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jswiontek (Mechanical) (OP)
19 Aug 02 10:32
I am trying to tap a 3/8" mild steel plate for a M10x1.5 thread but after the tapping process the screw is has too much play. We are using a .336 in. hole and a M10-1.5xD5 3 flute spiral tap. I am new to metric threads so am not familiar with the tolerance classes.  Could someone please suggest an appropriate process, drill, and tap?  
Paulcet (Industrial)
19 Aug 02 10:48
8.5mm is the suggested drill size.  Is the screw 60 degree thread?
MadMango (Mechanical)
19 Aug 02 10:55
I think your pre-tap size is too large, giving you that extra play.  Try a .332 pre-tap hole diameter.  The 3 flute spiral tap should work fine.

You didn't mention if this was being hand tapped, or done on a drill press or CNC machine.  If hand tapping, switch to a more stable process with a fixtured drill press, or CNC.

"The attempt and not the deed confounds us."

jswiontek (Mechanical) (OP)
19 Aug 02 11:23
The thread is a 60 degree thread.  We are currently using a floating head for our manual drill press while clamping the part to a table.  I will try a smaller pre-tap hole and will let you know how it turns out.  Thanks.
jswiontek (Mechanical) (OP)
21 Aug 02 8:06
We used the smaller pre-tapped hole but saw no improvement in the fit of the screw. The thread holds the appropriate torque when assembled but I am still concerned about the looseness of the fit over time.  Does anyone have any other suggestions?
MadMango (Mechanical)
21 Aug 02 10:51
Metric hardware is usually rated at class 6H/6G and has a looser fit when standard inch class 2A/2B threads are concerned.  I think you'll have to specify higher tolerance on your metric sizes if you want a tighter feel (on the order of 4G/6G).

Below are some links that might help.

http://www.boltscience.com/pages/screw8.htm
http://www.newsontool.com/01_16-17.html

"The attempt and not the deed confounds us."

scottmaha (Mechanical)
21 Aug 02 14:54
there may be some more variables at play here. are you actually measuring the hole after drilling to see if the drills are drilling the hole to the specified size? if not, you may want to check this.

are you drilling the hole using an alignment bushing? are you spot drilling the hole prior to drilling? if not, you could have a condition where the hole could be oversized for a bit at the top of the hole. since you are drilling 3/8" thick plate, the drill will not have enough material to "straighten" itself out. basically, your hole may be egg-shaped.

what type of tapping fluid, if any, are you using? sometimes when tapping low carbon steel, the material will gall or stick to the tap causing the threads to be oversized. you can fix this by using a tapping fluid with higher amounts of sulfer to help with the galling.

i would strongly suggest that you by a go-nogo gage to properly check the threads. standard metric threads are 6H/6G.

the 8.5mm drill you were using in the beginning is the proper drill size and i would not deviate from using this size. just double-check the hole size after drilling to see if is correct. we produce these threads every day with no size problems. we use a 8.5mm, solid carbide, through coolant guhring drill. we use YMW taps and our threads have an effective depth of 20mm(3/4").

hope this helps a little.
Vonmagnuson (Mechanical)
6 Sep 02 12:05
I agree with scottmaha.  Get your self a Go/NoGo and with a hole this size you could use a pin gage to look at the shape of the hole.  You really need to use a guide bushing.  Check the play in the spindle of your drill press.   Also use a center drill.  

Make sure your tap is sharp.I assume that you are using a "tapmatic" type tapper.  Make sure it is not wabling all over the place.  Make sure your clutch is set properly.


You are either "wallering" the hole during the drilling process or cutting too much material in the tapping process due to some other variable.

Reducing the tap drill size will only buy you so much.


 
 

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