wearing taps too fast, looking for 7H and 8H specs, thanks all

[Amear]

Would anyone be kind enough as to tell me the min and max dia for a M20x2.5 7H and 8H minor diameter. Tons of trouble looking for this information. Below is all Ive been able to come up with.
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Internal Metric Thread- M Profile Limiting Dimensions, ANSI/AMSE B1.13M-1983 (R1995)
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Basic thread minor diameters D1
designation min max
M20x2.5 6H 17.294 17.744
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Material is too hard and is wearing out our taps too fast, Im looking to use a larger drill that will hopefully meet the 7H or 8H specs, which I have yet to find. All thanks appreciated.
Alex
London ON

 

[unclesyd]

Take a look at this website.

http://www.mdmetric.com/fastindx/inxtst.htm

 

[Amear]

What I've listed in the initial post is for 6H tolerances. That is what we are curently using. But were going through a 50$ tap every part and a half(8 holes per part-600 parts to finish still). Since the material is really hard we are looking to use a larger drill and then when tapping there is not as much force on the tap.
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If anone can find a 7H or 8H spec thanks in advance.
M20 x 2.5 minor diameter.
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I took a look at unclesyd's link that was posted and didn't see anything useable. But thanks for your help. All appreciated.

 

[diamondjim]

I think the url did show that you could
use a larger diameter drill for these
part with the greater 7H and 8H taps.
I do think the drills might have to be
a special order, but certainly cheaper
than the taps. How hard is the material?
greater than 300 BHN? Hard is relative.
What size drill are you using now and
also how deep are you tapping? ie are
you tapping thru or using a bottom tap
in a blind hole?

 

[NickE]

You might also wnat to consider using carbide taps.

 

[strokersix]

Or resharpen the taps. This potentially could be used to alter the chip load per edge on the taps as well for improved life.

 

[unclesyd]

You could give these people a call for the information you require and they can help you select a tap that will improve your hole making.

http://www.newsontool.com/05_a05.html

 

[BillPSU]

What is this material?

Is this a through hole application or what? What is the length of thread? What are you presently using, spiral point, spiral flute, bottom tap, plug tap, standard tap, heavy duty? What RPM are you running? Are you running coolant or cutting oil? Coated or uncoated tap?

If it is a through hole have you tried a rollforming tap?

 

[Amear]

The material used is a Cast Alloy Steel similar to Catapilar Alloy used on costruction equipment. The holes have a depth of 1.38 min to 1.62 max and are full thread depth. We use coolant as well as standard taps. There are 4 holes to be tapped each side. Thanks for all the help guys

 

[diamondjim]

How much clearance does you tap drill
allow beyond the minimum depth? Is this
a one way conversation?

 

[BillPSU]

I would try a spiral flute heavy duty tap. I was searching for a supplier and found the link below.

http://www.cutting-tool-supply.com/Greenfield/Taps/HighPerformance/EM-NI/SpiralFlute/Home.htm

I do not want to recommend anybody but to illustrate the availability of such a tool. Your present tap supplier probably can supply you with such a tool. You didn't tell me the speed you are running but I would probably reduce the RPM from your current setting if the tool is burning up. If you are getting chipping on the cutting edges I would make sure the holes are free from chips before tapping. You may want to check the coolant concentration.

 

[Amear]

Mistake earlier, we use Spiral flute heavy duty taps. Just going to stick with it I guess. Thanks for the tips