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Does this represent the tap drill size?
mike_mcelhinney
Member Posts: 4 EDU
I thought the circled numbers in the images represented the tap drill size. When the tap size is selected, it auto-fills. Sometimes it matches according to all of the given charts but these are a few examples when they don't. Thanks for any help.
0
Answers
@mike_mcelhinney would you be expecting drill sizes of 21/32" ((0.656in) and 31/64" (0.484in) for those two holes?
Is this a thread class issue?
Seems to be incorrect acccording to.
https://www.americanfastener.com/tap-and-drill-size-chart/
Thanks for your response.
I'm a teacher and I explaining what a tap is, tap drill size etc. I'm specifically referencing a Starrett tap drill size chart and both numbers in my pic with the red "X" don't show up anywhere on the chart… coarse or fine thread. So if a student puts the desired tap size into the drop down menu for a threaded hole, its auto-filling with the wrong tap drill size. Does Onshape have a glitch or am I missing something?
Hmmm. That 0.669" is indeed a little odd. The closest size is 17mm, but in a purely inch shop, that wouldn't be available, so 21/32" as you said, or maybe 43/64" (0.671875 in for approximately 65–70% thread) or even 11/16" (0.6875 in if you want still easier tapping but a lower thread percentage).
FWIW, Machinery's Handbook agrees with 21/32" as a standard commercial tap drill size.
If you're really trying to control thread class and dealing with different hole depths or other tolerances, you might want to have a slightly larger hole. Indeed there are tables of sizes from 0.642" to 0.668" for that as well (class 1B or 2B with depths from 1/2d to 3d), but I don't understand why Onshape chose 0.669" as the default.
I wonder if it's a case of OS choosing a percentage for thread depth, and ignoring the closest drill size (wire, number, or fraction), for UNC/UNF threads? Whereas all the drill charts round up/down to the nearest wire/number/fractional drill available?
Even more reason that one request a couple months back of removing the drilled hole diameter callout from a tapped hole is a good idea. It's not the job of a 2D print to state to the machinist how to make a thread. Only the final thread spec should be called out.
Thanks for everyone's input. Since I'm just teaching the basics, I will have them check the tap drill size chart to make sure the correct value is entered. I really only need them to understand that they cannot drill a hole that is the same size as the bolt and expect it to screw in and the reason behind it. I have them create a nut and bolt and then 3D print them. We use hand taps and dies to cut the treads.
If anyone has a definitive answer to this question, I'm still open to it. Is there anyone from Onshape out there that has the answer?
While I agree with you on not controlling a machinist, there are a ton of shops that don't have someone with that level of experience that need this info on the drawings to control how the parts are made. For example, almost the entire shop that I work with has no clue what a drill/tap chart is or what the difference between allowance and tolerance is. If I were to show them GD&T symbols or specify a fit class/type, their heads would explode.
This sound like the common theme in all industry…can't find good/competent help. so the trend of placing that responsibility goes up the chain. Lots of 'experience' gone out the door to retirement or similar.