Home Improvement Asked by Maxwell55 on June 28, 2021
I purchased a used jointer. The fence should’ve been held by two M6x20 bolts (according to the manual) to the body of the jointer. When I went to attach the fence, I noticed two wood screws and the holes were filled with some unknown material. After I cleaned out the two holes I noticed they were stripped, that explained the missing M6 screws.
To which size can I re-tap the holes? I think M8 is perhaps too big, so what about SAE? Is there something close to M6 size?
By the way, I’m confused by the tap sizes, especially the SAE, UNC, UNF, etc. Besides that, how do I choose the right drill bit size for the new tap? I’ve spent a whole day trying to figure it out – I’m now frustrated.
Tap size chart and Drill Size Chart are the useful search terms. Most any useful tap size chart will have the drill size listed. A drill size chart will help you sort out the relative sizes of drills in fractional inch, number, letter, and metric sizes. Taps you can get at the hardware store may simplify the process by being packaged with the correct tap drill. Ordering on line from a "real metalwork supplier" (often less expensive, and far more selection) you'll likely have to order them separately.
What will work depends just how stripped out it is.
1/4-28 or 1/4-20 are the "next size up" with 1/4-28 offering a bit more allowance for hosed threads since it's a finer thread (and thus a bigger tap drill - #3 = 5.4102mm) but if the holes are stripped all the way to 6mm that's still a bit precarious. 1/4-20 uses a #7 tap drill = 5.1054mm)
The rather unusual but extant M7 is the first size that will work if the holes are ripped out to 6mm - 7x1 uses a 6mm tap drill, 7x0.75 uses a 6.2mm tap drill.
Next up from that is 5/16-18 (F tap drill = 6.5278mm) or -24 (I tap drill = 6.9088mm)
SAE (society of automotive engineers) is an older standard that has been incorporated into the Unified Thread Standard.
You can also repair the threads to M6x1 by using a Heli-Coil® thread repair kit (or possibly generic knockoffs if their patents have expired) where the hole is drilled out larger, larger threads are made in the larger hole, and a Helical Coil that fits in the new threads on the outside and has M6x1 threads on the inside is inserted in the hole. [I have used this product rather extensively, but am not associated in any other way.]
Answered by Ecnerwal on June 28, 2021
Depending on the construction of your jointer, a potential fix is to adhere a small plate to the table on one side of the stripped out hole, and drill and tap into that plate.
Answered by Tim Sparkles on June 28, 2021
Consider tapping to M7 if M8 is actually too big. If you're in the US you can get M7x1 20mm socket-head cap screws or hex-head screws, as well as a tap, from McMaster.
If you're in a metric country, it will probably be as available and cheaper.
The size of a stripped-out M6 hole (~6mm) is about right for an M7 tap (6.1mm)
Answered by Spehro Pefhany on June 28, 2021
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