Which hole size for tap?

rg12

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Hey Guys,

I want to send a 3d model to CNC and I need to have an M5 and M8 threads but in the model I don't design the threads but only the holes so I wanted to know what is the diameter I need to make the holes for M8 and M5.
 
rg12 said:
Hey Guys,

I want to send a 3d model to CNC and I need to have an M5 and M8 threads but in the model I don't design the threads but only the holes so I wanted to know what is the diameter I need to make the holes for M8 and M5.
Buy the thread tap kits, which will probably be 5 m/m X .80 m/m and 8 m/m X 1.25 m/m and the correct drills will be included(or at least the size will be on the package).
 
SlowCo said:
4,2mm for M5 and 6,8mm for M8. If the material is stainless steel add 0,1mm for ease of tapping.

I see in the chart posted above that there are different pitch sizes (meaning different hole size before tapping)
Does the sizes you wrote are the most common? meaning if I buy an M5 or M8 screw from eBay that it will screw right in with no pitch matching problems?
 
Normal metric thread bolts are, as motomech wrote, M5x0,8mm and M8x1,25mm. If you need to tap the holes for these standard metric sizes you need to drill 4,2mm and 6,8mm holes indeed.
 
Google is your friend. Lots of tap charts on the internet. The size of the hole also depends on the hardness of the material you are tapping, as well as the thread pitch which determines the minimum diameter of the bolt. The depth of the threads (percentage) desired also matters, if you want to get detailed. The Machinery Handbook is the reference on things like this.

Also - it is best to use HSS taps for cutting new threads, while carbon steel taps are fine for existing thread cleanup. After learning this I changed to HSS taps and there was a marked reduction in tapping torque. HSS seems to be "sharper".
 
The general rule of thumb is to subtract one thread pitch from the diameter. Metric fasteners make it super easy, because both those dimensions are right in the name. So the tap drill for M6x1.00 is 6mm minus 1mm. But it works just as well for inch sized fasteners, e.g. the tap drill for 3/8"-16 is 3/8" minus 1/16", or 5/16".

If you know your drilled hole is likely to be wallowed out, like if you're using a handheld drill, then maybe drop down a few thousandths to the closest smaller size.
 
Chalo said:
The general rule of thumb is to subtract one thread pitch from the diameter. Metric fasteners make it super easy, because both those dimensions are right in the name. So the tap drill for M6x1.00 is 6mm minus 1mm. But it works just as well for inch sized fasteners, e.g. the tap drill for 3/8"-16 is 3/8" minus 1/16", or 5/16".

If you know your drilled hole is likely to be wallowed out, like if you're using a handheld drill, then maybe drop down a few thousandths to the closest smaller size.

Thats a good rule to know but my problem is that I don't know what is the more common pitch for M6.
What is the pitch for M6 that are used on MTB?
 
rg12 said:
Hey Guys,

I want to send a 3d model to CNC and I need to have an M5 and M8 threads but in the model I don't design the threads but only the holes so I wanted to know what is the diameter I need to make the holes for M8 and M5.

Hi rg12.

1. If you are having the part made by a machine shop than you should specify the thread you need and have them drill and tap the threads. They know what hole size to use.

2. There are countless charts on the internet, for example:
http://www.carbidedepot.com/formulas-tap-metric.htm

3. The pitch for an M6 thread is 1mm, and the hole should be 5mm.

Avner.
 
ferret said:
rg12 said:
Hey Guys,

I want to send a 3d model to CNC and I need to have an M5 and M8 threads but in the model I don't design the threads but only the holes so I wanted to know what is the diameter I need to make the holes for M8 and M5.

Hi rg12.

1. If you are having the part made by a machine shop than you should specify the thread you need and have them drill and tap the threads. They know what hole size to use.

2. There are countless charts on the internet, for example:
http://www.carbidedepot.com/formulas-tap-metric.htm

3. The pitch for an M6 thread is 1mm, and the hole should be 5mm.

Avner.

Well, I am sending the 3D file to a CNC shop but I have to have holes so they will know where to make the threads so I rather have the holes the right size.

The most common M6 thread is 1mm? because I noticed in those charts that there are two sizes.
 
It is common practice to send the machine shop a drawing of the part which will have important dimensions marked appropriately even when the part will be manufactured by a CNC program based on a 3D model.

M6 pitch is 1 mm. This is the thread used in mountain bikes (and practically everywhere else).

There is a 6mm fine pitch metric thred (MF6) which has a 0.75mm pitch. It is almost never used.

Avner.
 
ferret said:
It is common practice to send the machine shop a drawing of the part which will have important dimensions marked appropriately even when the part will be manufactured by a CNC program based on a 3D model.

M6 pitch is 1 mm. This is the thread used in mountain bikes (and practically everywhere else).

There is a 6mm fine pitch metric thred (MF6) which has a 0.75mm pitch. It is almost never used.

Avner.

Thanks so much!
 
ferret said:
M6 pitch is 1 mm. This is the thread used in mountain bikes (and practically everywhere else).

There is a 6mm fine pitch metric thred (MF6) which has a 0.75mm pitch. It is almost never used.

Avner is right. I have only ever seen 6mm metric fine thread on a few old cable adjusting barrels and on clamp bolts in old road bike levers. It's so uncommon that my first impression is always that I'm looking at 1/4"-28 UNF thread.

M6x1, on the other hand, is one of the most common thread sizes in the world.
 
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