Machining Parts at Home

Discussion in 'Workshop Tips and Secrets / Tools' started by Winginit, Apr 23, 2017.

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  1. Apr 28, 2017 #61

    Little Scrapper

    Little Scrapper

    Little Scrapper

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    I'm a tool slut. Just to be clear.

    But I'm also on a homebuilt airplane forum and in 2017 we seen to be in the assembled airplane era not the built era.

    So I feel, given the demographics of today modern assembler, a good ole drill press is ideal. I also do not feel, based on my experience, a mill should be purchased instead of a drill press. I struggle with that thought process and money has nothing to do with it. I say buy both if machining is three road you want to go down.

    I will stand my ground on the fact that a drill press is cheaper. The theory of selling later doesn't really make sense. I could sell a mill, I'd never sell a drill press.

    But that's me. I have more tools than Carter has liver pills. Many don't get used, but when they do I'm always glad I have them.

    The original topic for this thread assumes the buyer doesn't already own a drill press. Think about that for a while. A guy who doesn't own a drill press buying a mill instead? Start with the drill press and see how it goes.

    Also. I don't like wood dust on a mill or lathe. A drill press is mandatory in my opinion.
     
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  2. Apr 28, 2017 #62

    Little Scrapper

    Little Scrapper

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    Finished a service call just now. Retired guy, medic in Vietnam and his retirement hobby is gunsmithing and making bullet for people.

    Little tiny house with a little tiny basement and his shop.was tiny in the basement. But he still had 4 drill presses, (those little bench top ones), a little bench mill, and two little bench lathes. The mill and lathes appeared to be old Grizzly ones, the little bench drill presses were older as well.

    It was totally under lit and freaking tooling everywhere. It was awesome, dirt and grease everywhere.

    It's truly amazing how people can cram little machines in such a tiny spot and put out so much work.

    Anyhow, thought you'd enjoy that.
     
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  3. Apr 29, 2017 #63

    Winginit

    Winginit

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    I believe they are two different terms that generally describe the same thing, but if used in specific situations one might split hairs to show some difference.

    The gage block is often used by machinists to quickly verify that their micrometer or calipers are reading correctly. Any calibration done this way is actually ONLY verifying the calibration at that one specific dimension. Anywhere along the range of the specific tools range of movement, the calibration can still be off do to wear. Generally wear will either occur near the smaller decimal readings because thats where most travel will occur, OR wear will occur at a specific spot in that range because the tool has been used in a production setting to verify a specific dimension repeatedly. The thing about a caliper is that it is a rack and gear as opposed to the thread used in a micrometer. It has less contact surface, and no lubrication. The gear and rack themselves have to have some play in them. They are handy for grassroots work because there is usually plenty of room for variation in the parts being made.When lathes and milling machines are professionally made, they are hand scraped (as you know). Even though they are precision parts for flatness and straightness, they are not assembled to rely on exact precision tolerances between the components. Saying that in a different way: The parts are precision straight and flat. They are not necessarily precise in how they have to fit together. Now, that being said, a few people must be asking themselves..."Huh?". When the machines are assembled they have a part(s) called "gibs". These gibs are actually built in shims that can be adjusted by a screw to take up the slack between the moving part and the stationary guide of the machine. So if a machine is assembled with parts that vary in size, they can still be adjusted . As they wear, they can also be used to return the machine to the correct clearance. Dial Calipers on the other hand come new with a certain amount of play built in and they only get worse as time goes on. They can be rebuilt, but almost never are.

    What you are referring to is "comparative inspection" where the actual dimension is irrelevant. Normally I don't like to talk about my personal qualifications for anything. You will just have to trust me that I know the hows and wheres, and whys of mechanical parts inspection. Here is the difference in what you and I are talking about. Yes, you can take a caliper and squeeze it against any object and compare the three readings you get. You will probably get pretty close looking readings depending on the thumb pressure you apply. If the shaft does happen to be the same diameter its whole length, the comparitive readings are also going to be pretty close, thumb pressure excepted.

    Here is what you are not considering. When making a rectangular hole in a piece of material maybe 2" wide and 5" long, you can't really use comparitive inspection because you have to make and hold actual dimensions. You not only have to maintain straightness, but parallelism, and perpendicularity in every case within .0005. This require both precision and accuracy as inseparable components of the process. If you watched the videos I attached earlier, you will see that the inner and outer measuring points on calipers are almost never the same, with the inside legs usually being inaccurate.

    Look Vtul, you seem like a nice guy and I'm really trying to be objective. I really don't want to debate your skills or knowledge. You have shown that you are capable of some remarkable things and I admire what you accomplished.


    I'd like to clarify one thing here. I'm glad that there are other opinions than mine on this thread and I think most of them have been pretty well stated and objective. I think that the give and take and the somewhat adamant stances from both views lets people look at what has been said and decide for themselves how they want to proceed and what they need to best meet their desires. I hope I haven't offended anyone, and I'm not offended by anyones comments either. In my mind its a good thread showing opposing viewpoints.:)
     
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  4. Apr 29, 2017 #64

    Winginit

    Winginit

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    I'm posting this separately cause I thought it might be amusing.

    Once upon a time in another galaxy (machine shop) I machined a part that had a groove with .001 diametrical tolerance. I was doing this on a lathe and using a flat blade micrometer. When I finally had the finished part ready I took it to the seasoned inspector for verification. He inspected it and promptly got his red pen out and wrote the part up as being out of tolerance. I took it back to my workstation and double checked it. Nope, it was still reading what I thought it should. Took it back to inspection again and he said it was still wrong. I asked if I could see his micrometer. I checked it using his micrometer and sure enough it read exactly the same as mine. I showed it to him and he replied that I wasn't allowing for wear. I asked him what he was talking about. He said that when using flat bladed groove mics the contact points of the mic were always the same and would wear slightly, so even though the mic said one thing, the actual dimension was incorrect. (anyone laughing yet?) I said , "well how do you know how much to allow for wear" He said ".001, you allow .001" (You gotta be laughing by now) Anyway I told him to go ahead and reject it and write it up because I wasn't taking another .001 off when I only had .001 tolerance. The part was later accepted by another inspector.

    2nd story: Running a machine that was putting 4 holes thru an aluminum casting. The holes on each end penetrated about 1 1/2" lugs all the way thru, and the two more central holes needed about 4 inches to penetrate all the way thru. The purpose of the holes was to accept some precision steel pins that were pressed into place. The tolerance on all 4 of the holes was +.0002/-.0000 . This was way over tolerancing for a simple pin locating hole. (Bout a 1/2" diameter pin) Anyway, the oil lubed reamer would hold the tolerance on the end holes but on the center holes about 3/4 of the way thru the holes would change size by .0001 and put me out of the required + .0002. The holes actually got smaller into the -.0001 range. The same finish reamer was used for all holes. If I adjusted it bigger the other holes would be wrong, so there really was nothing I could do. Remember after all the sole purpose of these holes was to accept a pressed in place steel pin.
    I talked to the inspector (a different one from the last story, and usually a pretty good guy). Nope, he wouldn't cut me any slack. Now these parts were being inspected with dedicated air gages, so there was no way to fudge. I took the rejected part back to my machine and later brought him another one which he accepted even though I had changed nothing.
    I waited a couple of days and then took the one he had rejected back to inspection with a blank new inspection record. Since he didn't realize it was the same part, he inspected it again and etched another serial number on it. Serial numbers don't always stand out on cast parts, so he didn't notice the first one. When he was done inspecting it, VIOLA..it was deemed a good part.
    I retrieved the original inspection record and took it along with the new record and the part back to the inspection station and asked him how come the same part was bad two days ago. In an effort to save face he accused me of bring a different part. I showed him the two serial numbers. He took the part and reinspected every dimension on it in an attempt to prove I had switched parts, but he had no luck because it was in fact the same part. Its one of those "winning the battle but losing the war situations". From that point on he went over every part with a fine tooth comb.

    Why am I boring you with these stories? My point simply is that even experts don't always get things right even though they have the very best tools. There are certain things that are manufactured that really don't need close tolerances to work, but there are other things that do require precision. As a rule of thumb, I generally consider most round things as usually requiring more precision than most square things. Not always true, but generally it works for me. Hope everyone got a laugh reading these.
     
    Last edited: Apr 29, 2017
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  5. Apr 29, 2017 #65

    vtul

    vtul

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    nevertheless...
     
  6. Apr 29, 2017 #66

    gtae07

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    And in those cases the engineer that put those tolerances down needs to be dragged out and beaten.


    I can say that because I was that engineer once :nervous:

    I was very fortunate to fall in the favor of the machine shop lead in my previous position. I was able to spend the next year working one day a week in the machine shop and learning from the machinists there (and oh man, did I learn a lot). On my second day there, they had me boring out holes that I'd dimensioned to a ±.00025 tolerance. Occasionally we actually needed that for certain parts that got bearings pressed in, but I didn't realize how small that really was and how much work it took to do it.

    Well, let me tell you, after boring 14 of those holes, I never dimensioned a part that tight again unless it was really, truly necessary.

    I did learn a little trick though; I once turned a bushing that wound up .0005 too small. Now, I had the authority to sign off that it was acceptable, but one of the guys told me to hold the bushing in my hand for a couple minutes, then measure again. It met the drawing that time, thanks to the 30°F worth of thermal expansion :ban:
     
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  7. Apr 29, 2017 #67

    Winginit

    Winginit

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    That was a great story. Its easy to talk about those small tolerances, but actually being in the position to really verify them, people really don't realize how difficult it can be, and how something as simple as temperature change can cause those small tolerances to change. You machine a part in a warm machine with warm oil cascading over it. Then you take it to the inspector in his air conditioned room and he lays it on his cold granite table and inspects it.

    Enjoyed the story.:gig:
     
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  8. Apr 29, 2017 #68

    Winginit

    Winginit

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    I thought I might try to give a little perspective to help people understand how small .0001 (1/10 of a thousandth of an inch) is. After looking on line, apparently even the experts don't seem to agree on the thickness of a human hair. Best I could determine is that it's probably somewhere between .004 and .007. (4-7 thousandths of an inch)
    If you were to divide a median .005 hair into tenths of a thousandth, you would have 50 pieces. If you just divided it into .0005 (5 ten thousandths or 1/2 of a thousandth),you would still have to be able to split the hair into 10 pieces. Hopefully that puts the concept of .0001 into some perspective.

    The thing the homebuilder should realize is that other than understanding the concept, for the most part this information is of little use to him. Usually the only time you need extremely close tolerances is when machining a shaft to install a bushing or bearing....something round. When and if that happens, thats where the "comparitive machining and measuring" comes into play. This is what Utul was refering to. If a shaft needs to be made that will be a press fit with (.001/.002)interference, you can use a micrometer and a caliper even if they don't provide accurate comparison readings. The dial caliper would be used to measure the internal diameter of the bushing or bearing. Then the micrometer would be used to measure the distance between the dial caliper tips. It does not matter what either one of them says on its scale or dial. It only matters that you need to make the shaft slightly (.001/.002) larger than what will fit in the micrometer. Write the dimension down and then loosen the micrometer and begin machining the shaft. Mic the shaft in multiple places to insure that you are cutting the same diameter all along the shaft. These are roughing cuts and the actual dimension does not matter. As you begin to near the dimension you wrote down, you will need to reduce the size of the cuts. I don't recommend trying to take .001 cuts, as the tool needs a little pressure to cut into the metal. Use maybe .003/.005 as you near the finished dimension you wrote down. Allow the shaft to cool if it has gotten warm during the cutting process. Remember that you are trying to make it slightly larger than what you wrote down. If you hit a point where you are .002/.003 above the dimension you want, it may be difficult to cut without going too far. In that case, turn the lathe on and use a file to lightly remove a little more metal, and maybe a little sandpaper. When you have reached the desired diameter (a few .0001s and maybe as much a whole .001) you should be ready to press the parts together. See if the two parts will slip together. If they do, you went too far. If they don't, (Hooray), then put the shaft in a freezer for a little while. Put the bearing out in the sun or use a heat gun to warm (not hot). Now see if they will slip together, or press together with a LITTLE pressure. If they don't seem to want to go together, put the shaft back in the lathe and file/polish it slightly again and check the new dimension against what you wrote down. Now try the hot/cold thing again. Comparative machining is used a lot in machine shops and shipyards when an old part is available but blueprints aren't. It works well in grassroots scenarios too.

    Other than something like the case above, most machining projects on an airplane can be done in the same vein. You don't have to make a part to a specific drawing specification, you only have to make something and then make a mating part to fit it. The comparative first part becomes your reference gage. So don't be frightened by all the "machinist speak" about holding finite tolerances and specifications.

    Another "food for thought" story. We were manufacturing some rather complicated gun bodies for a machine gun type grenade launcher. The tolerances were very precise and special steps were taken to insure both dimensional conformance and surface smoothness. In testing, the guns were plagued with jaming when used in desert climates.
    The solution...looser tolerances and more play in the moving parts.
     
    Last edited: Apr 30, 2017
  9. Apr 29, 2017 #69

    Hot Wings

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    I don't see what the fuss was about. After all .00025 inch is well over 50,000 atoms of iron. :whistle:
     
  10. Apr 29, 2017 #70

    vtul

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    Yup.
     
  11. Apr 30, 2017 #71

    lr27

    lr27

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    A downside of substantial milling machines: It's terrifying when you drop them. At a job, long ago, we were moving what I think was a Swedish milling machine. It tilted and fell off the cart. Fortunately, I moved my feet in time. As it turns out, those heavy duty handles you find on milling machines have limits. One of them punched a hole in the wooden floor and was itself bent. I wonder what it sounded like downstairs?

    I was once hired to fix a design by a guy who didn't get tolerances at all. Everything was just dimensioned exactly, so of course nothing fit. Also, later, had a plastic part that wasn't fitting any more because the part had been designed with no space to allow for tolerances and it was coming out maybe .001" off dimension. There was a slight problem with the hydraulic cylinder that closed the vendor's mold, which we were able to identify and fix. Considering that the official tolerance for everything on this part was +/- .005", that design was pretty bad. And I hate fudging around with strangely curved surfaces in CAD.

    ---------------
    Anyway, I don't have a lot to say about home machining. Except that if you have access to all sorts of fancy tools and then lose that access, you may decide that woodworking or machining isn't fun any more. So beware. Another hazard is jealousy. I have a friend who used to work at Draper Labs and managed to score a lathe, a milling machine, and a precision grinder of some sort. They are all in his basement now! No room for a full sized aircraft, though. It's probably appropriate, because he's very skilled. He once made me a mandrel for making carbon booms on. To make sure it didn't whip in the lathe, he made it in three pieces. However, the joints were pretty much invisible and I couldn't even feel them.
     
  12. May 19, 2017 #72

    donjohnston

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    I've been watching this thread and I think that this may be a good place to throw this out.

    I spent some time years ago as a design engineer and had to create some parts on my own. Fortunately, there was a nice machine shop with a machinist who helped with some of the finer points.

    But during my build, I've lusted after a mill and lathe. Lot's of small aluminum parts and handful of steel parts would have come out so much nicer if I would have had something more than my Delta drill press. But I don't have the space, money or requirements for a Bridgeport mill or Southport lathe. And while I normally avoid multi-task machines, sometimes it's the best (or only) option. :depressed

    With that said...

    I'm trying to decide between the Bolton AT750 and the BP250V. Now both are Morse Taper machines. I was once told "R8 or forget it. Only sissies use MT." Bolton 3-in-1's are all Morse Tapers except one so I'm going to have to go against that old guy.

    So for the experienced machinist out there, would one of these be considered better than the other?
     
  13. May 19, 2017 #73

    Hot Wings

    Hot Wings

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    Deja vous:

    I started out with it's father. Had no idea they were still available. Paid about twice the asking price long ago. Still have it. The only real problem is that the motor came with corroded contacts. Cleaned them and it worked for a while. I gave up on the mill motor and wrap a rope around on of the drive pulleys to spin up the mill when I use that. Just like an old pull start mower.

    It's not very ridged and the ways aren't hardened. Even with those limitations I managed to build a perfectly usable Benson rotor head with it. Most real machinist would call it a cheap toy, but for a hobbyist building a plans built plane = It's worth every penny they are asking.

    If you do get one go straight to eBay and buy an inexpensive DRO set.
     
  14. May 19, 2017 #74

    donjohnston

    donjohnston

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    Thanks!

    Could you offer any suggestions or examples of an inexpensive DRO set? They didn't exist (at least I don't think they did) back when I was using this type of equipment.
     
  15. May 19, 2017 #75

    Dan Thomas

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    Those contacts are silver-faced, and they burn and lose their conductivity. Some have enough silver that you can file them clean and keep going, and some have so little that once filed, they burn again real quick.

    You can use a double-pole, double-throw center-off toggle switch. Sit down with a piece of paper and pencil and work out the circuit. It ain't hard at all. Switch up to start, switch down to run, switch centered to stop. I have a handy small old direct-drive tablesaw I did that with after its start relay quit.
     
    Last edited: May 20, 2017
  16. May 19, 2017 #76

    Hot Wings

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    No, I haven't got any real world experience with them. I just know I lashed up some long throw dial indicators back in the dark ages to make repetitive projects soooo much faster. The 'real' lathe came with a DRO installed and I wouldn't do without now.

    Re: The silver contacts. I know I should fix it properly but I'm the only one that uses it and the rope is just so handy....
     
  17. May 19, 2017 #77

    Pops

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    Now the other end of the scale. Back when I was young I worked building several of the largest coal fired boilers in the world for electric power plants. After the building steel is erected the first part of the boiler is 2 steam headers that go at 318' elevation by punch marks on the building steel set with a water level. Crude, yes, but close enough because the boiler expands about 19" in length when up to temp. Each header that weights 157 tons is suspended by dozens of hanger rods about 50' long from the building steel above. As a header is coming up, someone has to slide down the hanger rod like a fireman coming down a pole and stop at the bottom with you feet on the large nut above the large clevis and when lined up insert the heavy clevis pin. I was young at the time so I usually volunteered so the older guys didn't have to do this.
    Once the header is hung, it has to be set on the proper elevation. We set the header correctly by the prints. Next morning it would be off beyond tolerances by the prints. So we had to redo the previous days work. By the time we got all the tools to the elevation and out on the building steel, the workers with the water levels would call us on the radios and tell us to stop , we had it right on elevation.
    It had warmed up to about the same temp as when we set it the day before and now on the correct elevation. Since the measurement on the prints had no notation about temperature it was called OK.
     
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  18. May 20, 2017 #78

    Turd Ferguson

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    I would check with Bolton to see if they can offer any advice on a DRO. Might not be practical (or possible) to fit a generic unit.

    None of that existed when I did machine shop work for a living. Heck, none of the machine work to send men to the moon was done with DRO equipped machine tools so I guess one should be able to get a plane airborne without it?
     
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  19. May 20, 2017 #79

    Jerry Lytle

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  20. May 20, 2017 #80

    Dana

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    I've had good results with the iGaging DROs on my mill.

    Dana
     

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