Machinest Question

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fredrosse
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Machinest Question

Post by fredrosse » Tue Jul 21, 2020 7:30 pm

I need to tap 72 holes, 5/16-18 NC into some mild steel plate, 3/8 inch thick for a large steam engine valve chest.
That is quite time consuming, but someone told me that a typical cordless drill, with the hammer type clutch, could make fast work of this.
Does this work?
Is there a special tap I can get to throw the chips forward since the plate is only 3/8 thick??
Any tricks that would help with this task?
Thanks in advance for actual experience here.
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barts
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Re: Machinest Question

Post by barts » Tue Jul 21, 2020 7:58 pm

This works like a treat.... if you have a spiral point 'gun' tap it helps, and go for tap drill size for 60% thread... and your favorite tapping fluid, of course.

I've tapped 1/2" holes in 1 1/4" aluminum plate this way.

- Bart
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Re: Machinest Question

Post by RGSP » Wed Jul 22, 2020 5:24 pm

Don't try and use hand taps for this: they may fatigue and break off with that many to do. As Bart says, though, with a machine tap, which will probably have spiral flutes, it's a doddle. Getting the tap etc. properly at right angles to the work piece is harder (for some reason) than with a hand tap wrench, and it's worth arranging a jig or some visual guides. I haven't tried it myself, but a friend swears by using a couple of small mirrors so that alignment is easy to check in two planes.
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barts
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Re: Machinest Question

Post by barts » Wed Jul 22, 2020 6:18 pm

This web page has some help with tap nomenclature, which can be confusing.

https://www.natool.com/tap-style-guide/

I've used spiral point taps by hand, in portable drills and in my lathe w/ excellent success on aluminum and mild steel. In the lathe, I use a tap wrench to hold the tap, and if the torque gets excessive I let go :) .The drill of course has built in torque limiting and I start out too low and increase it as needed. Blind holes need special care, and unless the tapped portion is quite short compared to the hole depth, I do these by hand.

As I'm sure you know, this is an area where it's worth buying quality US or Japanese tools.

- Bart
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DetroiTug
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Re: Machinest Question

Post by DetroiTug » Fri Jul 24, 2020 6:04 pm

Fred,

Yes, American made or good quality spiral point two flute gun taps. The two flute taps have a heavier spine and larger openings in between the flutes for chip accumulation. Use Tapmagic cutting oil on every hole.

I would avoid the impact drill, shock breaks taps. Have someone stand along side and make sure the tap is square to the work, hard to see looking straight down.

If it doesn't matter, use an m8-1.25 tap and bolts, same size and a bit finer pitch thread which means less likelihood of tap failure. English NC taps are born looking for a hole to snap off in, 10-24 being the worst of the bunch.

Adjust the chuck where it will slip if the tap binds. Go slow. A little over on the drill size never hurts neither. I think 5/16-18 is .257, .265 is okay. That is what I routinely use for 5/16-18 and m8-1.25.

The helical spiral taps are for running in tapping heads, they work excellent but only in a very controlled operation, which hand tapping with t-wrenches and cordless drills are not.

That is the most machinest information I can provide. :)

-Ron
Last edited by DetroiTug on Fri Jul 24, 2020 6:46 pm, edited 1 time in total.
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Re: Machinest Question

Post by DetroiTug » Fri Jul 24, 2020 6:36 pm

"In the lathe, I use a tap wrench to hold the tap, and if the torque gets excessive I let go''

Yeah, thats a recipe for injury. :D A lathe is very powerful.

Better to use tap T-handles with a center drilled in top of the stock then use a live center in the tailstock to hold, that supports it straight. Typically on small holes I just spin the lathe chuck by hand and catch it up with the tailstock. It allows the operator to feel whats happening.

Large taps I put in the drill chuck in the tailstock. Lowest RPM setting. They slip if they bind (hopefully).

Holes large enough for a boring bar, just thread cut the pitch inside, out a bit and then run the tap in, or just cut internal threads which I wind up doing when I can't justify a special 200 dollar tap on a fifty dollar job. I.e. 1-1/16-16 etc. I have several of those like new, used once, purchased many years ago. Live and learn.

-Ron
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Re: Machinest Question

Post by barts » Fri Jul 24, 2020 7:13 pm

> Better to use tap T-handles with a center drilled in top of the stock then use a live center in the tailstock to hold, that supports it straight. Typically on small holes I just spin the lathe chuck by hand and catch it up with the tailstock. It allows the operator to feel whats happening.

Yup; I support the T handle w/ the live center in the tailstock, push the handle so it only protrudes on one side and hang onto the protruding handle; at low speed there's plenty of time to let go if the torque gets excessive.

For small holes, either spinning the chuck or the t-handle works fine. For larger ones, this gets tedious and using the motor to do the cutting saves some RSI potential.

Thanks, I should have been more explicit.

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Re: Machinest Question

Post by fredrosse » Fri Jul 24, 2020 9:06 pm

Found a couple of "Gun" taps in storage, and made a "Tapped Tapping Guide" to hold the tap at right angles to the work. This is just a piece of 1/2 inch square x 4 inch long oil hardening steel, with a 5/16-18NC female threads, made with good alignment in the milling machine. Clamp the "Tapped Tapping Guide" over the drilled pilot hole, that gives good alignment of the tap, and starts the tap every time without any fiddling. Flooded with Sulfur oil, then power through.

The gun tap throws the chips forward, and everything went very fast once this was setup.


Thanks for all the advice.
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Re: Machinest Question

Post by DetroiTug » Sat Jul 25, 2020 1:12 am

"Yup; I support the T handle w/ the live center in the tailstock, push the handle so it only protrudes on one side and hang onto the protruding handle; at low speed there's plenty of time to let go if the torque gets excessive.''

Oh, I wasn't implying that the tailstock wasn't being used, rather driving the chuck by hand instead of using the headstock power. I just added the tailstock info for those that may not understand what we're discussing.

For large tapping dies, I use a cheater made from 3/4" emt conduit. It's a three-handed operation, but once the spindle is turning slowly, line the cheater up on the carriage and advance the die in to the work. Of course that is after cut a few passes to the pitch with a threading tool.

Fred, I've never heard of using a guide like that, but if it works.. That's sort of a cardinal sin of tapping, going through two plates at once, which generally results in a broken tap. But if that worked out okay, good deal.

Oh, another thing on taps, if they ever squeak or squelch, pitch em'.

-Ron

Bart, heard from Mike lately? I haven't seen him on here for a while.
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Re: Machinest Question

Post by Lopez Mike » Sat Aug 01, 2020 7:28 pm

Just got back from a couple of weeks in the woods of northern Idaho and Montana in my van. Got chased home by intense heat and zillions of idiots with no masks, Sigh.

In Fred's situation I use a tapping guide too. I don't bother threading it though. Just drill/ream it to a close fit on my tap. I've even bought drill bushing to make this easier and since they are hardened, last longer.
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