screw questions

[rdsii64]

Yesterday I asked a question about figuring out what size hole to drill so you could tap it for the screw you need to use. Well now that my DRO kit has arrived I have another screw related question. The documentation that came with my kit doesn't tell you the sizes of the various screws in the kit. So before I can use a reference chart to figure out the size of my clearance hole I have figure out the screw and have no idea how to ACCURATELY measure the screw to get the information I need.

Thanks in advance.

 

[JimDawson]

The good news is that screws are standard sizes and thread pitch. Screws will measure slightly smaller than it's nominal size, 1/4 inch will measure about 0.245 or so. Metric screws the same.

Unless you have special threads, which is not common, all screws will be either fine or course threads. Again, let's use the example of 1/4 inch. These come in two thread pitches, 1/4-20 (Course) and 1/4-28 (Fine). With some experience you will be able to judge the size and pitch just by looking at the screw. You can measure the screw with a caliper, then you could count the number of threads in an inch (or half inch) and compare what you find with a tap drill chart, to correct any errors in your count.

The same applies to metric screws, except that the thread pitch is measured in threads per mm, so counting the threads in 10mm and dividing by 10 is the easiest way to do it.

 

[jbollman]

1-19-15

Dear rdsii64

In regards to your question of what size drill for a particular thread I would suggest that you contact KAR tooling ( Canada )

Many years ago I obtained this chart which lists most all thread screw sizes and what drill(s) to use. Items listed are:

DECIMAL EQUIVALENTS w / TAP DRILL SIZES ( ENGLISH & METRIC ), TAPER PIPE, STRAIGHT PIPE, AND SHEET METAL SCREWS.

I use this chart many, many times in my shop. Number for KAR is 1800-387-3127 . Granted that company is in Canada & perhaps you
could contact USA providers & obtain a full depth chart.

In the interim if you have any questions feel free to contact me at any time on this web site,

Regards)

jbollman

 

[cathead]

Sometimes I run a nut on the bolt and then see if the tap fits the nut. Then I measure
on the end of the tap just where the threads are beginning and drill the hole that size.
You can tell if the hole is too small as the tap will have a lot of resistance. I know there
charts and letter and number drills for taps and a Machinery's Handbook to give you
all the details. I found that the requirements for a certain size hole was usually not
found in my shop or even locally available so I went to the above method for some
of my tapping. I do have a drill set with a chart that tells what bit to use for a given
thread size in standard coarse and fine threads. That I use a lot. I remember I
got a huge pile of hardware for my Mitutoyo DRO and most of it was not applicable
to my mill and was not ever used. I had to make a lot of cylindrical spacers fitted
to my application to get the fit correct. I'm sure the DRO people do their best but
it would be difficult to provide a hardware kit that fits every machine out there.
Good luck with your DRO install. Take the necessary time to get it running true
and it will serve you well.