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What's the Best Tap for my Application?

natewi

Plastic
Joined
May 2, 2019
Location
Durango CO
Hello all,

I am a blacksmith that makes cabinet hardware. I use a pneumatic tapping arm and tap blind holes 3/4" Deep with an 8-32 thread. Material is Mild Steel.
I currently use OSG, B2 H3, 3 spiral flute, bottoming, HSS or cobalt taps. I drill the hole on a drill press with a wire #26 bit prior to tapping. I use lots of quality cutting fluid, blow out the holes after drilling, and have a clutch drive on the tapping arm.

I have 2 main issues I run in to currently:

1. Screw does not want to go in all the way to bottom of hole. I often run in to this issue and am not sure why. Should I use a higher H factor? Would that mean
more room for the screw or less? I've read a lot on H values and pitch etc but I can't seem to wrap my head around whether or not a higher number means a tighter or looser fit. These are cabinet handles so I don't need a super tight fit, I'd rather have more room for error.

2. Taps break too often. Is HSS better for this than cobalt or another material? What do you sacfrifice with each option? What type of coating is best?

At the end of the day I'm looking for suggestions on the best type of tap for my application and why.

I respect you machinists greatly. What a science! I'm a humble blacksmith that prefers to bang on it till it fits lol.

Thanks in advance,

Nate
 

Larry Dickman

Titanium
Joined
Jan 30, 2014
Location
Temecula, Ca
your drill size s/b #29 (.136) For general purpose, I would use an OSG VA-3 tap. H3

Why would your screw go to the bottom of the hole? Your tap has a lead on it, so just tap it deeper
 

michiganbuck

Diamond
Joined
Jun 28, 2012
Location
Mt Clemens, Michigan 48035
I used to count my turns assembling broach bars. 7 to 11 turns are needed to make spec torque ..and not strip out the thread for most threads.
A good chance one should count the turns when tapping so to not bottom out the tap.
When possible I would tap parts in the drill press and then follow with the tap using tap ease, with counting turns I almost never had a broken tap. Having a tap wrench with a but end center and a center point on the drill chuck made a straight tapping.
 

natewi

Plastic
Joined
May 2, 2019
Location
Durango CO
your drill size s/b #29 (.136) For general purpose, I would use an OSG VA-3 tap. H3

Why would your screw go to the bottom of the hole? Your tap has a lead on it, so just tap it deeper
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.
 

natewi

Plastic
Joined
May 2, 2019
Location
Durango CO
I used to count my turns assembling broach bars. 7 to 11 turns are needed to make spec torque ..and not strip out the thread for most threads.
A good chance one should count the turns when tapping so to not bottom out the tap.
When possible I would tap parts in the drill press and then follow with the tap using tap ease, with counting turns I almost never had a broken tap. Having a tap wrench with a but end center and a center point on the drill chuck made a straight tapping.
I appreciate that, but this is a production setting. I need to stay efficient.
 

natewi

Plastic
Joined
May 2, 2019
Location
Durango CO
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. What does VA-3 stand for?
 

Nmbmxer

Hot Rolled
Joined
Jun 22, 2008
Location
VA
Larger H numbers are for materials that contract after cutting like plastics, or more commonly for finishes that add material (plating or hard anodizing.) Won’t change anything about breaking taps.
 

FamilyTradition

Aluminum
Joined
Feb 24, 2018
Location
Greenfield, Mass
1. Screw does not want to go in all the way to bottom of hole. I often run in to this issue and am not sure why. Should I use a higher H factor? Would that mean
more room for the screw or less? I've read a lot on H values and pitch etc but I can't seem to wrap my head around whether or not a higher number means a tighter or looser fit. These are cabinet handles so I don't need a super tight fit, I'd rather have more room for error.

Remember that even on a "bottoming" tap, there is 2 to 3 threads that are chamfered. Your screw will not go the same distance as the tap went because the last few threads way down in the hole are incomplete.

You could modify a tap to remove the chamfer and chase the threads to get those last few threads, but this is not ideal and adds labor to your product. You would not have any luck tapping a fresh hole with the modified tap, either.

If your hardware is not going together properly, the best thing would be to simply drill deeper and then tap deeper, that is assuming the thickness of your stock allows for it.

Bigger "H" number means the diameter of the tap is getting larger than the basic dimension. Your threads will have a looser fit the larger your H number is.

2. Taps break too often. Is HSS better for this than cobalt or another material? What do you sacfrifice with each option? What type of coating is best?
HSS is the most commonly used material for taps because it is the best option in most applications. Carbide is usually too brittle and can't spring like the steel. I don't know enough about cobalt to compare to HSS. There are also different grades of HSS to choose from.

How often is "too often"? You are bound to have breakage, especially with smaller taps. If you are looking for more life out of your taps, try a different brand or style. If you are simply trying to avoid scrapping due to broken taps, take note of typical tap life and change out before you think it will break.

As for coatings, TiN (titanium nitride, not the tin in "tin can") is cheap and generally better than uncoated.

I have had really good luck with taps that have the "steam oxide" surface treatment. Not technically a "coating" if I'm not mistaken , but a chemical process that makes the surface super slick.

As previously mentioned, you could try forming taps as well. You will have to drill your hole with a bigger drill because forming taps push the material into the shape of the thread, rather than removing material. They tend to be stronger because they don't have flutes, but very small grooves for oil to get in, resulting in a much larger core. This process requires more torque than a cutting tap, but at that small a size I can't see it being a problem.

Look up "Tru-Lede" forming taps from Widia if you want to try forming the threads. Made in the USA if that happens to be a factor in your choices. I believe you can get the Widia taps from MSC and McMaster-Carr.
 

Conrad Hoffman

Titanium
Joined
May 10, 2009
Location
Canandaigua, NY, USA
That seems almost twice the depth needed in steel. Aside from the annoyance of deep tapping, with that much engagement you have more possibility of binding due to accumulated pitch error. I'd try the form tap with a very good EP oil. If you have to cut, I'd think any quality spiral point tap would get the job done, again with a decent oil.
 

RJT

Titanium
Joined
Aug 24, 2006
Location
greensboro,northcarolina
Why are you tapping an 8-32 hole 3/4" deep...?
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.
 
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EPAIII

Diamond
Joined
Nov 23, 2003
Location
Beaumont, TX, USA
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.



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.
 

rogertoolmaker

Cast Iron
Joined
Jan 3, 2016
Male threads (screws) are made to a basic dimension which is about halfway between the root and the crest of the screw. The fit is obtained with the tap. Taps are made and measured in .0005 increments. Above the basic dimension 'H' and 'L' for below the basic dimension. H2 would be .002 above the basic dimension, H3 is .003 above the basic dimension for a clearance fit. If L1 would be .001 below the basic dimension for an interference fit.
Trying to thread all the way to bottom of a hole is a good way to break taps. Normal practice is to tap one and half times as deep as the diameter of the tap. This allows for the chamfer at the top of the hole and the lead of the tap. A spiral or plug tap pushed the chips in front of the tap to keep the chips out of the flutes of the tap to prevent breakage. Tapping to the bottom of a hole is asking for problems.
Roger
 

cyanidekid

Titanium
Joined
Jun 4, 2016
Location
Brooklyn NYC
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...
 
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atex57

Hot Rolled
Joined
Sep 6, 2006
Location
SW Wisconsin
Male threads (screws) are made to a basic dimension which is about halfway between the root and the crest of the screw. The fit is obtained with the tap. Taps are made and measured in .0005 increments. Above the basic dimension 'H' and 'L' for below the basic dimension. H2 would be .002 above the basic dimension, H3 is .003 above the basic dimension for a clearance fit. If L1 would be .001 below the basic dimension for an interference fit.
Trying to thread all the way to bottom of a hole is a good way to break taps. Normal practice is to tap one and half times as deep as the diameter of the tap. This allows for the chamfer at the top of the hole and the lead of the tap. A spiral or plug tap pushed the chips in front of the tap to keep the chips out of the flutes of the tap to prevent breakage. Tapping to the bottom of a hole is asking for problems.
Roger


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
 








 
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