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Tip: Transfer Punches

Rick6a

Rick6a

Well Known Member
Okay...You are faced with a build situation in which you need to match holes in a new part with existing holes in an assembly. An example: You have decided to install nutplates instead of pop riveting the floor in place. You have carefully laid out and pilot drilled the floor ribs for nutplate spacing, but how do you transfer that pattern to the undrilled sheet that lays atop it? Commonly, for this situation and similiar tasks we may employ the use of a strap duplicator such as this one.
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Less commonly, but the way I almost always prefer to accurately locate blind holes is with the use of transfer punches. Think of this type of transfer punch as a strap duplicator without the strap. Quality can vary among manufacturers and this little tool goes by many names including being referred to as "transfer pins," "sheet back markers" by Wicks and Spruce, and "blind-hole spotters" by Spellmaco http://www.rlspellmanmfg.com/blinds.html and Brown Tool http://www.browntool.com/productselect.asp?ProductID=480. Whatever you call them...they work.
transferpunchesfp5.jpg

You simply insert these little punches one or more at a time into the existing holes, and then lay the undrilled sheet (or replacement part) in place atop, and gently tap around the general area with a rubber mallet. The punch will leave an unmistakable mark upon the undrilled material assuring its precise mating alignment with the underlying structure. I keep a healthy stash of transfer punches stored in a 35mm film canister. Available in all common nominal diameters, I have loaded up an assembly with up to a couple dozen transfer punches at one time to make short work of transferring an entire hole pattern in one step. For sheet metal repair work, I consider these diminutive bits of hard steel an indispensable tool.

I once asked a major RV tool vendor why they did not sell transfer punches and replied at one time they did, but most builders did not know what transfer punches are and as a result, sold very few...a pity. Transfer punches are perfectly suited to work in difficult to reach areas that the average strap duplicator, limited by throat depth simply cannot reach. ;)
 
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If you can lay the undrilled sheet on top of the drilled sheet, can't you do the opposite? And then all you gotta do is back-drill. I'd much prefer back drilling, personally. Results will be more accurate imho. If you can't back-drill for some reason, then ok...
 
dan said:
........If you can't back-drill for some reason, then ok...
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That's the point.....situations in which back-drilling is not possible. If faced with a situation in which you need to use a strap duplicating tool, consider using transfer punches as an alternative. I mentioned installing floor panels placed atop pre-piloted floor ribs as one example. In repair work, you may have to remove and replace a skin but the underlying ribs or stiffeners remain in place. Fitting a new skin and lining up the existing holes in the structure is made far easier using transfer punches.
 
There is also a threaded version of the transfer punch--or center as I know them as----for use in threaded holes. The raised point has a hex shank so it can be screwed in and out.

Try machinest supply stores.

One other handy tool tidbit is Bristol key, also known as spline key wrench.
Very simmilar to Allen wrenches, commonly used in radio know set screws.

Mike
 
Mike S said:
There is also a threaded version of the transfer punch--or center as I know them as----for use in threaded holes. The raised point has a hex shank so it can be screwed in and out.

Try machinest supply stores.

One other handy tool tidbit is Bristol key, also known as spline key wrench.
Very simmilar to Allen wrenches, commonly used in radio know set screws.

Mike
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Mike,
Threaded transfer punches can be easily made, using a lathe. I simply cut the head off of a bolt, say 3/8" X 16 tpi (for the tie down ring skin holes). Mount the bolt in the lathe and machine the cut side down to a point. Cut a slotted notch, off center on the end with the point. Remove from the lathe and install the the required hole. In this case, that would be the tie down bracket. Use a straight slot screw driver to insert and remove the transfer punch.
Cleco the skin or part over the transfer punch to locate it. Smack the skin (or other part) with a plastic mallet above the punch. Remove the skin (or part) and drill out using your newly created punch mark.
If you don't have access to a lathe, mount the headless bolt in your electric drill. Spin the bolt in the drill and place it against the spinning stone of your bench grinder. With a steady hand, you can do a very nice, low tech job of creating that point.
Charlie Kuss
 
Charlie----------good tip. Might be a bit soft, but should be fine for aluminium.

I still have a set from a prior life as a machinest. Very useful, specially for blind holes.

Mike
 
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Mike S said:
Charlie----------good tip. Might be a bit soft, but should be fine for aluminium.

I still have a set from a prior life as a machinest. Very useful, specially for blind holes.

Mike
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Mike,
They are reasonably hard if you use grade 8 bolts. However, for use on aluminum, common grade 5 bolts work fine AND are much easier to cut a notch in with a hand hacksaw. I spent 4 years doing Aerospace machine assembly. That is where I learned this trick.
Charlie
 
Rick6a said:
Okay...You are faced with a build situation in which you need to match holes in a new part with existing holes in an assembly.
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Rick, is there a transfer punch that works with a dimpled hole? I've dimpled a few parts sooner than I should have, only to find that I needed to use the dimpled part to match-drill another part. Sure, you can stick a felt pen in the dimple and mark the new part, but it sure would be nice if there was something more accurate like a transfer punch that would nest inside the dimple.
 
rv9builder said:
Rick, is there a transfer punch that works with a dimpled hole? I've dimpled a few parts sooner than I should have, only to find that I needed to use the dimpled part to match-drill another part. Sure, you can stick a felt pen in the dimple and mark the new part, but it sure would be nice if there was something more accurate like a transfer punch that would nest inside the dimple.
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Yes, in the case of AD3 rivets, a standard #40 transfer punch will set inside the dimple. A #30 transfer punch will work in an AD4 rivet dimple. They may move about ever so slightly but for all intents and purposes will produce a reasonably accurate mark to match drill your new part. Because the punches will sit inside the dimple a bit, the mark left after tapping with a mallet may be a bit fainter than if the material was not dimpled, but will still work. Way more accurate than the felt pen approach.
 
Transfer punch

rv9builder said:
Rick, is there a transfer punch that works with a dimpled hole? I've dimpled a few parts sooner than I should have, only to find that I needed to use the dimpled part to match-drill another part. Sure, you can stick a felt pen in the dimple and mark the new part, but it sure would be nice if there was something more accurate like a transfer punch that would nest inside the dimple.
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Mark ... this might work....

http://www.aircraft-tool.com/shop/d...ID=TP-40&ReturnPage=/shop/flyer.aspx?PageNo=1

TP-x.jpg


Available in many sizes... should also be available from a machinists tool supply place near you....

gil in Tucson
 
Rick and Gil, thanks for the info.

Wouldn't a #40 transfer punch be slightly undersized, since a #40 hole will expand after dimpling? I'm assuming a #40 transfer punch is the same diameter as a #40 drill bit.
 
Many sizes...

rv9builder said:
Rick and Gil, thanks for the info.

Wouldn't a #40 transfer punch be slightly undersized, since a #40 hole will expand after dimpling? I'm assuming a #40 transfer punch is the same diameter as a #40 drill bit.
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Yes... but they are available from a machinists shop in the number drill sizes.

Use your drill index to see which size is best for your dimples... :)

gil in Tucson
 
rv9builder said:
Rick and Gil, thanks for the info.

Wouldn't a #40 transfer punch be slightly undersized, since a #40 hole will expand after dimpling? I'm assuming a #40 transfer punch is the same diameter as a #40 drill bit.
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Yes, that is why I mentioned it may move about the hole a bit but will settle on the center of the dimple well enough. As Gil mentioned, transfer punches "per se" are available in any number of sizes and shapes. The intent of my discussion was to describe the practical use of the small type illustrated at the start of this tread as an alternative to using a duplicator strap. In most situations, the use of standard nominal size transfer punches (like I illustrated) such as 3/32",1/8", 3/16" and even 1/4" transfer punches will normally suffice. In practice, I've never used any sizes or shapes other than those mentioned.
 
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I've run into a couple of cases in the last week where I THINK one of these tools would help me, perhaps you can confirm.

First case. I had to make a new elevator stop, but I'd already drilled the first one to the aft deck. No way can you get down underneath that aft deck to use the existing holes as a drill guide, so wouldn't this tool have helped me make the appropriate hole from the elevator stop down?

Second case. Similar deal. I need to make new rudder stops. The first version was already drilled to the fuse (I don't know why I did that, but I did it early in the fuselage process, I must've just got carried away by the plans). Now I need to drill the new stop to the old holes. but I can't use the fuselage skin interior as a drill guide.

Is this tool the answer?

I'll hang up and listen.
 
Bob Collins said:
I've run into a couple of cases in the last week where I THINK one of these tools would help me, perhaps you can confirm.....Is this tool the answer?
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Absolutely Bob.

Both cases are prime candidates where transfer punches can save the day. For example, in the case of making up a new rudder stop, let us assume for the sake of illustration, you can only afford to buy 1 transfer punch making sure it is the same diameter size as the holes in the fuselage are.

#1. Insert transfer punch into any one of the holes in the fuselage rudder stop pattern.
#2. Place the new rudder stop part generally into place, locating it as best you can, then tap it gently to assure a mark is left.
#3. Remove rudder stop, drill hole up full size using the mark the transfer punch left as your reference.
#4. Relocate transfer punch to the next hole location, then cleco the rudder stop back into place using the hole you just drilled into it. Tap again to generate mark at the next hole location.
#5. Repeat process until all the holes are drilled.

You described all too common situations when back drilling is not possible, the use of transfer punches is the next best way to proceed.

Good luck.
 
A couple thoughts -

Transfer punches are a great tool, I've been using them all my adult life. However, the typical punches have one issue which is pertinent to aircraft building - the point is too long to allow the tool to be used to transfer holes from a thin sheet to something else. This is because the cylindrical portion of the punch has not yet engaged the existing hole when the tip contacts the target surface. I suspect that short tip ones are available (for those of us without a metal lathe).

What I have done in cases like that is to tape some sort of shim between the surfaces, just next to the hole (a washer works well around the hole), to give the extra "thickness" needed.

Regarding simply drilling through the existing holes in a sheet - use caution if you want the holes to align and be the same size. The same problem exists for drills that exists for the transfer punches as described above, i.e., the cylindrical portion has not yet engaged the cylindrical part of the hole.
 
dimples up....

AlexPeterson said:
A couple thoughts -


Regarding simply drilling through the existing holes in a sheet - use caution if you want the holes to align and be the same size. The same problem exists for drills that exists for the transfer punches as described above, i.e., the cylindrical portion has not yet engaged the cylindrical part of the hole.
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But if it's a dimpled sheet, just lay the old sheet on top of the new one with the dimples facing up.... :)

Also the good quality transfer punches have a very small "pointy bit" ... the cheaper ones are just machined to a cone. Check out the picture in my earlier post. My #40 punch has an end as pictured. I believe it will sit inside a 0.025 sheet hole and contact the sides.

gil in Tucson - guess how I found this out.... :D
 
az_gila said:
gil in Tucson - guess how I found this out....
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Ok, let me guess, you looked at the sign on the road into town? :)

The idea of laying the dimpled sheet on top is great! I figured that somewhere out there one could find transfer punches with a shorter point. Thanks.
 
Almost

AlexPeterson said:
Ok, let me guess, you looked at the sign on the road into town? :)
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Not quite.... :)
It had a lot more to do with over-trimming the aft plexi on the tip up. The curve didn't match, so I have a slightly custom shape to my aft canopy with the replacement sheet.... :D
No pre-punched stuff in my kit,all the holes in the replacement sheet were transfer punched.....

gil in Tucson....handling the RV-6 plexiglas canopy was the hardest task so far...
 
AlexPeterson said:
Transfer punches are a great tool, I've been using them all my adult life. However, the typical punches have one issue which is pertinent to aircraft building - the point is too long to allow the tool to be used to transfer holes from a thin sheet to something else. This is because the cylindrical portion of the punch has not yet engaged the existing hole when the tip contacts the target surface........
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Alex,

I must confess to being initially confused by your comments. I do not doubt what your experience has been. I too have used transfer punches all my adult life and never encountered situations like you describe. The various tasks, (not counting the extensive workout they got while building my RV) include replacing a leading edge cuff on a friend's C-170 to replacing wing skins on a tired old C-182. Upon reflection, I now think we are comparing apples and oranges here. You must be describing a different type of transfer punch. The thread did go off into an unanticipated tangent with descriptions of a wholly unsuitable alternative tool for what sheet metal tasks I meant to address, and likely source of my initial confusion. The "typical" transfer punches I use have always worked perfectly, at least the specific tool type illustrated below and as originally described as the basis for my comments.
largebhsedited70bd6.jpg
 
This style

Rick... I'm talking of this simpler style...

TP-x.jpg


If the "pointy bit" is shallow enough, they can be used to transfer holes from one sheet (with holes) that is clamped against a new sheet without holes.

gil in Tucson
 
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