Threaded holes

[ajgepper]

I am a instructor at Plam Beach CC learning how to program and run a simple block form student project. My question is how do we program to check a 1/4-20 threaded hole? I am using 10 hits for diameter and position. I am thinking I should measure as a cylinder with even more hits to get enough points on the thread helix to establish position.

[Wes Cisco]

Use a cylinder, use 2 levels with 5 hits per level and enter .05 (1 divided by 20) for the pitch. The distance between your depth and height (the two rows of your cylinder) should be a multiple of the pitch. (example: hole is .5 deep with .4 thread, set depth @ .1 and height @ .3. The difference of .3-.1 = .2 which is = .05 x 4) I do this all time and it works well.HTH

[cmmguy]

How about using the thread feature in the auto circle???

[Wes Cisco]

How about using the thread feature in the auto circle???

You can, but if the hole was hand tapped crooked . . .

[cmmguy]

You can, but if the hole was hand tapped crooked . . .

The hand tap will only follow the minor diameter which is created using the drilled hole. Now if that is crooked...

[Matthew D. Hoedeman]

The hand tap will only follow the minor diameter which is created using the drilled hole. Now if that is crooked...

Naw, you can get the tap to go in skewed to the drilled hole, if you are hand tapping. I have seen it, I have done it. Usually (but not always) it will also break the tap, but it can be done.

[Wes Cisco]

The hand tap will only follow the minor diameter which is created using the drilled hole. Now if that is crooked...

Well that depends on if the tap is started straight and whether or not the gorilla spinning the tap uses a center, or even gives a chit. I have seen straight holes tapped quite crooked. Never underestimate the ability of the primates... But by and large if your machinist are worthy of the title, then an autocircle will suffice.

[kmcmm]

Use a cylinder, use 2 levels with 5 hits per level and enter .05 (1 divided by 20) for the pitch. The distance between your depth and height (the two rows of your cylinder) should be a multiple of the pitch. (example: hole is .5 deep with .4 thread, set depth @ .1 and height @ .3. The difference of .3-.1 = .2 which is = .05 x 4) I do this all time and it works well.HTH

OK Wes, what about metric threads, how do you figure pitch for those?

[#2#]

OK Wes, what about metric threads, how do you figure pitch for those?

I know but I'll let Wes explain.

[cmmguy]

Naw, you can get the tap to go in skewed to the drilled hole, if you are hand tapping. I have seen it, I have done it. Usually (but not always) it will also break the tap, but it can be done.

Well that depends on if the tap is started straight and whether or not the gorilla spinning the tap uses a center, or even gives a chit. I have seen straight holes tapped quite crooked. Never underestimate the ability of the primates... But by and large if your machinist are worthy of the title, then an autocircle will suffice.

Ok, if you predrill the hole and are able to start and tap crooked... how is the CMM going to pickup anything other that the minor diameter(predrilled hole) anyway??? You would need to use a thread locator if tapping the hole crooked is a problem.


Metric is well... metric.

[Wes Cisco]

Ok, if you predrill the hole and are able to start and tap crooked... how is the CMM going to pickup anything other that the minor diameter(predrilled hole) anyway??? You would need to use a thread locator if tapping the hole crooked is a problem.


Metric is well... metric.

If crooked, the tap will cut the minor diameter on one side, when you probe the hole as a cylinder you will see the skewed vector/size/location that results. I agree that for the most part an autocircle is fine for location, but a lot of the threaded holes I check also have a perpendicularity callout, so I am in the habit of using a cylinder so I can report perpendicualrity too.


Pitch is the distance from crest to crest or root to root whether English/Standard or Metric. I do not do a lot of metric work, so to find the pitch if it is not on the print I would pull out Machinery's Handbook.

[george frick]

my .02- i would teach students to just use the circle feature using maybe a 3mm ball & 4 or 5 hits to check the location of 1/4-20 tapped holes. on a cmm the simpler the geometry the better. when you start using geometrical shapes like cylinders you introduce what we call "projection error" which can give you erroroneous results as to the intended application. check the minor dia. with gage pins and the threads with threaded plug gages. also- be cognizant of your tolerances. look at your standard deviation or form also.

[ajgepper]

Use a cylinder, use 2 levels with 5 hits per level and enter .05 (1 divided by 20) for the pitch. The distance between your depth and height (the two rows of your cylinder) should be a multiple of the pitch. (example: hole is .5 deep with .4 thread, set depth @ .1 and height @ .3. The difference of .3-.1 = .2 which is = .05 x 4) I do this all time and it works well.HTH

I run V3.5mr2 and do not see height, only depth on my cylinder auto feature screen. Does the auto feature program "force" the programer to use the on clearance planes option? It does not seem to want to work when I step in to measure a hole or cylinder. Anyway, I manually measured two level with 5 hits each and the software recognized a cone. It is still not a very precise position, though.

I am having a blast learning to use this system. Some of my students took their reports away with long faces, lotsa red marks...

[#2#]

Pitch is the distance from crest to crest or root to root whether English/Standard or Metric. I do not do a lot of metric work, so to find the pitch if it is not on the print I would pull out Machinery's Handbook.

Ok, I'll have to explain then.

For english threads the designation will look something like 1/4-20 UNC-2B. The 1/4 is the major diameter of the thread. The 20 means 20 threads/inch. So, to get the pitch (inches/thread) of the thread you invert the fraction and get 1inch/20threads = .05. The UNC is for Unified National Coarse thread. The 2 is a tolerance class. The B designates that it is an internal thread (A is for external).

Now metric threads are designated a little different. Take for example, a M22X1.5 - 6h thread. The M is for metric, the 22 is the major diameter in millimeters. 1.5 IS the pitch in mm/thread. No math required - unless you are writing your program in english and then you will have to divide 1.5mm/thread by 25.4mm/inch to get .059055inches/thread. The 6 is a tolerance class designation. The h is also a tolerance class designation. The fact that the h is lower case means it is an external thread (capital means internal).

All of that is in Machinery's Handbook.

[#2#]

I run V3.5mr2 and do not see height, only depth on my cylinder auto feature screen. Does the auto feature program "force" the programer to use the on clearance planes option? It does not seem to want to work when I step in to measure a hole or cylinder. Anyway, I manually measured two level with 5 hits each and the software recognized a cone. It is still not a very precise position, though.

I am having a blast learning to use this system. Some of my students took their reports away with long faces, lotsa red marks...

The reason the position is not correct is because the thread is a helix. If you measure a circle with the hits all at one level, each hit will touch a different place on the thread. The first may fall on a crest and then the second just off of it, etc. If you use the pitch function in the autocircle/cylinder window, the hits will be generated such that they will spiral up or down the thread so that all hits will touch the same point on the thread. You will not know if all the hits fall on a thread crest or halfway between crests so hole size can not be measured but location can. One thing you will want to check is that the probe is spiraling in the correct direction. If for instance you have a hole with a vector of 0,0,1 and a right hand thread, and the probe moves in a counterclockwise direction around the hole, the z value of each hit should be increasing. Think about it, if you screw a bolt in and unscrew it counterclockwise, the z position of the bolt head will be increasing. Changing the pitch value from a positive to a negative will change which direction the probe spirals.

I'm not sure about 3.5 (I'm using 3.7) but on mine the box for depth is kind of centered in the window just under number of hits. The box for height is over on the right where you choose whether it is an internal (hole) or an external (stud) feature. See this for a good explaination for how to use the two boxes.