Modeling Machine Screws

I hope that one of the many fine Experts on this Newsgroup can point
me to a Powerpoint Presentation of modeling a machine screw. I need
to model several different size metric ball/screws, and V-Threads and
for some reason, thus far I never had occasion to use or learn how
to use the helicoil cutting tool.
I would be most grateful for any help anyone could give me

Karen

 

The Help section in SolidWorks covers helical cutting or helical
sweeps very well, and following the example steps in the Help &/or
Tutorial files will help. The examples of helical items in the user
submitted files on the SolidWorks web site will also let you follow
feature by feature to see how it is done.

You didn't bring up the most important question. The real issue is if
you want a "real" thread or not? Real helical threads in any
significant number of screws will cause your SolidWorks assembly files
to slow to below a crawl.

It might be better to experiment with Toolbox screws and threaded
holes and then prepare the same thing with exactly cut and formed
screw & threaded holes and see the difference.

Bo

 

I am sorry for not being specific enough. The models do require that
the threads be "real". The threads made for asthetic purposes are
easy enough to make, in that they are nothing more than annual
grooves. This method would be unacceptable for this job.
I am told that the V-Threads must be modeled to English Standard
dimensions. I would think that the dimensioning for that can be
obtained in an Engineering book of some sort. With regard to the
memory consumption, I did hear of this before, however it will have to
be lived with.

Karen

 

Karen,
I am unable to see the status of "Expert" at Solidworks with the
Hubble Telescope, but that being said, I believe I am able to help you
with the particular subject that you are having a problem with. I do
not have a Powerpoint Presentation, however I have an audio/video
tutorial that I made several months ago that will demonstrate the
method for making a machine screw . I have put it on my Website for
you, and if you have a Cable Connection or DSL, it shouldn't take you
very long to dowload it. I think it is about 16 MGB in size.
You can substitute the shape of the thread that you require with the
shape that is covered in the tutorial, (Ball-Screw), and if you need
exact shape and dimensions of a V- thread, they can be had by getting
your hands on the "Machinerys Handbook", published by the Industrial
Press. Any decent sized library would have it.
Here is the link: http://www.deangelistool.com/help/thread.zip

Good luck,
G. De Angelis

Valhalla Grafix www.valhallagrafix.com

 

Regardless of whether you use multiple annular grooves or a single
helical groove, the Help file contents and Tutorial that comes with
SolidWorks and the user & commercial submitted files at Solidworks at:

http://www.3dcontentcentral.com/3DContentCentral/

Bo

 

I really hate to publicly announce my stupidity like this, but after
finding that the machinery handbook is ambiguous on the issue (and boy
did I look), I feel I have to ask the question:

Is the profile of a screw thread supposed to be normal to the helix,
or perp to the axis of the screw?

This brought up enormous debate in my office, and when we talked to a
vendor it caused quite a bit of debate there,too. The vendor was
inclined to take a screw, grind it in half, and look at it under a
microscope to settle the debate (one which they also had), but I never
heard of any results.

All drawings that I can look up (and the drawings in the machinery
handbook) seem to indicate that the thread dims are perp to the axis,
but there is a school of thought that those drawings are a shorthand
from the 2D days where drawing an axial section would have been
incredibly complicated (for instance, there would be a slight cup on
what is represented as a straight edge) so they just represented the
perp-to-helix-profile in the axial section (this includes those
drawings in the machinery handbook)

We calculated that the difference would fall below tolerance for the
part we were doing so we could ignore it, but that does not make our
solution satisfactory. We still stand toe to toe in the office on the
subject, and I was wondering if anyone REALLY knew.

Ed

 

If you look at how they grind taps you see they put the axis of the
grinding wheel at the pitch angle with respect to the axis of the
screw.

Bo

 

Which could produce either form I think.
"Backgrinding" can generate many things.

 

Just thinking out loud here. If I remember correctly, when you manually
chase a thread on a lathe, you set the compound rest at half the included
angle. So that means on a 60° thread, you set it to 30°. You take
successive passes, cutting a bit each pass, and advancing the compound for
each pass. That way you are only cutting on one side of the tool for all
the passes. Then on the last pass, you run the cross slide in just a bit to
clean up both sides of the profile. The tool bit is set to be normal to the
axis of the screw. Does that then satisfy the question?

WT

 

The most common non-destructive way to clearly view a parts thread
form on an optical comparator.

All cut thread tools are made with the thread form being ground (made)
perpendicular to the thread axis exactly as they are specified in
machinery handbook* and other standards* and has a relief ground on
the sides to clear the helix angle.

http://www.mmsonline.com/articles/010302.html

Yes they are, no guessing involved, that's how it's drawn, that's how
the tools are made and how the threaded parts are made.

They are trying to add stuff that is not there. If that's what the
original designers meant then they would have specified/noted it.

Tom

* One variation being topping and non-topping inserts where the
"topping" is done by another tool.

* All standards I have ever seen

 

Specify the form how you like, but if you have a high helix angle, you
can not look at the thread with a comparator to identify the true
shape of a thread without sectioning the thread in whatever plane you
need to verify form.

If you have a high lead ball lead screw, the semi-circular ball groove
thread must be cut perpendicular to the helix or your balls are going
to have narrow line contact and BIG problems.

Normal screws with single pitch and low helix lead angle don't
represent a practical problem for cutting on a lathe, indeed, and I
have been cutting threads for 4 decades as noted with the flat tool
face parallel to the thread axis.

Bo

 

Is the profile of a screw thread supposed to be normal to the helix,

Perpendicular to the axis of the screw.

See machinists screw cutting tool gages which have 60 degree (or other
angle) notches for setting the lathe tool square to the work. Cutting
tools normally have a top rake angled cutting edge - which has a small
affect on the thread form. This is sometimes compensated for in fussy
work by adjusting the tool V angle.

Some thread grinders work with the grinding wheel set to the helix
angle of the thread, but the formed shape of the grinding wheel is
crushed in by a dressing wheel set parallel with the thread.

Accurate measurement by optical projector may require a specially
corrected lens or an adjusted form of comparison template.

briancad

 

Optical Comparator works just fine for both part and tool, nothing
special, with or without thread overlay chart.

Tom

 

And you might well need to tilt the thread at an angle to the axis
to see much of it <g>.

"The true profile of any thread form can only be obtained by viewing parallel
to the thread helix angle. Most Profile machines offer the option of a
mechanism for tilting the thread through the helix angle to give correct
alignment to the optics. This guarentees high accuracy measurement of many
different types of thread form."
http://www.brownandsharpe.com/optical_rpi_operating_principle.asp

Note the tilted insert ...


"Three wire" measurement of threads measures along the
helix angle.

http://www.vankeuren.com/threadmeasure3.htm
"Angle of Thread. This is the angle included between the sides of the thread
measured in an axial plane."

 

Cliff,

As I stated "The most common non-destructive way to clearly view a
parts thread form on an optical comparator."

Tom

 

Consider what you would have to do to make a high lead ballscrew on a
lathe & have the ball bearings nest properly in the helical raceway/
thread properly.

1. Lathe tool's flat face (assuming it wouldn't chatter which it most
certainly would do), must be positioned to give a true radius @ the
helix angle.

2. That means the round lathe tool's flat cutting face with a true
half round radius must be inclined @ the helix angle.

Yes, you could make an elliptical tool form parallel to the shaft
axis, but that represents forming and measurement issues that are too
tough for most shops.

Bo

 

Cliff,

I see your still cyberstalking me because you posted two lines of
G-Code in alt.machines.cnc where I keep pointing out your five
mistakes in those two lines of code.

Unlike you I also know a little about threading since I began my
machining carrier nearly 30 years ago in an aerospace fitting and
fastener threading job shop. I went on to manage that same shop and
eventually owned one as well.

In nearly 30 years of machining I have had ONE valid customer
rejection, it was an order where we used customer supplied gauges.

Unlike you I have actually inspected threads, ground my own threading
inserts and used them so I know a little about how they are made and
work.

As I stated above the tilted insert shown in figure 5 is ground
perpendicular to the thread axis.

No tilted inserts in any of the other 11 pictures/drawings all of
those views are perpendicular to the axis.

Cliff, here bonus questions for you;

http://www.mmsonline.com/articles/010302.html

Do you know why that tilted insert as shown in fig 5 drawing can
only be a single point threading inserts? (Can't be used on multiple
tooth inserts.)

Do you know why that tilted insert as shown in fig 5 drawing is
used for larger diameter (normally course) threads and not on small
diameter ones?

Tom

 

But not how ...

Which hardly answered the question, eh? LOL ....

 

There was no question pending on how to use a comparator, as usual you
are confused.

Tom

 

but Cliff asks "how" - which is quite pertinent.

I own two optical comparators, one has provision for viewing helix
angles. At the risk of going right off topic, I'm interested in Tom's
methodology at screw thread metrology. (Bo's ballscrews are a
different technology.)

briancad