Experienced Machinists and Machine Tool Designers Provide great tips and advice in this edition of Weld Fixture Design 101. 

Experienced Engineers share some tidbits in this lesson.


So pay attention!

There is a lot to learn about fasteners and drilling and tapping for weld jig and assembly fixture design.  You want to design your product holding fixtures so that they last and the parts stay firmly in place.


Sometimes when I complete a weld fixture design lesson, I usually post a link to it in several Linkedin Engineering and Mechanical Design groups.  Sometimes a discussion around the topic ensues.

My last blog Weld Fixture Design – Drilling and Tapping, 6 things the CAD Drafter should know – Machine Tool Design got several groups talking.  Which is so cool!  Because, now you can learn from a TEAM of experienced experts.

George Straley, Founder and Vice President of Rentapen was quick to move to CAD

What is wonderful is that in these groups we hear from beginning and seasoned Engineers and Machinists.  You can get a perspective from the CAD station, the class room and the shop.

I was so pleased with the quality of some of the expert tips and general contributions that I thought I should share them with you.


But before I do, I have 3 more tips to add to the 6 things CAD Drafters and Machine Designers should know about Drilling and Tapping…..

Hydraulic Manifold


1)      In applications of high pressure, such as a manifold or directional control valve, you want to make your taps deeper.  The rule of thumb is that the fastener should thread at least 2X the diameter of the screw.  So you will probably want to designate a tap of at least 2 ½ times the diameter of the screw.


Fasten high pressure valves with more thread engagement.







2)      If the piece you are tapping into is not at least 1 ½ times as thick as the diameter of the screw, if possible you should drill through the piece and use a nut (a lockwasher and a nut, a lock nut, two nuts.. depending on standards and the application.)  I am stating this as a fact.  However, it isn’t that simple.  Machine Design is both art and science.  And sometimes… I have seen those with years of experience in design and a Masters in Engineering not always follow this rule of thumb.

In fact in one discussion group on Linkedin,  Steven W. a Machine Designer from Canada said: 

 ” … The 2x diameter is a rule of thumb, but not everyone uses it. I’ve heard 1.5x diameter pretty often. And others have claimed that any more than 3 threads is redundant…

For example:

RAPid Plates locating a round tube in a fixture or jig.

This image shows two RAPid Plates in a stop and drop configuration.  The orange plate to the right of the tube is an RP03-312-2500-DS  (the DS stands for Drill thru and S indicates a hardened steel, 4140 PH).  The longer orange plate on the left is an RP03-312-3000-TS (the T stands for Tapped Thru).


In this case, the designer chose to use the thinner plates on both sides even though the plates are only 1 x the diameter of the screw.  The Machine Designer could have chosen to use RP02 for the tapped side to get 2 X the diameter of the screw for the area of possible thread engagement as you can see in the image from the 3D model site below.

The Designer can choose a thicker RP02 Plate over the thinner RP03 plate. What would you do?

This was this Engineers choice.
What would you have done?

RP02 with taps on the left, RP03 with drill throughs on the right.


It doesn’t appear that there is a space constraint.  Here the goal post is shown with the thicker RP02 plate on the left.  Allowing for 2 x the diameter of the screw for the tapped hole.




3)      In instances of high vibration, use a fine or extra-fine thread.


So here are some excerpts from the conversation about drills and taps from the Linkedin Mechanical Engineering and CAD Drafting and Machine Design Groups.

I hope you enjoy the conversation as much as I did.  And learn a thing or two about drilling and tapping in machine design.




Barry P., Bored Certified Ultracrepidarian residing in Florida – “I love it. There are not enough machine design basics being taught. … One possible mistake a beginner can make with the calculation on small inch numbered screw sizes is using the gage size instead of the screw size as instructed.

For example a #10-32 screw is .190″ OD and a #10 gage drill is .1935″ OD. If you use the gage size you get a .162″ drill for tapping which would not leave much thread, 50% maybe. If you use the .190″ screw size you get a .159″/#21 drill for tapping which gives you 68% thread. A .156″ or 5/32″ drill for tapping gives you 75% thread.

This can become an issue with soft material like aluminum and if material too thin to get two times screw OD of good thread.”


Randall W. President of a Machine Company in Indiana — “Something that goes along with this discussion is engagement length of the screw in different materials. I use, at a minimum 1 1/2 time the screw diameter for course threads in steel. Fine threads, soft material (aluminum, brass) require different engagement lengths. Tapped drill depth and good thread lengths have to be changed to accommodate”

Thanks, Randall, we will be talking more about how to fastener lengths in a future blog.  If you are interested in learning more on this topic make sure you are signed up for Education and Updates in the upper left of this website.  You will receive an email notification each time a new lesson is posted.

Tim G. of Milwaukee — “Great information. I worked for OSG Tap & Die for over six years and in that time you would not believe the number of times I ran into the most basic of misunderstandings concerning taps, their H-limit tolerances and the misuse of the several basic typed of taps in production today. If anyone wants to pick my brain on taps fire away.

Here is a link to OSG’s site with some more good reference material.

Then Tyler P.. asked Tim… Please help me out with the H limits, the difference between an H6 tap and an H4 is .001″ right? I do not think I have ever truly understood the H tolerance. I have worked on a few parts that require tapping oversize due to a future heat treat operation. If I recall correctly, some of the taps I used were labeled as “+0.003” and others used a higher H than the print. What is the correct way to order and use a tap on a part that will undergo heat treatment?

And James T. replied to Tyler with this… “Tyler – this may help:
Each jump in H limit puts the tap .0005 higher than basic pitch diameter. The second website has a nice drawing showing it. So an H4 to H6 is a .001 increase in size range.”


Merlin D. an entrepreneur in the Seattle, WA area said.. “Susan, this is a very informative group, nice to see.  … a standard 10-32 nut will have about 5 thread widths, so a 10-32 nut should be thinner than a 10-24 nut. However, when ordering nuts a designer needs to specify nut widths to satisfy the loads that will be on the machine screw.”

Here’s some comments from another Linkedin group..


Mike C. from the UK said: This article shows why designers that have come from a shop floor back ground produce better designs. Hands on and actually knowing what a set of taps look like and how to use them.”

Bernie D. President of a company in NY commented:  “Of course there will be some schools of thought on this. Seeing as how the article concentrated on machined parts I could see the taper tap as being used, possibly even as a starter tap for a hole to be finished with a bottoming tap.

Actually in all my 40 plus years of machining and precision sheet metal work, I have never used a bottoming tap to start and finish tapping a hole. It was only to finish the tapping to the bottom, so yes, actually two operations. On a closed bottom hole you do have to take in consideration where the chips will go. You absolutely do not want them at the bottom of the hole while you are tapping. …

The only true way to get a bottom tapped hole is with thread milling. I wholeheartedly agree with one of the images presented, specifically the note “BOTTOM TAPPED”. This calls attention to the process to be sure it is estimated and managed for in production.”

Shea P. an Engineering Manager in New Mexico responded to a criticism of the blog article by pointing out, “Mike, you’re right. The “taper tap” in the illustration is not intended to create a tapered thread. Three styles of taps (not a ‘set’ of three) are depicted in the image: a bottoming tap, a plug tap, and (yes) a taper tap. Don’t be confused by the descriptive name given the last tap. The name is not meant to describe the shape of the thread it will cut. Instead, it describes the shape of the tap itself at its leading end (or the tip) as being tapered. Ms. Straley stated quite well the differences in the styles of the taps in Section 5 of her article by answering the question “How many imperfect threads does each tapping tool create?”


James Tiedemann a Manufacturing Engineer in the Milwaukee area said: “Good work Susan! I’ve seen many prints in my career with thread callouts that are totally or nearly impossible to make.”

Kathy Bornheimer Career and Employment Specialists stated: It would be great if we had more people in education be not only members of this LinkedIn group, but actively participate. The “new” workforce coming into manufacturing need direct experience under the watchful eyes of the current practitioners.”

In a future blog we’ll be talking about fastener depth and how to calculate what length you need.   It’s easy!

In the mean time I would like to share some great news!  Our latest addition to our RAPid Tooling Components™ Line is now available.  Our Wolverine Clamp Risers.  What’s neat is the clamp riser can be ordered using the part number of the wolverine clamp. We already have riser for many other brands of clamps including De-sta-co, Te-co, All American, and Jergens.  3D models are available from our website to place in your designs.

Watch this video to learn how you can place your standard purchased-part parameters into a purchased component step file using Pro/E Wildfire 4.0.

‘Til next time,

The Queen of Lean Machine Design



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