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What's the Best Tap for my Application?

rogertoolmaker

Cast Iron
Joined
Jan 3, 2016
Roger, you know squat about threading dimensions.

H numbers are .0005 increments, not .001. Higher H numbers are usually used when heat treating and plating, not for final fit.

Pitch diameter of the male threads is the typical determining dimension.

Class 1A, 2A, 3A are the usual fits. 2A is the most common. 1A can be very loose and 3A has a tighter tolerance.
Example:3/4-10 pitch diameter.
1A .6744 to .6831
2A .6773 to .6832
3A .6806 to .6850
Your right, I made a Oops. H1 is .0005; H2 is .001, etc. In .0005 increments.
Thanks Roger
 

natewi

Plastic
Joined
May 2, 2019
Location
Durango CO
That was my first thought, you are gaining nothing by tapping that deep and putting a long screw in. 3/8 deep for an 8-32 is overkill. 1 1/2 times the diameter will give you full strength of the screw, 2 times is more than enough. You are at 4 1/2 times the diameter. Drill 3/4 deep and tap 5/16 - 3/8 deep with a standard spirol point tap.
These are for drawer pulls and i need a range that the screw can go in depending on the clients mounting surface. Basically 1" screws need to mount to 1/2 to 3/4" material.
 

natewi

Plastic
Joined
May 2, 2019
Location
Durango CO
cobalt can make steel more brittle. use cobalt only when the extra hardness is needed in materials like stainless and alloys if you are having breakage.

bottoming taps are in my mind to be used after a modified plug tap, to cut or form just the last few threads when needed.

TiCN, the purple color coating is really better I think. noticeably less torque required with that coating. (Titanium CarboNitride)

spiral flute cut taps lift the chip out of the hole, spiral point pushes them forward, and a chip remaining down in the hole can be stuck in there so that it can't be blown out and cause difficult threading. (but you do say spiral flute)

H3 should be fine, unless you are using shitty screws of course. quality screws are an important part of the equation.

as stated proper alignment is critical.

thread forming taps have more shank as stated, but your part needs to be able to be properly clamped to resist the extra force. in fact the shape and repeatable "clamability" of your parts may be an important factor here as I'd assume they are forged and of irregular nature. obviously if the part rocks or shifts in the setup your gonna have issues...

lube. very very important. surprised none has mentioned this. what "quality cutting fluid" are you using? when threading small holes some "cutting" fluids made for threading pipe by plumbers or other sorts of knuckle dragger juice may not provide enough lubrication as they are optimized for cutting something like a 4" pipe with 5 HP...
I use anchor lube. What's your opinion on that brand?
 

Milland

Diamond
Joined
Jul 6, 2006
Location
Hillsboro, New Hampshire
These are for drawer pulls and i need a range that the screw can go in depending on the clients mounting surface. Basically 1" screws need to mount to 1/2 to 3/4" material.
Keeping a few lengths of stainless 8-32 screws with the right head form in stock is cheaper and faster than trying to make extra-deep tapped holes. Just grab the size that gives you a 1/4" to 3/8" of engagement.
 

natewi

Plastic
Joined
May 2, 2019
Location
Durango CO
Guys, I have done carpentry work and cabinet work. The hardware, like knobs and handles is often supplied with long screws because the doors can easily be 3/4" thick or more. But then, some may be thinner. And if they are used on a sheet metal door, then the thickness may be less than 1/32". And carpenters or cabinet makers do not always have the means to cut a long screw down. AND, supplying two or three screws is an added expense which would make the product more expensive.

So a long screw with a deep hole is one answer to making it fit on whatever door or other thing it may be installed on.

What I find strange here is that no one is asking what STYLE of tap he is using: straight flute, spiral flute, or spiral point. He has a BLIND hole so he should be using spiral flute taps. At least that would be my first thought. I suspect he is using a straight flute because he says it is a bottoming tap. And a straight flute may be pushing some of the chips ahead of the tap. Or it could be a spiral point tap which is designed to push the chips ahead of the tap. Either could/would prevent it from going the full 3/4" into the hole. And if he tries to run it that deep, then it stands a good chance of breaking.

To the OP:

I would suggest a spiral flute tap as a first try.


You can start with a bottoming chamfer spiral flute and see if that works. Cabinet hardware is often made of softer metals and it may be OK.

But if you still have tap breakage you may need to start the hole with a plug chamfer spiral flute tap and then blow it out and finish with the bottoming chamfer one. This can be more efficiently done if a batch is first tapped with the plug chamfer and then switch to the bottoming chamfer.

Another point is that broken taps are often, dare I say most frequently, due to a misalignment when the tap is started. You do not say how the hardware is held in the drill press or if you drill a batch first then tap or drill and then tap on a one by one basis, but it is critical to ensure that the tap is aligned both in the X-Y directions and in the angular sense with the drilled hole. If you try to force it in off center or at an angle it may work for the first turns but the error will increase as you approach the bottom and breakage will become more likely. And at the end is where you say you are breaking them.

I provided a link to McMaster's page on spiral flute taps as a reference, but there are a number of good brands available. Whatever you do, DO buy a quality brand of GROUND thread tap.

A thread forming tap may also be a good choice. I have little experience with them so I will leave that part of the discussion to others.

HSS vs. cobalt? You can look it up but basically the addition of cobalt to HSS makes it harder, tougher, and stronger. And it can work at higher temperatures. Anyway, that is the general idea. Which is better for this may need to be answered by cut and try.
Exactly! Yes I do use spiral flute taps. I think my issue may be the x y axis as my tapping arm is a knockoff chinese one and is 4 years old. Next steps are to measure very carefully and ensure I have a square and level approach to the hole. Thanks!
 

natewi

Plastic
Joined
May 2, 2019
Location
Durango CO
That was my first thought, you are gaining nothing by tapping that deep and putting a long screw in. 3/8 deep for an 8-32 is overkill. 1 1/2 times the diameter will give you full strength of the screw, 2 times is more than enough. You are at 4 1/2 times the diameter. Drill 3/4 deep and tap 5/16 - 3/8 deep with a standard spirol point tap.
Need variance for difference mounting material thickness.
 

natewi

Plastic
Joined
May 2, 2019
Location
Durango CO
Keeping a few lengths of stainless 8-32 screws with the right head form in stock is cheaper and faster than trying to make extra-deep tapped holes. Just grab the size that gives you a 1/4" to 3/8" of engagement.
I sell these to builders and wholesale outlets. I doubt they'd like not having a standardized and versatile product but have to ask every customer what size screws they need and stock all those sizes. Not a bad thought, but not practical in a business sense.
 

PegroProX440

Hot Rolled
Joined
Mar 7, 2012
Location
Ormond Beach
Ive used one of thoes swing arm tapping heads with a clutch before. I could imagine them not working well with small taps. I used 10-32 for my smallest. They need to be relatively square to the work piece and the clutch system needs to be clean. They required quite a bit of maintnence because the tapping fluid gummed up the internals a lot.
 

natewi

Plastic
Joined
May 2, 2019
Location
Durango CO
I haven't used it, so can’t comment there. Howsit going, Have you had better results?
Somewhat. I think the biggest factor was cobalt bits. They are more brittle and prone to breakage. I just ordered several different hss with different coatings. excited to try one specifically that is a "lubricating" finish.
 

natewi

Plastic
Joined
May 2, 2019
Location
Durango CO
Ive used one of thoes swing arm tapping heads with a clutch before. I could imagine them not working well with small taps. I used 10-32 for my smallest. They need to be relatively square to the work piece and the clutch system needs to be clean. They required quite a bit of maintnence because the tapping fluid gummed up the internals a lot.
interesting. I've never cleaned the clutch. Will try this thanks!
 

michiganbuck

Diamond
Joined
Jun 28, 2012
Location
Mt Clemens, Michigan 48035
Bottoming taps have few lead-in threads, drilling a little deeper and using a plug tap can sometimes be an asst.
Plug taps usually push some of the chips forward so extra bottom hole space can be good.

Tap drills can drill smaller as they are shortened by resharpening..

The recommended tap drill for 8-32 is # 29 @ .136"
9/64 and # 28 drill are .140 so .002 on each side larger.
If having no screw strips/pull-outs you might try the .140" drill

Along with Tap Ease, thin oil or soapy water.
 
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rdenney

Plastic
Joined
Aug 10, 2022
Have you tried using a slightly larger tap drill for slightly less engagement?

Look at this document:


For tough materials, it's suggesting 55-65% thread engagement for deep holes (deep being defined as > 1.5 times the diameter, which this surely is), not the 75% nominal engagement usually baked into the standard tables. The #29 drill is sized for about 70% engagement. Try a 3.5mm drill, which corresponds to about 65% engagement. Also, 9/64 is about 58% and 3.6mm is about 54%.

For most machine or structural applications, we'd want a deeper thread engagement to get full strength, but that's a different objective. But we'd live with less thread depth.

A little experimentation should help you find the best balance between strength and the issues you are facing.

Others have mentioned the use of plug and bottoming taps in sequence, and the use of spiral flutes to evacuate chips.

Rick "deep holes in wrought iron" Denney
 
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cyanidekid

Titanium
Joined
Jun 4, 2016
Location
Brooklyn NYC
Somewhat. I think the biggest factor was cobalt bits. They are more brittle and prone to breakage. I just ordered several different hss with different coatings. excited to try one specifically that is a "lubricating" finish.
did you get plug style? the depth of thread difference is only 2 threads or so, but big difference in force, so general rule is don't use bottoming unless you have to. that and 9/64 drills, those can be cheaper too
 

rogertoolmaker

Cast Iron
Joined
Jan 3, 2016
I am drilling and tapping deeper. That's not really the point, it's more that I think there might be an issue with drill size/H factor/tap type or quality that is causing bad threads, which in turn don't allow the screw to go in like it should. Also I've found if I use a #29, the tap will either break or won't tap depending on the clutch setting.
Threads are made to a basic dimension. The fit is obtained with the tap by changing the size of the tap in .0005 increments; "L" below the basic dimension for an interference fit. "H" above the basic dimension for a clearance fit. L1 .0005 below the basic dimension. L2 .001 below etc. If H1 .0005 above the basic dimension. H3 .0015 etc. The strip-limits is controlled by the drilled hole size. Try # 27 or #28 drill. A #26 drill doesn't leave much thread quality. Why are you tapping so deep? Drill the hole the full depth and back-off on the tap depth. I would use an H3 Plug Tap HSS.
Roger
 

rdenney

Plastic
Joined
Aug 10, 2022
Yes, H-factor affects how loosely the bolt fits in the threaded hole, not how tightly the tap fits. In fact, a higher H-factor means a slightly larger thread diameter which will increase thread engagement and the amount of material the tap must cut when starting with the "standard" hole made using a #29 drill. That will make turning the tap more difficult, not easier, resulting in more broken taps.

H-factor for manufactured parts can also address diameter variability in the cheap screws bought at a price point to go with those cabinet hardware kits. A lot of the screws I've seen in such kits still have stamping flash on them. I've seen store-bought cabinet handles with a substantial thread clearance (high H-factor) that were made from pot metal and that stripped the first time Captain Torque put a little wrist in it, particularly if the cabinet door was on the thicker side. That should not be as much of a problem with wrought iron hardware (OP said he was a blacksmith). But with screws bought from a reliable source to a reasonable tolerance, extra thread clearance to account for screw variability shouldn't be needed.

I'm still thinking a standard-fit tap (or a standard-fit tap set used in sequence) and a 9/64 drill might be the solution.

Rick "nicknamed Captain Torque when working as a mechanic--it was not a compliment" Denney
 

rogertoolmaker

Cast Iron
Joined
Jan 3, 2016
Yes, H-factor affects how loosely the bolt fits in the threaded hole, not how tightly the tap fits. In fact, a higher H-factor means a slightly larger thread diameter which will increase thread engagement and the amount of material the tap must cut when starting with the "standard" hole made using a #29 drill. That will make turning the tap more difficult, not easier, resulting in more broken taps.

H-factor for manufactured parts can also address diameter variability in the cheap screws bought at a price point to go with those cabinet hardware kits. A lot of the screws I've seen in such kits still have stamping flash on them. I've seen store-bought cabinet handles with a substantial thread clearance (high H-factor) that were made from pot metal and that stripped the first time Captain Torque put a little wrist in it, particularly if the cabinet door was on the thicker side. That should not be as much of a problem with wrought iron hardware (OP said he was a blacksmith). But with screws bought from a reliable source to a reasonable tolerance, extra thread clearance to account for screw variability shouldn't be needed.

I'm still thinking a standard-fit tap (or a standard-fit tap set used in sequence) and a 9/64 drill might be the solution.

Rick "nicknamed Captain Torque when working as a mechanic--it was not a compliment" Denney
An "H2" fit is considered the "standard fit" for screws. If the workpiece is to be heat-treated or when plated use an "H3" tap to meet thread gage requirements.
Roger
 








 
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