Tapping blind holes in hot rolled steel

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jmarkwolf

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I need to tap just 5 holes, and I didn't have any 5/16-18 taps on hand, so I bought a plug tap from the local ACE hardware, intending to follow up with a bottoming tap later. Brand Irwin I think. I drilled the holes with a "F" size drill (0.257in dia) as indicated on my drill chart, and chased the holes directly after drilling each one, with a spring loaded tap guide.

I was only able to tap just a few threads into the hole before the tap handle got too tight and I was afraid I'd break the tap if I went any further. I was turning the tap with a forward and backing motion to break the chips etc. Plenty of oil, etc. I'm aware that blind holes take some care, but the holes are approx 1in deep and cutting just a few threads doesn't generate enough chips to pack the hole. I blew them out with compressed air anyway and retried. Even with clean holes, the tap just won't advance more than a few threads.

I did check that the thread count on the tap matches the bolts but it's almost as though the minor diameter of the tap was mis-ground and is too large for the "F" size hole.

I'll investigate further with plug gauges, etc., but am confused what might be at issue here. Is hot rolled steel any more difficult to tap than cold rolled steel?

Any thoughts?
 
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Try using a product like Rapid Tap or TapMagic, also hardware store taps are far from the best, usually cut, not ground threads and made of carbon steel, not HSS.
 
An F-drill is appropriate for aluminum and brass. For steel, a J-drill is the correct tap drill and unless the bolt has to handle high stress, I would go one size larger (K).
 
The performance difference between a hardware store tap and one from McMaster Carr is, I have painfully found, actually worth the price difference.
 
I agree with the above , a good quality tap this should be no effort at all .
 
cheap tools, aka poor quality are rarely worth what you pay for them, whereas more expensive ones aka better quality are often worth more.
 
Hardware store taps will work OK if you give them a chance by drilling a larger hole as mikey wrote. Binding will occur even with quality taps and they will break if the hole is too tight. Too tight being relative to the quality of the tap and the metal being tapped.

Good cutting lube is a must in all scenarios but it does not compensate for too small a hole. Tap drill sizes from charts are a good starting point but if binding occurs using the proper procedure, don't insist, remove the tap and drill out the hole to the next largest size and try again.

This chart is an example of different hole size for different material;
littlemachineshop.com

Tap & Clearance Drill Sizes

This tap and clearance drill chart shows tap and clearance drill sizes for standard threads. There is also a printable version that has both metric and inch threads.
littlemachineshop.com littlemachineshop.com
 
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I would also go with a spiral flute tap rather than a standard bottoming tap. The spiral flute taps force the swarf back up the hole so they don't accumulate in the flutes or migrate to the bottom of the hole.

Old school hand (plug, taper, or bottoming style) taps require the direction to be reversed each turn, or part of a turn to break the chips. Those chips then either fall to the bottom of the hole or are collected in the flutes. The only real difference between an old school plug tap and a bottoming tap is the number of tapered threads on the entrance end.

Also make sure the style tap you're using isn't a tapered point tap, also known as a "Gun". This style tap is designed for through holes. It forces the chips forward through the hole. If you use this style by mistake you'll quickly fill the hole with chips, and possibly break the tap.

Spiral flute taps are available through McMaster, other industrial suppliers, and even on eBay. They do cost more than the standard plug taps, but are well worth the money.

Standard Plug Tap:
Note the square end, deep flutes, and tapered threads at the entrance end of the tap.
www.ebay.com

Various Brands SAE Hand Tap Carbon Steel 1/4" to 1" NC & NF Taper Plug Bottom 2B | eBay

MORE INFORMATION: IDEAL FOR ROUTINE MAINTENANCE, HAND THREADING OR REPAIR WORK AND RETHREADING WHERE HIGH ACCURACY OR THREAD TOLERANCE ARE. THERE IS NO WARRANTY. YOU TRUST DAISY DON'T YOU. MOST HAND TOOLS ARE FULLY COVERED BY MANUFACTURER UNLESS OTHERWISE STATED.
www.ebay.com

Staandard Bottoming Tap:
Note the entrance end of the tap has fewer tapered threads allowing the tap to cut full threads closer to the bottom of the hole.
www.ebay.com

Greenfield Tap 34031 Hand Tap Bottom 1/4 Inx20 Bright for sale online | eBay

Find many great new & used options and get the best deals for Greenfield Tap 34031 Hand Tap Bottom 1/4 Inx20 Bright at the best online prices at eBay! Free shipping for many products!
www.ebay.com

Spiral Flute Bottoming Tap:
Note the "spiral flutes" for evacuation of the chips back up the tap.
www.ebay.com

1/4-20 H3 3 Spiral Flute Bottoming Tap HSS-V TiN Coated for Multi-Purpose F8403 | eBay

1/4-20 H3 3 Flute 45Deg Spiral Flute Modified Bottoming Tap HSS-V With TiN Coating For Multi-Purpose Use 45Deg Helix, Right Hand Spiral, ANSI Shank CNC Style Tap For Multi-Purpose Use YG-1 Part # F8403.
www.ebay.com

Spiral Point Tap:
Note the pointed end, and slightly curved flutes. This style tap is designed to push the chips forward through the hole.

OSG 2880601 Spiral Point Tap,1/4"-20,HSS for sale online

Find many great new & used options and get the best deals for OSG 2880601 Spiral Point Tap,1/4"-20,HSS at the best online prices at eBay! Free shipping for many products!
www.ebay.com

I realize you're tapping 5/16-18 holes. I just used the 1/4-20 taps as an example of the styles available.

As for drill size I have a different opinion than Mikey. I have used F drills for thousands of holes in 1018, and 316 stainless, as well as brass and aluminum. When working with tool steels such as the A,D, and O series I would agree with Mikey's selection of drill size.
 
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OP here:

Excellent discussion everybody.

I was shocked to see the 0.020in difference in suggested tap drill size chart from Little Machine Shop for brass vs steel.

I ordered a couple new taps from McMaster as suggested, designed for blind holes. I will see what the tap feels like with the existing holes. If it's tight, I'll open the holes a little and try again. Unfortunately, it's a fussy setup, that I had taken down to free up my vise, so I'll have to do it again.

Going 0.020in over the recommended tap drill size, however, gives me the heebie jeebies.
 
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