Advice on drilling and tapping frames for optic mounts?

[mmc45414]

This type of machine is a joke compared to a real Bridgeport, but they are fine for drilling and tapping holes.

Yup, and another big advantage is I will not have to scrape my car windows in the winter! Seriously, I got room in the basement for one of the mini mills, I would probably just replace my drill press and put it in the same spot.

That being said, I do have a small assortment of parts I have ruined through the years, but that's minor compared to the satisfaction of doing it myself.

So far I have not completely ruined anything. This was probably the most ambitious home gunsmithing project I took on, and it turned out great. My Trak-Lock install on the 870 wasn't perfect, but it was fine and I used it until I sold it.

If you're going to buy a new mini combo machine, I would trust someplace like Grizzly over Amazon. I've done no research on that, just a gut feeling. But it's always good to get out of the bottom of the barrel.

I have researched it a bit, and it is all Chinese, and the Harbor Freight stuff is not supposed to be that bad. It is one of those things that I to the "what do you get for $350/$700/$1400" level, so I will either figure that against a project (ETA: like finally getting around to converting my SR1911 LWC to a CCO...), or disposable income, or know a good deal if I trip over one.

Thanks for your reply, I was wanting to let to OP know that before he might spend $500 on a drill press there might be $800 alternatives that would be better.

[MickAK]

Thanks for humoring my noobness!

When drilling, I caliper measure the drill bit and the tap to make sure the drill bit is appropriate size, correct?

I don’t need a lot of thread engagement (the other side of the mount is structural, the side I’m tapping just kind of has to fit somewhat) and I thought slightly larger hole would make tapping easier.

Any general advice for drilling a hole for an M4x0.7 that would make tapping easier?
Am I using an M4 or M3.5 drill bit depending on how it caliper measures?

I debated doing trial tap and die runs on the frame with smaller hole and tap, then drilling larger and tapping for additional practice and work upwards but I understand the resistance feel of chip cutting would be different with different diameter and pitches.

Thanks in advance!

Taps have corresponding drill bits for their size. You shouldn't need to caliper measure the drill bit provided you're using the correct bit for that tap. https://drillsandcutters.com/metric-...d-drill-chart/

There are different types of taps for different holes. Since this will (I think) be a blind hole you want a bottom tap. https://www.cnccookbook.com/differen...uide-choosing/

Drill a smaller hole about the diameter of the divot you punched, then go up to the bit size that corresponds to your tap. This will help keep your hole centered. It looks like the mount itself can be used as a jig provided it's clamped properly but you might want a bushing to ensure you're new thread is perfectly centered. In fabrication it's often better to just make the job easier than to use more skill.

Mark your drill bit at the correct depth, or use a stop if you're using a press.

[JCN]

Taps have corresponding drill bits for their size. You shouldn't need to caliper measure the drill bit provided you're using the correct bit for that tap. https://drillsandcutters.com/metric-...d-drill-chart/

There are different types of taps for different holes. Since this will (I think) be a blind hole you want a bottom tap. https://www.cnccookbook.com/differen...uide-choosing/

Drill a smaller hole about the diameter of the divot you punched, then go up to the bit size that corresponds to your tap. This will help keep your hole centered. It looks like the mount itself can be used as a jig provided it's clamped properly but you might want a bushing to ensure you're new thread is perfectly centered. In fabrication it's often better to just make the job easier than to use more skill.

Mark your drill bit at the correct depth, or use a stop if you're using a press.

Thank you! For simplicity I think not a blind hole. The factory holes in the other side go all the way through so I was going to do that on this side too.

[MickAK]

Thank you! For simplicity I think not a blind hole. The factory holes in the other side go all the way through so I was going to do that on this side too.

That makes things easier.

[JCN]

Taps have corresponding drill bits for their size. You shouldn't need to caliper measure the drill bit provided you're using the correct bit for that tap. https://drillsandcutters.com/metric-...d-drill-chart/

One of the drill and tap sets I bought has a 3.3mm drill bit.
One of the drill sets has 10 of the 3.2mm drill bits.
I ordered a few more taps.
I have a vise for the platform. Sharp punches too.

Drill press, cutting oil, compressed air all coming Tuesday.

I was thinking of using the mount in place to locate center to punch a dimple to start.

I might print out some tiny concentric circles on paper to help locate absolute center.

If I can slip the paper under the mount then it could help me really get dead center for the dimple.

[JCN]

Label maker. Clip art CD image 24 pt font.

[MickAK]

Label maker. Clip art CD image 24 pt font.

Good thinking outside of the box. Having the center punch be physically unable to punch the divot anywhere but dead center is preferable but a lot more time consuming.

[JCN]

Good thinking outside of the box. Having the center punch be physically unable to punch the divot anywhere but dead center is preferable but a lot more time consuming.

Hmm. I have extra M4 screws.

I could drill the center out of one, cut off the threads and use it as a jig.

[MickAK]

Hmm. I have extra M4 screws.

I could drill the center out of one, cut off the threads and use it as a jig.

The threads probably don't have an interference fit with the outside of the screw hole in the mount, so that probably wouldn't get you much more accuracy than what you have made up for a lot more effort.

Use the closest diameter to the screw hole center punch that you have and I think you'll be good, just make sure the gap is even all around. Really in a frame mount application a slight off centering and side loading of the screw isn't likely to matter like it might in a slide mount. Way less force involved.

If for some reason you bugger punching the divot you can usually fix it if you've got a good idea where center is, which you do with your label maker idea.

[MistWolf]

Go easy on the center punch. Center punching can create micro fractures which is why center punches are not allowed for use on aircraft structures.

HSS drills are bad for drilling metal. Cobalt is good. Carbide is better than cobalt. Use split tip bits. They center and cut better.

Use cutting fluid liberally.

Don't rush. Make every move a conscious decision.

Drill a little and back off to break the chip. Repeat.

Tap a little and back off to break the chip. Repeat.

Lightly chamfer the holes to deburr.