its no big deal as long as you get what you want when doing a job. View attachment 44192 Note the Centerdrill has a short ridgid body. It doesn't "walk" off location as a two flute drill bit might. Its kinda like a precision centerpunch and the two flute bit will then drill at the correct location with no "walk".
    View attachment 44193 The Core Drill has more flutes and is more rigid. It is somewhat like a reamer and usually is used to follow an existing hole. The tip is sometimes ground in the shape of a round pilot to insure it follows the existing hole. It usually doesn't have the ability to initiate drilling a hole from scratch because the tip is blunt.

    View attachment 44194 The counterbore tool has a removable pilot shown next to it. The pilot is selected to fit an existing hole size, and the counterbore tool then cuts a perfectly concentric counterbore around it.

    View attachment 44195 The pilot drill is similar to the a coredrill with a ground pilot more so than a centerdrill, but the similarities often cause people to interchange the terminology.

    Hope some of the readers out there leaned a few helpful things from all this............:)[/QUOTE]


    Thanks for the update on 'center drills'. What I know as a 'piloted drill' is what our vendor referred to as 'double-margin drills'. Basically a step-drill with only one step. They are great for opening matched holes. As a lot of the stuff we built used 1/8 rivets, the most common was #40-#30.
     
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  19. Oct 15, 2015 #19

    ekimneirbo

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    Like to make a couple of General Statements here, as nothing is ever without exceptions.

    Generally holes and there locations need to be consistent in size and location to assure parts will assemble properly and be strong when assembled.

    Dimensions for rib blank contours do not have to be extremely precise. The holes drilled into the blanks for location on the template should be as
    consistent as possible, so they should probably be drilled in a stack with the template. Once the blanks are attached to the template and formed, the
    any small variation in the original external size will be of no consequence as the folded edge will vary somewhat and an exact dimension
    is pretty much irrelevant at that point.

    Dimensions for rib forming templates should mirror the drawing as closely as possible but in most wings deviation of the contour up to maybe .060
    probably won't have much effect. The main thing is that the ribs which are formed around the template be consistant with each other.

    If someone buys a premade kit, the manufacturer will have all the parts fabbed with exacting tolerances.

    If you go to any decent machine shop they will as a matter of course be able to make the exact hole sizes you want at the exact locations you need.
    You might find a shop that is not equipped with a machine that will do the work you want, but if they do have a machine with that capability,
    holding your size and location tolerances will be simple for them.

    If you are doing the work to build a kit from scratch, you will have variation in size. You are not equipped to make and verify many things as being nearly
    perfect. What you have to do is realize when close is good enough, and when it has to be exact. By this I mean, if you are drilling a 1,000 rivet
    holes........If you match drill them to the mating part a slight variance between hole locations in a line will not hurt because you are match drilling
    it. On the other hand, you do need to be as consistent as possible on the size of the holes.....but an occasional oversize here or there won't ruin the
    whole wing and you can use an oversize rivet if needed.
     
    Last edited: Oct 15, 2015
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  20. Oct 15, 2015 #20

    Angusnofangus

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    Well said. On the subject of oversize holes; even brand new, factory-built airplanes will have the odd oversize hole.I know because I have done it. Mess up a hole, engineering tells you to put in the next size or whatever, and it's done. Except for the paperwork. Every deviation gets documented.
     
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