Monitor Steam Engine

[mklotz]

When I get a new die I thread a piece of nominal size brass with the die in fully open and fully tight positions.  I drop this in the pill vial where resides the corresponding tap and the threaded nut made with it.  In addition to providing sizing hints, the male threaded pieces are handy to use as gages on unknown threads.

The white plastic tops on the vials can be written on with Sharpies.  Use one color for metric and another for inferial.  I put a big purple 'LH' on the left-handed ones.  Stand the vials upright in a wooden box from the craft store and you can store an amazing amount of threading tackle in a very small space.

[zeeprogrammer]

Worthwhile idea Don and Frazer. Thanks.

Frazer..."drill the right size tapping hole"...well that's the question. I should have done a deeper dive on investigation. I had conflicting information.
One reference was giving one size while another reference was giving another.
The first reference was for 5/16-32 and specified 9/32. It didn't go to 5/16-40.
But I have a 5/16-40 and the 2nd reference specified an 'L' drill.

I just noticed the 1st reference specified an 'L' drill but for steel. The 9/32 was for aluminum...which is what the steam chest is.
The 2nd reference didn't make a difference between aluminum and steel.

'L' is slight bigger.

I'll try opening the die first and see if that gets me there.
Otherwise...I have to go to 3/8-40...which luckily I have.

Thanks guys. That's a big help.

My frustration level has been building.

Enjoy your eating out Don.

And the two bobs may be priceless.

Just saw your post Marv. Yes. A bit more testing would have helped.
But isn't that what I'm doing. 

[mklotz]

The generalized formula for 60 degree threads is:

TD = MD - 0.013*DOT/P

where:

TD = tap drill size
MD = major diameter of thread
DOT = desired depth of thread expressed as a percentage
P = thread pitch expressed as TPI

TD = 0.3125 - 0.013*75/40 = 0.288

which implies an 'L' drill [damned inferial nomenclature]

We've been over this before.  You should have it in your notebook.  It's worth doing this calculation for every possibly critical thread, if even only as a check on whoever wrote the suggested size.

[crueby]

Marv, what are the rules of thumb for the DOT percentage? I've got some tables that use different values for brass and steel, is there a set of good numbers to use for the metals we commonly use in the models? From what I've seen the harder ones use a smaller value. Have broken a few taps when using the brass value on stainless.

[zeeprogrammer]

Well 'L' is what I used.
That would imply the die was too tight.
But I'm not convinced.

Question: As I mentioned, one reference for 5/16-32 was 9/32. But the reference for 5/16-40 was 'L' (a slightly larger drill).

Why would a 32 thread be different from a 40 thread for the tapping drill?

In the one reference...9/32 was specified for aluminum and 'L' for steel. The 2nd reference didn't distinguish...it just said 'L'.

I'm thinking I should have used 9/32...but I don't know if that is/was the problem.

[EDIT] I would think the tables provide the information the calculation would do. As in this case...it seems too. That is, 'L'. If that's correct, and is the one I used, then what went wrong?

[b.lindsey]

Zee, it sounds to me like the die was cutting on the small side. Assuming it is adjustable, screw the adjusting screw in to open the die more resulting in a slightly larger thread OD. It is common for finer threads to specify a larger tap drill at least as far as the tables I have seen/used.

Bill

[Pete49]

Zee you're going well both with the build and the general banter which makes for a good read and a giggle.
Marv thanks for the tip. Now to get some vials and capitalise on it.
Pete

[Don1966]

Well 'L' is what I used.
That would imply the die was too tight.
But I'm not convinced.

Question: As I mentioned, one reference for 5/16-32 was 9/32. But the reference for 5/16-40 was 'L' (a slightly larger drill).

Why would a 32 thread be different from a 40 thread for the tapping drill?

In the one reference...9/32 was specified for aluminum and 'L' for steel. The 2nd reference didn't distinguish...it just said 'L'.

I'm thinking I should have used 9/32...but I don't know if that is/was the problem.

[EDIT] I would think the tables provide the information the calculation would do. As in this case...it seems too. That is, 'L'. If that's correct, and is the one I used, then what went wrong?

Zee use this spread sheet if you haven't already. It gives you the tapping drill size for 75% engagement for each different thread values you enter. Also in the sheet you can find the minor diameter for nuts at any thread engagement percentage you enter. Make sure you get the Rev1 sheet......http://www.modelenginemaker.com/index.php/topic,4655.0.html

Don

[philjoe5]

Don,
That's a good reference especially for the uncommon thread pitches like 1/4" - 40 etc.

Zee, I just picked up this thread and will need to find the time to pick it up from the start.  Ray HasBrouck had a fine example of this engine and when he was alive, exhibited it at the shows.  It was very impressive and, of course, Ray's model was beautifully done.

Good luck with your build.  Threading, internal or external, had a steep learning curve for me, like learning how to properly use boring bars.  Given enough practice, it all seems so simple now.  Keep at it

Cheers,
Phil

[PStechPaul]

An easy way to calculate a nominal tap drill size is simply to use MD (Major Diameter) - P (Pitch), so for 5/16-40 it would be

0.3125 - 0.025 = 0.2875

This is between 9/32" (0.2812") and "L" (0.2900")

The actual root diameter for sharp edged 60 degree threads would be MD - 2 * sin(60) * P. That would be:

0.3125 - 0.0433 = 0.2692

More information:
http://guhring.com/documents/Tech/Formulas/Tap.pdf

The actual diameter formed by a drill can vary from -0.001" to +0.006" for the tap drill size for this thread. So it's probably a good idea to drill a bit smaller and then try tapping. If excessive torque is needed, then use a slightly larger drill, or even allow the drill to remain in the hole a bit longer. However, this can also cause heating and work hardening. When I tapped 1/4"-20 blind holes in a piece of stainless steel for my wobbler motor project, I had to increase the drill size from the specified #7 (0.2010") to #5 (0.2055") and even then it was tough going. Granted, I was using cheap taps that were also probably dull, but when I went back later with a new high quality spiral flute tap, it was also quite difficult.

http://engineersedge.com/manufacturing/drill-mechanical-tolerances.htm

[zeeprogrammer]

Thanks everyone for the support.

As Marv mentioned, his formula indicates an 'L' bit to be used. Which I did.
Unfortunately I don't have a 5/16-40 with which to test and see if the the problem is in the steam chest or the die used to make the gland.

Bill...same lines I'm thinking. I'll try opening the die.

Don...I took a look at your spreadsheet. Thanks for that. To be honest, I'm not sure how to use it. But in playing around with it I noticed some formatting problems. You may want to reformat the cells for numbers with whatever decimal place you want. For example, when I entered 5/16 in (I think it was cell D9) it formatted it as a date. However, it doesn't look like all the cells have this problem. (And it may be some setting I have that's changing things.)

Now I'm getting pretty worried about the tapping I did on the cylinder cover. 

Hope I can get to this soon. A lot of life is going to get in the way for the next couple of months. 

[mklotz]

Here are the suggested DOT percentages used in my DRILL program, which has an option to determine tap drill sizes for ordinary and forming taps.


 

Code: [Select]

MATERIAL                                          % DOT

MILD AND UNTREATED STEELS                         60-65
HIGH CARBON STEEL                                  50
HIGH SPEED STEEL                                  55
STAINLESS STEEL                                  50
FREE CUTTING STAINLESS STEEL                      60
CAST IRON                                        70-75
WROUGHT ALUMINUM                                  65
CAST ALUMINUM                                    75
WROUGHT COPPER                                    60
FREE CUTTING YELLOW BRASS            70
DRAWN BRASS                                        65
MANGANESE BRONZE                                  55
MONEL METAL                                      55-60
NICKEL SILVER (GERMAN SILVER)                     50-60

The lazy man's formula

TD = MD - 1/P

makes the approximation that

0.013*DOT = 1

or

DOT = 1/0.013 = 77 %

which isn't that bad for many materials but way too (unnecessarily) tight for tough materials.

[Doc]

I usually shoot for 60% to 70% but that also depends on material and initial  thread size.
Marv I like your little chart and it looks to be a good starting point!
   As far as die adjustment they don't adjust much they are meant to go from one class of threads to another like a 2A to a 3A and vis-versa which is only like a thou and half or so.

[mklotz]

Question: As I mentioned, one reference for 5/16-32 was 9/32. But the reference for 5/16-40 was 'L' (a slightly larger drill).

Why would a 32 thread be different from a 40 thread for the tapping drill?

5/16 - .013*75/32 = 0.282 which is much closer to 9/32 than it is to 'L'

Take the time to do the calculations whenever you do a critical threading job.

OTOH, how critical is a gland?  You're going to stuff it full of graphited thread anyway to provide sealing so a limited amount of thread sloppiness may be tolerable.

[mklotz]

Marv, what are the rules of thumb for the DOT percentage? I've got some tables that use different values for brass and steel, is there a set of good numbers to use for the metals we commonly use in the models? From what I've seen the harder ones use a smaller value. Have broken a few taps when using the brass value on stainless.

As you can see from my reply above, percentages typically vary between 50 and 75 %.

I always use 50 % for stainless.  Even with a value that low, thread strength is not much compromised and tapping is much easier.  Besides, thread strength is seldom an issue in modelmaking.