Auto Circle/Cylinder Pitch

[TAC45]

I am programming offline currently and have been asked to program location for threaded nuts and bolts on an assembly. I do not have access to the drawings and CAD doesn't include the actual threads on model. I am not sure whether or not RH or LH threads. I will need to know this before I can correctly program the measurement, correct? The pitch value will need to positive or negative based on handedness of thread?

[Hi-Tech Redneck]

I feel the value will be the same either way....the direction will be different (CW vs CCW)

[mckenzie]

you can do it either way

[mckenzie]

if they're locations for bolted connections to hold together an assembly wouldn't that just be clearance holes for the bolts, no pitch needed?

[TAC45]

Sorry if wasn't clear. The assembly I am dealing with has exposed bolts and nuts which will be used to assemble additional components at customer.

[kingsld1]

Might be able to match up the nominal hole size on the cad with the thread size. CAD should give you the minor diameter. This of course assumes that the engineer is using the more common pitches for a given minor diameter. YMMV; Buyer beware

Other possibility is to get the Assembly Cad, open up the CAD assembly dialog box and look for the nuts and bolts that go in your part. Might be able to matchup the names in the assembly dialog with a part number and from that get the thread size.

[TAC45]

My supplier quality was able to get the thread length and pitch for me. I don't have much experience with measuring threads in the past. So I just need to enter pitch value and then either change +/- of pitch or CW/CCW to account for direction of thread? As I noted before I do not know handed of threads but may be able to get hold of the physical components to verify. Thanks

[TAC45]

Thank you so much for the help!!! Sorry to be hard headed here but I want to make sure I don't miss anything, I will still ultimately have to know thread direction correct? Or are you saying that I will be able to measure correctly for location executing the steps you provided without knowing direction? Thanks!!

[mckenzie]

no worries
no you definitely need to know thread direction.

if you don't want to wait to hear back from your client you could program them all as RH threads. (99% of threads i've ever seen are RH) then if some do turn out to be LH you could just edit those specific features and flip the direction or make the pitch negative.

[TAC45]

Got it! Thanks!

[Ego Murphy]

My supplier quality was able to get the thread length and pitch for me. I don't have much experience with measuring threads in the past. So I just need to enter pitch value and then either change +/- of pitch or CW/CCW to account for direction of thread? As I noted before I do not know handed of threads but may be able to get hold of the physical components to verify. Thanks

Best thing you can do is add extra cylindricity and roundness dimensions to verify the Form of each threaded inspection. These will save your butte when the Position shows out-of-spec but a bad Form result clues you that your probe didn't march down the spiral correctly and instead bonked all over the thread peaks, flanks, and valleys - meaning the reported centroid is not to be believed.

[TAC45]

Best thing you can do is add extra cylindricity and roundness dimensions to verify the Form of each threaded inspection. These will save your butte when the Position shows out-of-spec but a bad Form result clues you that your probe didn't march down the spiral correctly and instead bonked all over the thread peaks, flanks, and valleys - meaning the reported centroid is not to be believed.

Thanks, makes sense to confirm results. My positional tolerance thankfully is rather large in each axis: +/- 1 MM (.040").

[SingularitY]

Like the people mentioned before, the pitch will be the same whether LH or RH but it will either be CCW or CW. I'm not sure how experienced you are with pitch, so here are a few things I learned.

1- If you are using an Auto Circle, pitch is all you need. You can adjust the depth either with your axis location or depth with no I'll results. I do recommend a higher than normal point density. I used to use Maximum inscribed but lately I've stuck with Least Squared since the pitch should theoretically hit the same area of the thread so inscribed, I feel, could actually cause more error in a circle.

2- If you choose Auto Cylinder then there's a bit more work. I keep the depth and ending offset set to zero for ease of use. I enter the pitch, (let's say it is 1.25,) and I figure out the length of the threaded cylinder. Usually it tells you the length by default but you can't probe that entire length because of lead in and out plus other slight variables. Let's say the thread depth is 25mm. I figure I can probably probe at least 18.75mm of this thread comfortably so I change my cylinder length to 18.75mm. Divide the length by the pitch, luckily this comes out evenly and equals 15, but if it isn't a nice even number I will usually round down to the nearest whole number to be safe unless it's super close to the next whole number. If you end up rounding then you have to work backwards to figure out the cylinder length again and change the value accordingly (rounded number x pitch= length.) Add 1 to my previous number of 15 and that becomes how many levels, adding 1 because you must include the first thread. So now for a 18.75mm long cylinder with a pitch of 1.25mm I would have 16 levels. Since that is overkill, you can reduce the number of levels by dividing by a number that produces a whole number, like 4. The result would be 4 levels, each level would probe in the same area of the threads and would give the least amount of error. I still use least squared but I'm sure inscribed is ok too.

EDIT: After #2 is complete, you would want to change the location of the cylinder along it's axis so it does not probe on the start or end of the thread. You could change the depth and ending offset the same direction and the same amount, but changing the axis location is easier and has less room for error.

Sorry for the long reply.

[TAC45]

I have very little experience with pitch, your information is much appreciated, thank you!