Which hole size for tap?

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rg12   10 kW

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Which hole size for tap?

Post by rg12 » Apr 02 2018 11:12am

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.
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teklektik   10 GW

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Re: Which hole size for tap?

Post by teklektik » Apr 02 2018 11:45am

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motomech   1 GW

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Re: Which hole size for tap?

Post by motomech » Apr 02 2018 2:25pm

rg12 wrote:
Apr 02 2018 11:12am
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).
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SlowCo   10 kW

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Re: Which hole size for tap?

Post by SlowCo » Apr 02 2018 3:19pm

4,2mm for M5 and 6,8mm for M8. If the material is stainless steel add 0,1mm for ease of tapping.

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rg12   10 kW

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Re: Which hole size for tap?

Post by rg12 » Apr 03 2018 10:51am

SlowCo wrote:
Apr 02 2018 3:19pm
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?
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Re: Which hole size for tap?

Post by SlowCo » Apr 03 2018 1:04pm

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.

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rg12   10 kW

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Re: Which hole size for tap?

Post by rg12 » Apr 04 2018 6:37am

Thanks alot :)
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Alan B   100 GW

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Re: Which hole size for tap?

Post by Alan B » Apr 04 2018 7:03am

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".

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Re: Which hole size for tap?

Post by rg12 » May 20 2018 1:02pm

Which is a more common hole size for M6, 5.00mm or 5.25mm?
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Chalo   100 GW

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Re: Which hole size for tap?

Post by Chalo » May 20 2018 3:09pm

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.
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Re: Which hole size for tap?

Post by rg12 » May 21 2018 4:04am

Chalo wrote:
May 20 2018 3:09pm
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?
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Re: Which hole size for tap?

Post by ferret » May 21 2018 7:31am

rg12 wrote:
Apr 02 2018 11:12am
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.
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rg12   10 kW

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Re: Which hole size for tap?

Post by rg12 » May 21 2018 8:45am

ferret wrote:
May 21 2018 7:31am
rg12 wrote:
Apr 02 2018 11:12am
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.
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ferret   1 kW

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Re: Which hole size for tap?

Post by ferret » May 21 2018 9:39am

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.
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rg12   10 kW

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Re: Which hole size for tap?

Post by rg12 » May 21 2018 9:41am

ferret wrote:
May 21 2018 9:39am
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!
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Chalo   100 GW

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Re: Which hole size for tap?

Post by Chalo » May 21 2018 12:30pm

ferret wrote:
May 21 2018 9:39am
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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